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Nuclear starbursts and AGN fueling
We argue that radiation pressure from star formation can support agalactic disk against its own self-gravity. This model is appropriatewhen the disk is optically thick to its own infrared radiation, as inthe central regions of Ultraluminous Infrared Galaxies and in the outerparts of accretion disks in Active Galactic Nuclei. We review theproperties of radiation-pressure supported disks and discuss theconditions under which AGN can be fueled by gas stored in an outerstarburst disk.
| Mid-Infrared Diagnostics of LINERS
We report results from the first mid-infrared spectroscopic study of acomprehensive sample of 33 LINERs, observed with the Spitzer SpaceTelescope. We compare the properties of two different LINER populations:infrared-faint LINERs, with LINER emission arising mostly in compactnuclear regions, and infrared-luminous LINERs, which often showspatially extended (non-AGN) LINER emission. We show that these twopopulations can be easily distinguished by their mid-infrared spectra inthree different ways: (1) their mid-IR spectral energy distributions(SEDs), (2) the emission features of polycyclic aromatic hydrocarbons(PAHs), and (3) various combinations of IR fine-structure line ratios.IR-luminous LINERs show mid-IR SEDs typical of starburst galaxies, whilethe mid-IR SEDs of IR-faint LINERs are much bluer. PAH flux ratios aresignificantly different in the two groups. Fine-structure emission linesfrom highly excited gas, such as [O IV], are detected in bothpopulations, suggesting the presence of an additional AGN in a largefraction of IR-bright LINERs as well, which contributes little to thecombined mid-IR light. The two LINER groups occupy different regions ofmid-infrared emission-line excitation diagrams. The positions of thevarious LINER types in our diagnostic diagrams provide important cluesregarding the power source of each LINER type. Most of thesemid-infrared diagnostics can be applied at low spectral resolution,making AGN- and starburst-excited LINERs distinguishable at highredshifts as well.
| Dynamical Properties of Ultraluminous Infrared Galaxies. II. Traces of Dynamical Evolution and End Products of Local Ultraluminous Mergers
We present results from our Very Large Telescope large program to studythe dynamical evolution of local ultraluminous infrared galaxies(ULIRGs) and QSOs. This paper is the second in a series presenting thestellar kinematics of 54 ULIRGs, derived from high-resolution, long-slitH- and K-band spectroscopy. The data presented here, includingobservations of 17 new targets, are mainly focused on sources that havecoalesced into a single nucleus. The stellar kinematics, extracted fromthe CO ro-vibrational band heads in our spectra, indicate that ULIRGremnants are dynamically heated systems with a mean dispersion of 161 kms-1. The combination of kinematic, structural, andphotometric properties of the remnants indicate that they mostlyoriginate from major mergers and that they result in the formation ofsystems supported by random motions, i.e., elliptical galaxies. The peakof the velocity dispersion distribution and the locus of ULIRGs on thefundamental plane of early-type galaxies indicate that the end productsof ultraluminous mergers are typically moderate-mass ellipticals (ofstellar mass ~1010-1011 Msolar).Converting the host dispersion into black hole mass with the aid of theMBH-σ relation yields black hole mass estimates of theorder 107-108 Msolar and high accretionrates with Eddington efficiencies often >0.5.Based on observations at the European Southern Observatory (ESO)(171.B-044).
| Dynamical Masses in Luminous Infrared Galaxies
We have studied the dynamics and masses of a sample of 10 nearbyluminous and ultraluminous infrared galaxies (LIRGs and ULIRGs,respectively) using 2.3 μm 12CO-absorption linespectroscopy and near-infrared H- and Ks-band imaging. Bycombining velocity dispersions derived from the spectroscopy, disk scalelengths obtained from the imaging, and a set of likely model densityprofiles, we calculate dynamical masses for each LIRG. For the majorityof the sample, it is difficult to reconcile our mass estimates with thelarge amounts of gas derived from millimeter observations and from astandard conversion between 12CO emission and H2mass. Our results imply that LIRGs do not have huge amounts of moleculargas (1010-1011 Msolar) at their centersand support previous indications that the standard conversion of12CO to H2 probably overestimates the gas massesand cannot be used in these environments. This in turn suggests muchmore modest levels of extinction in the near-infrared for LIRGs thanpreviously predicted (AV~10-20 vs. AV~100-1000).The lower gas mass estimates indicated by our observations imply thatthe star formation efficiency in these systems is very high and istriggered by cloud-cloud collisions, shocks, and winds rather than bygravitational instabilities in circumnuclear gas disks.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| 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.
