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A Chandra Survey of Early-Type Galaxies. I. Metal Enrichment in the Interstellar Medium
We present a Chandra study of the emission-weighted metal abundances in28 early-type galaxies, spanning ~3 orders of magnitude in X-rayluminosity (LX). We report constraints for Fe, O, Ne, Mg, Si,S, and Ni. We find no evidence of the very subsolar Fe abundance(ZFe) historically reported, confirming a trend in recentobservations of bright galaxies and groups, nor do we find anycorrelation between ZFe and luminosity. Excepting one case,the ISM is single-phase, indicating that multitemperature fits foundwith ASCA reflected temperature gradients that we resolve with Chandra.We find no evidence that ZFe (ISM) is substantially lowerthan the stellar metallicity estimated from simple stellar populationmodels. In general, these quantities are similar, which is inconsistentwith galactic wind models and recent hierarchical chemical enrichmentsimulations. Our abundance ratio constraints imply that 66%+/-11% of theISM Fe was produced in SNe Ia, similar to the solar neighborhood,indicating similar enrichment histories for elliptical galaxies and theMilky Way. Although these values are sensitive to the considerablesystematic uncertainty in the supernova yields, they are in agreementwith observations of more massive systems. This indicates considerablehomology in the enrichment process operating from cluster scales tolow-to-intermediate-LX galaxies. The data uniformly exhibitlow ZO/ZMg ratios, which have been reported insome clusters, groups, and galaxies. This is inconsistent with standardSN II metal yield calculations and may indicate an additional source ofenrichment, such as Population III hypernovae.
| Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton
We investigated the dynamical structure of 53 elliptical galaxies usingthe Chandra archival X-ray data. In X-ray-luminous galaxies, temperatureincreases with radius and gas density is systematically higher at theoptical outskirts, indicating the presence of a significant amount ofthe group-scale hot gas. In contrast, X-ray-dim galaxies show a flat ordeclining temperature profile against radius and the gas density isrelatively lower at the optical outskirts. Thus, it is found thatX-ray-bright and faint elliptical galaxies are clearly distinguished bythe temperature and gas density profile. The mass profile is well scaledby a virial radius r200 rather than an optical half-radiusre, is quite similar at (0.001-0.03)r200 betweenX-ray-luminous and dim galaxies, and smoothly connects to those profilesof clusters of galaxies. At the inner region of(0.001-0.01)r200 or (0.1-1)re, the mass profilewell traces a stellar mass with a constant mass-to-light ratio ofM/LB=3-10 Msolar/Lsolar. TheM/LB ratio of X-ray-bright galaxies rises up steeply beyond0.01r200 and thus requires a presence of massive dark matterhalo. From the deprojection analysis combined with the XMM-Newton data,we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 alsohave a high M/LB ratio of 20-30 at 20 kpc, comparable to thatof X-ray-luminous galaxies. Therefore, dark matter is indicated to becommon in elliptical galaxies; their dark matter distribution, as wellas that of galaxy clusters, almost follows the NFW profile.
| Peculiarities and populations in elliptical galaxies. III. Dating the last star formation event
Using 6 colours and 4 Lick line-indices we derive two-component modelsof the populations of ellipticals, involving a "primary" and a"juvenile" population. The first component is defined by the regressionsof indices against the central velocity dispersion found in Papers I andII for the {Nop} sample of non-peculiar objects. The second one isapproximated by an SSP, and the modeling derives its age A, metallicityZ and fractional V-luminosity q_V, the fractional mass qMbeing found therefrom. The model is designed for "blueish" peculiargalaxies, i.e. the {Pec} sample and NGC 2865 family in the terminologyof Paper I. The morphological peculiarities and the population anomalyare then believed to involve the same event, i.e. a merger plusstarburst. It is possible to improve the models in a few cases byintroducing diffuse dust (as suggested by far IR data), and/or by takinginto account the fact that Lick- and colour indices do not relate toidentical galaxy volumes. In most of the cases, the mass ratio of youngstars qM seems too small for the product of a recent majormerger: the events under consideration might be minor mergers bringing"the final touch" to the build-up of the structure of the E-type object.The same modeling has been successfully applied to blueish galaxies ofthe {Nop} sample, without morphological peculiarities however, tosupport the occurence of a distinct perturbing event. A few reddishobjects of the {Pec} sample (NGC 3923 family) and of the {Nop} sampleare also modeled, in terms of an excess of high metallicity stars, ordiffuse dust, or both, but the results are inconclusive.
