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Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.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.
| 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.
| Late-Time Radio Observations of 68 Type Ibc Supernovae: Strong Constraints on Off-Axis Gamma-Ray Bursts
We present late-time radio observations of 68 local Type Ibc supernovae,including six events with broad optical absorption lines(``hypernovae''). None of these objects exhibit radio emissionattributable to off-axis gamma-ray burst jets spreading into our line ofsight. Comparison with our afterglow models reveals the followingconclusions. (1) Less than ~10% of Type Ibc supernovae are associatedwith typical gamma-ray bursts initially directed away from our line ofsight; this places an empirical constraint on the GRB beaming factor of<~104, corresponding toan average jet opening angle, θj>~0.8d. (2) Thisholds in particular for the broad-lined supernovae (SNe 1997dq, 1997ef,1998ey, 2002ap, 2002bl, and 2003jd), which have been argued to host GRBjets. Our observations reveal no evidence for typical (or evensubenergetic) GRBs and rule out the scenario in which every broad-linedSN harbors a GRB at the 84% confidence level. Their large photosphericvelocities and asymmetric ejecta (inferred from spectropolarimetry andnebular spectroscopy) appear to be characteristic of the nonrelativisticSN explosion and do not necessarily imply the existence of associatedGRB jets.
| The relation between mergers and AGN activity. Results from radio galaxy and luminous infrared galaxy studies
There is morphological evidence that the activity in powerful radiogalaxies could be triggered by mergers and galaxy interactions. However,nothing is known about the timescales, order of events, and the type ofinteraction involved. It is not yet known whether there exists anevolutionary link between radio galaxies and other merger systems suchas very luminous and ultraluminous infrared galaxies (VLIRGs andULIRGs). Here, we report preliminary results obtained from the analysisof near-ultraviolet and optical spectroscopic observations of samples ofRadio Galaxies and VLIRGs-ULIRGs to investigate, through age-dating oftheir young stellar population, whether an evolutionary link existsbetween VLIRGs/ULIRGs-Radio Galaxies-normal elliptical galaxies. Theseresults will help to understand the genesis events that lead to theformation of radio jet and quasar activity, and they will allow us toplace radio galaxies in the context of hierarchical evolution models forthe population of giant elliptical 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.
| SINFONI adaptive optics integral field spectroscopy of the Circinus Galaxy
Aims.We investigate the star formation activity and the gas and stellardynamics on scales of a few parsecs in the nucleus of the CircinusGalaxy. Methods: .Using the adaptive optics near infrared integralfield spectrometer SINFONI on the VLT, we have obtained observations ofthe Circinus galaxy on scales of a few parsecs and at a spectralresolution of 70 km s-1 FWHM. The physical properties of thenucleus are analyzed by means of line and velocity maps extracted fromthe SINFONI datacube. Starburst models are constrained using theBrγ flux, stellar continuum (as traced via the CO absorptionbandheads longward of 2.3 μm), and radio continuum. Results:.The similarity of the morphologies of the H2 1-0 S(1) 2.12 μm andBrγ 2.17 μm lines to the stellar continuum and also theirkinematics, suggest a common origin in star formation. Within 8 pc ofthe AGN we find there has been a recent starburst in the last 100 Myr,which currently accounts for 1.4% of the galaxy's bolometric luminosity.The similarity of the spatial scales over which the stars and gas existindicates that this star formation is occuring within the torus; andcomparison of the gas column density through the torus to the maximumpossible optical depth to the stars implies the torus is a clumpymedium. The coronal lines show asymmetric profiles with a spatiallycompact narrow component and a spatially extended blue wing. Thesecharacteristics are consistent with some of the emission arising inclouds gravitationally bound to the AGN, and some outflowing incloudlets which have been eroded away from the bound clouds.
