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The hot, warm and cold gas in Arp 227 - an evolving poor group
Arp 227 represents a prototypical example of an interacting mixed pairof galaxies located in a low-density environment. We investigate the gasproperties of the pair in the X-ray, Hα, HI and CO bands. We alsodetect two additional members of the group in HI which indicates thatthe pair constitutes the dominant members of a loose group.The HI distribution shows a tail of gas that connects the spiral member,NGC 470, to the lenticular, NGC 474, showing that the two main membersare currently undergoing interaction. The Hα emission reveals thepresence of secondary components at the centre of NGC 470, superposed onthe main component tracing the rotation of the galaxy. This latter mapsa nearly unperturbed velocity field. The dominant, nearly unperturbedtrend of the kinematics is confirmed by CO observations, althoughrestricted to the centre of the galaxy. The X-ray luminosity of NGC 470is comparable with that of a `normal' spiral galaxy. NGC 474 on theother hand is very gas-poor and has not been detected in Hα. ItsX-ray luminosity is consistent with the low end of the expected emissionfrom discrete sources.Arp 227 as a loose group shows several signatures of galaxy-galaxyinteraction. Our observations suggest the presence of signatures ofinteraction in the overall kinematics of the spiral companion. Theongoing interaction is clearly visible only in the outer HI halo of NGC470. While the large shell system of NGC 474 could be associated with anaccretion event, the secondary components in the Hα profile in thecentre of NGC 470 could be due to the interaction with the companion.The low X-ray luminosity of NGC 470 seems to be a characteristic ofdynamically young systems. All the above evidence suggest that Arp 227is an evolving group in the early phase of its evolution and that itsdrivers are the accretion of faint galaxies and the ongoing large-scaleinteraction between NGC 470 and 474.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations
We use very deep Spitzer MIPS 24 μm observations to examine thebolometric luminosities (Lbol) and UV extinction propertiesof more than 200 spectroscopically identified, optically selected(UnGR) z~2 galaxies, supplemented with near-IR-selected(``BzK'' and ``DRG'') and submillimeter galaxies at similar redshifts,in the GOODS-N field. Focusing on redshifts 1.51012 Lsolar, with a mean~=2×1011 Lsolar. Using24 μm observations as an independent probe of dust extinction, wefind that, as in the local universe, the obscurationLIR/L1600 is strongly dependent on Lboland ranges in value from <1 to ~1000 within the sample considered.However, the obscuration is generally ~10 times smaller at a givenLbol at z~2 than at z~0. We show that the values ofLIR and obscuration inferred from the UV spectral slopeβ generally agree well with the values inferred fromL5-8.5μm for Lbol<1012Lsolar. Using the specific SFRs of galaxies as a proxy forcold gas fraction, we find a wide range in the evolutionary state ofgalaxies at z~2, from galaxies that have just begun to form stars tothose that have already accumulated most of their stellar mass and areabout to become, or already are, passively evolving.Based, in part, on data obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation. Alsobased in part on observations made with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with NASA.
| The Rest-Frame Far-Ultraviolet Morphologies of Star-forming Galaxies at z ~ 1.5 and 4
We apply a new approach to quantifying galaxy morphology and identifyinggalaxy mergers to the rest-frame far-ultraviolet images of 82 z~4 Lymanbreak galaxies (LBGs) and 55 1.22.5 and Petrosian radii >0.3". Ten of the 82 LBGs haveM20>=-1.1 and possess bright double or multiple nuclei,implying a major-merger fraction of star-forming galaxies ~10%-25% atMFUV<-20, depending on our incompleteness corrections.Galaxies with bulge-like morphologies (G>=0.55,M20<-1.6) make up ~30% of the z~4 LBG sample, while theremaining ~50% have G- and M20-values higher than expectedfor smooth bulges and disks and may be star-forming disks, minormergers, or postmergers. The star-forming z~1.5 galaxy sample has amorphological distribution that is similar to the UDF z~4 LBGs, with anidentical fraction of major-merger candidates but fewer spheroids. Theobserved morphological distributions are roughly consistent with currenthierarchical model predictions for the major-merger rates andminor-merger-induced starbursts at z~1.5 and ~4. We also examine therest-frame FUV-NUV and FUV-B colors as a function of morphology and findno strong correlations at either epoch.