| The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies
The Hubble Space Telescope has revealed a dichotomy in the centralsurface brightness profiles of early-type galaxies, which havesubsequently been grouped into two families: core, boxy, anisotropicsystems; and cuspy (`power-law'), discy, rotating ones. Here weinvestigate whether a dichotomy is also present in the X-ray propertiesof the two families. We consider both their total soft emission(LSX,tot), which is a measure of the galactic hot gascontent, and their nuclear hard emission (LHX,nuc), mostlycoming from Chandra observations, which is a measure of the nuclearactivity. At any optical luminosity, the highest LSX,totvalues are reached by core galaxies; this is explained by their beingthe central dominant galaxies of groups, subclusters or clusters, inmany of the logLSX,tot (ergs-1) >~ 41.5 cases.The highest LHX,nuc values, similar to those of classicalactive galactic nuclei (AGNs), in this sample are hosted only by core orintermediate galaxies; at low luminosity AGN levels, LHX,nucis independent of the central stellar profile shape. The presence ofoptical nuclei (also found by HST) is unrelated to the level ofLHX,nuc, even though the highest LHX,nuc are allassociated with optical nuclei. The implications of these findings forgalaxy evolution and accretion modalities at the present epoch arediscussed.
| Group, field and isolated early-type galaxies - II. Global trends from nuclear data
We have derived ages, metallicities and enhanced-element ratios[α/Fe] for a sample of 83 early-type galaxies essentially ingroups, the field or isolated objects. The stellar-population propertiesderived for each galaxy correspond to the nuclear re/8aperture extraction. The median age found for Es is 5.8+/-0.6 Gyr andthe average metallicity is +0.37+/-0.03 dex. For S0s, the median age is3.0+/-0.6 Gyr and [Z/H]= 0.53+/-0.04 dex. We compare the distribution ofour galaxies in the Hβ-[MgFe] diagram with Fornax galaxies. Ourelliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster.We find that the galaxies lie in a plane defined by [Z/H]= 0.99logσ0- 0.46 log(age) - 1.60, or in linear terms Z~σ0× (age) -0.5. More massive (largerσ0) and older galaxies present, on average, large[α/Fe] values, and therefore must have undergone shorterstar-formation time-scales. Comparing group against field/isolatedgalaxies, it is not clear that environment plays an important role indetermining their stellar-population history. In particular, ourisolated galaxies show ages differing by more than 8 Gyr. Finally weexplore our large spectral coverage to derive log(O/H) metallicity fromthe Hα and NIIλ6584 and compare it with model-dependent[Z/H]. We find that the O/H abundances are similar for all galaxies, andwe can interpret it as if most chemical evolution has already finishedin these galaxies.
| Group, field and isolated early-type galaxies - I. Observations and nuclear data
This is the first paper of a series on the investigation of stellarpopulation properties and galaxy evolution of an observationallyhomogeneous sample of early-type galaxies in groups, field and isolatedgalaxies.Here we present high signal-to-noise ratio (S/N) long-slit spectroscopyof 86 nearby elliptical and S0 galaxies. Eight of them are isolated,selected according to a rigorous criterion, which guarantees a genuinelow-density subsample. The present survey has the advantage of coveringa larger wavelength range than normally found in the literature, whichincludes [OIII]λ5007 and Hα, both lines important foremission correction. Among the 86 galaxies with S/N >= 15 (perresolution element, for re/8 central aperture), 57 have theirHβ-index corrected for emission (the average correction is 0.190Åin Hβ) and 42 galaxies reveal [OIII]λ5007 emission,of which 16 also show obvious Hα emission. Most of the galaxies inthe sample do not show obvious signs of disturbances nor tidal featuresin the morphologies, although 11 belong to the Arp catalogue of peculiargalaxies; only three of them (NGC 750, 751 and 3226) seem to be stronglyinteracting. We present the measurement of 25 central line-strengthindices calibrated to the Lick/IDS system. Kinematic information isobtained for the sample. We analyse the line-strength index versusvelocity dispersion relations for our sample of mainly low-densityenvironment galaxies, and compare the slope of the relations withcluster galaxies from the literature. Our main findings are that theindex-σ0 relations presented for low-density regionsare not significantly different from those of cluster E/S0s. The slopeof the index-σ0 relations does not seem to change forearly-type galaxies of different environmental densities, but thescatter of the relations seems larger for group, field and isolatedgalaxies than for cluster galaxies.
| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| Chandra Study of X-Ray Point Sources in the Early-Type Galaxy NGC 4552 (M89)
We present a Chandra ACIS study of the early-type galaxy NGC 4552. Wedetect 47 X-ray point sources, most of which are likely low-mass X-raybinaries (LMXBs), within four effective radii (Re). Thebrightest X-ray source coincides with the optical, UV, and radio centerof the galaxy and shows variability on >1 hr timescales, indicatingthe possible existence of a low-luminosity active galactic nucleus(AGN). The 46 off-center sources and the unresolved point sourcescontribute about 29% and 20% to the total luminosity of the galaxy,respectively. We find that after correcting for the incompleteness atthe low-luminosity end, the observed cumulative X-ray luminosityfunction (XLF) of the off-center sources is best fitted by a brokenpower-law model with a break atLb=4.4+2.0-1.4×1038ergs s-1. We identified 210 globular cluster (GC) candidatesin a HST WFPC2 optical image of the galaxy's central region. Of the 25off-center LMXBs that fall within the WFPC2 field of view, 10 sourcesare coincident with a GC. Thus, the fraction of the GCs hosting brightLMXBs and the fraction of the LMXBs associated with GCs are 4.8% and40%, respectively. In the V and I bands, the GCs hosting bright LMXBsare typically 1-2 mag brighter than the GCs with no detected LMXBs.There are about 1.9+/-0.4 times as many LMXBs in the red, metal-rich GCsas there are in the blue, metal-poor ones. We find no obvious differencebetween the luminosity distributions of LMXBs in GCs and in the field,but the cumulative spectrum of the LMXBs in GCs tends to be softer thanthat of the LMXBs in field. We detected three X-ray sources that haveisotropic luminosities larger than 1039 ergs s-1.Only one of these is located in the joint Chandra-HST field and is foundto be associated with a GC. By studying its ACIS spectra we infer thatthe this may be a candidate black hole system with a mass of 15-135Msolar. One of the other sources with a luminosity brighterthan 1039 ergs s-1 reveals temporal variations inbrightness on timescales greater than 1 hr.
| Nuclear Accretion in Galaxies of the Local Universe: Clues from Chandra Observations
In order to find an explanation for the radiative quiescence ofsupermassive black holes in the local universe, the most accurateestimates for a sample of nearby galaxies are collected for the mass ofa central black hole (MBH), the nuclear X-ray luminosityLX,nuc, and the circumnuclear hot gas density andtemperature, by using Chandra data. The nuclear X-ray luminosityLX,nuc varies by ~3 orders of magnitude and does not show arelationship with MBH or with the Bondi mass accretion rateM˙B LX,nuc is always much lower than expectedif M˙B ends in a standard accretion disk with highradiative efficiency (this instead can be the case of the active nucleusof Cen A). Radiatively inefficient accretion as in the standardadvection-dominated accretion flow (ADAF) modeling may explain the lowluminosities of a few cases; for others, the predicted luminosity isstill too high, and, in terms of Eddington-scaled quantities, it isincreasingly higher than that observed for increasingM˙B. Variants of the simple radiatively inefficientscenario including outflow and convection may reproduce the low emissionlevels observed, since the amount of matter actually accreted is reducedconsiderably. However, the most promising scenario includes feedbackfrom accretion on the surrounding gas; this has the important advantagesof naturally explaining the observed lack of relationship amongLX,nuc, MBH, and M˙B, and evadingthe problem of the fate of the material accumulating in the centralgalactic regions over cosmological times.
| The Epochs of Early-Type Galaxy Formation as a Function of Environment
The aim of this paper is to set constraints on the epochs of early-typegalaxy formation through the ``archaeology'' of the stellar populationsin local galaxies. Using our models of absorption-line indices thataccount for variable abundance ratios, we derive ages, totalmetallicities, and element ratios of 124 early-type galaxies in high-and low-density environments. The data are analyzed by comparison withmock galaxy samples created through Monte Carlo simulations taking thetypical average observational errors into account, in order to eliminateartifacts caused by correlated errors. We find that all threeparameters, age, metallicity, and α/Fe ratio, are correlated withvelocity dispersion. We show that these results are robust againstrecent revisions of the local abundance pattern at high metallicities.To recover the observed scatter we need to assume an intrinsic scatterof about 20% in age, 0.08 dex in [Z/H], and 0.05 dex in [α/Fe].All low-mass objects withM*<~1010Msolar (σ<~130kms-1) show evidence for the presence of intermediate-agestellar populations with low α/Fe ratios. About 20% of theintermediate-mass objects with1010<~M*/Msolar<~1011[110<~σ/(kms-1)<~230 both elliptical andlenticular galaxies] must have either a young subpopulation or a bluehorizontal branch. On the basis of the above relationships, valid forthe bulk of the sample, we show that the Mg-σ relation is mainlydriven by metallicity, with similar contributions from the α/Feratio (23%) and age (17%). We further find evidence for an influence ofthe environment on the stellar population properties. Massive early-typegalaxies in low-density environments seem on average ~2 Gyr younger andslightly (~0.05-0.1 dex) more metal-rich than their counterparts inhigh-density environments. No offsets in the α/Fe ratios areinstead detected. With the aid of a simple chemical evolution model, wetranslate the derived ages and α/Fe ratios into star formationhistories. We show that most star formation activity in early-typegalaxies is expected to have happened between redshifts ~3 and 5 inhigh-density environments and between redshifts 1 and 2 in low-densityenvironments. We conclude that at least 50% of the total stellar massdensity must have already formed at z~1, in good agreement withobservational estimates of the total stellar mass density as a functionof redshift. Our results suggest that significant mass growth in theearly-type galaxy population below z~1 must be restricted to lessmassive objects, and a significant increase of the stellar mass densitybetween redshifts 1 and 2 should be present, caused mainly by the fieldgalaxy population. The results of this paper further imply the presenceof vigorous star formation episodes in massive objects at z~2-5 andevolved elliptical galaxies around z~1, both observationally identifiedas SCUBA galaxies and extremely red objects, respectively.
