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The Spitzer Survey of Stellar Structure in Galaxies ( S4 G )
The Spitzer Survey of Stellar Structure in Galaxies ( 4 G )is an Exploration Science Legacy Program approved for the Spitzerpost-cryogenic mission. It is a volume-, magnitude-, and size-limited (d < 40 Mpc , | b | > 30 ° , mBcorr < 15.5 , andD25 > 1') survey of 2331 galaxies using the Infrared ArrayCamera (IRAC) at 3.6 and 4.5 ?m. Each galaxy is observed for 240 sand mapped to ? 1.5 × D25 . The final mosaickedimages have a typical 1? rms noise level of 0.0072 and 0.0093 MJysr-1 at 3.6 and 4.5 ?m, respectively. Our azimuthallyaveraged surface brightness profile typically traces isophotes at?3.6 ?m ( AB ) ( 1 ? ) ˜ 27 magarcsec-2 , equivalent to a stellar mass surface density of˜ 1 M? pc-2. S4 G thusprovides an unprecedented data set for the study of the distribution ofmass and stellar structures in the local universe. This large, unbiased,and extremely deep sample of all Hubble types from dwarfs to spirals toellipticals will allow for detailed structural studies, not only as afunction of stellar mass, but also as a function of the localenvironment. The data from this survey will serve as a vital testbed forcosmological simulations predicting the stellar mass properties ofpresent-day galaxies. This article introduces the survey and describesthe sample selection, the significance of the 3.6 and 4.5 ?m bandsfor this study, and the data collection and survey strategies. Wedescribe the S4 G data analysis pipeline and presentmeasurements for a first set of galaxies, observed in both the cryogenicand warm mission phases of Spitzer. For every galaxy we tabulate thegalaxy diameter, position angle, axial ratio, inclination at?3.6 ?m ( AB ) = 25.5 , and 26.5 magarcsec-2 (equivalent to ? ?B ( AB ) = 27.2and 28.2 mag arcsec-2 , respectively). These measurementswill form the initial S4 G catalog of galaxy properties. Wealso measure the total magnitude and the azimuthally averaged radialprofiles of ellipticity, position angle, surface brightness, and color.Finally, using the galaxy-fitting code GALFIT, we deconstruct eachgalaxy into its main constituent stellar components: the bulge/spheroid,disk, bar, and nuclear point source, where necessary. Together, thesedata products will provide a comprehensive and definitive catalog ofstellar structures, mass, and properties of galaxies in the nearbyuniverse and will enable a variety of scientific investigations, some ofwhich are highlighted in this introductory S4 G survey paper.
| The Calibration of Monochromatic Far-Infrared Star Formation Rate Indicators
Spitzer data at 24, 70, and 160 ?m and ground-based H? imagesare analyzed for a sample of 189 nearby star-forming and starburstgalaxies to investigate whether reliable star formation rate (SFR)indicators can be defined using the monochromatic infrared dust emissioncentered at 70 and 160 ?m. We compare recently published recipes forSFR measures using combinations of the 24 ?m and observed H?luminosities with those using 24 ?m luminosity alone. From thesecomparisons, we derive a reference SFR indicator for use in ouranalysis. Linear correlations between SFR and the 70 ?m and 160 ?mluminosity are found for L(70) >~ 1.4 × 1042 ergs-1 and L(160) >~ 2 × 1042 ergs-1, corresponding to SFR >~ 0.1-0.3 Msun yr-1, and calibrations of SFRs based onL(70) and L(160) are proposed. Below those two luminosity limits, therelation between SFR and 70 ?m (160 ?m) luminosity is nonlinearand SFR calibrations become problematic. A more important limitation isthe dispersion of the data around the mean trend, which increases forincreasing wavelength. The scatter of the 70 ?m (160 ?m) dataaround the mean is about 25% (factor ~2) larger than the scatter of the24 ?m data. We interpret this increasing dispersion as an effect ofthe increasing contribution to the infrared emission of dust heated bystellar populations not associated with the current star formation.Thus, the 70 (160) ?m luminosity can be reliably used to trace SFRsin large galaxy samples, but will be of limited utility for individualobjects, with the exception of infrared-dominated galaxies. Thenonlinear relation between SFR and the 70 and 160 ?m emission atfaint galaxy luminosities suggests a variety of mechanisms affecting theinfrared emission for decreasing luminosity, such as increasingtransparency of the interstellar medium, decreasing effective dusttemperature, and decreasing filling factor of star-forming regionsacross the galaxy. In all cases, the calibrations hold for galaxies withoxygen abundance higher than roughly 12 +log(O/H) ~ 8.1. At lowermetallicity, the infrared luminosity no longer reliably traces the SFRbecause galaxies are less dusty and more transparent.Based on observations obtained with the Spitzer Space Telescope, whichis operated by JPL, CalTech, under NASA Contract 1407.
| A Warm Molecular Hydrogen Tail due to Ram-pressure Stripping of a Cluster Galaxy
We have discovered a remarkable warm (130-160 K) molecular hydrogen tailwith a H2 mass of approximately 4 × 107 Msun extending 20 kpc from a cluster spiral galaxy, ESO137-001, in Abell 3627. At least half of this gas is lost permanently tothe intracluster medium, as the tail extends beyond the tidal radius ofthe galaxy. We also detect a hot (400-550 K) component in the tail thatis approximately 1% of the mass. The large H2 line to IRcontinuum luminosity ratio in the tail indicates that star formation isnot a major excitation source and that the gas is possibly shock-heated.This discovery confirms that the galaxy is currently undergoingram-pressure stripping, as also indicated by its previously discoveredX-ray and H? tails. We estimate that the galaxy is losing its warmH2 gas at a rate of ~2-3 M sunyr–1. The true mass-loss rate is likely higher if weaccount for cold molecular gas and atomic gas. We predict that thegalaxy will lose most of its gas in a single pass through the core andplace a strong upper limit on the ram-pressure timescale of 1 Gyr. Wealso study the star-forming properties of the galaxy and its tail. Weidentify most of the previously discovered external H? sourceswithin the tail in our 8 ?m data but not in our 3.6 ?m data; IRSspectroscopy of the region containing these H? sources alsoreveals aromatic features typically associated with star formation. Fromthe positions of these H II regions, it appears that star formation isnot occurring throughout the molecular hydrogen tail but onlyimmediately downstream of the galaxy. Some of these H II regions lieoutside the tidal radius of the galaxy, indicating that ram-pressurestripping can be a source of intracluster stars.
| Optical Spectroscopy and Nebular Oxygen Abundances of the Spitzer/SINGS Galaxies
We present intermediate-resolution optical spectrophotometry of 65galaxies obtained in support of the Spitzer Infrared Nearby GalaxiesSurvey (SINGS). For each galaxy we obtain a nuclear, circumnuclear, andsemi-integrated optical spectrum designed to coincide spatially withmid- and far-infrared spectroscopy from the Spitzer Space Telescope. Wemake the reduced, spectrophotometrically calibrated one-dimensionalspectra, as well as measurements of the fluxes and equivalent widths ofthe strong nebular emission lines, publically available. We use opticalemission-line ratios measured on all three spatial scales to classifythe sample into star-forming, active galactic nuclei (AGNs), andgalaxies with a mixture of star formation and nuclear activity. We findthat the relative fraction of the sample classified as star formingversus AGN is a strong function of the integrated light enclosed by thespectroscopic aperture. We supplement our observations with a largedatabase of nebular emission-line measurements of individual H IIregions in the SINGS galaxies culled from the literature. We use theseancillary data to conduct a detailed analysis of the radial abundancegradients and average H II-region abundances of a large fraction of thesample. We combine these results with our new integrated spectra toestimate the central and characteristic (globally averaged) gas-phaseoxygen abundances of all 75 SINGS galaxies. We conclude with an in-depthdiscussion of the absolute uncertainty in the nebular oxygen abundancescale.
| Mid-infrared Galaxy Morphology from the Spitzer Survey of Stellar Structure in Galaxies (S4G): The Imprint of the De Vaucouleurs Revised Hubble-Sandage Classification System at 3.6 ?m
Spitzer Space Telescope Infrared Array Camera imaging provides anopportunity to study all known morphological types of galaxies in themid-IR at a depth significantly better than ground-based near-infraredand optical images. The goal of this study is to examine the imprint ofthe de Vaucouleurs classification volume in the 3.6 ?m band, which isthe best Spitzer waveband for galactic stellar mass morphology owing toits depth and its reddening-free sensitivity mainly to older stars. Forthis purpose, we have prepared classification images for 207 galaxiesfrom the Spitzer archive, most of which are formally part of the SpitzerSurvey of Stellar Structure in Galaxies (S4G), a Spitzerpost-cryogenic ("warm") mission Exploration Science Legacy Programsurvey of 2331 galaxies closer than 40 Mpc. For the purposes ofmorphology, the galaxies are interpreted as if the images are bluelight, the historical waveband for classical galaxy classificationstudies. We find that 3.6 ?m classifications are well correlated withblue-light classifications, to the point where the essential features ofmany galaxies look very similar in the two very different wavelengthregimes. Drastic differences are found only for the most dusty galaxies.Consistent with a previous study by Eskridge et al., the main differencebetween blue-light and mid-IR types is an ?1 stage intervaldifference for S0/a to Sbc or Sc galaxies, which tend to appear"earlier" in type at 3.6 ?m due to the slightly increased prominenceof the bulge, the reduced effects of extinction, and the reduced (butnot completely eliminated) effect of the extreme population I stellarcomponent. We present an atlas of all of the 207 galaxies analyzed hereand bring attention to special features or galaxy types, such as nuclearrings, pseudobulges, flocculent spiral galaxies, I0 galaxies,double-stage and double-variety galaxies, and outer rings, that areparticularly distinctive in the mid-IR.