| Ultraviolet-to-Far-Infrared Properties of Local Star-forming Galaxies
We present the results of a multiwavelength study of nearby galaxiesaimed at understanding the relation between the ultraviolet andfar-infrared emission in star-forming galaxies. The data set comprisesnew ultraviolet (from HST STIS), ground-based Hα, and radiocontinuum observations, together with archival infrared data (from IRASand ISO). The local galaxies are used as benchmarks for comparison ofthe infrared-to-ultraviolet properties with two populations ofhigh-redshift galaxies: the submillimeter star-forming galaxies detectedby SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). Inaddition, the long wavelength baseline covered by the present dataenables us to compare the star formation rates (SFRs) derived from theobserved ultraviolet, Hα, infrared, and radio luminosities and togauge the impact of dust opacity in the local galaxies. We also derive anew calibration for the nonthermal part of the radio SFR estimator,based on the comparison of 1.4 GHz measurements with a new estimator ofthe bolometric luminosity of the star-forming regions. We find that moreactively star-forming galaxies show higher dust opacities, which is inline with previous results. We find that the local star-forming galaxieshave a lower Fλ(205 μm)/Fλ(UV)ratio by 2-3 orders of magnitude than the submillimeter-selectedgalaxies and may have a similar or somewhat higherFλ(205 μm)/Fλ(UV) ratio thanLBGs. The Fλ(205 μm)/Fλ(UV) ratioof the local galaxy population may be influenced by the cool dustemission in the far-infrared heated by nonionizing stellar populations,which may be reduced or absent in the LBGs.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Is HCN a True Tracer of Dense Molecular Gas in Luminous and Ultraluminous Infrared Galaxies?
We present the results of the first HCO+ survey probing thedense molecular gas content of a sample of 16 luminous and ultraluminousinfrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN (1-0)observations, had shown that LIRGs and ULIRGs possess a significantlyhigher fraction of dense molecular gas compared to normal galaxies.While the picture issued from HCO+ partly confirms thisresult, we have discovered an intriguing correlation between the HCN(1-0)/HCO+ (1-0) luminosity ratio and the IR luminosity ofthe galaxy (LIR). This trend casts doubts on the use of HCNas an unbiased quantitative tracer of the dense molecular gas content inLIRGs and ULIRGs. A plausible scenario explaining the observed trendimplies that X-rays coming from an embedded active galactic nucleus mayplay a dominant role in the chemistry of molecular gas atLIR>=1012 Lsolar. We discuss theimplications of this result for the understanding of LIRGs, ULIRGs, andhigh-redshift gas-rich galaxies.
| A Survey of Merger Remnants. II. The Emerging Kinematic and Photometric Correlations
This paper is the second in a series exploring the properties of 51optically selected, single-nuclei merger remnants. Spectroscopic datahave been obtained for a subsample of 38 mergers and combined withpreviously obtained infrared photometry to test whether mergers exhibitthe same correlations as elliptical galaxies among parameters such asstellar luminosity and distribution, central stellar velocity dispersion(σ0), and metallicity. Paramount to the study is totest whether mergers lie on the fundamental plane. Measurements ofσ0 have been made using the Ca triplet absorption lineat 8500 Å for all 38 mergers in the subsample. Additionalmeasurements of σ0 were made for two of the mergers inthe subsample using the CO absorption line at 2.29 μm. The resultsindicate that mergers show a strong correlation among the parameters ofthe fundamental plane but fail to show a strong correlation betweenσ0 and metallicity (Mg2). In contrast toearlier studies, the σ0 values of the mergers areconsistent with objects that lie somewhere between intermediate-mass andluminous giant elliptical galaxies. However, the discrepancies withearlier studies appear to correlate with whether the Ca triplet or COabsorption lines are used to derive σ0, with the latteralmost always producing smaller values. Finally, the photometric andkinematic data are used to demonstrate for the first time that thecentral phase-space densities of mergers are equivalent to those inelliptical galaxies. This resolves a long-standing criticism of themerger hypothesis.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| 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.