| Near infra-red and optical colour gradients in E-type galaxies. Inferences on dust content
Colour gradients are considered for a sample of circa 50 E-type galaxiesin the Local Supercluster. The new data includes isophotal colourprofiles in J-H, J-K, V-J and V-K, measured using 2MASS frames mostlyfrom the Large Galaxies Atlas, V frames from previous work and Vprofiles from the literature. This is supplemented by U-B, B-V, B-R, V-Icolour gradients obtained anew from published photometric data. Colourgradients in E galaxies show remarkably large variations from object toobject and do not correlate with other properties. Metallicity gradientsare the primary cause as shown before. Age gradients with oppositeeffects are possibly needed to explain objects with small colourgradients. Some empirical evidence of such age effects has been foundfor a subset of objects with morphological peculiarities and youngerstars mixed. Dust has only modest effects on colour gradients, as shownby the fact that objects with zero IRAS 100 μ flux have the sameaverage values of the gradients, except in V-J and V-K, as those withnon zero flux (cf. Table 7). This last subsample however exhibits poorbut definite correlations between IRAS flux and gradients, which mightbe caused by the presence of a few relatively dusty galaxies in thesample. Given the absence of a correlation between any gradients andgalaxy velocity dispersion (and hence mass), the observations do notagree with the predictions of the monolithic scenario for the formationof E galaxies. Simulated datasets of “dummy” objectsmimicking the hierarchical scenario have been obtained, and used to testa technique for estimating the dust content of E-galaxies from thecomparison of the V-K (or V-J) colour gradients with the U-B (or B-V)ones: the contents of diffuse dust, gauged in terms of published models,are obtained for a dozen objects.
| Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles
We present a study of the optical brightness profiles of early typegalaxies, using a number of samples of radio galaxies and opticallyselected elliptical galaxies. For the radio galaxy samples - B2 ofFanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined anumber of parameters that describe a "Nuker-law" profile, which werecompared with those already known for the optically selected objects. Wefind that radio active galaxies are always of the "core" type (i.e. aninner Nuker law slope γ < 0.3). However, there are core-typegalaxies which harbor no significant radio source and which areindistinguishable from the radio active galaxies. We do not find anyradio detected galaxy with a power law profile (γ > 0.5). Thisdifference is not due to any effect with absolute magnitude, since in aregion of overlap in magnitude the dichotomy between radio active andradio quiescent galaxies remains. We speculate that core-type objectsrepresent the galaxies that have been, are, or may become, radio activeat some stage in their lives; active and non-active core-type galaxiesare therefore identical in all respects except their eventualradio-activity: on HST scales we do not find any relationship betweenboxiness and radio-activity. There is a fundamental plane, defined bythe parameters of the core (break radius rb and breakbrightness μ_b), which is seen in the strong correlation betweenrb and μ_b. The break radius is also linearly proportionalto the optical Luminosity in the I band. Moreover, for the few galaxieswith an independently measured black hole mass, the break radius turnsout to be tightly correlated with MBH. The black hole masscorrelates even better with the combination of fundamental planeparameters rb and μ_b, which represents the centralvelocity dispersion.