| Examining the Seyfert-Starburst Connection with Arcsecond-Resolution Radio Continuum Observations
We compare the arcsecond-scale circumnuclear radio continuum propertiesof five Seyfert and five starburst galaxies, concentrating on the searchfor any structures that could imply a spatial or causal connectionbetween the nuclear activity and a circumnuclear starburst ring. Noevidence is found in the radio emission for a link between thetriggering or feeding of nuclear activity and the properties ofcircumnuclear star formation. Conversely, there is no clear evidence ofnuclear outflows or jets triggering activity in the circumnuclear ringsof star formation. Interestingly, the difference in the angle betweenthe apparent orientation of the most elongated radio emission and theorientation of the major axis of the galaxy is on average larger inSeyfert galaxies than in starburst galaxies, and Seyfert galaxies appearto have a larger physical size scale of the circumnuclear radiocontinuum emission. The concentration, asymmetry, and clumpinessparameters of radio continuum emission in Seyfert galaxies andstarbursts are comparable, as are the radial profiles of radio continuumand near-infrared line emission. The circumnuclear star formation andsupernova rates do not depend on the level of nuclear activity. Theradio emission usually traces the near-infrared Brγ andH2 1-0 S(1) line emission on large spatial scales, butlocally their distributions are different, most likely because of theeffects of varying local magnetic fields and dust absorption andscattering.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
| The evolution of actively star-forming galaxies in the mid-infrared
In this paper we analyze the evolution of actively star-forming galaxiesin the mid-infrared (MIR). This spectral region, characterized bycontinuum emission by hot dust and by the presence of strong emissionfeatures generally ascribed to polycyclic aromatic hydrocarbon (PAH)molecules, is the most strongly affected by the heating processesassociated with star formation and/or active galactic nuclei (AGNs).Following the detailed observational characterization of galaxies in theMIR by the Infrared Space Observatory (ISO), we have updated themodelling of this spectral region in our spectrophotometric modelGRASIL. In the diffuse component we have updated the treatment of PAHsaccording to the model by Li & Draine. As for the dense phase of theinterstellar medium associated with the star-forming regions, themolecular clouds, we strongly decrease the abundance of PAHs as comparedto that in the cirrus, based on the observational evidence of the lackor weakness of PAH bands close to the newly formed stars, possibly dueto the destruction of the molecules in strong ultraviolet fields. Therobustness of the model is checked by fitting near-infrared to radiobroad-band spectra and the corresponding detailed MIR spectra of a largesample of galaxies, at once. With this model, we have analyzed thelarger sample of actively star-forming galaxies by Dale et al. We showthat the observed trends of galaxies in the ISO-IRAS-radio colour-colourplots can be interpreted in terms of the different evolutionary phasesof star formation activity, and the consequent different dominance inthe spectral energy distribution of the diffuse or dense phase of theISM. We find that the observed colours indicate a surprising homogeneityof the starburst phenomenon, allowing only a limited variation of themost important physical parameters, such as the optical depth of themolecular clouds, the time-scale of the escape of young stars from theirfor mation sites, and the gas consumption time-scale. In this paper wedo not attempt to reproduce the far-infrared coolest region in thecolour-colour plots, as we concentrate on models meant to reproduceactive star-forming galaxies, but we discuss possible requirements of amore complex modelling for the coldest objects.
