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Stellar kinematics and populations of early-type galaxies with the SAURON and OASIS integral-field spectrographs
We summarise the results and achievements of integral-field spectroscopyof early-type galaxies, observed as part of a survey using both theSAURON and OASIS spectrographs. From the perspective of integral-fieldspectroscopy, these otherwise smooth and featureless objects show awealth of structure, both in their stellar kinematics and populations.We focus on the stellar content, and examine properties on bothkiloparsec scales with SAURON, and scales of 100’s of parsecs withOASIS. These complementary studies reveal two types of kinematicallydistinct components (KDCs), differing primarily in their intrinsicsizes. In previous studies, KDCs and their host galaxies have generallybeen found to be unremarkable in other aspects. We show that large KDCs,typical of the well-studied cases, indeed show little or no agedifferences with their host galaxy. The KDCs detected with the higherspatial-resolution of OASIS are intrinsically smaller and include, incontrast, a significant fraction of young stars. We speculate on therelationship between KDCs and their host galaxies, and the implicationsfor young populations in early-type galaxies.
| The SAURON project - VI. Line strength maps of 48 elliptical and lenticular galaxies
We present absorption line strength maps of 48 representative ellipticaland lenticular galaxies obtained as part of a survey of nearby galaxiesusing our custom-built integral-field spectrograph, SAURON, operating onthe William Herschel Telescope. Using high-quality spectra, spatiallybinned to a constant signal-to-noise ratio, we measure four key age,metallicity and abundance ratio sensitive indices from the Lick/IDSsystem over a two-dimensional field extending up to approximately oneeffective radius. A discussion of calibrations and offsets is given,along with a description of error estimation and nebular emissioncorrection. We modify the classical Fe5270 index to define a new index,Fe5270S, which maximizes the useable spatial coverage ofSAURON. Maps of Hβ, Fe5015, Mgb and Fe5270S arepresented for each galaxy. We use the maps to compute average linestrengths integrated over circular apertures of one-eighth effectiveradius, and compare the resulting relations of index versus velocitydispersion with previous long-slit work. The metal line strength mapsshow generally negative gradients with increasing radius roughlyconsistent with the morphology of the light profiles. Remarkabledeviations from this general trend exist, particularly the Mgb isoindexcontours appear to be flatter than the isophotes of the surfacebrightness for about 40 per cent of our galaxies without significantdust features. Generally, these galaxies exhibit significant rotation.We infer from this that the fast-rotating component features a highermetallicity and/or an increased Mg/Fe ratio as compared to the galaxy asa whole. The Hβ maps are typically flat or show a mild positiveoutwards radial gradient, while a few galaxies show strong central peaksand/or elevated overall Hβ strength likely connected to recent starformation activity. For the most prominent post-starburst galaxies, eventhe metal line strength maps show a reversed gradient.
| The SAURON project - V. Integral-field emission-line kinematics of 48 elliptical and lenticular galaxies
We present the emission-line fluxes and kinematics of 48 representativeelliptical and lenticular galaxies obtained with our custom-builtintegral-field spectrograph, SAURON, operating on the William HerschelTelescope. Hβ, [OIII]λλ4959,5007 and[NI]λλ5198,5200 emission lines were measured using a newprocedure that simultaneously fits both the stellar spectrum and theemission lines. Using this technique we can detect emission lines downto an equivalent width of 0.1 Å set by the current limitations indescribing galaxy spectra with synthetic and real stellar templates,rather than by the quality of our spectra. Gas velocities and velocitydispersions are typically accurate to within 14 and 20 kms-1, respectively, and at worse to within 25 and 40 kms-1. The errors on the flux of the [OIII] and Hβ linesare on average 10 and 20 per cent, respectively, and never exceed 30 percent. Emission is clearly detected in 75 per cent of our samplegalaxies, and comes in a variety of resolved spatial distributions andkinematic behaviours. A mild dependence on the Hubble type and galacticenvironment is observed, with higher detection rates in lenticulargalaxies and field objects. More significant is the fact that only 55per cent of the galaxies in the Virgo cluster exhibit clearly detectedemission. The ionized-gas kinematics is rarely consistent with simplecoplanar circular motions. However, the gas almost never displayscompletely irregular kinematics, generally showing coherent motions withsmooth variations in angular momentum. In the majority of the cases, thegas kinematics is decoupled from the stellar kinematics, and in half ofthe objects this decoupling implies a recent acquisition of gaseousmaterial. Over the entire sample however, the distribution of the meanmisalignment values between stellar and gaseous angular momenta isinconsistent with a purely external origin. The distribution ofkinematic misalignment values is found to be strongly dependent on theapparent flattening and the level of rotational support of galaxies,with flatter, fast rotating objects hosting preferentially corotatinggaseous and stellar systems. In a third of the cases, the distributionand kinematics of the gas underscore the presence of non-axisymmetricperturbations of the gravitational potential. Consistent with previousstudies, the presence of dust features is always accompanied by gasemission while the converse is not always true. A considerable range ofvalues for the [OIII]/Hβ ratio is found both across the sample andwithin single galaxies. Despite the limitations of this ratio as anemission-line diagnostic, this finding suggests either that a variety ofmechanisms is responsible for the gas excitation in E and S0 galaxies orthat the metallicity of the interstellar material is quiteheterogeneous.