| Molecular Gas and Nuclear Activity in Radio Galaxies Detected by IRAS
This paper reports the latest results from a millimeter-wave (CO)spectroscopic survey of IRAS-detected radio galaxies withL1.4GHz~1023-1028 W Hz-1 inthe redshift range z~0.02-0.15. The IRAS flux-limited sample contains 33radio galaxies with different radio morphologies and a broad range ofinfrared luminosities (LIR=109-1012Lsolar), allowing for an investigation of (1) whether low-zradio-selected active galactic nuclei (AGNs) reside in moleculargas-rich host galaxies and (2) whether the CO properties are correlatedwith the properties of the host galaxy or the AGN. All of the radiogalaxies in Mazzarella et al. and Mirabel et al. have been reobserved.Three new CO detections have been made, raising the total number of COdetections to nine and setting the survey detection rate at ~25%. Manyof the CO lines have double-peaked profiles, and the CO line widths arebroad (average ΔvFWHM~500+/-130 km s-1),exceeding the average CO widths of both ultraluminous infrared galaxies(300+/-90 km s-1) and Palomar-Green QSOs (260+/-160 kms-1), and thus being indicative of massive host galaxies. TheCO luminosities translate into molecular gas masses of~(0.4-7)×109 Msolar, however, the 3 σCO upper limits for nondetections do not rule out a molecular gas massas high as that of the Milky Way (~3×109Msolar). Optical images of eight out of nine moleculargas-rich radio galaxies show evidence of close companions and/or tidalfeatures. Finally, there is no obvious correlation between radio powerand molecular gas mass. However, it is notable that only one F-R IIgalaxy out of 12 is detected in this CO survey; the remaining detectionsare of galaxies hosting F-R I and compact radio jets.
| A General Theory of Turbulence-regulated Star Formation, from Spirals to Ultraluminous Infrared Galaxies
We derive an analytic prediction for the star formation rate inenvironments ranging from normal galactic disks to starbursts and ULIRGsin terms of the observables of those systems. Our calculation is basedon three premises: (1) star formation occurs in virialized molecularclouds that are supersonically turbulent; (2) the density distributionwithin these clouds is lognormal, as expected for supersonic isothermalturbulence; and (3) stars form in any subregion of a cloud that is sooverdense that its gravitational potential energy exceeds the energy inturbulent motions. We show that a theory based on this model isconsistent with simulations and with the observed star formation rate inthe Milky Way. We use our theory to derive the Kennicutt-Schmidt lawfrom first principles and make other predictions that can be tested byfuture observations. We also provide an algorithm for estimating thestar formation rate that is suitable for inclusion in numericalsimulations.
| Radiation Pressure-supported Starburst Disks and Active Galactic Nucleus Fueling
We consider the structure of marginally Toomre-stable starburst disksunder the assumption that radiation pressure on dust grains provides thedominant vertical support against gravity. This assumption isparticularly appropriate when the disk is optically thick to its owninfrared radiation, as in the central regions of ULIRGs. We argue thatbecause the disk radiates at its Eddington limit (for dust), the``Schmidt law'' for star formation changes in the optically thick limit,with the star formation rate per unit area scaling asΣ˙*~Σg/κ, whereΣg is the gas surface density and κ is the meanopacity of the disk. Our calculations further show that optically thickstarburst disks have a characteristic flux, star formation rate per unitarea, and dust effective temperature of F~1013Lsolar kpc-2,Σ˙*~103 Msolaryr-1 kpc-2, and Teff~90 K,respectively. We compare our model predictions with observations ofULIRGs and find good agreement. We extend our model of starburst disksfrom many hundred parsec scales to subparsec scales and address theproblem of fueling AGNs. We assume that angular momentum transportproceeds via global torques (e.g., spiral waves, winds, or a centralbar) rather than a local viscosity. We consistently account for theradial depletion of gas due to star formation and find a strongbifurcation between two classes of disk models: (1) solutions with astarburst on large scales that consumes all of the gas with little or nofueling of a central AGN and (2) models with an outer large-scalestarburst accompanied by a more compact starburst on 1-10 pc scales anda bright central AGN. The luminosity of the latter models is in manycases dominated by the AGN, although these disk solutions exhibit abroad mid- to far-infrared peak from star formation. We show that thevertical thickness of the starburst disk on parsec scales can approachh~r, perhaps accounting for the nuclear obscuration in some type 2 AGNs.We also argue that the disk of young stars in the Galactic center may bethe remnant of such a compact nuclear starburst.