| Dense and Warm Molecular Gas between Double Nuclei of the Luminous Infrared Galaxy NGC 6240
High spatial resolution observations of the 12CO (1-0),HCN(1-0), HCO+(1-0), and 13CO(1-0) molecular linestoward the luminous infrared merger NGC 6240 have been performed usingthe Nobeyama Millimeter Array and the RAINBOW Interferometer. All of theobserved molecular emission lines are concentrated in the region betweenthe double nuclei of the galaxy. However, the distributions of both HCNand HCO+ emissions are more compact compared with that of12CO, and they are not coincident with the star-formingregions. The HCN/12CO line intensity ratio is 0.25; thissuggests that most of the molecular gas between the double nuclei isdense. A comparison of the observed high HCN/13CO intensityratio, 5.9, with large velocity gradient calculations suggests that themolecular gas is dense [n(H2) = 104--6cm-3] and warm (Tkin > 50 K). The observedstructure in NGC 6240 may be explained by time evolution of themolecular gas and star formation, which was induced by an almost head-oncollision or very close encounter of the two galactic nuclei accompaniedwith the dense gas and star-forming regions.
| Molecular hydrogen and [FeII] in active galactic nuclei - II. Results for Seyfert 2 galaxies
Near-infrared spectroscopy is used to study the kinematics andexcitation mechanisms of H2 and [FeII] lines in a sampledominated by Seyfert 2 galaxies. The spectra simultaneously cover theJHK bands, allowing us to compare line fluxes emitted in the interval0.8-2.4 μm and avoiding aperture and seeing effects. TheH2 lines are systematically narrower than the narrow-lineregion lines, suggesting that, very likely, the H2 does notoriginate from the same parcel of gas that forms the narrow-line region.Emission-line ratios between H2 lines favour thermalexcitation mechanisms for the molecular gas in active galactic nuclei.It was found that non-thermal excitation contributes, at most, 30 percent of the observed H2. Thermal excitation is also confirmedby the rather similar vibrational and rotational temperatures in theobjects (~2000 K). The mass of hot H2 ranges from102 to 103Msolar, with nearly half ofobjects showing values of <500 Msolar. It shows that thefraction of molecular mass present in the nuclear region and emitting inthe near-infrared is a very small fraction of the warm molecular masspresent in the centre. A diagnostic diagram composed of the line ratiosH2/Brγ and [FeII]/Paβ proves to be a useful toolin the near-infrared for separating emission-line objects by theirdegree of nuclear activity. We found that active galactic nuclei arecharacterized by H2 2.121 μm/Brγ and [FeII] 1.257μm/Paβ flux ratios between 0.6 and 2. Starburst/HII galaxiesdisplay line ratios <0.6 while low-ionization nuclear emission-lineregions are characterized by values larger than 2 in either ratio.
| From young massive star cluster to old globular: the LV-σ0 relationship as a diagnostic tool
We present a new analysis of the properties of the young massive starclusters (YMCs) forming profusely in intense starburst environments,which demonstrates that these objects are plausible progenitors of theold globular clusters (GCs) seen abundantly in the Local Group. Themethod is based on the tight relationship for old GCs between theirV-band luminosities, LV, and (central) velocity dispersions,σ0. We improve the significance of the relationship byincreasing the GC sample size and find that its functional form,LV/Lsolar~σ1.57+/-0.100(km s-1), is fully consistent with previous determinationsfor smaller Galactic and M31 GC samples. The tightness of therelationship for a GC sample drawn from environments as diverse as thosefound in the Local Group implies that its origin must be sought inintrinsic properties of the GC formation process itself. We evolve theluminosities of those YMCs in the local Universe which have velocitydispersion measurements to an age of 12 Gyr, adopting a variety ofinitial mass function (IMF) descriptions, and find that most YMCs willevolve to loci close to, or to slightly fainter luminosities than theimproved GC relationship. In the absence of significant externaldisturbances, this implies that these objects may potentially survive tobecome old GC-type objects over a Hubble time. The main advantage of ournew method is its simplicity. Whereas alternative methods, based ondynamical mass estimates, require one to obtain accurate size estimatesand to make further assumptions, the only observables required here arethe system's velocity dispersion and luminosity. The most importantfactor affecting the robustness of our conclusions is the adopted formof the IMF. We use the results of N-body simulations to confirm thatdynamical evolution of the clusters does not significantly alter ourconclusions about the likelihood of individual clusters surviving tolate times. Finally, we find that our youngest observed clusters areconsistent with having evolved from a relation of the form . Thisrelation may actually correspond to the origin of the GC fundamentalplane.