| A Chandra view of the anomalous half-merger NGC 520
High spatial and spectral resolution Chandra X-ray observations of theanomalous merging galaxy NGC 520, a similarly evolved system to thewell-known Antennae galaxies, are presented here. Of great interest isthe fact that NGC 520, on account of it being supposedly due (as seen invarious multiwavelength studies) to the result of an encounter betweenone gas-rich disc and one gas-poor disc, appears in X-rays to be only`half a merger' whereas an ultraluminous X-ray source (ULX) lies at theprimary (south-eastern), more-massive nucleus, no sources are seen atthe secondary nucleus. Whereas what appears to be a starburst-drivengalactic wind is seen outflowing perpendicular to the molecular discsurrounding the primary nucleus, no such diffuse structure is seenanywhere near the secondary nucleus. Comparing the X-ray properties withthose of other merging galaxies, including famous gas-rich-gas-richmergers such as the Mice and the Antennae, one sees that, relative toits star formation rate, the number of ULXs seen within the system israther small. Similarly, the total X-ray luminosity and the fraction ofthis emission that appears diffuse are both a factor of ~2 less thanthat expected based on NGC 520's evolutionary merger stage.Although only half of NGC 520 appears in X-rays as other mergers do,there is still a wealth of structure and detail: 15 X-ray sources aredetected within the system, many of them showing long-term variability,including a small number of bright ULXs that flatten the source X-rayluminosity function to a level similar to that of the Antennae and othermergers. Lastly, to see what appears to be a starburst-driven diffusegalactic wind, with a spectrum entirely consistent with that of otherknown galactic winds, although unusually, emanating from only one of thenuclei, is a surprise, given that one might have expected suchstructures to have distorted very quickly in such a rapidly evolvingenvironment. The wind is larger and more massive than structures seen inevolutionarily earlier systems (e.g. the Mice), but smaller and lessmassive than as seen in later systems (e.g. the Antennae) or classicstarbursts. Perhaps these structures can survive for longer than waspreviously thought.
| The local submillimetre luminosity functions and predictions from Spitzer to Herschel
We present new determinations of the local submillimetre (submm)luminosity functions, solving the `submm Olbers' paradox'. We alsopresent predictions of source counts and luminosity functions in currentand future far-infrared to submm surveys. Using the submm colourtemperature relations from the Submillimetre Common User Bolometer Array(SCUBA) Local Universe Galaxy Survey, and the discovery of 450-μmexcess emission in these galaxies, we interpolate and extrapolate theIRAS detections to make predictions of the spectral energy distributionsof all 15411 PSCz galaxies from 50 to 1300 μm. Despite the longextrapolations, we find excellent agreement with (i) the 90-μmluminosity function of Serjeant et al., (ii) the 850-μm luminosityfunction of Dunne et al., (iii) the millimetre-wave photometry ofAndreani and Franceschini, and (iv) the asymptotic differential andintegral source count predictions at 50-1300 μm by Rowan-Robinson. Wefind that the local 850-μm submm luminosity density converges to 7.3+/- 0.2 × 1019h65 W Hz-1Mpc-3. Remarkably, the local spectral luminosity density andthe extragalactic background light together strongly constrain thecosmic star formation history for a wide class of evolutionaryassumptions. We find that the extragalactic background light, the850-μm 8-mJy source counts and the Ω* constraintsall independently point to a decline in the comoving star formation rateat z > 1. In order to reconcile this with direct determinations, wesuggest that either there is a top-heavy initial mass function at highredshifts, and/or there is stronger evolution in the more luminousfar-infrared galaxies than seen in the population as a whole.
| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| Outflows in Infrared-Luminous Starbursts at z < 0.5. II. Analysis and Discussion1,
We have performed an absorption-line survey of outflowing gas in 78starburst-dominated, infrared-luminous galaxies. This is the largeststudy of superwinds at z<~3. Superwinds are found in almost allinfrared-luminous galaxies, and changes in detection rate with SFR-windsare found twice as often in ultraluminous infrared galaxies (ULIRGs) asin less-luminous galaxies-reflect different wind geometries. The maximumvelocities we measure are 600 km s-1, though most of theoutflowing gas has lower velocities (100-200 km s-1). (Onegalaxy has velocities exceeding 1000 km s-1.) Velocities inLINERs are higher than in H II galaxies, and outflowing ionized gasoften has higher velocities than the neutral gas. Wind properties(velocity, mass, momentum, and energy) scale with galaxy properties(SFR, luminosity, and galaxy mass), consistent with ram-pressure drivingof the wind. Wind properties increase strongly with increasing galacticmass, contrary to expectation. These correlations flatten at high SFR(>~10-100 Msolar yr-1), luminosities, andmasses. This saturation is due to a lack of gas remaining in the wind'spath, a common neutral gas terminal velocity, and/or a decrease in theefficiency of thermalization of the supernovae energy. It means thatmass entrainment efficiency, rather than remaining constant, declines ingalaxies with SFR>10 Msolar yr-1 andMK<-24. Half of our sample consists of ULIRGs, which hostas much as half of the star formation in the universe at z>~1. Thepowerful, ubiquitous winds we observe in these galaxies imply thatsuperwinds in massive galaxies at redshifts above unity play animportant role in the evolution of galaxies and the intergalacticmedium.Some of the observations reported here were obtained at the MMTObservatory, which is a joint facility of the Smithsonian Institutionand the University of Arizona.Some of the observations reported here were obtained at the Kitt PeakNational Observatory, National Optical Astronomy Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc. (AURA), under cooperative agreement with the National ScienceFoundation.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope
Full low-resolution (65
| EGRET Upper Limits and Stacking Searches of Gamma-Ray Observations of Luminous and Ultraluminous Infrared Galaxies
We present a stacking analysis of EGRET γ-ray observations at thepositions of luminous and ultraluminous infrared galaxies. The latterwere selected from the recently presented HCN survey, which is thoughtto contain the most active star-forming regions of the universe.Different sorting criteria are used, and since there is no positivecollective detection of γ-ray emission from these objects, wedetermined both collective and individual upper limits. The uppermostexcess we find appears in the case of ULIRGs ordered by redshift, at avalue of 1.8 σ.
| Ultraviolet Emission from Stellar Populations within Tidal Tails: Catching the Youngest Galaxies in Formation?
New Galaxy Evolution Explorer (GALEX) observations have detectedsignificant far-UV (FUV; 1530 Å) and near-UV (NUV; 2310 Å)emission from stellar substructures within the tidal tails of fourongoing galaxy mergers. The UV-bright regions are optically faint andare coincident with H I density enhancements. FUV emission is detectedat any location where the H I surface density exceeds ~2Msolar pc-2, and it is often detected in theabsence of visible wavelength emission. UV luminosities of the brighterregions of the tidal tails imply masses of 106 to~109 Msolar in young stars in the tails, and H Iluminosities imply similar H I masses. UV-optical colors of the tidaltails indicate stellar populations as young as a few megayears, and inall cases ages under 400 Myr. Most of the young stars in the tailsformed in single bursts, rather than resulting from continuous starformation, and they formed in situ as the tails evolved. Star formationappears to be older near the parent galaxies and younger at increasingdistances from the parent galaxy. This could be because the starformation occurs progressively along the tails, or because the starformation has been inhibited near the galaxy/tail interface. Theyoungest stellar concentrations, usually near the ends of long tidaltails, have masses comparable to confirmed tidal dwarf galaxies and maybe newly forming galaxies undergoing their first burst of starformation.