| The SAURON project - IV. The mass-to-light ratio, the virial mass estimator and the Fundamental Plane of elliptical and lenticular galaxies
We investigate the well-known correlations between the dynamicalmass-to-light ratio (M/L) and other global observables of elliptical (E)and lenticular (S0) galaxies. We construct two-integral Jeans andthree-integral Schwarzschild dynamical models for a sample of 25 E/S0galaxies with SAURON integral-field stellar kinematics to about oneeffective (half-light) radius Re. They have well-calibratedI-band Hubble Space Telescope WFPC2 and large-field ground-basedphotometry, accurate surface brightness fluctuation distances, and theirobserved kinematics is consistent with an axisymmetric intrinsic shape.All these factors result in an unprecedented accuracy in the M/Lmeasurements. We find a tight correlation of the form (M/L) = (3.80 +/-0.14) ×(σe/200kms-1)0.84+/-0.07 betweenthe M/L (in the I band) measured from the dynamical models and theluminosity-weighted second moment σe of the LOSVDwithin Re. The observed rms scatter in M/L for our sample is18 per cent, while the inferred intrinsic scatter is ~13 per cent. The(M/L)-σe relation can be included in the remarkableseries of tight correlations between σe and othergalaxy global observables. The comparison of the observed correlationswith the predictions of the Fundamental Plane (FP), and with simplevirial estimates, shows that the `tilt' of the FP of early-typegalaxies, describing the deviation of the FP from the virial relation,is almost exclusively due to a real M/L variation, while structural andorbital non-homology have a negligible effect. When the photometricparameters are determined in the `classic' way, using growth curves, andthe σe is measured in a large aperture, the virial massappears to be a reliable estimator of the mass in the central regions ofgalaxies, and can be safely used where more `expensive' models are notfeasible (e.g. in high-redshift studies). In this case the best-fittingvirial relation has the form (M/L)vir= (5.0 +/- 0.1)×Reσ2e/(LG), in reasonableagreement with simple theoretical predictions. We find no differencebetween the M/L of the galaxies in clusters and in the field. Thecomparison of the dynamical M/L with the (M/L)pop inferredfrom the analysis of the stellar population, indicates a median darkmatter fraction in early-type galaxies of ~30 per cent of the total massinside one Re, in broad agreement with previous studies, andit also shows that the stellar initial mass function varies little amongdifferent galaxies. Our results suggest a variation in M/L at constant(M/L)pop, which seems to be linked to the galaxy dynamics. Wespeculate that fast-rotating galaxies have lower dark matter fractionsthan the slow-rotating and generally more-massive ones. If correct, thiswould suggest a connection between the galaxy assembly history and thedark matter halo structure. The tightness of our correlation providessome evidence against cuspy nuclear dark matter profiles in galaxies.
| Optical Counterparts of Ultraluminous X-Ray Sources Identified from Archival HST WFPC2 Images
We present a systematic analysis of archival HST WFPC2 ``Association''data sets that correlate with the Chandra positions of a set of 44ultraluminous X-ray sources (ULXs) of nearby galaxies. The mainmotivation is to address the nature of ULXs by searching for opticalcounterparts. Sixteen of the ULXs are found in early-type galaxies (RC3Hubble type <3). We have improved the Chandra/HST relative astrometrywhenever possible, resulting in errors circles of 0.3"-1.7" in size.Disparate numbers of potential ULX counterparts are found, and in somecases none are found. The lack of or low number of counterparts in somecases may be due to insufficient depth in the WFPC2 images. Particularlyin late-type galaxies, the HST image in the ULX region was often complexor crowded, requiring source detection to be performed manually. Wetherefore address various scenarios for the nature of the ULX since itis not known which, if any, of the sources found are true counterparts.The optical luminosities of the sources are typically in the range104-106 Lsolar, with (effective) Vmagnitudes typically in the range 22-24. In several cases colorinformation is available, with the colors roughly tending to be more redin early-type galaxies. This suggests that, in general, the (potential)counterparts found in early-type galaxies are likely to be older stellarpopulations and are probably globular clusters. Several early-typegalaxy counterparts have blue colors, which may be due to youngerstellar populations in the host galaxies, however, these could also bebackground sources. In spiral galaxies the sources may also be due tolocalized structure in the disks rather than bound stellar systems.Alternatively, some of the counterparts in late-type galaxies may beisolated supergiant stars. The observed X-ray/optical flux ratio isdiluted by the optical emission of the cluster in cases where the systemis an X-ray binary in a cluster, particularly in the case of a low-massX-ray binaries in an old cluster. If any of the counterparts are boundsystems with ~104-106 stars and are the truecounterparts to the ULX sources, then the X-ray luminosities of the ULXare generally well below the Eddington limit for a black hole with mass~0.1% of the cluster mass. Finally, we find that the optical flux of thecounterparts is consistent with being dominated by emission from anaccretion disk around an intermediate-mass black hole if the black holehappens to have a mass >~102 Msolar and isaccreting at close to the Eddington rate, unless the accretion disk isirradiated (which would result in high optical disk luminosities atlower black hole masses).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. This project isassociated with Archival proposal 9545.