| Infrared mergers and infrared quasi-stellar objects with galactic winds - III. Mrk 231: an exploding young quasi-stellar object with composite outflow/broad absorption lines (and multiple expanding superbubbles)
We present a study of outflow (OF) and broad absorption line (BAL)systems in Mrk 231, and in similar infrared (IR) quasi-stellar objects(QSOs). This study is based mainly on one-dimensional andtwo-dimensional spectroscopy (obtained at La Palma/William HerschelTelescope, Hubble Space Telescope, International Ultraviolet Explorer,European Southern Observatory/New Technology Telescope, Kitt PeakNational Observatory, Apache Point Observatory and Complejo AstronomicoEl Leoncito observatories) plus Hubble Space Telescope images. For Mrk231, we report evidence that the extreme nuclear OF process has at leastthree main components on different scales, which are probably associatedwith: (i) the radio jet, at parsec scale; (ii) the extreme starburst atparsec and kiloparsec scale. This OF has generated at least fourconcentric expanding superbubbles and the BAL systems.Specifically, inside and very close to the nucleus the two-dimensionalspectra show the presence of an OF emission bump in the blendHα+[NII], with a peak at the same velocity of the main BAL-Isystem (VEjectionBAL-I~-4700 km s-1). This bumpwas more clearly detected in the area located at 0.6-1.5 arcsec(490-1220 pc), to the south-west of the nucleus core, showing a strongand broad peak. In addition, in the same direction [at position angle(PA) ~-120°, i.e. close to the PA of the small-scale radio jet] at1.7-2.5 arcsec, we also detected multiple narrow emission-linecomponents, with `greatly' enhanced [NII]/Hα ratio (very similarto the spectra of jets bow shocks). These results suggest that the BAL-Isystem is generated in OF clouds associated with the parsec-scale jet.The Hubble Space Telescope images show four (or possibly five) nuclearsuperbubbles or shells with radii r~ 2.9, 1.5, 1.0, 0.6 and 0.2 kpc. Forthese bubbles, the two-dimensional Hα velocity field map andtwo-dimensional spectra show the following. (i) At the border of themore extended bubble (S1), a clear expansion of the shell withblueshifted velocities (with circular shape and at a radius r~ 5.0arcsec). This bubble shows a rupture arc - to the south - suggestingthat the bubble is in the blowout phase. The axis of this rupture orejection (at PA ~ 00°) is coincident with the axis of theintermediate and large-scale structures detected at radio wavelengths.(ii) In addition, in the three more external bubbles (S1, S2, S3), thetwo-dimensional William Herschel Telescope spectra show multipleemission-line components with OF velocities, of S1, S2 and S3 =[-(650 - 420) +/- 30], [-500+/- 30] and [-230 +/- 30] km s-1. (iii) In the wholecircumnuclear region (1.8 < r < 5 arcsec), the [NII]/Hα and[SII]/Hα narrow emission-line ratios show high values (>0.8),which are consistent with low-ionization nuclear emission-line region/OFprocesses associated with fast velocity shocks. Therefore, we suggestthat these giant bubbles are associated with the large-scale nuclear OFcomponent, which is generated - at least in part - by the extremenuclear starburst: giant supernova/hypernova explosions.The variability of the short-lived BAL-III NaI D system was studied,covering almost all the period in which this system appeared (between~1984 and 2004). We have found that the BAL-III light curve is clearlyasymmetric with a steep increase, a clear maximum and an exponentialfall (similar to the shape of a supernova light curve). The origin ofthis BAL-III system is discussed, mainly in the framework of an extremeexplosive event, probably associated with giant supernova/hypernovaexplosions.Finally, the IR colour diagram and the ultraviolet BAL systems of IR +GW/OF + FeII QSOs are analysed. This study shows two new BAL IR QSOs andsuggests/confirms that these objects could be nearby young BAL QSOs,similar to those detected recently at z~ 6.0. We propose that the phaseof young QSOs is associated with accretion of a large amount of gas (bythe supermassive black hole) + extreme starbursts + extreme compositeOFs/BALs.
