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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 Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions
The mean ages of early-type galaxies obtained from the analysis ofoptical spectra give a mean age of 8 Gyr at z=0, with 40% being youngerthan 6 Gyr. Independent age determinations are possible by usinginfrared spectra (5-21 μm), which we have obtained with the InfraredSpectrograph on Spitzer. This age indicator is based on the collectivemass-loss rate of stars, in which mass loss from AGB stars produces asilicate emission feature at 9-12 μm. This feature decreases morerapidly than the shorter wavelength continuum as a stellar populationages, providing an age indicator. From observations of 30 nearbyearly-type galaxies, 29 show a spectral energy distribution dominated bystars, and one has significant emission from the ISM and is excluded.The infrared age indicators for the 29 galaxies show them all to be old,with a mean age of about 10 Gyr and a standard deviation of only a fewGyr. This is consistent with the ages inferred from the values ofM/LB, but is inconsistent with the ages derived from theoptical line indices, which can be much younger. All of these ageindicators are luminosity weighted and should be correlated, even ifmultiple-age components are considered. The inconsistency indicates thatthere is a significant problem with either the infrared and theM/LB ages, which agree, or with the ages inferred from theoptical absorption lines.
| 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.
| Star Formation Histories of Nearby Elliptical Galaxies. II. Merger Remnant Sample
This work presents high signal-to-noise spectroscopic observations of asample of six suspected merger remnants, selected primarily on the basisof H I tidal debris detections. Single stellar population analysis ofthese galaxies indicates that their ages, metallicities, andα-enhancement ratios are consistent with those of a representativesample of nearby elliptical galaxies. The expected stellar population ofa recent merger remnant, a young age combined with low [α/Fe], isnot seen in any H I-selected galaxy. However, one galaxy (NGC 2534) isfound to deviate from the Z-plane in the sense expected for a mergerremnant. Another galaxy (NGC 7332), selected by other criteria, bestmatches the merger remnant expectations.
| 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.
| Stellar populations in a complete sample of local radio galaxies
We investigate the nature of the continuum emission and stellarpopulations in the inner 1-3 kpc of a complete sample of 24 southernradio galaxies, and we compare the results with a control sample of 18non-active early-type galaxies. 12 of the radio galaxies are classifiedas Fanaroff-Riley type I (FR I), eight as FR II and four as intermediateor undefined type (FR x). Optical long-slit spectra are used to performspectral synthesis as a function of distance from the nucleus at anaverage sampling of 0.5-1.0 kpc and to quantify the relativecontributions of a blue featureless continuum and stellar populationcomponents of different ages. Our main finding is a systematicdifference between the stellar populations of the radio and controlsample galaxies: the former have a larger contribution from anintermediate-age (1 Gyr) component, suggesting a connection between thepresent radio activity and a starburst which occurred ~1 Gyr ago. Inaddition, we find a correlation between the contribution of the 1-Gyrcomponent and the radio power, suggesting that more massive starburstshave led to more powerful radio emission. A similar relation is foundbetween the radio power and the mean age of the stellar population, inthe sense that stronger nuclear activity is found in younger galaxies.We also find that the stellar populations of FR I galaxies are, onaverage, older and more homogeneous than those of FR IIs. Significantpopulation gradients were found in only four radio galaxies, which arealso those with more than 10 per cent of their total flux at 4020Åcontributed by age components younger than 100 Myr and/or afeatureless continuum (indistinguishable from a 3-Myr-old stellarpopulation).
| 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.
| Dark matter in early-type galaxies: dynamical modelling of IC 1459, IC 3370, NGC 3379 and NGC 4105
We analyse long-slit spectra of four early-type galaxies which extendfrom ~1 to 3 effective radii: IC 1459; IC 3370; NGC 3379 and NGC 4105.We have extracted the full line-of-sight velocity distribution (in thecase of NGC 3379 we also used data from the literature), which we modelusing the two-integral approach. Using two-integral modelling, we findno strong evidence for dark haloes, but the fits suggest thatthree-integral modelling is necessary. We also find that the inferredconstant mass-to-light ratio in all the four cases is typical forearly-type galaxies. Finally, we also discuss the constraints on themass-to-light ratio, which can be obtained using X-ray haloes in thecase of IC 1459, NGC 3379 and NGC 4105, and compare the estimated valueswith the predictions from the dynamical modelling.
| Spatially resolved stellar populations in the isolated elliptical NGC 821
We present the analysis of Lick absorption-line indices from threeseparate long-slit spectroscopic observations of the nearby isolatedelliptical galaxy NGC 821. The three data sets present a consistentpicture of the stellar population within one effective radius, in whichstrong gradients are evident in both luminosity-weighted age andmetallicity. The central population exhibits a young age of ~4 Gyr and ametallicity ~3 times solar. At one effective radius the age has risen to~12 Gyr and the metallicity fallen to less than ~1/3 times solar. Thelow-metallicity population around one effective radius appears to havean exclusively red horizontal branch (RHB), with no significantcontribution from the blue horizontal branch evident in some globularclusters of the same age and metallicity. Despite the strong central agegradient, we demonstrate that only a small fraction (<=10 per cent)of the galaxy's stellar mass can have been created in recent starformation events. We consider possible star formation histories for NGC821 and find that the most likely cause of the young central populationwas a minor merger or tidal interaction that caused NGC 821 to consumeits own gas in a centrally concentrated burst of star formation 1-4 Gyrago.