| The Cool ISM in S0 Galaxies. II. A Survey of Atomic Gas
The place of lenticular galaxies within the range of types of galaxiesremains unclear. We previously reported the mass of molecular hydrogenfor a volume-limited sample of lenticular galaxies, where we saw thatthe amount of gas was less than that predicted by the return of stellarmass to the interstellar medium. Here we report observations of atomichydrogen (H I) for the same sample. Detections in several galaxies makemore compelling the case presented in our earlier paper that the mass ofcool gas in S0 galaxies cuts off at ~10% of what is expected fromcurrent models of gas return from stellar evolution. The molecular andatomic phases of the gas in our sample galaxies appear to be separateand distinct, both spatially and in velocity space. We propose that themolecular gas arises mostly from the stellar mass returned to thegalaxy, while the atomic hydrogen is mainly accumulated from externalsources (infall, captured dwarfs, etc.). While this proposal fits mostof the observations, it makes the presence of the upper mass cutoff evenmore mysterious.
| Stellar Populations in Nearby Lenticular Galaxies
We have obtained two-dimensional spectral data for a sample of 58 nearbyS0 galaxies with the Multi-Pupil Fiber/Field Spectrograph of the 6 mtelescope of the Special Astrophysical Observatory of the RussianAcademy of Sciences. The Lick indices Hβ, Mg b, and arecalculated separately for the nuclei and for the bulges taken as therings between R=4'' and 7", and the luminosity-weighted ages,metallicities, and Mg/Fe ratios of the stellar populations are estimatedby comparing the data to single stellar population (SSP) models. Fourtypes of galaxy environments are considered: clusters, centers ofgroups, other places in groups, and the field. The nuclei are found tobe on average slightly younger than the bulges in any type ofenvironment, and the bulges of S0 galaxies in sparse environments areyounger than those in dense environments. The effect can be partlyattributed to the well-known age correlation with the stellar velocitydispersion in early-type galaxies (in our sample the galaxies in sparseenvironments are on average less massive than those in denseenvironments), but for the most massive S0 galaxies, withσ*=170-220 km s-1, the age dependence on theenvironment is still significant at the confidence level of 1.5 σ.Based on observations collected with the 6 m telescope (BTA) at theSpecial Astrophysical Observatory (SAO) of the Russian Academy ofSciences (RAS).
| Neutral hydrogen in radio galaxies: Results from nearby, importance for far away
The study of neutral hydrogen emission and absorption in radio galaxiesis giving new and important insights on a variety of phenomena observedin these objects. Such observations are helping to understand the originof the host galaxy, the effects of the interaction between the radio jetand the ISM, the presence of fast gaseous outflows as well asjet-induced star formation. Recent results obtained on these phenomenaare summarized in this review. Although the {H I observationsconcentrate on nearby radio galaxies, the results also have relevancefor the high-z objects as all these phenomena are important, and likelyeven more common, in high-redshift radio sources.
| Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field
We present Hubble Space Telescope Advanced Camera for Surveys images andcolor-magnitude diagrams for 25 nearby galaxies with radial velocitiesVLG<500 km s-1. Distances are determined basedon the luminosities of stars at the tip of the red giant branch thatrange from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, arefound to be the nearest known representatives of blue compact dwarfobjects. Using high-quality data on distances and radial velocities of110 nearby field galaxies, we derive their mean Hubble ratio to be 68 kms-1 Mpc-1 with a standard deviation of 15 kms-1 Mpc-1. Peculiar velocities of most of thegalaxies, Vpec=VLG-68D, follow a Gaussiandistribution with σv=63 km s-1 but with atail toward high negative values. Our data display the known correlationbetween peculiar velocity and galaxy elevation above the LocalSupercluster plane. The small observed fraction of galaxies with highpeculiar velocities, Vpec<-500 km s-1, may beunderstood as objects associated with nearby groups (Coma I, Eridanus)outside the local volume.