| Radio Identifications of Markarian Galaxies and the Correlation between Radio and Far-Infrared Properties
By checking DSS optical images and NVSS radio images, 782 Markariangalaxies were identified to be NVSS radio sources. A comparison of theradio luminosity at 1.4 GHz and the far-infrared (FIR) luminosity for468 ``normal" galaxies shows a tight correlation. Most of the Seyfertgalaxies and quasars follow the radio-FIR relation deduced from the``normal" galaxy sample, but with a somewhat larger scatter. A total 167Markarian galaxies, comprising 100 ``normal" galaxies, 66 Seyfertgalaxies and one quasar, have either excess radio emission or much lowerFIR spectral index α (25μm, 60μm). These galaxies may beclassified as ``AGN-powered". For ``normal" galaxies, the average qvalue (defined as the log ratio between FIR and radio luminosities) is2.3. There seems a trend for q to slightly decrease with increasingradio luminosity. This may imply that the ongoing active star formationin galaxies with higher radio luminosities is more efficient in heatingthe cosmic-ray electrons.
| 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.
| Outflows in Infrared-Luminous Starbursts at z < 0.5. I. Sample, Na I D Spectra, and Profile Fitting1,
We have conducted a spectroscopic survey of 78 starburstinginfrared-luminous galaxies at redshifts up to z=0.5. We usemoderate-resolution spectroscopy of the Na I D interstellar absorptionfeature to directly probe the neutral phase of outflowing gas in thesegalaxies. Over half of our sample are ultraluminous infrared galaxiesthat are classified as starbursts; the rest have infrared luminositiesin the range log(LIR/Lsolar)=10.2-12.0. The sampleselection, observations, and data reduction are described here. Theabsorption-line spectra of each galaxy are presented. We also discussthe theory behind absorption-line fitting in the case of a partiallycovered, blended absorption doublet observed at moderate-to-highresolution, a topic neglected in the literature. A detailed analysis ofthese data is presented in a companion paper.Some of the observations reported here were obtained at the MMTObservatory, a joint facility of the Smithsonian Institution and theUniversity 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.
| Comparison of Star Clusters With and Without Wolf-Rayet Stars in Wolf-Rayet Galaxies
We compare the properties of young star clusters with and withoutWolf-Rayet (W-R) stars in W-R galaxies using optical, near-infraredimagery and optical spectroscopy. Our work identifies the clusters withW-R stars in these galaxies for the first time. With this information,comparisons of clusters with and without W-R stars are now possible,enabling us to understand the chemical and morphological impact ofmassive stars on their environment and to constrain the parameters formodeling these systems. We find that clusters with W-R stars (W-Rclusters) are systematically younger, bluer clusters. Knowing this agedifference between the two cluster sets, we use an evolutionary scenarioto interpret their other properties. Young clusters, typically W-Rclusters, have a Strömgren sphere-like gas configuration. They alsotend to have H-K colors redder than those of theoretical models. Weinterpret the H-K excess as a combination of thermal emission from hotdust, nebular emission, and molecular emission. Older clusters,typically clusters without W-R stars, have ionized gas in a superbubbleconfiguration caused by the prolonged influence of stellar winds andsupernovae. The H-K excess is generally absent for these clusters. Thenitrogen-to-oxygen abundance ratio (N/O) does not appear to increase asa function of age over the first 10 Myr. Systems without W-R stars doappear to have a significant, elevated N/O over systems with W-R starsin the metallicity range 12+log(O/H)=7.7-7.9. For the entire metallicityrange in our sample, this finding is only marginally significant. Weconcur with previous studies, which find no correlation between thesulfur-to-oxygen abundance ratio and metallicity.
| 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.