| Simulating the Spitzer Mid-Infrared Color-Color Diagrams
We use a simple parameterization of the mid-IR spectra of a wide rangeof galaxy types in order to predict their distribution in the InfraredArray Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 μm and MultibandPhotometer for Spitzer 24 μm color-color diagrams. We distinguishthree basic spectral types by the energetically dominant component inthe 3-12 μm regime: stellar-dominated, polycyclic aromatichydrocarbon (PAH)-dominated, and continuum-dominated. We use a Markovchain Monte Carlo approach to arrive at a more systematic and robustrepresentation of the mid-IR spectra of galaxies than do moretraditional approaches. We find that IRAC color-color plots are wellsuited to distinguishing the above spectral types, while the addition of24 μm data allows us to suggest practical three-color cuts thatpreferentially select higher redshift sources of a specific type. Wecompare our simulations with the color-color plot obtained by theSpitzer First Look Survey and find reasonable agreement. Lastly, wediscuss other applications as well as future directions for this work.
| 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.
| Spectroscopic study of blue compact galaxies. V. Oxygen abundance and the metallicity-luminosity relation
This is the fifth paper in a series studying the stellar components,star formation histories, star formation rates and metallicities of ablue compact galaxy (BCG) sample. Based on our high-quality ground-basedspectroscopic observations, we have determined the electrontemperatures, electron densities, nitrogen abundances and oxygenabundances for 72 star-forming BCGs in our sample, using differentoxygen abundance indicators. The oxygen abundance covers the range 7.15< 12 + log (O/H)< 9.0, and nitrogen is found to be mostly aproduct of secondary nucleosynthesis for 12 + log (O/H)>8.2 andapparently a product of primary nucleosynthesis for 12 + log (O/H)<8.2. To assess the possible systematic differences among differentoxygen abundance indicators, we have compared oxygen abundances of BCGsobtained with the Te method, R23 method, P method,N2 method and O3N2 method. The oxygen abundances derived from theTe method are systematically lower by 0.1-0.25 dex than thosederived from the strong line empirical abundance indicators, consistentwith previous studies based on region samples. We confirm the existenceof the metallicity-luminosity relation in BCGs over a large range ofabundances and luminosities. Our sample of galaxies shows that the slopeof the metallicity-luminosity relation for the luminous galaxies(~-0.05) is slightly shallower than that for the dwarf galaxies(~-0.17). An offset was found in the metallicity-luminosity relation ofthe local galaxies and that of the intermediate redshift galaxies. Itshows that the metallicity-luminosity relation for the emission linegalaxies at high redshift is displaced to lower abundances, higherluminosities, or both.
| Galaxy mergers with various mass ratios: Properties of remnants
We study galaxy mergers with various mass ratios using N-bodysimulations, with an emphasis on the unequal-mass mergers in therelatively unexplored range of mass-ratios 4:1-10:1. Our recent work(Bournaud et al. 2004) shows that the above range of mass ratio resultsin hybrid systems with spiral-like luminosity profiles but withelliptical-like kinematics, as observed in the data analysis for asample of mergers by Jog & Chitre 2002). In this paper, we study themerger remnants for mass ratios from 1:1 to 10:1 while systematicallycovering the parameter space. We obtain the morphological andkinematical properties of the remnants, and also discuss the robustnessand the visibility of disks in the merger remnants with a randomline-of-sight. We show that the mass ratios 1:1-3:1 give rise toelliptical remnants whereas the mass ratios 4.5:1-10:1 produce hybridsystems with mixed properties. We find that the transition betweendisk-like and elliptical remnants occurs between a narrow mass range of4.5:1-3:1. The unequal-mass mergers are more likely to occur than thestandard equal-mass mergers studied in the literature so far, and wediscuss their implications for the evolution of galaxies.
| Mid-infrared luminosity as an indicator of the total infrared luminosity of galaxies
The infrared (IR) emission plays a crucial role in understanding thestar formation in galaxies hidden by dust. We first examined fourestimators of the IR luminosity of galaxies, LFIR [CITE],LTIR [CITE], revised version of LTIR [CITE] (wedenote LTIR2), and LIR [CITE] by using theobserved SEDs of well-known galaxies. We found that LIRprovides excellent estimates of the total IR luminosity for a variety ofgalaxy SEDs. The performance of LTIR2 was also found to bevery good. Using LIR, we then statistically analyzed the IRASPSCz galaxy sample [CITE] and found useful formulae relating the MIRmonochromatic luminosities [ L(12 μm) and L(25 μm)] andLIR. For this purpose we constructed a subsample of 1420galaxies with all four IRAS band (12, 25, 60, and 100 μm) fluxdensities. We found linear relations between LIR and MIRluminosities, L(12 μm) and L(25 μm). The prediction error with a95% confidence level is a factor of 4-5. Hence, these formulae areuseful for the estimation of the total IR luminosity only from 12 μmor 25 μm observations. We further tried to make an“interpolation” formula for galaxies at 0
| 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.