| Measuring shapes of galaxy images - II. Morphology of 2MASS galaxies
We study a sample of 112 galaxies of various Hubble types imaged in theTwo Micron All Sky Survey (2MASS) in the near-infrared (NIR; 1-2 μm)J, H and Ks bands. The sample contains (optically classified)32 ellipticals, 16 lenticulars and 64 spirals acquired from the 2MASSExtended Source Catalogue (XSC).We use a set of non-parametric shape measures constructed from theMinkowski functionals (MFs) for galaxy shape analysis. We useellipticity (ɛ) and orientation angle (Φ) as shapediagnostics. With these parameters as functions of area within theisophotal contour, we note that the NIR elliptical galaxies withɛ > 0.2 show a trend of being centrally spherical andincreasingly flattened towards the edge, a trend similar to images inoptical wavelengths. The highly flattened elliptical galaxies showstrong change in ellipticity between the centre and the edge. Thelenticular galaxies show morphological properties resembling eitherellipticals or disc galaxies. Our analysis shows that almost half of thespiral galaxies appear to have bar-like features while the rest arelikely to be non-barred. Our results also indicate that almost one-thirdof spiral galaxies have optically hidden bars.The isophotal twist noted in the orientations of elliptical galaxiesdecreases with the flattening of these galaxies, indicating that twistand flattening are also anticorrelated in the NIR, as found in opticalwavelengths. The orientations of NIR lenticular and spiral galaxies showa wide range of twists.
| A Sample of Field Ellipticals
Using well-defined selection criteria derived from Zaritsky et al.applied to the LEDA galaxy catalog, we have constructed a sample ofelliptical galaxies that can be taken to lie in the field. Such criteriacan easily be applied to theoretical simulations for direct comparisonwith observations. The variation of the number of ``isolated''ellipticals with selection criteria is also investigated. A preliminarystudy of the environment of the field ellipticals shows that, in themean, they are surrounded by a population of dwarf galaxies, out toprojected radii of at least 500 kpc, with a radial density profile ofr-0.6+/-0.2 and a luminosity function slope of α~-1.8.The results are compared and contrasted to the satellite populationaround isolated spiral galaxies.
| Chandra Observations of Diffuse Gas and Luminous X-Ray Sources around the X-Ray-bright Elliptical Galaxy NGC 1600
We observed the X-ray-bright E3 galaxy NGC 1600 and nearby members ofthe NGC 1600 group with the Chandra X-Ray Observatory ACIS-S3 to studytheir X-ray properties. Unresolved emission dominates the observation;however, we resolved some of the emission into 71 sources, most of whichare low-mass X-ray binaries associated with NGC 1600. Twenty-one of thesources have LX>2×1039 ergss-1 (0.3-10.0 keV; assuming they are at the distance of NGC1600), marking them as ultraluminous X-ray point source (ULX)candidates; we expect that only 11+/-2 are unrelatedforeground/background sources. NGC 1600 may have the largest number ofULX candidates in an early-type galaxy to date; however, cosmic variancein the number of background active galactic nuclei cannot be ruled out.The spectrum and luminosity function (LF) of the resolved sources aremore consistent with sources found in other early-type galaxies thanwith sources found in star-forming regions of galaxies. The source LFand the spectrum of the unresolved emission both indicate that there area large number of unresolved point sources. We propose that thesesources are associated with globular clusters (GCs) and that NGC 1600has a large GC specific frequency. Observations of the GC population inNGC 1600 would be very useful for testing this prediction. Approximately50%-75% of the unresolved flux comes from diffuse gaseous emission. Thespectral fits, hardness ratios, and X-ray surface brightness profile allpoint to two gas components. We interpret the soft inner component(a<~25'', kT~0.85 keV) as the interstellar medium of NGC1600 and the hotter outer component (a>~25'', kT~1.5 keV)as the intragroup medium of the NGC 1600 group. The X-ray image showsseveral interesting structures. First, there is a central region ofexcess emission that is roughly cospatial with Hα and dustfilaments immediately west of the center of NGC 1600. There appear to beholes in the X-ray emission to the north and south of the galaxy centerthat are roughly coincident with the lobes of the NGC 1600 radio source.On larger scales, there is excess emission to the northeast, which wesuggest may indicate the center of the group potential. The group galaxyNGC 1603 shows a tail of X-ray emission to its west that is probably dueto ram pressure stripping.
| Revised Rates of Stellar Disruption in Galactic Nuclei
We compute rates of tidal disruption of stars by supermassive blackholes in galactic nuclei, using downwardly revised black hole massesfrom the MBH-σ relation. In galaxies with steep nucleardensity profiles, which dominate the overall event rate, the disruptionfrequency varies inversely with assumed black hole mass. We compute atotal rate for nondwarf galaxies of ~10-5 yr-1Mpc-3, about a factor of 10 higher than in earlier studies.Disruption rates are predicted to be highest in nucleated dwarfgalaxies, assuming that such galaxies contain black holes. Monitoring ofa rich galaxy cluster for a few years could rule out the existence ofintermediate-mass black holes in dwarf galaxies.