| Spitzer 70 ?m Emission as a Star Formation Rate Indicator for Sub-galactic Regions
We use Spitzer 24 ?m, 70 ?m and ground-based H? data for asample of 40 SINGS galaxies to establish a star formation rate (SFR)indicator using 70 ?m emission for sub-galactic (~0.05-2 kpc)line-emitting regions and to investigate limits in application. A linearcorrelation between 70 ?m and SFR is found and a star formationindicator SFR(70) is proposed for line-emitting sub-galactic regions as?(SFR)(Msun yr-1 kpc-2 ) =9.4 × 10-44 ?(70)(erg s-1kpc-2), for regions with 12 + log(O/H) >~ 8.4 and?(SFR) >~ 10-3(M sunyr-1 kpc-2), with a 1? dispersionaround the calibration of ~0.16 dex. We also discuss the influence ofmetallicity on the scatter of the data. Comparing with the SFR indicatorat 70 ?m for integrated light from galaxies, we find that there is~40% excess 70 ?m emission in galaxies, which can be attributed tostellar populations not involved in the current star formation activity.
| Ultraviolet+Infrared Star Formation Rates: Hickson Compact Groups with Swift and Spitzer
We present Swift UVOT ultraviolet (UV; 1600-3000 Å) data withcomplete three-band UV photometry for a sample of 41 galaxies in 11nearby (<4500 km s–1) Hickson Compact Groups (HCGs)of galaxies. We use UVOT uvw2-band (2000 Å) photometry to estimatethe dust-unobscured component, SFRUV, of the total starformation rate, SFRTOTAL. We use Spitzer MIPS 24 ?mphotometry to estimate SFRIR, the component ofSFRTOTAL that suffers dust extinction in the UV and isre-emitted in the IR. By combining the two components, we obtainSFRTOTAL estimates for all HCG galaxies. We obtain totalstellar mass, M *, estimates by means of Two Micron All SkySurvey Ks -band luminosities, and use them to calculatespecific star formation rates, SSFR ? SFRTOTAL/M*. SSFR values show a clear and significant bimodality, witha gap between low (lsim3.2 × 10–11yr–1) and high-SSFR (gsim1.2 ×10–10 yr–1) systems. We compare thisbimodality to the previously discovered bimodality in?IRAC, the MIR activity index from a power-law fit tothe Spitzer IRAC 4.5-8 ?m data for these galaxies. We find that allgalaxies with ?IRAC <= 0 ( >0) are in the high-(low-) SSFR locus, as expected if high levels of star-forming activitypower MIR emission from polycyclic aromatic hydrocarbon molecules and ahot dust continuum. Consistent with this finding, all elliptical/S0galaxies are in the low-SSFR locus, while 22 out of 24spirals/irregulars are in the high-SSFR locus, with two borderlinecases. We further divide our sample into three subsamples (I, II, andIII) according to decreasing H I richness of the parent galaxy group towhich a galaxy belongs. Consistent with the SSFR and?IRAC bimodality, 12 out of 15 type I (11 out of 12type III) galaxies are in the high- (low-) SSFR locus, while type IIgalaxies span almost the full range of SSFR values. We use the SpitzerInfrared Nearby Galaxy Survey (SINGS) to construct a comparisonsubsample of galaxies that (1) match HCG galaxies in J-band total galaxyluminosity and (2) are not strongly interacting and largely isolated.This selection eliminates mostly low-luminosity dwarfs and galaxies withsome degree of peculiarity, providing a substantially improved,quiescent control sample. Unlike HCG galaxies, galaxies in thecomparison SINGS subsample are continuously distributed both in SSFR and?IRAC, although they show ranges in SFRTOTALvalues, morphologies and stellar masses similar to those for HCGsystems. We test the SSFR bimodality against a number of uncertainties,and find that these can only lead to its further enhancement. Excludinggalaxies belonging to HCGs with three giant galaxies (triplets) leavesboth the SSFR and the ?IRAC bimodality completelyunaffected. We interpret these results as further evidence that anenvironment characterized by high galaxy number densities and low galaxyvelocity dispersions, such as the one found in compact groups, plays akey role in accelerating galaxy evolution by enhancing star formationprocesses in galaxies and favoring a fast transition to quiescence.
| The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. II. Warm Molecular Gas and Star Formation in Three Field Spiral Galaxies
We present the results of large-area 12CO J = 3-2 emissionmapping of three nearby field galaxies, NGC 628, NGC 3521, and NGC 3627,completed at the James Clerk Maxwell Telescope as part of the NearbyGalaxies Legacy Survey. These galaxies all have moderate to strong12CO J = 3-2 detections over large areas of the fieldsobserved by the survey, showing resolved structure and dynamics in theirwarm/dense molecular gas disks. All three galaxies were part of theSpitzer Infrared Nearby Galaxies Survey sample, and as such haveexcellent published multiwavelength ancillary data. These data setsallow us to examine the star formation properties, gas content, anddynamics of these galaxies on sub-kiloparsec scales. We find that theglobal gas depletion time for dense/warm molecular gas in these galaxiesis consistent with other results for nearby spiral galaxies, indicatingthis may be independent of galaxy properties such as structures, gascompositions, and environments. Similar to the results from The H INearby Galaxy Survey, we do not see a correlation of the star formationefficiency with the gas surface density consistent with theSchmidt-Kennicutt law. Finally, we find that the star formationefficiency of the dense molecular gas traced by 12CO J = 3-2is potentially flat or slightly declining as a function of molecular gasdensity, the 12CO J = 3-2/J = 1-0 ratio (in contrast to thecorrelation found in a previous study into the starburst galaxy M83),and the fraction of total gas in molecular form.
| An observational estimate for the mean secular evolution rate in spiral galaxies
We have observationally quantified the effect of gravitational torqueson stars in disc galaxies due to the stellar distribution itself andexplored whether these torques are efficient at transporting angularmomentum within a Hubble time. We derive instantaneous torque maps for asample of 24 spiral galaxies, based on stellar mass maps that werederived using the pixel-by-pixel mass-to-light estimator by Zibetti, Rixand Charlot. In conjunction with an estimate of the rotation velocity,the mass maps allow us to determine the torque-induced instantaneousangular momentum flow across different radii, resulting from the overallstellar distributions for each galaxy in the sample. By stacking thesample, which effectively replaces a time average by an ensembleaverage, we find that the torques due to the stellar disc act totransport angular momentum outwards over much of the disc (within threedisc scalelengths). The strength of the ensemble-averaged gravitationaltorques within one disc scalelength has a time-scale of 4 Gyr forangular momentum redistribution.The individual torque profiles show that only a third of our sampleexhibit torques strong enough to redistribute angular momentum within aHubble time, mostly those with strong bars. However, advective angularmomentum transport is another source of angular momentum redistribution,especially in the presence of long-lived spiral arms, but is notaccessible to direct observations. The torque-driven angular momentumredistribution is thus observed to be effective, either in one-third ofdisc galaxies or in most disc galaxies one third of the time and shouldlead to either changes in the mass density profile or the orbitalshapes.We use a set of self-consistent disc, bulge and halo simulated isolateddisc galaxies with realistic cold gas fractions to verify that thetorques exerted by the stellar distribution, such as spiral arms or abar, exceed those of the gas and halo, as assumed in the analysis of theobservations.This study is the first to observationally determine the strength oftorque-driven angular momentum flow of stars for a sample of spiralgalaxies, providing an important empirical constraint on secularevolution.
| On the nature of star formation at large galactic radii
We have compared far-ultraviolet (FUV), near-ultraviolet (NUV) andH? measurements for star-forming regions in 21 galaxies, in orderto characterize the properties of their discs at radii beyond the mainoptical radius (R25).In our representative sample of extended and non-extended ultraviolet(UV) discs, we find that half of the extended UV discs also exhibitextended H? emission. We find that extended UV discs fall into twocategories: those with a sharp truncation in the H? disc close tothe optical edge (R25), and those with extended emission inH? as well as in the UV. Although most galaxies with strongH? truncations near R25 show a significantcorresponding falloff in UV emission (a factor of 10-100), thetransition tends to be much smoother than in H?, and significantUV emission often extends well beyond this radius, confirming earlierresults by Thilker et al. and others.After correcting for dust attenuation the median fraction of total FUVemission from regions outside of R25 is 1.7 per cent, but itcan be as high as 35 per cent in the most extreme cases. Thecorresponding fractions of H? emission are approximately half aslarge on average. This difference reflects both a slightly lower ratioof H? to UV emission in the HII regions in the outer discs and alower fraction of star clusters showing HII regions. Most HII regions inthe extended disc have fluxes consistent with small numbers of ionizingO-type stars, and this poor sampling of the upper initial mass function(IMF) in small clusters can probably account for the differences in theemission properties, consistent with earlier conclusions by Zaritsky& Christlein, without needing to invoke a significant change in thestellar IMF itself. Consistent H?/FUV ratios and brightest HIIregion to total H? fluxes in the inner and extended discs acrossour whole galaxy sample demonstrate no evidence for a change in thecluster luminosity function or the IMF in the low gas density outerdisc.