| On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback from Momentum-driven Winds
We investigate large-scale galactic winds driven by momentum deposition.Momentum injection is provided by (1) radiation pressure produced by thecontinuum absorption and scattering of photons on dust grains and (2)supernovae (momentum injection by supernovae is important even if thesupernova energy is radiated away). Radiation can be produced by astarburst or active galactic nucleus (AGN) activity. We argue thatmomentum-driven winds are an efficient mechanism for feedback during theformation of galaxies. We show that above a limiting luminosity,momentum deposition from star formation can expel a significant fractionof the gas in a galaxy. The limiting, Eddington-like luminosity isLM~=(4fgc/G)σ4, where σ isthe galaxy velocity dispersion and fg is the gas fraction;the subscript M refers to momentum driving. A starburst that attainsLM moderates its star formation rate and its luminosity doesnot increase significantly further. We argue that elliptical galaxiesattain this limit during their growth at z>~1 and that this is theorigin of the Faber-Jackson relation. We show that Lyman break galaxiesand ultraluminous infrared galaxies have luminosities nearLM. Since these starbursting galaxies account for asignificant fraction of the star formation at z>~1, this supports ourhypothesis that much of the observed stellar mass in early-type galaxieswas formed during Eddington-limited star formation. Star formation isunlikely to efficiently remove gas from very small scales in galacticnuclei, i.e., scales much smaller than that of a nuclear starburst. Thisgas is available to fuel a central black hole (BH). We argue that a BHclears gas out of its galactic nucleus when the luminosity of the BHitself reaches ~LM. This shuts off the fuel supply to the BHand may also terminate star formation in the surrounding galaxy. As aresult, the BH mass is fixed to beMBH~=(fgκes/πG2)σ4,where κes is the electron scattering opacity. Thislimit is in accord with the observed MBH-σ relation.
| Infrared mergers and infrared quasi-stellar objects with galactic winds - II. NGC5514: two extranuclear starbursts with LINER properties and a supergiant bubble in the rupture phase
A study of the morphology, kinematics and ionization structure of theinfrared (IR) merger NGC5514 is presented. This study is based mainly onINTEGRAL two-dimensional (2D) spectroscopy (obtained at the 4.2-mWilliam Herschel Telescope, WHT), plus optical and near-IR images. Clearevidence of two extranuclear starbursts with young outflows (OFs) andlow-ionization nuclear emission region (LINER) activity are reported.One of these OFs has generated a supergiant bubble and the other isassociated with an extended complex of HII regions.In the galactic bubble it was found that: (i) the [SII], Hα,[NII], [OI] and [OIII] emission-line maps show a bubble with a distortedellipsoidal shape, with major and minor axes of ~6.5 kpc [13.6 arcsec;at position angle (PA) = 120°+/- 10°] and ~4.5 kpc (9.6 arcsec);(ii) these maps depict four main knots, a very strong one and threeothers more compact and located at the border; (iii) the centre of thebubble is located at ~4.1 kpc (8.5 arcsec) to the west of the mainnucleus; (iv) the WHT spectra show, in this area, two strong components:blue and red emission-line systems, probably associated with emissionfrom the near and far side of the external shell, for which the mean OFvelocities were measured as VOFblue= (-320 +/- 20)kms-1 and VOFred= (+265 +/- 25) kms-1(v) these two components depict LINER properties, probably associatedwith large-scale OF + shocks; (vi) at the east border, the kinematics ofthe ionized gas and the [SII] emission-line maps show an extendedejection of 4 kpc aligned with the PA of the major axis; (vii) threeother ejections were found, two of them perpendicular to the extendedone. Each ejection starts in one of the knots. These results suggestthat the bubble is in the rupture phase.For the complex of giant HII regions it was found that: (i) theHα, [NII] and [SII] emission-line maps show a compact strongemission area (peaking at ~810 pc ~1.7 arcsec, to the east of the secondnucleus) and faint extended emission with an elongated shape, and majorand minor axes of ~5.1 kpc (10.8 arcsec; at PA ~20°) and ~2.9 kpc(6.0 arcsec); (ii) inside this complex, the spectra show HII region andtransition LINER/HII characteristics; (iii) at the border of thisextended HII area the spectra have outflow components and LINERproperties.INTEGRAL 2D [NII], Hα, [SII] and [OIII] velocity fields (VFs) arepresented. These VF maps show results consistent with an expansion ofthe bubble, plus four ejections of ionized gas. The U, B, V, I, J, H andKS images show a pre-merger morphology, from which faintfilaments of emission emerge, centred on the bubble. The ionizationstructure and the physical conditions were analysed using the following2D emission-line ratio and width maps: [SII]/Hα, [NII]/Hα,[OI]/Hα, [OIII]/Hβ and FWHM-[NII]. In the region of thebubble, 100 per cent of the [NII]/Hα and [SII]/Hα ratiosshow very high values (>0.8) consistent with LINER processesassociated with high-velocity shocks. These new results support theprevious proposition that extreme nuclear and `extranuclear' starburstswith galactic winds + shocks play an important role in the evolution ofIR mergers/quasi-stellar objects.