| The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties
In a recently completed survey of the stellar population properties oflow-ionization nuclear emission-line regions (LINERs) and LINER/HIItransition objects (TOs), we have identified a numerous class ofgalactic nuclei which stand out because of their conspicuous108-9 yr populations, traced by high-order Balmer absorptionlines and other stellar indices. These objects are called `young-TOs',because they all have TO-like emission-line ratios. In this paper weextend this previous work, which concentrated on the nuclear properties,by investigating the radial variations of spectral properties inlow-luminosity active galactic nuclei (LLAGNs). Our analysis is based onhigh signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distancesof typically up to 850 pc from the nucleus with a resolution of ~100 pc(~1 arcsec) and S/N ~ 30. Stellar population gradients are mapped by theradial profiles of absorption-line equivalent widths and continuumcolours along the slit. These variations are further analysed by meansof a decomposition of each spectrum in terms of template galaxiesrepresentative of very young (<=107 yr), intermediate age(108-9 yr) and old (1010 yr) stellar populations.This study reveals that young-TOs also differ from old-TOs andold-LINERs in terms of the spatial distributions of their stellarpopulations and dust. Specifically, our main findings are as follows.(i) Significant stellar population gradients are found almostexclusively in young-TOs. (ii) The intermediate age population ofyoung-TOs, although heavily concentrated in the nucleus, reachesdistances of up to a few hundred pc from the nucleus. Nevertheless, thehalf width at half-maximum of its brightness profile is more typically100 pc or less. (iii) Objects with predominantly old stellar populationspresent spatially homogeneous spectra, be they LINERs or TOs. (iv)Young-TOs have much more dust in their central regions than otherLLAGNs. (v) The B-band luminosities of the central <~1 Gyr populationin young-TOs are within an order of magnitude of MB=-15,implying masses of the order of ~107-108Msolar. This population was 10-100 times more luminous in itsformation epoch, at which time young massive stars would have completelyoutshone any active nucleus, unless the AGN too was brighter in thepast.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.
| The Molecular Interstellar Medium of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies
We present a new survey for CO in dwarf galaxies using the ARO Kitt Peak12 m telescope. This survey consists of observations of the centralregions of 121 northern dwarfs with IRAS detections and no known COemission. We detect CO in 28 of these galaxies and marginally detectanother 16, increasing by about 50% the number of such galaxies known tohave significant CO emission. The galaxies we detect are comparable instellar and dynamical mass to the Large Magellanic Cloud, althoughsomewhat brighter in CO and fainter in the far-IR. Within dwarfs, wefind that the CO luminosity LCO is most strongly correlatedwith the K-band and the far-infrared luminosities. There are also strongcorrelations with the radio continuum (RC) and B-band luminosities andlinear diameter. Conversely, we find that far-IR dust temperature is apoor predictor of CO emission within the dwarfs alone, although a goodpredictor of normalized CO content among a larger sample of galaxies. Wesuggest that LCO and LK correlate well because thestellar component of a galaxy dominates the midplane gravitational fieldand thus sets the pressure and density of the atomic gas, which controlthe formation of H2 from H I. We compare our sample with moremassive galaxies and find that dwarfs and large galaxies obey the samerelationship between CO and the 1.4 GHz RC surface brightness. Thisrelationship is well described by a Schmidt law withΣRC~Σ1.3CO. Therefore,dwarf galaxies and large spirals exhibit the same relationship betweenmolecular gas and star formation rate (SFR). We find that this result isrobust to moderate changes in the RC-to-SFR and CO-to-H2conversion factors. Our data appear to be inconsistent with large (orderof magnitude) variations in the CO-to-H2 conversion factor inthe star-forming molecular gas.
| New distances of unresolved dwarf elliptical galaxies in the vicinity of the Local Group
We present Surface Brightness Fluctuation distances of nine early-typedwarf galaxies and the S0 galaxy NGC 4150 in the Local Volume based ondeep B- and R-band CCD images obtained with the 2.56 m Nordic OpticalTelescope. Typically, six stellar fields at various galactocentricdistances have been chosen for each galaxy as appropriately free offoreground stars and other contaminants, and Fourier analysed todetermine the distances, which are found to lie in the range of 3 to 16Mpc. The SBF method is thus demonstrated to efficiently measuredistances from the ground with mid-aperture telescopes for galaxies forwhich only the tip of the red giant branch method in combination withthe Hubble Space Telescope has been available until now. We obtained thefollowing distance moduli: 28.11 ± 0.15 mag (or 4.2 ± 0.3Mpc) for UGC 1703, 27.61 ± 0.17 mag (or 3.3 ± 0.3 Mpc) forKDG 61, 29.00 ± 0.27 mag (or 6.3 ± 0.8 Mpc) for UGCA 200,27.74 ± 0.18 mag (or 3.5 ± 0.3 Mpc) for UGC 5442, 30.22± 0.17 mag (or 11.1 ± 0.9 Mpc) for UGC 5944, 30.79± 0.11 mag (or 14.4 ± 0.7 Mpc) for NGC 4150, 31.02± 0.25 mag (or 16.0 ± 1.9 Mpc) for BTS 128, 29.27 ±0.16 mag (or 7.1 ± 0.6 Mpc) for UGC 7639, 30.19 ± 0.23 mag(or 10.9 ± 1.2 Mpc) for UGC 8799 with an alternative distance of30.61 ± 0.26 mag (or 13.2 ± 1.7 Mpc), and 29.60 ±0.20 mag (or 8.3 ± 0.8 Mpc) for UGC 8882.