| Unobscured Type 2 Active Galactic Nuclei
Type 2 active galactic nuclei (AGNs) with intrinsically weak broademission lines (BELs) would be exceptions to the unified model. Afterexamining a number of proposed candidates critically, we find that thesample is contaminated significantly by objects with BELs of strengthsindicating that they actually contain intermediate-type AGNs, plus a fewCompton-thick sources as revealed by extremely low ratios of X-ray tonuclear IR luminosities. We develop quantitative metrics that show two(NGC 3147 and NGC 4594) of the remaining candidates to have BELs 2-3orders of magnitude weaker than those of typical type 1 AGNs. Severalmore galaxies remain as candidates to have anomalously weak BELs, butthis status cannot be confirmed with the existing information. Althoughthe parent sample is poorly defined, the two confirmed objects are wellunder 1% of its total number of members, showing that the absence of aBEL is possible, but very uncommon in AGN. We evaluate these two objectsin detail using multi-wavelength measurements including new IR dataobtained with Spitzer and ground-based optical spectropolarimetericobservations. They have little X-ray extinction with N H <~1021 cm-2. Their IR spectra show strongsilicate emission (NGC 4594) or weak aromatic features on a generallypower-law continuum with a suggestion of silicates in emission (NGC3147). No polarized BEL is detected in NGC 3147. These results indicatethat the two unobscured type 2 objects have circumnuclear tori that areapproximately face-on. Combined with their X-ray and optical/UVproperties, this behavior implies that we have an unobscured view of thenuclei and thus that they have intrinsically weak BELs. We compare theirproperties with those of the other less-extreme candidates. We thencompare the distributions of bolometric luminosities and accretion ratesof these objects with theoretical models that predict weak BELs.
| X-ray and multiwavelength view of NGC 4278. A LINER-Seyfert connection?
Context. The emission mechanism responsible for the bulk of energy fromradio to X-rays in low ionization emission line regions (LINERs) and lowluminosity active galactic nuclei (LLAGN) has been long debated. Basedon UV to X-ray and radio to UV flux ratios, some argue that LINERs/LLAGNare a scaled-down version of their more luminous predecessors Seyfertgalaxies. Others, based on the lack of X-ray short (hours) time-scalevariability, the non detection of an iron line at 6.4 keV, and the faintUV emission compared to typical AGNs, suggest the truncation of theclassical thin accretion disk in the inner regions of the AGN where aradiatively inefficient accretion flow (RIAF) structure forms. Aims: We investigate the LINER-Seyfert connection by studying theunabsorbed LINER galaxy NGC 4278 that accretes at a low rate(Lbol/Edd ~ 7 × 10-6) but exhibits a broadH? line, and a point-like nucleus in radio, optical, UV andX-rays. Methods: We analyzed one XMM-Newton and seven ChandraX-ray observations of NGC 4278 spread over a three year period, allowingthe study of the X-ray variability at different time-scales (hours,months, years). We also examined the radio to X-ray spectral energydistribution to constrain the accretion mode in the nucleus of NGC 4278. Results: Long time-scale (months) variability is observed wherethe flux increased by a factor of ~3 on a time-scale of a few months andby a factor of 5 between the faintest and the brightest observationseparated by ~3 years. During the XMM-Newton observation, where thehighest flux level is detected, we found a 10% flux increase on a shorttime-scale of a few hours, while the light curves for the differentChandra observations do not show short time-scale (minutes to hours)variability. A combination of an absorbed power law (NH ?1020 cm-2, ? =2.2+0.1-0.2) plus a thermal component (kT ? 0.6keV) were able to fit the Chandra spectra. The XMM-Newton spectra, wherethe highest X-ray flux is detected, are well fitted with an absorbedpower-law with no need for a thermal component as the emission from thepower-law component is dominant. The power-law photon index is˜2.1 and the hydrogen column density is of the order of1020 cm-2. Neither a narrow nor a broad FeK? emission line at 6.4 keV are detected with a 22 eV and 118 eVupper limits derived on their equivalent widths. We derive opticalfluxes from archival HST ACS observations and detected opticalvariability on time-scales of years. For the first time for this source,thanks to the optical/UV monitor on board XMM-Newton, we obtainedsimultaneous UV and X-ray flux measurements. We constructed SEDs basedon simultaneous or quasi simultaneous observations and compared them toLINER, radio-loud, and radio-quiet quasar SEDs. We find that at a lowX-ray flux the NGC 4278 SED resembles that of typical LINER sourceswhere the radio to X-ray emission can be considered as originating froma jet and/or RIAF, whereas at a high X-ray flux, NGC 4278 SED is morelike a low luminosity Seyfert SED. Consequently, NGC 4278 could exhibitboth LINER and Seyfert nuclear activity depending on the strength of itsX-ray emission.
| Is BL Lacertae an ``orphan'' AGN?. Multiband and spectroscopic constraints on the parent population
Aims: We have analysed optical spectra of BL Lacertae, theprototype of its blazar subclass, to verify the broad H? emissionline detected more than a decade ago and its possible flux variation. Weused the spectroscopic information to investigate the question of the BLLacertae parent population. Methods: Low- and high-resolutionoptical spectra of BL Lacertae were acquired with the DOLORESspectrograph at the 3.58 m telescopio nazionale Galileo (TNG) duringfour nights in 2007-2008, when the source was in a relatively faintstate. In three cases we were able to fit the complex H? spectralrange with multiple line components and to measure both the broadH? and several narrow emission line fluxes. Results: Acritical comparison with previous results suggests that the broadH? flux has increased by about 50% in ten years. This might be dueto an addition of gas in the broad line region (BLR), or to astrengthening of the disc luminosity, but such flux changes are notunusual in Broad Lined active nuclei. We estimated the BL Lacertae blackhole mass by means of its relation with the bulge luminosity, finding4-6 × 108 M?. The virial mass estimatedfrom the spectroscopic data gives instead a value 20-30 times lower. Ananalysis of the disc and BLR properties in different AGNs suggests thatthis discrepancy is due to an underluminosity of the BL Lacertae BLR.Finally, we addressed the problem of the BL Lacertae parent population,comparing its isotropic quantities with those of other AGN classes. Fromthe point of view of the narrow emission line spectrum, the source islocated close to low-excitation radio galaxies. When one also considersits diffuse radio power, an association with FR I radio galaxies isseverely questioned due to the lower radio luminosity (at a given lineluminosity) of BL Lacertae. The narrow line and radio luminosities of BLLacertae instead match those of a sample of miniature radio galaxies,which however do not show a BLR. Yet, if existing, “misaligned BLLacertae” objects should have entered that sample. We also ruleout the possibility that they were excluded because of a QSO opticalappearance. Conclusions: The observational constraints suggestthat BL Lacertae is caught in a short term transient stage, which doesnot leave a detectable evolutionary “trace” in the AGNpopulation. We present a scenario that can account for the observedproperties.Based on observations made with the Italian Telescopio Nazionale Galileooperated on the island of La Palma by the Centro Galileo Galilei of INAF(Istituto Nazionale di Astrofisica) at the Spanish Observatorio delRoque del los Muchachos of the Instituto de Astrofisica de Canarias.