| Peculiarities and populations in elliptical galaxies. I. An old question revisited
Morphological peculiarities, as defined from isophote asymmetries andnumber of detected shells, jets or similar features, have been estimatedin a sample of 117 E classified galaxies, and qualified by an ad hocΣ2 index. The overall frequency of ``peculiar'' objects(Pec subsample) is 32.5%. It decreases with the cosmic density of theenvironment, being minimal for the Virgo cluster, the densestenvironment in the sampled volume. This environmental effect is strongerfor galaxies with relatively large Σ2.The Pec subsample objects are compared with ``normal'' objects (Nopsubsample) as regards their basic properties. Firstly, theysystematically deviate from the Fundamental Plane and the Faber-Jacksonrelation derived for the Nop subsample, being too bright for their mass.Secondly, the dust content of galaxies, as estimated from IRAS fluxes,are similar in both subsamples. Third, the same is true of the frequencyof Kinematically Distinct cores (KDC), suggesting that KDC andmorphological peculiarities do not result from the same events in thehistory of E-galaxies.Using the Nop sample alone, we obtain very tight reference relationsbetween stellar population indicators (U-B, B-V, B-R, V-I,Mg2, Hβ, , Mgb) and the central velocitydispersion σ0. The discussion of the residuals of theserelations allows us to classify the Pec galaxies in two families i.e.the YP or NGC 2865 family, and the NP or NGC 3923 one. Galaxies in thefirst group show consistent evidence for a younger stellar populationmixed with the old one, in agreement with classical results (Schweizeret al. \cite{Schweizer1990}; Schweizer & Seitzer\cite{Schweizer1992}). The second group, however, has ``normal``, orreddish, populations. It is remarkable that a fraction (circa 40%) ofmorphologically perturbed objects do not display any signature of ayoung population, either because the event responsible for thepecularity is too ancient, or because it did not produce significantstar formation (or eventually that the young sub-population has highmetallicity).A preliminary attempt is made to interpret the populations of Pecobjects by combining a young Single Stellar Population with a Nopgalaxy, with only limited success, perhaps largely due to uncertaintiesin the SSP indices used.Based in part on observations collected at the Observatoire deHaute-Provence.Figures \ref{fig1}-\ref{fig3} are only available in electronic form athttp://www.edpsciences.orgTable 10 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/423/833
| Stellar collisions in galactic centres: black hole growth and colour gradients
We study the effects of stellar collisions, particularly on feedingmassive black holes (BHs) and colour gradients, in realistic galacticcentres. We find that the mass released by stellar collisions is notsufficient to account for the present BH mass in galactic centres,especially in bright galaxies. This study, together with the study byMagorrian & Tremaine on tidal disruption of stars by massive BHs,implies that the material for BH growth (especially in galaxies brighterthan ~109 Lsolar) can only come from othersources, for example, the mass released by stellar evolution in theinitial ~1 Gyr of the lifetime of the galaxy, or the gas that sinks tothe galactic centre in a galaxy merger. We also analyse how the colourof a stellar system is affected by collisions of stars. We find thatcollisions between main-sequence stars cannot cause observable colourgradients in the visible bands at projected radius R>~ 0.1 arcsec inM31, M32 and other nearby galactic centres. This result is consistentwith the lack of an observable colour gradient in M32 at R >~ 0.1arcsec. At even smaller radii, the colour differences caused bycollisions between main-sequence stars are at most 0.08 mag at R = 0.02arcsec. The averaged bluing caused by stellar collisions in the region R< 0.1 arcsec of M32 should not be larger than 0.06 mag in colourindex U - V and 0.02 mag in V - I. The observed bluing in the centre ofthe galaxy M31 (in a 0.14 × 0.14 arcsec2 box) must becaused by some mechanism other than collisions between main-sequencestars.
| 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.
| Redshift-Distance Survey of Early-Type Galaxies: Spectroscopic Data
We present central velocity dispersions and Mg2 line indicesfor an all-sky sample of ~1178 elliptical and S0 galaxies, of which 984had no previous measures. This sample contains the largest set ofhomogeneous spectroscopic data for a uniform sample of ellipticalgalaxies in the nearby universe. These galaxies were observed as part ofthe ENEAR project, designed to study the peculiar motions and internalproperties of the local early-type galaxies. Using 523 repeatedobservations of 317 galaxies obtained during different runs, the dataare brought to a common zero point. These multiple observations, takenduring the many runs and different instrumental setups employed for thisproject, are used to derive statistical corrections to the data and arefound to be relatively small, typically <~5% of the velocitydispersion and 0.01 mag in the Mg2 line strength. Typicalerrors are about 8% in velocity dispersion and 0.01 mag inMg2, in good agreement with values published elsewhere.