| A radio monitoring survey of ultra-luminous X-ray sources
We present the results of a radio monitoring campaign to search forradio emission from nearby ultra-luminous X-ray sources (ULXs). Thesesources are bright off-nuclear X-ray point sources with luminositiesexceeding LX > 1039 erg s-1. Awell-defined sample of the 9 nearest ULXs has been monitored eight timesover 5 months with the Very Large Array in A and B configuration. Ourlimiting sensitivity is ≈0.15 mJy (4σ) for radio flares and≈60 μJy for continuous emission. In M 82 two ULXs seem to havecoincident compact radio sources, which are probably supernova remnants.No continuous or flaring radio emission has been detected from any otherULX. Thus, ULXs do not generally emit steady-state radio emission aboveradio powers of 1.5 × 1017 W/Hz. The non-detections ofthe continuous emission are consistent with beamed or unbeamed radioemission from accreting black holes of ≤ 103 Mȯ based on the radio/X-ray correlation. Other publishedradio detections (M 82, NGC 5408) are also discussed in this context.Both detections are significantly above our detection limit. If ULXshave flaring radio emission above 4 × 1017 W/Hz we cangive an upper limit on the duty cycle of the flares of 6%. This upperlimit is in agreement with the observed number of flares in Galacticradio transients. Additionally we present a yet unreported radio doublestructure in the nearby low-luminosity AGN NGC 4736.
| Exceptional H2 emission in the Antennae galaxies: Pre-starburst shocks from the galaxy collision
The collision of gas-rich galaxies is believed to produce strong shocksbetween their gas clouds which cause the onset of the observed bursts ofextended star formation. However, the so far observed shock signaturesin colliding galaxies can be explained essentially by winds from alreadyexisting massive stars and supernovae and thus do not give any evidencefor an outstanding pre-starburst phase. Either pre-starburst gas shocksare too short-lived to be detected or one has to modify our perceptionof colliding galaxies. A dedicated analysis of ISOCAM-CVF mid-infraredspectral maps led us to the discovery of exceptional H2 v = 0-0 S(3)λ = 9.66 μm line emission from the “Antennae”galaxy pair, which is at an early stage of galaxy collision. Its H2 lineluminosity, normalized by the far-infrared luminosity, exceeds that ofall other known galaxies and the strongest H2 emission is spatiallydisplaced from the known starbursts regions. This implies that most ofthe excited H2 gas in the Antennae must be shocked due to the collisionof the two galaxies. These observations indicate that the outstandingphase of pre-starburst shocks exists, and that they might be a key toour understanding of the formation of the first proto-galaxies.Based onobservations with the Infrared Space ObservatoryISO, an ESA project with instrumentsfunded by ESA Member States (especially the PI countries: France,Germany, the Netherlands and the United Kingdom) andwith the participation of ISAS and NASA.
| Cold and warm dust along a merging galaxy sequence
We investigate the cold and warm dust properties during galaxyinteractions using a merging galaxy sample ordered into a chronologicalsequence from pre- to post-mergers. Our sample comprises a total of 29merging systems selected to have far-infrared and submillimetreobservations. The submillimetre data are mainly culled from theliterature, while for five galaxies (NGC 3597, 3690, 6090, 6670 and7252) the submillimetre observations are presented here for the firsttime. We use the 100- to 850-μm flux density ratio,f100/f850, as a proxy for the mass fraction of thewarm and cold dust in these systems. We find evidence for an increase inf100/f850 along the merging sequence from early toadvanced mergers, and interpret this trend as an increase of the warmrelative to the cold dust mass. We argue that the two key parametersaffecting the f100/f850 flux ratio is the starformation rate and the dust content of individual systems relative tothe stars. Using a sophisticated model for the absorption andre-emission of the stellar ultraviolet radiation by dust, we show thatthese parameters can indeed explain both the increase and the observedscatter in f100/f850 along the merging galaxysequence. We also discuss our results under the hypothesis thatelliptical galaxies are formed via disc galaxy mergers.