| Internal kinematics of spiral galaxies in distant clusters. IV. Gas kinematics of spiral galaxies in intermediate redshift clusters and in the field
Aims: We trace the interaction processes of galaxies atintermediate redshift by measuring the irregularity of their ionized gaskinematics, and investigate these irregularities as a function of theenvironment (cluster versus field) and of morphological type (spiralversus irregular). Methods: We obtain the gas velocity fields byplacing three parallel and adjacent VLT/FORS2 slits on each galaxy. Toquantify irregularities in the gas kinematics, we use three indicators:the standard deviation of the kinematic position angle(?PA), the mean deviation of the line of sight velocityprofile from the cosine form which is measured using high order Fourierterms (k3,5/k1) and the average misalignmentbetween the kinematical and photometric major axes (??).These indicators are then examined together with some photometric andstructural parameters (measured from HST and FORS2 images in theoptical) such as the disk scale length, rest-frame colors, asymmetry,concentration, Gini coefficient and M20. Our sample consistsof 92 distant galaxies. 16 cluster (z ~ 0.3 and z ~ 0.5) and 29 fieldgalaxies (0.10 ? z ? 0.91, mean z = 0.44) of these have velocityfields with sufficient signal to be analyzed. To compare our sample withthe local universe, we also analyze a sample from the SINGS survey. Results: We find that the fraction of galaxies that have irregulargas kinematics is remarkably similar in galaxy clusters and in the fieldat intermediate redshifts (according to ?PA ? 10%,k3,5/k1 ? 30%, ?? ? 70%). Thedistribution of the field and cluster galaxies in (ir)regularityparameters space is also similar. On the other hand galaxies with smallcentral concentration of light, that we see in the field sample, areabsent in the cluster sample. We find that field galaxies atintermediate redshifts have more irregular velocity fields as well asmore clumpy and less centrally concentrated light distributions thantheir local counterparts. Comparison with a SINS sample of 11 z ~ 2galaxies shows that these distant galaxies have more irregular gaskinematics than our intermediate redshift cluster and field sample. Wedo not find a dependence of the irregularities in gas kinematics onmorphological type. We find that two different indicators of starformation correlate with irregularity in the gas kinematics. Conclusions: More irregular gas kinematics, also more clumpy and lesscentrally concentrated light distributions of spiral field galaxies atintermediate redshifts in comparison to their local counterpartsindicate that these galaxies are probably still in the process ofbuilding their disks via mechanisms such as accretion and mergers. Onthe other hand, they have less irregular gas kinematics compared togalaxies at z ~ 2.Based on observations collected at the European Southern Observatory(ESO), Cerro Paranal, Chile (ESO Nos. 74.B-0592 & 75.B-0187) andobservations of the Hubble Space Telescope (HST No 10635).Appendices areonly available in electronic form at http://www.aanda.org
| Long-term Profile Variability in Active Galactic Nucleus with Double-peaked Balmer Emission Lines
An increasing number of active galactic nuclei (AGNs) exhibit broad,double-peaked Balmer emission lines, which represent some of the bestevidence for the existence of relatively large-scale accretion disks inAGNs. A set of 20 double-peaked emitters have been monitored for nearlya decade in order to observe long-term variations in the profiles of thedouble-peaked Balmer lines. Variations generally occur on timescales ofyears, and are attributed to physical changes in the accretion disk.Here we characterize the variability of a subset of seven double-peakedemitters in a model independent way. We find that variability is causedprimarily by the presence of one or more discrete "lumps" of excessemission; over a timescale of a year (and sometimes less) these lumpschange in amplitude and shape, but the projected velocity of these lumpschanges over much longer timescales (several years). We also find thatall of the objects exhibit red peaks that are stronger than the bluepeak at some epochs and/or blueshifts in the overall profile, contraryto the expectations for a simple, circular accretion disk model, thusemphasizing the need for asymmetries in the accretion disk. Comparisonswith two simple models, an elliptical accretion disk and a circular diskwith a spiral arm, are unable to reproduce all aspects of the observedvariability, although both account for some of the observed behaviors.Three of the seven objects have robust estimates of the black holemasses. For these objects the observed variability timescale isconsistent with the expected precession timescale for a spiral arm, butincompatible with that of an elliptical accretion disk. We suggest thatwith the simple modification of allowing the spiral arm to befragmented, many of the observed variability patterns could bereproduced.
| FIR colours and SEDs of nearby galaxies observed with Herschel
We present infrared colours (in the 25-500 ?m spectral range) and UVto radio continuum spectral energy distributions of a sample of 51nearby galaxies observed with SPIRE on Herschel. The observed sampleincludes all morphological classes, from quiescent ellipticals to activestarbursts. Active galaxies have warmer colour temperatures than normalspirals. In ellipticals hosting a radio galaxy, the far-infrared (FIR)emission is dominated by the synchrotron nuclear emission. The colourtemperature of the cold dust is higher in quiescent E-S0a than instar-forming systems probably because of the different nature of theirdust heating sources (evolved stellar populations, X-ray, fastelectrons) and dust grain properties. In contrast to the colourtemperature of the warm dust, the f350/f500 index sensitive to the colddust decreases with star formation and increases with metallicity,suggesting an overabundance of cold dust or an emissivity parameter? < 2 in low metallicity, active systems.Herschel is an ESA space observatory with science instruments providedby Principal Investigator consortia. It is open for proposals forobserving time from the worldwide astronomical community.
| Total Infrared Luminosity Estimation of Resolved and Unresolved Galaxies
The total infrared (TIR) luminosity from galaxies can be used to examineboth star formation and dust physics. We provide here new relations toestimate the TIR luminosity from various Spitzer bands, in particularfrom the 8 ?m and 24 ?m bands. To do so, we use data for 45''subregions within a subsample of nearby face-on spiral galaxies from theSpitzer Infrared Nearby Galaxies Survey (SINGS) that have known oxygenabundances as well as integrated galaxy data from the SINGS, the LocalVolume Legacy survey (LVL), and Engelbracht et al. samples. Taking intoaccount the oxygen abundances of the subregions, the star formation rateintensity, and the relative emission of the polycyclic aromatichydrocarbons at 8 ?m, the warm dust at 24 ?m, and the cold dust at70 ?m and 160 ?m, we derive new relations to estimate the TIRluminosity from just one or two of the Spitzer bands. We also show thatthe metallicity and the star formation intensity must be taken intoaccount when estimating the TIR luminosity from two wave bands,especially when data longward of 24 ?m are not available.
| The Herschel Virgo Cluster Survey . II. Truncated dust disks in H I-deficient spirals
By combining Herschel-SPIRE observations obtained as part of theHerschel Virgo Cluster Survey with 21 cm Hi data from the literature, weinvestigate the role of the cluster environment on the dust content ofVirgo spiral galaxies. We show for the first time that the extent of thedust disk is significantly reduced in Hi-deficient galaxies, followingremarkably well the observed “truncation” of the Hi disk.The ratio of the submillimetre-to-optical diameter correlates with theHi-deficiency, suggesting that the cluster environment is able to stripdust as well as gas. These results provide important insights not onlyinto the evolution of cluster galaxies but also into the metalenrichment of the intra-cluster medium.Herschel is an ESA space observatory with science instruments providedby European-led Principal Investigator consortia and with importantparticipation from NASA.
| Infrared Spectral Energy Distributions of Seyfert Galaxies: Spitzer Space Telescope Observations of the 12 μm Sample of Active Galaxies
The mid-infrared spectral energy distributions (SEDs) of 83 activegalaxies, mostly Seyfert galaxies, selected from the extended 12 μmsample are presented. The data were collected using all threeinstruments, Infrared Array Camera (IRAC), Infrared Spectrograph (IRS),and Multiband Imaging Photometer for Spitzer (MIPS), aboard the SpitzerSpace Telescope. The IRS data were obtained in spectral mapping mode,and the photometric data from IRAC and IRS were extracted from matched,20'' diameter circular apertures. The MIPS data were obtained in SEDmode, providing very low-resolution spectroscopy (R ~ 20) between ~55and 90 μm in a larger, 20'' × 30'' synthetic aperture. Wefurther present the data from a spectral decomposition of the SEDs,including equivalent widths and fluxes of key emission lines; silicate10 μm and 18 μm emission and absorption strengths; IRACmagnitudes; and mid-far-infrared spectral indices. Finally, we examinethe SEDs averaged within optical classifications of activity. We findthat the infrared SEDs of Seyfert 1s and Seyfert 2s with hidden broadline regions (HBLRs, as revealed by spectropolarimetry or othertechnique) are qualitatively similar, except that Seyfert 1s showsilicate emission and HBLR Seyfert 2s show silicate absorption. Theinfrared SEDs of other classes within the 12 μm sample, includingSeyfert 1.8-1.9, non-HBLR Seyfert 2 (not yet shown to hide a type 1nucleus), LINER, and H II galaxies, appear to be dominated by starformation, as evidenced by blue IRAC colors, strong polycyclic aromatichydrocarbon emission, and strong far-infrared continuum emission,measured relative to mid-infrared continuum emission.
| Spectral Energy Distributions of Weak Active Galactic Nuclei Associated with Low-Ionization Nuclear Emission Regions
We present a compilation of spectral energy distributions (SEDs) of 35weak active galactic nuclei (AGNs) in low-ionization nuclear emissionregions (LINERs) using recent data from the published literature. Wemake use of previously published compilations of data, aftercomplementing and extending them with more recent data. The mainimprovement in the recent data is afforded by high-spatial-resolutionobservations with the Chandra X-Ray Observatory andhigh-spatial-resolution radio observations utilizing a number offacilities. In addition, a considerable number of objects have beenobserved with the Hubble Space Telescope in the near-IR through near-UVbands since the earlier compilations were published. The data includeupper limits resulting from either non-detections or observations at lowspatial resolution that do not isolate the AGN. For the sake ofcompleteness, we also compute and present a number of quantities fromthe data, such as optical-to-X-ray spectral indices(αox), bolometric corrections, bolometric luminosities,Eddington ratios, and the average SED. We anticipate that these datawill be useful for a number of applications. In a companion paper, weuse a subset of these data ourselves to assess the energy budgets ofLINERs.
| An Assessment of the Energy Budgets of Low-Ionization Nuclear Emission Regions
Using the spectral energy distributions (SEDs) of the weak activegalactic nuclei (AGNs) in 35 low-ionization nuclear emission regions(LINERs) presented in a companion paper, we assess whetherphotoionization by the weak AGN can power the emission-line luminositiesmeasured through the large (few-arcsecond) apertures used inground-based spectroscopic surveys. Spectra taken through such aperturesare used to define LINERs as a class and constrain non-stellarphotoionization models for LINERs. Therefore, our energy budget test isa self-consistency check of the idea that the observed emission linesare powered by an AGN. We determine the ionizing luminosities and photonrates by integrating the observed SEDs and by scaling a template SED. Wefind that even if all ionizing photons are absorbed by the line-emittinggas, more than half of the LINERs in this sample suffer from a deficitof ionizing photons. In 1/3 of LINERs the deficit is severe. If only 10%of the ionizing photons are absorbed by the gas, there is an ionizingphoton deficit in 85% of LINERs. We disfavor the possibility thatadditional electromagnetic power, either obscured or emitted in theunobservable far-UV band, is available from the AGN. Therefore, weconsider other power sources such as mechanical heating by compact jetsfrom the AGN and photoionization by either young or old stars.Photoionization by young stars may be important in a small fraction ofcases. Mechanical heating can provide enough power in most cases but itis not clear how this power would be transferred to the emission-linegas. Photoionization by post asymptotic giant branch stars is animportant power source; it provides more ionizing photons than the AGNin more than half of the LINERs and enough ionizing photons to power theemission lines in 1/3 of the LINERs. It appears likely that theemission-line spectra of LINERs obtained from the ground include the sumof emission from different regions where different power sourcesdominate.