| 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
| Abundance gradients in elliptical galaxies
Abundance gradients predicted by dissipative models of galaxy formationare studied with the aid of mass models of spherical galaxies obeyingthe Sersic R1/n law in projection. The link betweenmetallicity and stellar binding energies and angular momenta is derivedfrom the ``concentration model'' by Lynden-Bell (\cite{Lyn75}) with theaddition of a terminal wind and an age spread among the stellarpopulations of individual ellipticals. By using the calibration of theMg2 index as a function of metallicity and age fromSingle-burst Stellar Populations models, the radial profiles ofMg2 are computed for a variety of models to illustrate thegeneral predictions of the present approach. We also matched theobserved radial profiles of Mg2 in a sample of 11ellipticals. We find that observations are reproduced reasonably well inour adopted approximation and require a considerable degree ofdissipation and a metal yield above the solar value. A terminal wind oran age spread make it easier to fit the Mg2 profiles of theellipticals with shallow Mg2 profiles in the innermostregions. However, model results (with or without an age spread) showthat the exact determination of the free parameters suffers fromdegeneracy of their effects. Moreover, model results also suffer fromthe poorly known effects of non-solar abundance ratios on indexMg2. Other spectral indices, besides Mg2, andbroad-band colours observed in elliptical galaxies can be examined inthe same way with the present scheme; their study is postponed to afurther investigation.
| A SAURON study of M32: measuring the intrinsic flattening and the central black hole mass
We present dynamical models of the nearby compact elliptical galaxy M32,using high-quality kinematic measurements, obtained with theintegral-field spectrograph SAURON mounted on the William HerschelTelescope on La Palma. We also include STIS data obtained previously byJoseph et al. We find a best-fitting black hole mass ofM•= (2.5 +/- 0.5) × 106Msolar and a stellar I-band mass-to-light ratio of (1.85 +/-0.15) Msolar/Lsolar. For the first time, we arealso able to constrain the inclination along which M32 is observed to70°+/- 5°. Assuming that M32 is indeed axisymmetric, theaveraged observed flattening of 0.73 then corresponds to an intrinsicflattening of 0.68 +/- 0.03. These tight constraints are mainly causedby the use of integral-field data. We show this quantitatively bycomparing with models that are constrained by multiple slits only. Weshow the phase-space distribution and intrinsic velocity structure ofthe best-fitting model and investigate the effect of regularization onthe orbit distribution.
| Evolution of massive binary black holes
Since many or most galaxies have central massive black holes (BHs),mergers of galaxies can form massive binary black holes (BBHs). In thispaper we study the evolution of massive BBHs in realistic galaxy models,using a generalization of techniques used to study tidal disruptionrates around massive BHs. The evolution of BBHs depends on BH mass ratioand host galaxy type. BBHs with very low mass ratios (say, <~0.001)are hardly ever formed by mergers of galaxies, because the dynamicalfriction time-scale is too long for the smaller BH to sink into thegalactic centre within a Hubble time. BBHs with moderate mass ratios aremost likely to form and survive in spherical or nearly sphericalgalaxies and in high-luminosity or high-dispersion galaxies; they aremost likely to have merged in low-dispersion galaxies (line-of-sightvelocity dispersion <~90kms-1 ) or in highly flattened ortriaxial galaxies. The semimajor axes and orbital periods of survivingBBHs are generally in the range 10-3 -10pc and10-105 yr they are also larger in high-dispersion galaxiesthan in low-dispersion galaxies, larger in nearly spherical galaxiesthan in highly flattened or triaxial galaxies, and larger for BBHs withequal masses than for BBHs with unequal masses. The orbital velocitiesof surviving BBHs are generally in the range 102-104 kms-1 . The methods of detecting survivingBBHs are also discussed. If no evidence of BBHs is found in AGNs, thismay be either because gas plays a major role in BBH orbital decay orbecause nuclear activity switches on soon after a galaxy merger, andends before the smaller BH has had time to spiral to the centre of thegalaxy.
| A catalogue and analysis of local galaxy ages and metallicities
We have assembled a catalogue of relative ages, metallicities andabundance ratios for about 150 local galaxies in field, group andcluster environments. The galaxies span morphological types from cD andellipticals, to late-type spirals. Ages and metallicities were estimatedfrom high-quality published spectral line indices using Worthey &Ottaviani (1997) single stellar population evolutionary models. Theidentification of galaxy age as a fourth parameter in the fundamentalplane (Forbes, Ponman & Brown 1998) is confirmed by our largersample of ages. We investigate trends between age and metallicity, andwith other physical parameters of the galaxies, such as ellipticity,luminosity and kinematic anisotropy. We demonstrate the existence of agalaxy age-metallicity relation similar to that seen for local galacticdisc stars, whereby young galaxies have high metallicity, while oldgalaxies span a large range in metallicities. We also investigate theinfluence of environment and morphology on the galaxy age andmetallicity, especially the predictions made by semi-analytichierarchical clustering models (HCM). We confirm that non-clusterellipticals are indeed younger on average than cluster ellipticals aspredicted by the HCM models. However we also find a trend for the moreluminous galaxies to have a higher [Mg/Fe] ratio than the lowerluminosity galaxies, which is opposite to the expectation from HCMmodels.