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.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/429/1125
| The SAURON project - III. Integral-field absorption-line kinematics of 48 elliptical and lenticular galaxies
We present the stellar kinematics of 48 representative elliptical andlenticular galaxies obtained with our custom-built integral-fieldspectrograph SAURON operating on the William Herschel Telescope. Thedata were homogeneously processed through a dedicated reduction andanalysis pipeline. All resulting SAURON data cubes were spatially binnedto a constant minimum signal-to-noise ratio. We have measured thestellar kinematics with an optimized (penalized pixel-fitting) routinewhich fits the spectra in pixel space, via the use of optimal templates,and prevents the presence of emission lines to affect the measurements.We have thus generated maps of the mean stellar velocity V, the velocitydispersion σ, and the Gauss-Hermite moments h3 andh4 of the line-of-sight velocity distributions. The mapsextend to approximately one effective radius. Many objects displaykinematic twists, kinematically decoupled components, central stellardiscs, and other peculiarities, the nature of which will be discussed infuture papers of this series.
| The Stellar Populations of Low-Luminosity Active Galactic Nuclei. II. Space Telescope Imaging Spectrograph Observations
We present a study of the stellar populations of low-luminosity activegalactic nuclei (LLAGNs). Our goal is to search for spectroscopicsignatures of young and intermediate-age stars and to investigate theirrelationship with the ionization mechanism in LLAGNs. The method used isbased on the stellar population synthesis of the optical continuum ofthe innermost (20-100 pc) regions in these galaxies. For this purpose,we have collected high spatial resolution optical (2900-5700 Å)STIS spectra of 28 nearby LLAGNs that are available in the Hubble SpaceTelescope archive. The analysis of these data is compared with a similaranalysis also presented here for 51 ground-based spectra of LLAGNs. Ourmain findings are as follows: (1) No features due to Wolf-Rayet starswere convincingly detected in the STIS spectra. (2) Young starscontribute very little to the optical continuum in the ground-basedaperture. However, the fraction of light provided by these stars ishigher than 10% in most of the weak-[O I] ([OI]/Hα<=0.25) LLAGNSTIS spectra. (3) Intermediate-age stars contribute significantly to theoptical continuum of these nuclei. This population is more frequent inobjects with weak than with strong [O I]. Weak-[O I] LLAGNs that haveyoung stars stand out for their intermediate-age population. (4) Most ofthe strong-[O I] LLAGNs have predominantly old stellar population. A fewof these objects also show a featureless continuum that contributessignificantly to the optical continuum. These results suggest that youngand intermediate-age stars do not play a significant role in theionization of LLAGNs with strong [O I]. However, the ionization inweak-[O I] LLAGNs with young and/or intermediate-age populations couldbe due to stellar processes. A comparison of the properties of theseobjects with Seyfert 2 galaxies that harbor a nuclear starburst suggeststhat weak-[O I] LLAGNs are the lower luminosity counterparts of theSeyfert 2 composite nuclei.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. Based on observations made with the Nordic OpticalTelescope (NOT), operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrofísica deCanarias.