| Infrared Diagnostics for the Extended 12 μm Sample of Seyferts
We present an analysis of Spitzer IRS spectroscopy of 83 active galaxiesfrom the extended 12 μm sample. We find rank correlations betweenseveral tracers of star formation which suggest that (1) the polycyclicaromatic hydrocarbon feature is a reliable tracer of star formation, (2)there is a significant contribution to the heating of the cool dust bystars, and (3) the H2 emission is also primarily excited bystar formation. The 55-90 versus 20-30 spectral index plot is also adiagnostic of the relative contribution of starburst to active galacticnuclei (AGNs). We see there is a large change in spectral index acrossthe sample: Δα ~ 3 for both indices. Thus, the contributionto the IR spectrum from the AGN and starburst components can becomparable in magnitude but the relative contribution also varies widelyacross the sample. We find rank correlations between several AGNtracers. We find correlations of the ratios [O III]λ5007/[O IV]26 μm and [O III]λ5007/[Ne V] 14 μm with the silicatestrength which we adopt as an orientation indicator. This suggests thatsome of the [O III]λ5007 emission in these Seyferts is subject toorientation dependent obscuration as found by Haas et al. for radiogalaxies and quasars. There is no correlation of [Ne V] equivalent widthwith the silicate 10 μm strength, indicating that the [Ne V] emissionis not strongly orientation dependent. This suggests that the obscuringmaterial (e.g., torus) is not very optically thick at 14 μmconsistent with the results of Buchanan et al. We search forcorrelations between AGN and starburst tracers and we conclude that theAGN and starburst tracers are not correlated. This is consistent withour conclusion that the relative strength of the AGN and starburstcomponents varies widely across the sample. Thus, there is no simplelink between AGN fueling and black hole growth and star formation inthese galaxies. The density diagnostic [Ne V] 14/24 μm and [S III]18/33 μm line ratios are consistent with the gas being near the lowdensity limit, i.e., ~103 cm–3 for [Ne V]and ne ~ few hundred cm–3 for [S III]. Thedistribution of silicate 10 μm and 18 μm strengths is consistentwith the clumpy torus models of Sirocky et al. We find a rankcorrelation between the [Ne V] 14 μm line and the 6.7 μm continuumwhich may be due to an extended component of hot dust. The Sy 2's with ahidden broad-line region (HBLR) have a higher ratio of AGN-to-starburstcontribution to the spectral energy distribution than Sy 2's without anHBLR. This may contribute to the detection of the HBLR in polarizedlight. The Sy 2's with an HBLR are more similar to the Sy 1's than theyare to the Sy 2's without an HBLR.
| Molecular gas in NUclei of GAlaxies (NUGA) XIII. The interacting Seyfert 2/LINER galaxy NGC 5953
We present 12CO(1-0) and 12CO(2-1) maps of theinteracting Seyfert 2/LINER galaxy NGC 5953 obtained with the IRAMinterferometer at resolutions of 2.1 arcsec × 1.4 arcsec and 1.1arcsec × 0.7 arcsec, respectively. We also present single-dishIRAM 30 m observations of the central region of NGC 5953 for the12CO(1-0), 12CO(2-1), and HCN(1-0) transitions atresolutions of 22´´, 12´´, and 29'',respectively. The CO emission is distributed over a disk of diameter~16´´(~2.2 kpc), within which are several, randomlydistributed peaks. The strongest peak does not coincide with thenucleus, but is instead offset from the center, ~2-3´´(~340pc) toward the west/southwest. The kinematics of the molecular componentare quite regular, as is typical of a rotating disk. We also comparedthe 12CO distribution of NGC 5953 with observations at otherwavelengths in order to study correlations between different tracers ofthe interstellar medium. The HST/F606W WFPC2 images show flocculentspiral structures and an “S-shape” feature ?60 pc inradius, possibly associated with a nuclear bar or with the radio jet. Atwo-dimensional bulge/disk decomposition of the H-band (HST/F160W) and3.6 ?m (Spitzer/IRAC) images reveals a circumnuclear“ring” ~10-14´´ in diameter, roughly coincidentin size with the CO disk and with a star-forming ring previouslyidentified in ionized gas. This ring is not present in the near-infrared(NIR) J-K color image, nor is it present in the “dust-only”image constructed from the 8 ?m IRAC map. The implication is that theexcess residual ring is stellar, with colors similar to the surroundingdisk. We interpret this ring, visible in ionized gas, which appears asstars in the NIR, and with no sign of hot dust, as due to a red supergiant population at least 10-15 Myr old. However, star formation isstill ongoing in the disk and in the ring itself. Using NIR images, wecomputed the gravity torques exerted by the stellar potential on thegas. The torques are predominantly positive in both 12CO(1-0)and 12CO(2-1), suggesting that gas is not flowing into thecenter, and less than 5% of the gas angular momentum is exchanged ineach rotation. This comes from the regular and almost axisymmetric totalmass and gas distributions in the center of the galaxy. In NGC 5953, theAGN is apparently not being actively fueled in the current epoch.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by the INSU/CNRS (France), MPG(Germany), and IGN (Spain).
| Variability and spectral energy distributions of low-luminosity active galactic nuclei: a simultaneous X-ray/UV look with Swift
We have observed four low-luminosity active galactic nuclei (AGNs)classified as type 1 Low-Ionization Nuclear Emission-Line Regions(LINERs) with the X-Ray Telescope (XRT) and the Ultraviolet-OpticalTelescope (UVOT) onboard Swift, in an attempt to clarify the mainpowering mechanism of this class of nearby sources. Among our targets,we detect X-ray variability in NGC3998 for the first time. The lightcurves of this object reveal variations of up to 30 per cent amplitudein half a day, with no significant spectral variability on thistime-scale. We also observe a decrease of ~30 per cent over 9d, withsignificant spectral softening. Moreover, the X-ray flux is ~40 per centlower than observed in previous years. Variability is detected in M81 aswell, at levels comparable to those reported previously: a flux increasein the hard X-rays (1-10keV) of 30 per cent in ~3h and variations by upto a factor of 2 within a few years. This X-ray behaviour is similar tothat of higher luminosity, Seyfert-type objects. Using previoushigh-angular-resolution imaging data from the Hubble Space Telescope(HST), we evaluate the diffuse UV emission due to the host galaxy andisolate the nuclear flux in our UVOT observations. All sources aredetected in the UV band, at levels similar to those of the previousobservations with HST. The XRT (0.2-10keV) spectra are well described bysingle power laws and the UV-to-X-ray flux ratios are again consistentwith those of Seyferts and radio-loud AGNs of higher luminosity. Thesimilarity in X-ray variability and broad-band energy distributionssuggests the presence of similar accretion and radiation processes inlow- and high-luminosity AGNs.
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| On the Curvature of Dust Lanes in Galactic Bars
We test the theoretical prediction that the straightest dust lanes inbars are found in strongly barred galaxies, or more specifically, thatthe degree of curvature of the dust lanes is inversely proportional tothe strength of the bar. The test uses archival images of barredgalaxies for which a reliable nonaxisymmetric torque parameter (Qb) and the radius at which Q b has been measured(r(Q b)) have been published in the literature. Our resultsconfirm the theoretical prediction but show a large spread that cannotbe accounted for by measurement errors. We simulate 238 galaxies withdifferent bar and bulge parameters in order to investigate the origin ofthe spread in the dust lane curvature versus Q b relation.From these simulations, we conclude that the spread is greatly reducedwhen describing the bar strength as a linear combination of the barparameters Q b and the quotient of the major and minor axesof the bar, a/b. Thus, we conclude that the dust lane curvature ispredominantly determined by the parameters of the bar.