| 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.
| 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.
| HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies
We report systematic HCN J=1-0 (and CO) observations of a sample of 53infrared (IR) and/or CO-bright and/or luminous galaxies, including sevenultraluminous infrared galaxies, nearly 20 luminous infrared galaxies,and more than a dozen of the nearest normal spiral galaxies. This is thelargest and most sensitive HCN survey of galaxies to date. All galaxiesobserved so far follow the tight correlation between the IR luminosityLIR and the HCN luminosity LHCN initially proposedby Solomon, Downes, & Radford, which is detailed in a companionpaper. We also address here the issue of HCN excitation. There is noparticularly strong correlation between LHCN and the 12 μmluminosity; in fact, of all the four IRAS bands, the 12 μm luminosityhas the weakest correlation with the HCN luminosity. There is also noevidence of stronger HCN emission or a higher ratio of HCN and COluminosities LHCN/LCO for galaxies with excess 12μm emission. This result implies that mid-IR radiative pumping, orpopulating, of the J=1 level of HCN by a mid-IR vibrational transitionis not important compared with the collisional excitation by densemolecular hydrogen. Furthermore, large velocity gradient calculationsjustify the use of HCN J=1-0 emission as a tracer of high-densitymolecular gas (>~3×104/τcm-3) andgive an estimate of the mass of dense molecular gas from HCNobservations. Therefore, LHCN may be used as a measure of thetotal mass of dense molecular gas, and the luminosity ratioLHCN/LCO may indicate the fraction of moleculargas that is dense.
| A Keck/HIRES Study of Kinematics of the Cold Interstellar Medium in Dwarf Starburst Galaxies
We have obtained high-resolution Echelle spectra (R=30,000-50,000) ofthe Na D absorption doublet (λλ5890, 5896) for six dwarfstarburst galaxies and two more luminous starbursts: M82 and NGC 1614.The absorption features were separated into multiple components andseparated into stellar and interstellar parts on the basis ofkinematics. We find that three of the dwarfs show outflows, with anaverage blueshift of 27 km s-1. This is small compared withthe highest velocity components in NGC 1614 and M82 (blueshifted by 150and 91 km s-1, respectively); these two brighter galaxiesalso show more complex absorption profiles than the dwarfs. None of theoutflow speeds clearly exceed the escape velocity of the host galaxy.Sight lines in NGC 2363 and NGC 4214 apparently intersect expandingshells. We compare the shocked gas velocity (vNaD) with theionized gas velocity (vHα) and interpret the velocitydifference as either a trapped ionization front (NGC 4214) or a leaky HII region (NGC 2363). The dwarfs showNNaD=1011.8-1013.7cm-2,while the Na D columns in M82 and NGC 1614 are 1013.7 and1014.0 cm-2, respectively. The mass of expelledgas is highly sensitive to outflow geometry, dust depletion, andionization fraction, but with a simple shell model we estimate neutraloutflow gas masses from ~106 to ~1010Msolar.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| 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.