| A high-resolution study of the interstellar medium in the luminous IRAS galaxy Arp 193
We present high-resolution multi-element radio-linked interferometernetwork (MERLIN) and Very Large Array (VLA) observations of the H Iabsorption towards the starburst nucleus of Arp 193 (IC 883, UGC 08387),a luminous IRAS merger. We compare the distribution of atomic gas withthat of radio continuum emission, high-resolution CO data andnear-infrared Hubble Space Telescope (HST) Near-Infrared Camera andMulti-object Spectrometer (NICMOS) data. We find, that for a constantspin temperature and CO flux to H2 mass conversion factor,XCO, the interstellar medium (ISM) becomes progressively moreH2 rich towards the centre of the source. We also find thatthe projected velocity distribution of the atomic and molecular gascomponents are not the same. We discuss possible reasons for theobserved differences and find that the principle difference is probablyin the spatial distribution of the molecular and atomic ISM, althoughthe addition of real dynamical differences provides a better explanationof the data. Evidence for free-free absorption towards the centre of thestarburst nucleus is found, implying a large mass of ionized gas, andtherefore young stars. Arp 193 is one of the very few IRAS luminousobjects in which such a detailed study is possible.
| Neutral hydrogen gas in interacting galaxies: the NGC 6221/6215 galaxy group
Neutral hydrogen observations of the spiral galaxies NGC 6221 and 6215with the Australia Telescope Compact Array (ATCA) reveal a wide,two-stranded bridge of at least 3 × 108Msolar which can be traced between the two galaxies over aprojected distance of 100 kpc. The velocity gradient of the HI bridgeprovides a rough estimate for the time since the encounter of 500 Myr.For NGC 6221, the brightest and most massive galaxy of the group, wederive a dynamical mass of Mtot= 8 × 1010Msolar, while its companion NGC 6215 has a mass of onlyMtot~ 2 × 109 Msolar. Further, wefind three low-surface-brightness dwarf galaxies (Dwarfs 1, 2 and 3) inthe neighbourhood of NGC 6221/15 with HI masses of 3.3, 0.6 and 0.3× 108 Msolar, respectively. The smallest,previously uncatalogued galaxy, Dwarf 3, lies between NGC 6221 and 6215,and may have formed out of bridge material.The brightest part of the HI bridge lies roughly halfway between theinteracting galaxies, indicating that bridge gas close to NGC 6221 and6215 may have fallen back to the galaxies. The asymmetric extensions tothe HI envelope of NGC 6221 are likely to be reaccreted gas, stillsettling in. Also, the peculiar velocity field of NGC 6215 may beexplained by accreted bridge material settling into a plane offset fromthe old disc.
| Infrared mergers and infrared quasi-stellar objects with galactic winds - I. NGC 2623: nuclear outflow in a proto-elliptical candidate
We present the first results of a study of the morphology, kinematicsand ionization structure of infrared (IR) mergers/quasi-stellar objects(QSOs) with galactic winds. This study is based mainly on INTEGRALtwo-dimensional (2D) fibre spectroscopy [obtained on the 4.2-m WilliamHerschel Telescope (WHT), La Palma] combined with high-resolution HubbleSpace Telescope (HST) observations.Clear evidence of outflow (OF) from the nucleus of the luminous infraredmerger NGC 2623 is reported. Specifically: (i) the INTEGRAL 2D Hα,[N II] and [S II] emission line maps depict a cone-shaped extendednebula that emerges from the nucleus, with an aperture angle θ=100°+/- 5° and reaching a distance of ~3.2 kpc from the nucleus;(ii) inside the nebula and in the central region, all the emission-lineWHT spectra show low velocity blue/OF components, with= (-405 +/- 35) km s-1 and (iii) in theOF nebula, the emission line ratios are consistent with ionization by adusty nuclear starburst plus shock heating. These results are consistentwith a galactic wind process powered mainly by a nuclear starburst.The INTEGRAL 2D Hα and [N II]λ6583 velocity field (VF) mapsfor the main body of NGC 2623 (16.4 × 12.3 arcsec2;~5.9 × 4.4 kpc2) show outflow motion in the nuclear andthe Hα+[N II] nebular regions superposed on a general circularmotion. This circular