| VLA Imaging of Virgo Spirals in Atomic Gas (VIVA). I. The Atlas and the H I Properties
We present the results of a new VLA H I Imaging survey of Virgogalaxies, the VLA Imaging survey of Virgo galaxies in Atomic gas (VIVA).The survey includes high-resolution H I data of 53 carefully selectedlate type galaxies (48 spirals and five irregular systems). The goal isto study environmental effects on H I gas properties of cluster galaxiesto understand which physical mechanisms affect galaxy evolution indifferent density regions, and to establish how far out the impact ofthe cluster reaches. As a dynamically young cluster, Virgo containsexamples of galaxies experiencing a variety of environmental effects.Its nearness allows us to study each galaxy in great detail. We haveselected Virgo galaxies with a range of star formation properties in lowto high density regions (at projected distances from M87, d87 = 0.3-3.3 Mpc). Contrary to previous studies, more thanhalf of the galaxies in the sample (~60%) are fainter than 12 mag inBT . Overall, the selected galaxies represent the late typeVirgo galaxies (S0/a to Sd/Irr) down to mp lsim 14.6 fairlywell in morphological type, systemic velocity, subcluster membership, HI mass, and deficiency. The H I observations were done in C short (CS)configuration of the VLA radio telescope, with a typical spatialresolution of 15'' and a column density sensitivity of ≈3-5 ×1019 cm–2 in 3σ per 10 kms–1 channel. The survey was supplemented with data ofcomparable quality from the NRAO archive, taken in CS or Cconfiguration. In this paper, we present H I channel maps, totalintensity maps, velocity fields, velocity dispersions, global/radialprofiles, position-velocity diagrams and overlays of H I/1.4 GHzcontinuum maps on the optical images. We also present H I propertiessuch as total flux (S H I ), H I mass (M H I ),linewidths (W 20 and W 50), velocity (V H I), deficiency (def H I ), and size (D effH I and D iso H I ), and describe theH I morphology and kinematics of individual galaxies in detail. Thesurvey has revealed details of H I features that were never seen before.In this paper, we briefly discuss differences in typical H I morphologyfor galaxies in regions of different galaxy densities. We confirm thatgalaxies near the cluster core (d 87 lsim 0.5 Mpc) have H Idisks that are smaller compared to their stellar disks (D H I/D 25 < 0.5). Most of these galaxies in the corealso show gas displaced from the disk, which is either currently beingstripped or falling back after a stripping event. At intermediatedistances (d 87 ~ 1 Mpc) from the center, we find aremarkable number of galaxies with long one-sided H I tails pointingaway from M87. In a previous letter, we argue that these galaxies arerecent arrivals, falling into the Virgo core for the first time. In theoutskirts, we find many gas-rich galaxies, with gas disks extending farbeyond their optical disks. Interestingly, we also find some galaxieswith H I disks that are smaller compared to their stellar disks at largeclustercentric distances.
| The ultraviolet flare at the center of the elliptical galaxy NGC 4278
Context: A large fraction of otherwise normal galaxies shows a weaknuclear activity. One of the signatures of the low-luminosity activegalactic nuclei (LLAGNs) is ultraviolet variability which wasserendipitously discovered in the center of some low-ionization nuclearemission-line region (LINER) galaxies. Aims: There is a pressingneed to acquire better statistics about UV flaring and variability ingalaxy nuclei, both in terms of the number and monitoring of targets.The Science Data Archive of the Hubble Space Telescope was queried tofind all the elliptical galaxies with UV images obtained in differentepochs with the Wide Field Planetary Camera 2 (WFPC2) and possibly withnuclear spectra obtained with the Space Telescope Imaging Spectrograph(STIS) in the region of the Hα emission line. These data werefound only for the elliptical radiogalaxy NGC4278. Methods: The UV flux of the nuclear source of NGC4278 was measured by means of aperture photometry on the WFPC2/F218Wimages obtained between June 1994 and January 1995. The mass of thecentral supermassive black hole (SBH) was estimated by measuring thebroad components of the emission lines observed in the STIS/G750Mspectrum and assuming that the gas is uniformly distributed in asphere. Results: The nucleus of NGC 4278 hosts a barely resolvedbut strongly variable UV source. Its UV luminosity increased by a factorof 1.6 in a period of 6 months. The amplitude and scale time of the UVflare in NGC 4278 are remarkably similar to those of the brightest UVnuclear transients which were found earlier in other LLAGNs. The mass ofthe SBH was found to be in the range between 7 × 107and 2 × 109 Mȯ. This is in agreementwith previous findings based on different assumptions about the gasdistribution and with the predictions based on the galaxy velocitydispersion. Conclusions: All the LINER nuclei with availablemulti-epoch UV observations and a detected radio core are characterizedby a UV variable source. This supports the idea that the UV variabilityis a relatively common phenomenon in galaxy centers, perhaps providingthe missing link between LINERs and true AGN activity.
| Analysis of galaxy spectral energy distributions from far-UV to far-IR with CIGALE: studying a SINGS test sample
Aims: Photometric data of galaxies covering the rest-frame wavelengthrange from far-UV to far-IR make it possible to derive galaxy propertieswith a high reliability by fitting the attenuated stellar emission andthe related dust emission at the same time. Methods: Forthis purpose we wrote the code CIGALE (Code Investigating GALaxyEmission) that uses model spectra composed of the Maraston (or PEGASE)stellar population models, synthetic attenuation functions based on amodified Calzetti law, spectral line templates, the Dale & Heloudust emission models, and optional spectral templates of obscured AGN.Depending on the input redshifts, filter fluxes were computed for themodel set and compared to the galaxy photometry by carrying out aBayesian-like analysis. CIGALE was tested by analysing 39 nearbygalaxies selected from SINGS. The reliability of the different modelparameters was evaluated by studying the resulting expectation valuesand their standard deviations in relation to the input model grid.Moreover, the influence of the filter set and the quality of photometricdata on the code results was estimated. Results: For up to17 filters with effective wavelengths between 0.15 and 160 μm, wefind robust results for the mass, star formation rate, effective age ofthe stellar population at 4000 Å, bolometric luminosity,luminosity absorbed by dust, and attenuation in the far-UV. Details ofthe star formation history (excepting the burst fraction) and the shapeof the attenuation curve are difficult to investigate with the availablebroad-band UV and optical photometric data. A study of the mutualrelations between the reliable properties confirms the dependence ofstar formation activity on morphology in the local Universe andindicates a significant drop in this activity at about 1011Mȯ towards higher total stellar masses. The dustiestgalaxies in the SINGS sample are present in the same mass range. On theother hand, the far-UV attenuation of our sample galaxies does notappear to show a significant dependence on star formation activity.Conclusions: The results for our SINGS test sample demonstratethat CIGALE can be a valuable tool for studying basic properties ofgalaxies in the near and distant Universe if UV-to-IR data areavailable.
| The ring galaxy HRG 54 103: a first study
Aims: We report the first study of the peculiar ring galaxy HRG54103 which was previously classified as a Saturn-like typegalaxy. Methods: The study is based on low resolutionspectroscopy and photometric observations in the optical band tohighlight the characteristics of this almost isolated galaxy. The colourdistribution of HRG 54103 was examined through directCCD BVRI Kron-Cousins system imagery. Color-color diagrams of the bulgeand ring are displayed and further compared with the star-forming ringgalaxy HRG 2302. Results: The results of image enhancementof the morphological structure of this galaxy are discussed. The nuclearemission-line spectrum resembles that of a Seyfert2/LINER object, with z= 0.022 and heliocentric V = 6483 ± 18 km s-1, inagreement with the literature. The nuclear, bulge and ring sectionradial velocities along the ring major axis show a peculiardistribution, which together with the [N ii]/Hα and [Sii]/Hα ratios and image enhancement suggest an offset nucleus andan internal tilted ring or shell. Conclusions: HRG54103 is a peculiar galaxy with an intermediate activityregion, probably due to a residual excitation effect through the centralAGN phenomenon. The individual sections of the color-color map areredder than a typical star-forming ring galaxy, in agreement with thebehavior of the [S ii]/Hα versus [N ii]/Hα diagnosticdiagram. The two bulge satellites, the plume-like appendix, and thedisk-ring asymmetry suggest a possible merger event in the recent pastof this object, which could also have caused its formation.Based on observations made at: (a) Observatório do Pico dos Dias,operated by MCT/Laboratório Nacional de Astrofísica,Brazil, and (b) Cerro Tololo Inter-American Observatory, NationalOptical Astronomy Observatories, which are operated by AURA, Inc., undercontract to the National Science Foundation.
| The Herschel Reference Survey
The Herschel Reference Survey is a Herschel guaranteed time key projectand will be a benchmark study of dust in the nearby universe. The surveywill complement a number of other Herschel key projects including largecosmological surveys that trace dust in the distant universe. We willuse Herschel to produce images of a statistically-complete sample of 323galaxies at 250, 350, and 500 ?m. The sample is volume-limited,containing sources with distances between 15 and 25 Mpc and flux limitsin the band to minimize the selection effects associated with dust andwith young high-mass stars and to introduce a selection in stellar mass.The sample spans the whole range of morphological types (ellipticals tolate-type spirals) and environments (from the field to the center of theVirgo Cluster) and as such will be useful for other purposes than ourown. We plan to use the survey to investigate (i) the dust content ofgalaxies as a function of Hubble type, stellar mass, and environment;(ii) the connection between the dust content and composition and theother phases of the interstellar medium; and (iii) the origin andevolution of dust in galaxies. In this article, we describe the goals ofthe survey, the details of the sample and some of the auxiliaryobserving programs that we have started to collect complementary data.We also use the available multifrequency data to carry out an analysisof the statistical properties of the sample.
| The Mid-Infrared Continua of Seyfert Galaxies
An analysis of archival mid-infrared (mid-IR) spectra of Seyfertgalaxies from the Spitzer Space Telescope observations is presented. Wecharacterize the nature of the mid-IR active nuclear continuum bysubtracting a template starburst spectrum from the Seyfert spectra. Thelong wavelength part of the spectrum contains a strong contribution fromthe starburst-heated cool dust; this is used to effectively separatestarburst-dominated Seyferts from those dominated by the active nuclearcontinuum. Within the latter category, the strength of the activenuclear continuum drops rapidly beyond ~20 μm. On average, type 2Seyferts have weaker short-wavelength active nuclear continua ascompared to type 1 Seyferts. Type 2 Seyferts can be divided into twotypes, those with strong polycyclic aromatic hydrocarbon (PAH) bands andthose without. The latter type show polarized broad emission lines intheir optical spectra. The PAH-dominated type 2 Seyferts and Seyfert1.8/1.9s show very similar mid-IR spectra. However, after thesubtraction of the starburst component, there is a striking similarityin the active nuclear continuum of all Seyfert optical types.PAH-dominated Seyfert 2s and Seyfert 1.8/1.9s tend to show weak activenuclear continua in general. A few type 2 Seyferts with weak/absent PAHbands show a bump in the spectrum between 15 and 20 μm. We suggestthat this bump is the peak of a warm (~200 K) blackbody dust emission,which becomes clearly visible when the short-wavelength continuum isweaker. This warm blackbody emission is also observed in other Seyfertoptical subtypes, suggesting a common origin in these active galacticnuclei.