| 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.
| Galaxies with a Central Minimum in Stellar Luminosity Density
We used Hubble Space Telescope WFPC2 images to identify six early-typegalaxies with surface brightness profiles that decrease inward over alimited range of radii near their centers. The inferred luminositydensity profiles of these galaxies have local minima interior to theircore break radii. NGC 3706 harbors a high surface brightness ring ofstarlight with radius ~20 pc. Its central structure may be related tothat in the double-nucleus galaxies M31 and NGC 4486B. NGC 4406 and NGC6876 have nearly flat cores that, on close inspection, are centrallydepressed. Colors for both galaxies imply that this is not due to dustabsorption. The surface brightness distributions of both galaxies areconsistent with stellar tori that are more diffuse than the sharplydefined system in NGC 3706. The remaining three galaxies are thebrightest cluster galaxies in A260, A347, and A3574. Color informationis not available for these objects, but they strongly resemble NGC 4406and NGC 6876 in their cores. The thin ring in NGC 3706 may have formeddissipatively. The five other galaxies resemble the endpoints of somesimulations of the merging of two gas-free stellar systems, eachharboring a massive nuclear black hole. In one version of this scenario,diffuse stellar tori are produced when stars initially bound to oneblack hole are tidally stripped away by the second black hole.Alternatively, some inward-decreasing surface brightness profiles mayreflect the ejection of stars from a core during the hardening of thebinary black hole created during the merger. Based on observations madewith the NASA/ESA Hubble Space Telescope, obtained at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with GO proposals 5454, 5512,6099, 6587, and 8683.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
| Hot dust in normal star-forming galaxies: JHKL' photometry of the ISO Key Project sample
We present JHK and 3.8 mu m (L') photometry of 26 galaxies in theInfrared Space Observatory (ISO) Normal Galaxy Key Project (KP) sampleand of seven normal ellipticals with the aim of investigating the originof the 4 mu m emission. The majority of the KP galaxies, and all theellipticals, have K-L<~ 1.0, consistent with stellar photospheresplus moderate dust extinction. Ten of the 26 KP galaxies have K-L>~1.0, corresponding to a flat or rising 4 mu m continuum, consistent withsignificant emission from hot dust at 600-1000 K. K-L is anticorrelatedwith ISO flux ratio F6.75/F15, weakly correlatedwith line ratio [O I]/[C II], but not with [C II]/FIR or IRAS ratioF60/F100. Photodissociation-region models forthese galaxies show that the hot dust responsible for red K-L resides inregions of high pressure and intense far-ultraviolet radiation field.Taken together, these results suggest that star formation in normalstar-forming galaxies can assume two basic forms: an ``active'',relatively rare, mode characterized by hot dust, suppressed AromaticFeatures in Emission (AFEs), high pressure, and intense radiation field;and the more common ``passive'' mode that occurs under more quiescentphysical conditions, with AFEs, and without hot dust. The occurrence ofthese modes appears to only weakly depend on the star-formation rate perunit area. Passive star formation over large scales makes up the bulk ofstar-forming activity locally, while the ``active'' regime may dominateat high redshifts. Based on data obtained at TIRGO, Gornergrat,Switzerland.
| Near-infrared surface photometry of early-type galaxies
CCD infrared (JHKs) photometry was performed on a sample of10 elliptical and 2 lenticular galaxies. Isophotal parameters,brightness profiles, integrated colors and color gradients arepresented. Color gradients found are very weak, showing bluer colorstowards the outer regions. The colors of the sample galaxies arecompatible with stellar populations like those found in metal-richclusters of the Galaxy; objects NGC 7192, NGC 7562 and NGC 7619 arecompatible with less metal-rich populations. The brightness profile ofmost galaxies is well described by the r1/4 law. The profilesof NGC 1600 and NGC 720 are described by Sérsic's law with n ~1.5 and n ~ 1.8 respectively. The infrared effective radius of theobjects studied is typically one half of its counterpart in the B band,which can be an indication that the stellar population that dominatesthe infrared emission is more concentrated in the central regions. Weshow that the sample satisfies the Fundamental Plane relation ofelliptical galaxies in the infrared, with an rms scatter of 0.20 for Jand H and 0.23 for Ks.
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