motion prevails inside r~ 1.5 kpc, and for largerradii we detected non-circular motions. In the central region, theaverage observed rotation curve was fitted with a model corresponding toa single-component Plummer spherical potential. After the subtraction ofthe Plummer and an axisymmetric polynomial model, the residues of the VFin both cases indicate ejection as the origin of the cone nebula. Thefitted Plummer model implies a total mass of MT= 1.5 ×1010 Msolar and a spherical distribution of matterin the central region.The high-resolution HST WFPC2 F555W (~V) and F814W (~I) broad-bandimages display a strongly obscured nucleus in the apex of a smallnuclear cone, an asymmetrical clumpy spiral arm located to the east ofthe nucleus, a ring plus an arc to the west and several large-scalefilaments of dust. A good r1/4-law fit to the HST WFPC2 Iband luminosity profile was found.In 85 per cent of the INTEGRAL 2D field we measure very high values(>1) of the [N II]λ6583/Hα and [S II]λ6717 +31/Hα ratios, suggesting that shocks are important on large scales(in almost all the main body). Furthermore, the 2D full width at halfmaximum FWHM-[N II] and VF residual maps show a good spatialcorrelation, suggesting that the OF shocks ionize the gas and broadenthe emission lines. However, close to the nucleus, the OF nebula showslow values of the [N II]λ6583/Hα ratio (in the range0.1-0.4), indicating that photoionization by a dusty nuclear starburstalso plays a significant role in the excitation of the nebula. Thecharacteristics of the nucleus of NGC 2623 could be associated with astarburst-related LINER.The properties found in IR mergers/QSOs with galactic winds mainlyunderline the importance of studying the possible link between IRmergers with starburst + galactic wind -> IR QSOs with compositenature + galactic wind, and elliptical galaxies.
| HST Observations of the Toomre Sequence of Merging Galaxies
We discuss our ongoing multi-instrument HST investigation of the nuclearregions of the 11 interacting and merging galaxies in the ToomreSequence. We are studying the nuclear kinematics using STIS (G750M)spectra, the nuclear stellar populations using STIS (G430L) spectra, andthe nuclear morphology using NICMOS and WFPC2 images. The results willprovide new insight into the physical processes that operate duringgalaxy interactions.
| PAH features in Infrared Luminous Galaxies: Results from Michelle
We present recent results from mid-infrared observations of a sample ofnearby, infrared luminous starbursts and AGN made with the newmid-infrared instrument, ``Michelle'', on UKIRT. Narrow band imaging inthe 7-13 micron range with sub-arcsec resolution has been used to studythe spatial distribution of polycyclic aromatic hydrocarbon (PAH)features. The comparison of these sub-arcsecond resolution data withradio continuum data at similar resolution can be used to determine, 1)the sources of excitation required for PAH emission, and 2) whether PAHfeatures are a measure of the relative contribution of star formationand AGN to the bolometric energy output of a galaxy. Unlike thefar-infrared emission from dust, that in the mid-infrared can be used todiscriminate between different heating sources.
| Studies of Extragalactic Formaldehyde and Radio Recombination Lines
We present the most sensitive and extensive survey yet performed ofextragalactic H2CO 6 cm (4.829 GHz) emission/absorption.Sixty-two sources were observed with the C-band system of the AreciboTelescope to a 1 σ rms noise level of ~0.3 mJy. We report a newdetection of H2CO 6 cm absorption toward NGC 520 and theconfirmation of H2CO 6 cm absorption toward several sources.We report confirmation of H2CO 6 cm emission toward the OHmegamasers Arp 220, IC 860, and IRAS 15107+0724. At present these arethe only extragalactic H2CO 6 cm emitters independentlyconfirmed. A characterization of the properties of formaldehydeabsorbers and emitters based on infrared properties of the galaxies isdiscussed. We also conducted a simultaneous survey of the H110αhydrogen recombination line toward a sample of 53 objects. We report thedetection of H110α toward the giant extragalactic H II region NGC604 in M33.