| H I in nearby low-luminosity QSO host galaxies
We searched for 21 cm H I emission in a sample of 27 previously COdetected nearby galaxies hosting low-luminosity quasi - stellar objects(QSOs). In this paper we investigate the relationship between the H Iand infrared properties of these host galaxies, compare the atomic andmolecular gas content and look for connections to the optical and FIRproperties. The single dish observations have been made with theEffelsberg 100-m telescope with a beam size of 9.5´. The sampleobjects have been drawn from a wide-angle survey for optically brightQSOs (HES), which have declinations δ >-30° and redshiftsup to z = 0.06. 12 host galaxies from the sample have been detected inthe H I 21 cm emission line. Eight of them have a spiral geometry,whereas the other four are bulge dominated and probably of ellipticaltype (E/S0). Three of the objects seem to be in a phase ofmerging/interaction. The neutral atomic gas masses range from 1.1× 10^9 Msun up to 3.8 × 10^10 Msun.The median H I gas mass in the whole sample is of the order of 11.4× 10^9 Msun, which is a factor of two higher than the HI content of our galaxy. We find no strong correlation between H I massand IR luminosity. The objects agree well within the expectations fromthe Tully-Fisher relation. In the color-color diagram we find allsources in the estimated locations. With the non-detected sources weclearly sample an upper envelope of this mass distribution.
| An X-ray view of 82 LINERs with Chandra and XMM-Newton data
We present the results of a homogeneous X-ray analysis for 82 nearbylow-ionisation, narrow emission-line regions (LINERs) selected from thecatalogue of Carrillo et al. (1999, Rev. Mex. Astron. Astrofis., 35,187). All sources have available Chandra (68 sources) and/or XMM-Newton(55 sources) observations. This is the largest sample of LINERs withX-ray spectral data (60 out of the 82 objects), and it significantlyimproves our previous analysis based on Chandra data for 51 LINERs(Gonzalez-Martin et al. 2006b, A&A, 460, 45). It both increases thesample size and adds XMM-Newton data. New models permit the inclusion ofdouble absorbers in the spectral fits. Nuclear X-ray morphology isinferred from the compactness of detected nuclear sources in the hardband (4.5-8.0 keV). Sixty per cent of the sample shows a compact nuclearsource and are classified as active galactic nucleus (AGN) candidates.The spectral analysis indicates that best fits involve a compositemodel: 1) absorbed primary continuum and 2) soft spectrum below 2 keVdescribed by an absorbed scatterer and/or a thermal component. Theresulting median spectral parameters and their standard deviations are<Γ> = 2.11 ± 0.52, < kT> = 0.54 ± 0.30keV, < log(NH1) > } = 21.32 {± 0.71 and < log(NH2)> }= 21.93 {± 1.36. We complement our X-ray results with an analysisof HST optical images and literature data on emission lines, radiocompactness, and stellar population. After adding all thesemultiwavelength data, we conclude that evidence supports the AGN natureof their nuclear engine for 80% of the sample (66 out of 82 objects).Tables 1 to 15, and Appendices are only available in electronic form athttp://www.aanda.org
| The anticorrelation between the hard X-ray photon index and the Eddington ratio in low-luminosity active galactic nuclei
We find a significant anticorrelation between the hard X-ray photonindex Γ and the Eddington ratio Lbol/LEddfor a sample of low-ionization nuclear emission-line regions and localSeyfert galaxies, compiled from literature with Chandra or XMM-Newtonobservations. This result is in contrast with the positive correlationfound in luminous active galactic nuclei (AGN), while it is similar tothat of X-ray binaries (XRBs) in the low/hard state. Our result isqualitatively consistent with the spectra produced fromadvection-dominated accretion flows (ADAFs). It implies that the X-rayemission of low-luminosity active galactic nuclei (LLAGN) may originatefrom the Comptonization process in ADAF, and the accretion process inLLAGN may be similar to that of XRBs in the low/hard state, which isdifferent from that in luminous AGN.
| 12CO(J = 1 – 0) On-the-Fly Mapping Survey of the Virgo Cluster Spirals. I. Data and Atlas
We have performed an On-The-Fly (OTF) mapping survey of12CO(J = 1-0) emission in 28 Virgo cluster spiral galaxiesusing the Five College Radio Astronomy Observatory (FCRAO) 14 mtelescope. This survey aims to characterize the CO distribution,kinematics, and luminosity of a large sample of galaxies covering thefull extents of stellar disks, rather than sampling only the inner disksor the major axis as was done by many previous single dish andinterferometric CO surveys. CO emission is detected in 20 galaxies amongthe 28 Virgo spirals observed. An atlas consisting of global measures,radial measures, and maps is presented for each detected galaxy. A notesummarizing the CO data is also presented along with relevantinformation from the literature. The CO properties derived from our OTFobservations are presented and compared with the results from the FCRAOExtragalactic CO Survey by Young et al. which utilizedposition-switching observations along the major axis and a model fittingmethod. We find that our OTF-derived CO properties agree well with theYoung et al. results in many cases, but the Young et al.measurements are larger by a factor of 1.4-2.4 for seven (out of 18)cases. We will explore further the possible causes for the discrepancyin the analysis paper currently under preparation.
| Fitting Liner Nuclei within the Active Galactic Nucleus Family: A Matter of Obscuration?
In this paper, we study the nuclear obscuration of galaxies hosting lowionization narrow emission regions (LINERs) based on their X-ray andoptical emission. They show column densities at soft energies (0.5-2keV) mostly related to the diffuse emission around the active galacticnucleus (AGN), showing a correlation with the optical extinction. Columndensities at hard energies (2-10 keV) seem to be much higher than whatwould be expected from the optical extinction. They might be associatedwith the inner regions of the AGN, buried at optical wavelengths. Themain result of this paper is that around 50% of our LINER sample showssignatures of Compton-thickness according to the most common tracers:the X-ray spectral index, {F_{X}(2\--10\;keV)/F([\mbox{O}\,\mathsc{iii}])} ratio, and FeKα equivalent width (EW). However, the EWsof the Compton-thick LINERs are significantly lower than in theCompton-thick Seyferts (sime200 eV against >=500 eV), suggesting thatthe 2-10 keV emission is dominated by electron scattering of theotherwise invisible AGN, or by emission from shocked gas associated withstar formation rather than by reflection from the inner wall of thetorus. However, no clear relation seems to exist between galaxies withoptical dust lanes and X-ray classified Compton-thick objects. This maysuggest that Compton-thick sources should be related to absorbingmaterial located at the very inner regions of the AGN, maybe in theputative dusty torus. Larger black hole masses and lower Eddingtonratios than Seyfert galaxies have been found. This effect can be betterattributed to LINER nuclei being hosted by earlier morphological typesthan Seyfert nuclei. However, it has to be noted that, once a propercorrection to the X-ray luminosity is applied, LINERs show Eddingtonratios overlapping those of type 2 Seyferts. We speculate with apossible scenario for LINER nuclei: an inner obscuring matter similar tothat of type 2 Seyfert, and an external obscuring matter responsible forthe optical extinction. Compton-thick sources appear to be more commonamong LINERs than Seyferts.
| Dust-corrected Star Formation Rates of Galaxies. I. Combinations of Hα and Infrared Tracers
We combine Hα emission-line and infrared (IR) continuummeasurements of two samples of nearby galaxies to derive dustattenuation-corrected star formation rates (SFRs). We use a simpleenergy balance based method that has been applied previously to H IIregions in the Spitzer Infrared Nearby Galaxies Survey, and extend themethodology to integrated measurements of galaxies. We find that ourcomposite Hα + IR based SFRs are in excellent agreement withattenuation-corrected SFRs derived from integrated spectrophotometry,over the full range of SFRs (0.01-80 Msun yr–1) and attenuations (0-2.5 mag)studied. We find that the combination of Hα and total IRluminosities provides the most robust SFR measurements, but combinationsof Hα measurements with monochromatic luminosities at24 μm and 8 μm perform nearly as well. The calibrationsdiffer significantly from those obtained for H II regions, with thedifference attributable to a more evolved population of stars heatingthe dust. Our results are consistent with a significant component ofdiffuse dust (the "IR cirrus" component) that is heated by anon-star-forming population. The same methodology can be applied to [OII]λ3727 emission-line measurements, and the radio continuumfluxes of galaxies can be applied in place of IR fluxes when the latterare not available. We assess the precision and systematic reliability ofall of these composite methods.