| The Stellar Populations of Low-Luminosity Active Galactic Nuclei. I. Ground-based Observations
We present a spectroscopic study of the stellar populations oflow-luminosity active galactic nuclei (LLAGNs). Our main goal is todetermine whether the stars that live in the innermost (100 pc scale)regions of these galaxies are in some way related to the emission-lineproperties, which would imply a link between the stellar population andthe ionization mechanism. High signal-to-noise ratio, ground-basedlong-slit spectra in the 3500-5500 Å interval were collected for60 galaxies: 51 LINERs and LINER/H II transition objects, two starburstgalaxies, and seven nonactive galaxies. In this paper, the first of aseries, we (1) describe the sample; (2) present the nuclear spectra; (3)characterize the stellar populations of LLAGNs by means of an empiricalcomparison with normal galaxies; (4) measure a set of spectral indices,including several absorption-line equivalent widths and colorsindicative of stellar populations; and (5) correlate the stellar indiceswith emission-line ratios that may distinguish between possibleexcitation sources for the gas. Our main findings are as follows: (1)Few LLAGNs have a detectable young (<~107 yr) starburstcomponent, indicating that very massive stars do not contributesignificantly to the optical continuum. In particular, no features dueto Wolf-Rayet stars were convincingly detected. (2) High-order Balmerabsorption lines of H I (HOBLs), on the other hand, are detected in ~40%of LLAGNs. These features, which are strongest in108-109 yr intermediate-age stellar populations,are accompanied by diluted metal absorption lines and bluer colors thanother objects in the sample. (3) These intermediate-age populations arevery common (~50%) in LLAGNs with relatively weak [O I] emission([OI]/Hα<=0.25) but rare (~10%) in LLAGNs with stronger [O I].This is intriguing since LLAGNs with weak [O I] have been previouslyhypothesized to be ``transition objects'' in which both an AGN and youngstars contribute to the emission-line excitation. Massive stars, ifpresent, are completely outshone by intermediate-age and old stars inthe optical. This happens in at least a couple of objects whereindependent UV spectroscopy detects young starbursts not seen in theoptical. (4) Objects with predominantly old stars span the whole rangeof [O I]/Hα values, but (5) sources with significant young and/orintermediate-age populations are nearly all (~90%) weak-[O I] emitters.These new findings suggest a link between the stellar populations andthe gas ionization mechanism. The strong-[O I] objects are most likelytrue LLAGNs, with stellar processes being insignificant. However, theweak-[O I] objects may comprise two populations, one where theionization is dominated by stellar processes and another where it isgoverned by either an AGN or a more even mixture of stellar and AGNprocesses. Possible stellar sources for the ionization include weakstarbursts, supernova remnants, and evolved poststarburst populations.These scenarios are examined and constrained by means of complementaryobservations and detailed modeling of the stellar populations inforthcoming communications.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canárias.
| Inner-truncated Disks in Galaxies
We present an analysis of the disk brightness profiles of 218 spiral andlenticular galaxies. At least 28% of disk galaxies exhibit innertruncations in these profiles. There are no significant trends oftruncation incidence with Hubble type, but the incidence among barredsystems is 49%, more than 4 times that for nonbarred galaxies. However,not all barred systems have inner truncations, and not allinner-truncated systems are currently barred. Truncations represent areal dearth of disk stars in the inner regions and are not an artifactof our selection or fitting procedures nor the result of obscuration bydust. Disk surface brightness profiles in the outer regions are wellrepresented by simple exponentials for both truncated and nontruncateddisks. However, truncated and nontruncated systems have systematicallydifferent slopes and central surface brightness parameters for theirdisk brightness distributions. Truncation radii do not appear tocorrelate well with the sizes or brightnesses of the bulges. Thissuggests that the low angular momentum material apparently missing fromthe inner disk was not simply consumed in forming the bulge population.Disk parameters and the statistics of bar orientations in our sampleindicate that the missing stars of the inner disk have not simply beenredistributed azimuthally into bar structures. The sharpness of thebrightness truncations and their locations with respect to othergalactic structures suggest that resonances associated with diskkinematics, or tidal interactions with the mass of bulge stars, might beresponsible for this phenomenon.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| The Cool Interstellar Medium in S0 Galaxies. I. A Survey of Molecular Gas
Lenticular galaxies remain remarkably mysterious as a class.Observations to date have not led to any broad consensus about theirorigins, properties, and evolution, although they are often thought tohave formed in one big burst of star formation early in the history ofthe universe and to have evolved relatively passively since then. Inthat picture, current theory predicts that stellar evolution returnssubstantial quantities of gas to the interstellar medium; most isejected from the galaxy, but significant amounts of cool gas might beretained. Past searches for that material, though, have provided unclearresults. We present results from a survey of molecular gas in avolume-limited sample of field S0 galaxies selected from the NearbyGalaxies Catalog. CO emission is detected from 78% of the samplegalaxies. We find that the molecular gas is almost always located insidethe central few kiloparsecs of a lenticular galaxy, meaning that ingeneral it is more centrally concentrated than in spirals. We combineour data with H I observations from the literature to determine thetotal masses of cool and cold gas. Curiously, we find that, across awide range of luminosity, the most gas-rich galaxies have ~10% of thetotal amount of gas ever returned by their stars. That result isdifficult to understand within the context of either monolithic orhierarchical models of evolution of the interstellar medium.
| Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations
To empirically calibrate the IR surface brightness fluctuation (SBF)distance scale and probe the properties of unresolved stellarpopulations, we measured fluctuations in 65 galaxies using NICMOS on theHubble Space Telescope. The early-type galaxies in this sample includeelliptical and S0 galaxies and spiral bulges in a variety ofenvironments. Absolute fluctuation magnitudes in the F160W (1.6 μm)filter (MF160W) were derived for each galaxy using previouslymeasured I-band SBF and Cepheid variable star distances. F160W SBFs canbe used to measure distances to early-type galaxies with a relativeaccuracy of ~10%, provided that the galaxy color is known to ~0.035 magor better. Near-IR fluctuations can also reveal the properties of themost luminous stellar populations in galaxies. Comparison of F160Wfluctuation magnitudes and optical colors to stellar population modelpredictions suggests that bluer elliptical and S0 galaxies havesignificantly younger populations than redder ones and may also be moremetal-rich. There are no galaxies in this sample with fluctuationmagnitudes consistent with old, metal-poor (t>5 Gyr, [Fe/H]<-0.7)stellar population models. Composite stellar population models implythat bright fluctuations in the bluer galaxies may be the result of anepisode of recent star formation in a fraction of the total mass of agalaxy. Age estimates from the F160W fluctuation magnitudes areconsistent with those measured using the Hβ Balmer-line index. Thetwo types of measurements make use of completely different techniquesand are sensitive to stars in different evolutionary phases. Bothtechniques reveal the presence of intermediate-age stars in theearly-type galaxies of this sample.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS 5-26555.
| Lensing and the Centers of Distant Early-Type Galaxies
Gravitational lensing provides a unique probe of the inner 10-1000 pc ofdistant galaxies (z~0.2-1). Theoretical studies have predicted that eachstrong lens system should have a faint image near the center of the lensgalaxy, which should, in principle, be visible in radio lenses but hasnever been detected. We study the predicted ``core'' images using modelsderived from the stellar distributions in nearby early-type galaxies. Wefind that realistic lens galaxies produce a remarkably wide range ofcore images, with magnifications spanning some 6 orders of magnitude.More concentrated galaxies produce fainter core images, although notwith any model-independent relation between the galaxy properties andthe core images. Some real galaxies have diffuse cores that should yieldbright core images (magnification μcore>~0.1), but morecommon are galaxies that yield faint core images(μcore<~0.001). Thus, stellar mass distributions aloneare probably concentrated enough to explain the lack of observed coreimages. Observational sensitivity may need to improve by an order ofmagnitude before detections of core images become common. Two-imagelenses should tend to have brighter core images than four-image lenses,so they will be the better targets for finding core images andexploiting these tools for studying the central mass distributions ofdistant galaxies.
| Dust and the Infrared Kinematic Properties of Early-Type Galaxies
We have obtained spectra and measured the stellar kinematics in a sampleof 25 nearby early-type galaxies (with velocity dispersions from lessthan 100 km s-1 to over 300 km s-1) using thenear-infrared CO absorption band head at 2.29 μm. Our medianuncertainty for the dispersions is ~10%. We examine the effects of duston existing optical kinematic measurements. We find that thenear-infrared velocity dispersions are, in general, smaller than opticalvelocity dispersions, with differences as large as 30%. The mediandifference is 11% smaller, and the effect is of greater magnitude forhigher dispersion galaxies. The lenticular galaxies (18 out of 25)appear to be causing the shift to lower dispersions, while the classicalelliptical galaxies (7 out of 25) are consistent between the twowavelength regimes. If uniformly distributed dust causes thesedifferences, we would expect to find a correlation between the relativeamount of dust in a galaxy and the fractional change in dispersion, butwe do not find such a correlation. We do see correlations between bothvelocity dispersion and CO band head equivalent width, and velocitydispersion and Mg2 index. The differences in dispersion arenot well explained by current models of dust absorption. The lack ofcorrelation between the relative amount of dust and shift in dispersionpossibly suggests that dust does not have a similar distribution fromgalaxy to galaxy. The CO equivalent widths of these galaxies are quitehigh (>~10 Å for almost all), requiring the light at thesewavelengths to be dominated by very cool stars.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| Galaxy flow in the Canes Venatici I cloud