| Evidence for Interactions in H I Imaging of Seyfert Galaxies
We still do not know the primary trigger of local active galacticnuclei. Tidal torques and nuclear bars are often invoked to explain gasinfall onto the central supermassive black hole, but as yet neither hasdefinitive observational support. Here we exploit the sensitivity andendurance of H I to trace the strength and prevalence of tidalinteractions among Seyfert galaxies. To minimize selection biases, wesurvey the 27 northern Seyferts compiled in the comprehensive opticalcatalog of Véron-Cetty & Véron lying in the narrowredshift range 0.015-0.017. This paper is a detailed presentation of asubsample, including channel maps for all systems. In addition to theprogram objects, we also investigate galaxies imaged in the field.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations
We investigate the detailed response of gas to the formation oftransient and long-lived dynamical structures induced in the earlystages of a disk-disk collision and identify observational signatures ofradial gas inflow through a detailed examination of the collisionsimulation of an equal-mass bulge-dominated galaxy. Our analysis anddiscussion mainly focuses on the evolution of the diffuse and dense gasin the early stages of the collision, when the two disks are interactingbut have not yet merged. Stars respond to the tidal interaction byforming both transient arms and long-lived m=2 bars, but the gasresponse is more transient, flowing directly toward the central regionswithin about 108 yr after the initial collision. The rate ofinflow declines when more than half of the total gas supply reaches theinner few kiloparsecs, where the gas forms a dense nuclear ring insidethe stellar bar. The average gas inflow rate to the central 1.8 kpc is~7 Msolar yr-1 with a peak rate of 17Msolar yr-1. Gas with high volume density is foundin the inner parts of the postcollision disks at size scales close tothe spatial resolution of the simulations, and this may be a directresult of shocks traced by the discontinuity in the gas velocity field.The evolution of gas in a bulgeless progenitor galaxy is also discussed,and a possible link to the ``chain galaxy'' population observed at highredshifts is inferred. The evolution of the structural parameters suchas asymmetry and concentration of both stars and gas are studied indetail. Further, a new structure parameter (the compactness parameter K)that traces the evolution of the size scale of the gas relative to thestellar disk is introduced, and this may be a useful tracer to determinethe merger chronology of colliding systems. Noncircular gas kinematicsdriven by the perturbation of the nonaxisymmetric structure can producedistinct emission features in the ``forbidden velocity quadrants'' ofthe position-velocity diagram (PVD). The dynamical mass calculated usingthe rotation curve derived from fitting the emission envelope of the PVDcan determine the true mass to within 20%-40%. The evolution of themolecular fraction(MH2/MH2+HI) is a potentialtracer to quantitatively assign the age of the interaction, but theapplication to real systems may require additional observationaldiagnostics to properly assess the exact chronology of the mergerevolution.
| Luminous Infrared Galaxies as Plausible Gamma-Ray Sources for the Gamma-Ray Large Area Space Telescope and the Imaging Atmospheric Cerenkov Telescopes
We argue that luminous infrared galaxies may constitute a newlydetectable population of γ-ray sources for the next generation ofground- and space-based high-energy telescopes. In addition, we reportfor the first time upper limits on their fluxes using data obtained withthe EGRET telescope.
| The Star Formation Rate and Dense Molecular Gas in Galaxies
HCN luminosity is a tracer of dense molecular gas,n(H2)>~3×104cm-3, associatedwith star-forming giant molecular cloud (GMC) cores. We present theresults and analysis of our survey of HCN emission from 65 infraredgalaxies, including nine ultraluminous infrared galaxies (ULIGs,LIR>~1012Lsolar), 22 luminousinfrared galaxies (LIGs,1011Lsolar0.06 are LIGs or ULIGs. Normal spiralsall have similar and low dense gas fractionsLHCN/LCO=0.02-0.05. The global star formationefficiency depends on the fraction of the molecular gas in a densephase.
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