| Radial Distribution of Stars, Gas, and Dust in SINGS Galaxies. I. Surface Photometry and Morphology
We present ultraviolet through far-infrared (FIR) surface brightnessprofiles for the 75 galaxies in the Spitzer Infrared Nearby GalaxiesSurvey (SINGS). The imagery used to measure the profiles includes GalaxyEvolution Explorer UV data, optical images from Kitt Peak NationalObservatory, Cerro Tololo Inter-American Observatory, and Sloan DigitalSky Survey, near-IR data from Two Micron All Sky Survey, and mid- andFIR images from Spitzer. Along with the radial profiles, we also providemulti-wavelength asymptotic magnitudes and several nonparametricindicators of galaxy morphology: the concentration index (C42), the asymmetry (A), the Gini coefficient (G), and thenormalized second-order moment of the brightest 20% of the galaxy's flux(\overline{M}_{20}). In this paper, the first of a series, we describethe technical aspects regarding the surface photometry, and present abasic analysis of the global and structural properties of the SINGSgalaxies at different wavelengths. The homogeneity in the acquisition,reduction, and analysis of the results presented here makes these dataideal for multiple unanticipated studies on the radial distribution ofthe properties of stars, dust, and gas in galaxies. Our radial profilesshow a wide range of morphologies and multiple components (bulges,exponential disks, inner and outer disk truncations, etc.) that vary notonly from galaxy to galaxy but also with wavelength for a given object.In the optical and near-IR, the SINGS galaxies occupy the same regionsin the C 42-A-G-\overline{M}_{20} parameter space as othernormal galaxies in previous studies. However, they appear much lesscentrally concentrated, more asymmetric, and with larger values of Gwhen viewed in the UV (due to star-forming clumps scattered across thedisk) and in the mid-IR (due to the emission of polycyclic aromatichydrocarbons at 8.0 μm and very hot dust at 24 μm). In anaccompanying paper by Muñoz-Mateos et al., we focus on theradial distribution of dust properties in the SINGS galaxies, providinga detailed analysis of the radial variation of the attenuation, the dustcolumn density, the dust-to-gas ratio, the abundance of PAHs, and theintensity of the heating starlight.
| Revisiting the "Fundamental Plane" of Black Hole Activity at Extremely Low Luminosities
We investigate the origin of the X-ray emission in low-luminosity activegalactic nuclei (LLAGNs). Yuan and Cui predicted that the X-ray emissionshould originate from jets rather than an advection-dominated accretionflow (ADAF) when the X-ray luminosity L X of the source isbelow a critical value of L X,crit ≈ 10–6L Edd. This prediction implies that the X-ray spectrum insuch sources should be fitted by jets rather than ADAFs. Furthermore,below L X,crit the correlation between radio (L R)and X-ray (L X) luminosities and the black hole mass(M)—the so-called fundamental plane of black holeactivity—should deviate from the general correlation obtained byMerloni et al. and become steeper. The Merloni et al. correlation isdescribed by logL R = 0.6logL X + 0.78logM + 7.33,while the predicted correlation is logL R = 1.23logLX + 0.25logM – 13.45. We collect data from theliterature to check the validity of these two expectations. We find thatamong the 16 LLAGNs with good X-ray and radio spectra, 13 are consistentwith the Yuan and Cui prediction. For the 22 LLAGNs with L X< L X,crit, the fundamental plane correlation is describedby logL R = 1.22logL X + 0.23logM – 12.46,also in excellent agreement with the prediction.
| Fractal Dimension of Galaxy Isophotes
In this paper we investigate the use of the fractal dimension of galaxyisophotes in galaxy classification. We have applied two differentmethods for determining fractal dimensions to the isophotes ofelliptical and spiral galaxies derived from CCD images. We conclude thatfractal dimension alone is not a reliable tool but that combined withother parameters in a neural net algorithm the fractal dimension couldbe of use. In particular, we have used three parameters to segregate theellipticals and lenticulars from the spiral galaxies in our sample.These three parameters are the correlation fractal dimension Dcorr, the difference between the correlation fractaldimension and the capacity fractal dimension D corr – Dcap, and, thirdly, the B – V color of the galaxy.
| Radial Distribution of Stars, Gas, and Dust in Sings Galaxies. II. Derived Dust Properties
We present a detailed analysis of the radial distribution of dustproperties in the SINGS sample, performed on a set of ultraviolet (UV),infrared (IR), and H I surface brightness profiles, combined withpublished molecular gas profiles and metallicity gradients. The internalextinction, derived from the total-IR (TIR)-to-far-UV (FUV) luminosityratio, decreases with radius, and is larger in Sb-Sbc galaxies. TheTIR-to-FUV ratio correlates with the UV spectral slope β, followinga sequence shifted to redder UV colors with respect to that ofstarbursts. The star formation history (SFH) is identified as the maindriver of this departure. Both L TIR/L FUV andβ correlate well with metallicity, especially in moderately face-ongalaxies. The relation shifts to redder colors with increased scatter inmore edge-on objects. By applying physical dust models to our radialspectral energy distributions, we have derived radial profiles of thetotal dust mass surface density, the fraction of the total dust masscontributed by polycyclic aromatic hydrocarbons (PAHs), and theintensity of the radiation field heating the grains. The dust profilesare exponential, their radial scale length being constant from Sb to Sdgalaxies (only ~10% larger than the stellar scale length). Many S0/a-Sabgalaxies have central depressions in their dust radial distributions.The PAH abundance increases with metallicity for 12 + log(O/H) < 9,and at larger metallicities the trend flattens and even reverses, withthe SFH being a plausible underlying driver for this behavior. Thedust-to-gas ratio is also well correlated with metallicity and thereforedecreases with galactocentric radius. Although most of the total emittedIR power (especially in the outer regions of disks) is contributed bydust grains heated by diffuse starlight with a similar intensity as thelocal Milky Way radiation field, a small amount of the dust mass (~1%)is required to be exposed to very intense starlight in order toreproduce the observed fluxes at 24 μm, accounting for ~10% ofthe total integrated IR power.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Spitzer/IRS 5-35 μm Low-resolution Spectroscopy of the 12 μm Seyfert Sample
We present low-resolution 5.5-35 μm spectra for 103 galaxiesfrom the 12 μm Seyfert sample, a complete unbiased 12 μmflux limited sample of local Seyfert galaxies selected from the IRASFaint Source Catalog, obtained with the Infrared Spectrograph (IRS)on-board Spitzer Space Telescope. For 70 of the sources observed in theIRS mapping mode, uniformly extracted nuclear spectra are presented forthe first time. We performed an analysis of the continuum emission, thestrength of the polycyclic aromatic hydrocarbon (PAH) and astronomicalsilicate features of the sources. We find that on average, the15-30 μm slope of the continuum islangα15–30rang = –0.85 ± 0.61 forSeyfert 1s and –1.53 ± 0.84 for Seyfert 2s, and there issubstantial scatter in each type. Moreover, nearly 32% of the Seyfert1s, and 9% of the Seyfert 2s, display a peak in the mid-infraredspectrum at 20 μm, which is attributed to an additional hot dustcomponent. The PAH equivalent width decreases with increasing dusttemperature, as indicated by the global infrared color of the hostgalaxies. However, no statistical difference in PAH equivalent width isdetected between the two Seyfert types, 1 and 2, of the same bolometricluminosity. The silicate features at 9.7 and 18 μm in Seyfert 1galaxies are rather weak, while Seyfert 2s are more likely to displaystrong silicate absorption. Those Seyfert 2s with the highest silicateabsorption also have high infrared luminosity and high absorption(hydrogen column density NH > 1023cm–2) as measured from the X-rays. Finally, we proposea new method to estimate the active galactic nucleus contribution to theintegrated 12 μm galaxy emission, by subtracting the "starformation" component in the Seyfert galaxies, making use of the tightcorrelation between PAH 11.2 μm luminosity and 12 μmluminosity for star-forming galaxies.
| Molecular gas in NUclei of GAlaxies (NUGA). XII. The head-on collision in NGC 1961
We present high-resolution maps of the CO(1-0) and CO(2-1) emission fromthe LINER 2 galaxy NGC 1961. This galaxy is unusual among late-type (Sc)disk galaxies in having a very large radial extent and inferreddynamical mass. We propose a head-on collision scenario to explain theperturbed morphology of this galaxy - both the off-centered rings andthe inflated radius. This scenario is supported by the detection of asteep velocity gradient in the CO(1-0) map at the position of asouthwest peak in radio continuum and near-infrared emission. This peakwould represent the remnant of the disrupting companion. We usenumerical models to demonstrate the plausibility of the scenario. Whileram pressure stripping could in principle be important for shocking theatomic gas and produce the striking head-tail morphology, the nondetection of this small galaxy group in X-ray emission suggests that anyhot intragroup medium has too low a density. A prediction of thecollision model is the propagation of ring waves from the center to theouter parts, superposed on a probable pre-existing m=2 barred spiralfeature, accounting for the observed complex structure of rings andspokes. This lopsided wave accounts for the sharp boundary observed inthe atomic gas on the southern side. Through dynamical friction, thecollision finishes quickly in a minor merger, the best fit being for acompanion with a mass ratio 1:4. We argue that NGC 1961 has a stronglywarped disk, which gives the false impression of a nearly face-onsystem; the main disk is actually more edge-on, and this error in thetrue inclination has led to the surprisingly high dynamical mass for amorphologically late-type galaxy. In addition, the outwardly propagatingring artificially enlarges the disk. The collision de-stabilizes theinner disk and can provide gas inflow to the active nucleus.Based on observations conducted at the IRAM Plateau de BureInterferometer. IRAM is supported by the INSU/CNRS (France), the MPG(Germany), and the IGN (Spain).
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