| 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.
| 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.
| 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).
| 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.
| A Deep K-Band Photometric Survey of Merger Remnants
We present K-band photometry for 51 candidate merger remnants to assessthe viability of whether spiral-spiral mergers can produce bona fideelliptical galaxies. Using both the de Vaucouleurs r1/4 andSérsic r1/n fitting laws, it is found that the stellarcomponent in a majority of the galaxies in the sample has undergoneviolent relaxation. However, the sample shows evidence for incompletephase mixing. The analysis also indicates the presence of ``excesslight'' in the surface brightness profiles of nearly one-third of themerger remnants. Circumstantial evidence suggests that this is due tothe effects of a starburst induced by the dissipative collapse of thegas. The integrated light of the galaxies also shows that mergers canmake L* elliptical galaxies, in contrast to earlier infrared studies.The isophotal shapes and related structural parameters are alsodiscussed, including the fact that 70% of the sample show evidence fordisky isophotes. The data and results presented are part of a largerphotometric and spectroscopic campaign to thoroughly investigate a largesample of mergers in the local universe.
| Spectroscopic study of blue compact galaxies. IV. Star formation rates and gas depletion timescales
This is the fourth paper in a series studying star formation rates,stellar components, metallicities, and star formation histories of ablue compact galaxy (BCG) sample. Using Hα, [O II]λ3727,infrared (IR), radio (1.4 GHz) luminosities and neutral hydrogen (H I)gas masses, we estimated star formation rates and gas depletiontimescales of 72 star-forming BCGs. The star formation rates of the BCGsin our sample span nearly four orders of magnitude, from approximately10-2 to 102 Mȯ yr-1,with a median star formation rate of about 3 Mȯyr-1. The typical gas depletion timescale of BCGs is aboutone billion years. Star formation could be sustained at the currentlevel only on a timescale significantly lower than the age of theuniverse before their neutral gas reservoir is completely depleted. Toassess the possible systematic differences among different starformation rate indicators, we compared the star formation rates derivedfrom Hα, [O II]λ3727, IR, and radio luminosities, andinvestigated the effects from underlying stellar absorption and dustextinction. We found that subtracting underlying stellar absorption isvery important to calculate both dust extinction and star formation rateof galaxies. Otherwise, the intrinsic extinction will be overestimated,the star formation rates derived from [O II]λ3727 and Hαwill be underestimated (if the underlying stellar absorption and theinternal extinction were not corrected from the observed luminosity) oroverestimated (if an overestimated internal extinction were used forextinction correction). After both the underlying stellar absorption andthe dust extinction were corrected, a remarkably good correlationemerges among Hα, [O II]λ3727, IR and radio star formationrate indicators. Finally, we find a good correlation between themeasured star formation rate and the absolute blue magnitude,metallicity, interstellar extinction of BCGs. Our results indicate thatfaint, low-mass BCGs have lower star formation rates.Star formation rates and gas depletion timescales of BCGs are availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/425/417
| The gas content of peculiar galaxies: Strongly interacting systems
A study of the gas content in 1038 interacting galaxies, essentiallyselected from Arp, Arp & Madore, Vorontsov-Velyaminov catalogues andsome of the published literature, is presented here. The data on theinterstellar medium have been extracted from a number of sources in theliterature and compared with a sample of 1916 normal galaxies. The meanvalues for each of the different ISM tracers (FIR, 21 cm, CO lines,X-ray) have been estimated by means of survival analysis techniques, inorder to take into account the presence of upper limits. From the datait appears that interacting galaxies have a higher gas content thannormal ones. Galaxies classified as ellipticals have both a dust and gascontent one order of magnitude higher than normal. Spirals have in mostpart a normal dust and HI content but an higher molecular gas mass. TheX-ray luminosity also appears higher than that of normal galaxies ofsame morphological type, both including or excluding AGNs. We consideredthe alternative possibilities that the molecular gas excess may derivefrom the existence of tidal torques which produce gas infall from thesurrounding regions or from a different metallicity which affects the Xconversion factor between the observed CO line luminosity and the H_2calculated mass. According to our tests, it appears that interactinggalaxies possess a higher molecular mass than normal galaxies but with asimilar star formation efficiency.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/941