| Missing Massive Stars in Starbursts: Stellar Temperature Diagnostics and the Initial Mass Function
Determining the properties of starbursts requires spectral diagnosticsof their ultraviolet radiation fields, to test whether very massivestars are present. We test several such diagnostics, using new models ofline ratio behavior combining CLOUDY, Starburst99, and up-to-datespectral atlases. For six galaxies we obtain new measurements of He I1.7 μm/Br10, a difficult to measure but physically simple (andtherefore reliable) diagnostic. We obtain new measurements of He I 2.06μm/Brγ in five galaxies. We find that He I 2.06 μm/Brγand [O III]/Hβ are generally unreliable diagnostics in starbursts.The heteronuclear and homonuclear mid-infrared line ratios (notably [NeIII] 15.6 μm/[Ne II] 12.8 μm) consistently agree with each otherand with He I 1.7 μm/Br10 this argues that the mid-infrared lineratios are reliable diagnostics of spectral hardness. In a sample of 27starbursts, [Ne III]/[Ne II] is significantly lower than modelpredictions for a Salpeter initial mass function (IMF) extending to 100Msolar. Plausible model alterations strengthen thisconclusion. By contrast, the low-mass and low-metallicity galaxies II Zw40 and NGC 5253 show relatively high neon line ratios, compatible with aSalpeter slope extending to at least ~40-60 Msolar. Onesolution for the low neon line ratios in the high-metallicity starburstswould be that they are deficient in >~40 Msolar starscompared to a Salpeter IMF. An alternative explanation, which we prefer,is that massive stars in high-metallicity starbursts spend much of theirlives embedded within ultracompact H II regions that prevent the near-and mid-infrared nebular lines from forming and escaping. Thishypothesis has important consequences for starburst modeling andinterpretation.
| 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.
| Catalog of Double Nucleus Disk Galaxies
We have compiled a catalog of disk galaxies that have a double nucleus,through systematic examination of existing catalogs and publications.The Catalog of Double Nucleus Disk Galaxies includes 107 objects,together with their basic data. The aim of the catalog is to provide amore systematic and homogeneous basis for the study of the relevance ofgalaxy interactions and minor mergers in the formation of these doublenuclei. We have also investigated possible correlations betweengeometric and photometric parameters of the double nuclei and their hostgalaxies. The preliminary results indicate the presence of severalsignificant correlations that should be considered in any theoreticalscenario describing minor mergers and disk galaxy evolution.
| A Study of the Distribution of Star-forming Regions in Luminous Infrared Galaxies by Means of Hα Imaging Observations
We performed Hα imaging observations of 22 luminous infraredgalaxies to investigate how the distribution of star-forming regions inthese galaxies is related to galaxy interactions. Based on correlationdiagrams between Hα flux and continuum emission for individualgalaxies, a sequence for the distribution of star-forming regions wasfound: very compact (~100 pc) nuclear starbursts with almost nostar-forming activity in the outer regions (type 1), dominant nuclearstarbursts <~1 kpc in size with a negligible contribution from theouter regions (type 2), nuclear starbursts >~1 kpc in size with asignificant contribution from the outer regions (type 3), and extendedstarbursts with relatively faint nuclei (type 4). These classes ofstar-forming regions were found to be strongly related to globalstar-forming properties, such as star formation efficiency, far-infraredcolor, and dust extinction. There was a clear tendency for the objectswith more compact distributions of star-forming regions to show a higherstar formation efficiency and hotter far-infrared color. An appreciablefraction of the sample objects were dominated by extended starbursts(type 4), which is unexpected in the standard scenario ofinteraction-induced starburst galaxies. We also found that thedistribution of star-forming regions was weakly but clearly related togalaxy morphology: severely disturbed objects had a more concentrateddistribution of star-forming regions. This suggests that the propertiesof galaxy interactions, such as dynamical phase and orbital parameters,play a more important role than the internal properties of progenitorgalaxies, such as dynamical structure or gas mass fraction. We alsodiscuss the evolution of the distribution of star-forming regions ininteracting galaxies.
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