We present an analysis of Hubble Space Telescope/WFPC2 images ofeighteen galaxies in the Canes Venatici I cloud. We derive theirdistances from the luminosity of the tip of the red giant branch starswith a typical accuracy of ~ 12%. The resulting distances are 3.9 Mpc(UGC 6541), 4.9 Mpc (NGC 3738), 3.0 Mpc (NGC 3741), 4.5 Mpc (KK 109),>6.3 Mpc (NGC 4150), 4.2 Mpc (UGC 7298), 4.5 Mpc (NGC 4244), 4.6 Mpc(NGC 4395), 4.9 Mpc (UGC 7559), 4.2 Mpc (NGC 4449), 4.4 Mpc (UGC 7605),4.6 Mpc (IC 3687), 4.7 Mpc (KK 166), 4.7 Mpc (NGC 4736), 4.2 Mpc (UGC8308), 4.3 Mpc (UGC 8320), 4.6 Mpc (NGC 5204), and 3.2 Mpc (UGC 8833).The CVn I cloud has a mean radial velocity of 286 +/- 9 kms-1, a mean distance of 4.1 +/- 0.2 Mpc, a radial velocitydispersion of 50 km s-1, a mean projected radius of 760 kpc,and a total blue luminosity of 2.2 x 1010 Lsun .Assuming virial or closed orbital motions for the galaxies, we estimatedtheir virial and their orbital mass-to-luminosity ratio to be 176 and 88Msun /Lsun , respectively. However, the CVn Icloud is characterized by a crossing time of 15 Gyr, and is thus farfrom a state of dynamical equilibrium. The large crossing time for thecloud, its low content of dSph galaxies (<6%), and the almost``primordial'' shape of its luminosity function show that the CVn Icomplex is in a transient dynamical state, driven rather by the freeHubble expansion than by galaxy interactions.Based on observations made with the NASA/ESA Hubble Space Telescope. TheSpace Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. under NASA contract NAS5-26555.Figures 1 and 2 are only available in electronic form athttp://www.edpsciences.org
| The SAURON project - II. Sample and early results
Early results are reported from the SAURON survey of the kinematics andstellar populations of a representative sample of nearby E, S0 and Sagalaxies. The survey is aimed at determining the intrinsic shape of thegalaxies, their orbital structure, the mass-to-light ratio as a functionof radius, the age and metallicity of the stellar populations, and thefrequency of kinematically decoupled cores and nuclear black holes. Theconstruction of the representative sample is described, and itsproperties are illustrated. A comparison with long-slit spectroscopicdata establishes that the SAURON measurements are comparable to, orbetter than, the highest-quality determinations. Comparisons arepresented for NGC 3384 and 4365, where stellar velocities and velocitydispersions are determined to a precision of 6kms-1, and theh3 and h4 parameters of the line-of-sight velocitydistribution to a precision of better than 0.02. Extraction of accurategas emission-line intensities, velocities and linewidths from the datacubes is illustrated for NGC 5813. Comparisons with published linestrengths for NGC 3384 and 5813 reveal uncertainties of <~0.1Åon the measurements of the Hβ, Mg b and Fe5270 indices.Integral-field mapping uniquely connects measurements of the kinematicsand stellar populations to the galaxy morphology. The maps presentedhere illustrate the rich stellar kinematics, gaseous kinematics, andline-strength distributions of early-type galaxies. The results includethe discovery of a thin, edge-on, disc in NGC 3623, confirm theaxisymmetric shape of the central region of M32, illustrate the LINERnucleus and surrounding counter-rotating star-forming ring in NGC 7742,and suggest a uniform stellar population in the decoupled core galaxyNGC 5813.
| The New Galaxy: Signatures of Its Formation
The formation and evolution of galaxies is one of the great outstandingproblems of astrophysics. Within the broad context of hierachicalstructure formation, we have only a crude picture of how galaxies likeour own came into existence. A detailed physical picture whereindividual stellar populations can be associated with (tagged to)elements of the protocloud is far beyond our current understanding.Important clues have begun to emerge from both the Galaxy (near-fieldcosmology) and the high redshift universe (far-field cosmology). Here wefocus on the fossil evidence provided by the Galaxy. Detailed studies ofthe Galaxy lie at the core of understanding the complex processesinvolved in baryon dissipation. This is a necessary first step towardachieving a successful theory of galaxy formation.
| The Radio Properties of Composite LINER/H II Galaxies
Arcsecond-resolution VLA observations-newly obtained as well aspublished-of 40 nearby galaxies are discussed, completing a study of theradio properties of a magnitude-limited sample of nearby galaxies of thecomposite LINER/H II type. Our results reveal an overall detection rateof at least 25% active galactic nucleus (AGN) candidates among thesecomposite sources. The general properties of these AGN candidates, ascompared to non-AGN composite sources and H II galaxies, are discussed.
| Nuclear Cusps and Cores in Early-Type Galaxies as Relics of Binary Black Hole Mergers
We present an analysis of the central cusp slopes and core parameters ofearly-type galaxies using a large database of surface brightnessprofiles obtained from Hubble Space Telescope observations. We examinethe relation between the central cusp slopes, core parameters, and blackhole masses in early-type galaxies, in light of two models that attemptto explain the formation of cores and density cusps via the dynamicalinfluence of black holes. Contrary to the expectations fromadiabatic-growth models, we find that the cusp slopes do not steepenwith increasing black hole mass fraction. Moreover, a comparison ofkinematic black hole mass measurements with the masses predicted by theadiabatic models shows that they overpredict the masses by a factor of~3. Simulations involving binary black hole mergers predict that boththe size of the core and the central mass deficit correlate with thefinal black hole mass. These relations are qualitatively supported bythe present data.
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