| A joint mid-infrared spectroscopic and X-ray imaging investigation of LINER galaxies
We present a comprehensive comparative high resolution mid-IRspectroscopic and X-ray imaging investigation of LINERs using archivalobservations from the ISO-SWS and the Chandra Advanced CCD ImagingSpectrometer. Although the sample is heterogenous and incomplete, thisis the first comprehensive study of the mid-infrared fine structure lineemission of LINERs. These results have been compared with similarobservations of starburst galaxies and AGN. We find that LINERs veryclearly fall between starbursts and AGN in their mid-IR fine structureline spectra, showing L[OIV]26 μm/LFIR andL[OIV]26 μm/L[NeII]12.8 μm ratios, bothmeasures of the dominant nuclear energy source in dust-enshroudedgalaxies, intermediate between those of AGN and starbursts. Chandraimaging observations of the LINERs reveal hard nuclear point sourcesmorphologically consistent with AGN in most (67%) of the sample, with aclear trend with IR-brightness. Most LINERs that show a single dominanthard compact X-ray core are IR-faint (LFIR/LB <1), whereas most LINERs that show scattered X-ray sources are IR-bright.A comparative X-ray/mid-IR spectroscopic investigation of LINERs revealssome puzzling results. Objects that display strong hard nuclear X-raycores should also display high excitation lines in the IR. However, wefind two LINERs disagree with this expectation. The galaxy NGC 404 showsweak soft X-ray emission consistent with a starburst but has the mostprominent highest excitation mid-IR spectrum of our entire sample. UsingIR emission line diagnostics alone, this galaxy would be classified ashosting a dominant AGN. Conversely, the IR luminous LINER NGC 6240 hasan extremely luminous binary AGN as revealed by the X-rays but showsweak IR emission lines. With the advent of SIRTF, and future IR missionssuch as Herschel and JWST, it is increasingly critical to determine theorigin of these multiwavelength anomalies.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/825Table 3 is only available in electronic form athttp://www.edpsciences.org
| Detailed structure of the ISM in the luminous infrared galaxy Arp193.
We describe high resolution observations of the neutral hydrogenabsorption towards the nuclear regions of the luminous infrared galaxyArp 193 made with MERLIN and the VLA. We find a rotating ring of neutralgas of radius ~ 400 tmspace + thickmuskip .2777empc. The radiocontinuum spectral index is consistent with a scenario where the mostrecent star formation is occurring at the centre of this ring.
| A high-resolution radio study of neutral gas in the starburst galaxy NGC 520
We present subarcsec angular resolution observations of the neutral gasin the nearby starburst galaxy NGC 520. The central kpc region of NGC520 contains an area of significantly enhanced star formation. The radiocontinuum structure of this region resolves into ~10 continuumcomponents. By comparing the flux densities of the brightest of thesecomponents at 1.4 GHz with published 15-GHz data we infer that thesecomponents detected at 1.4 and 1.6 GHz are related to the starburst andare most likely to be collections of several supernova remnants withinthe beam. None of these components is consistent with emission from anactive galactic nuclei. Both neutral hydrogen (HI) and hydroxyl (OH)absorption lines are observed against the continuum emission, along witha weak OH maser feature probably related to the star formation activityin this galaxy. Strong HI absorption (NH~ 1022atoms cm-2) traces a velocity gradient of 0.5 kms-1 pc-1 across the central kpc of NGC 520. The HIabsorption velocity structure is consistent with the velocity gradientsobserved in both the OH absorption and in CO emission observations. Theneutral gas velocity structure observed within the central kpc of NGC520 is attributed to a kpc-scale ring or disc. It is also noted that thevelocity gradients observed for these neutral gas components appear todiffer with the velocity gradients observed from optical ionizedemission lines. This apparent disagreement is discussed and attributedto the extinction of the optical emission from the actual centre of thissource hence implying that optical ionized emission lines are onlydetected from regions with significantly different radii to thosesampled by the observations presented here.
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