| The fundamental plane of isolated early-type galaxies
Here we present new measurements of effective radii, surfacebrightnesses and internal velocity dispersions for 23 isolatedearly-type galaxies. The photometric properties are derived from newmulticolour imaging of ten galaxies, whereas the central kinematics forseven galaxies are taken from forthcoming work by Hau & Forbes.These are supplemented with data from the literature. We reproduce thecolour-magnitude and Kormendy relations and strengthen the result of therecent work of Reda et al. that isolated galaxies follow the samephotometric relations as galaxies in high-density environments. We alsofind that some isolated galaxies reveal fine structure indicative of arecent merger, while others appear undisturbed. We examine theFundamental Plane both in the traditional Re,μe, σ space and also in κ space. Most isolatedgalaxies follow the same Fundamental Plane tilt and scatter for galaxiesin high-density environments. However, a few galaxies notably deviatefrom the Plane in the sense of having smaller M/L ratios. This can beunderstood in terms of their younger stellar populations, which arepresumably induced by a gaseous merger. Overall, isolated galaxies havesimilar properties to those in groups and clusters with a slightenhancement in the frequency of recent mergers/interactions.
| 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.
| Mass-to-light ratio gradients in early-type galaxy haloes
Owing to the fact that the near future should see a rapidly expandingset of probes of the halo masses of individual early-type galaxies, weintroduce a convenient parameter for characterizing the halo masses fromboth observational and theoretical results:∇lΥ, the logarithmic radial gradient of themass-to-light ratio. Using halo density profiles from Λ-cold darkmatter (CDM) simulations, we derive predictions for this gradient forvarious galaxy luminosities and star formation efficienciesɛSF. As a pilot study, we assemble the available∇lΥ data from kinematics in early-type galaxies- representing the first unbiased study of halo masses in a wide rangeof early-type galaxy luminosities - and find a correlation betweenluminosity and ∇lΥ, such that the brightestgalaxies appear the most dark-matter dominated. We find that thegradients in most of the brightest galaxies may fit in well with theΛCDM predictions, but that there is also a population of faintergalaxies whose gradients are so low as to imply an unreasonably highstar formation efficiency ɛSF > 1. This difficultyis eased if dark haloes are not assumed to have the standard ΛCDMprofiles, but lower central concentrations.
| 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.
| Gemini and Chandra Observations of Abell 586, A Relaxed Strong-lensing Cluster
We analyze the mass content of the massive strong-lensing cluster Abell586 (z=0.17). We use optical data (imaging and spectroscopy) obtainedwith the Gemini Multi-Object Spectrograph (GMOS) mounted on the 8 mGemini North telescope, together with publicly available X-ray datataken with the Chandra space telescope. Employing differenttechniques-velocity distribution of galaxies, weak gravitationallensing, and spatially resolved X-ray spectroscopy-we derive mass andvelocity dispersion estimates from each of them. All estimates agreewell with each other, within a 68% confidence level, indicating avelocity dispersion of 1000-1250 km s-1. The projected massdistributions obtained through weak lensing and X-ray emission arestrikingly similar, having nearly circular geometry. We suggest thatAbell 586 is probably a truly relaxed cluster whose last major mergeroccurred more than ~4 Gyr ago.
| 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.
| Redshifts in the Southern Abell Redshift Survey Clusters. I. The Data
The Southern Abell Redshift Survey (SARS) contains 39 clusters ofgalaxies with redshifts in the range 0.021h (while avoiding theLMC and SMC), with |b|>40°. Cluster locations were chosen fromthe Abell and Abell-Corwin-Olowin catalogs, while galaxy positions wereselected from the Automatic Plate Measuring Facility galaxy catalog withextinction-corrected magnitudes in the range 15<=bJ<19.SARS used the Las Campanas 2.5 m du Pont telescope, observing either 65or 128 objects concurrently over a 1.5 deg2 field. Newredshifts for 3440 galaxies are reported in the fields of these 39clusters of galaxies.
| The Centers of Early-Type Galaxies with Hubble Space Telescope. V. New WFPC2 Photometry
We present observations of 77 early-type galaxies imaged with the PC1CCD of the Hubble Space Telescope (HST) WFPC2. ``Nuker-law'' parametricfits to the surface brightness profiles are used to classify the centralstructure into ``core'' or ``power-law'' forms. Core galaxies aretypically rounder than power-law galaxies. Nearly all power-law galaxieswith central ellipticities ɛ>=0.3 have stellar disks,implying that disks are present in power-law galaxies withɛ<0.3 but are not visible because of unfavorable geometry. Afew low-luminosity flattened core galaxies also have disks; these may betransition forms from power-law galaxies to more luminous core galaxies,which lack disks. Several core galaxies have strong isophote twistsinterior to their break radii, although power-law galaxies have interiortwists of similar physical significance when the photometricperturbations implied by the twists are evaluated. Central colorgradients are typically consistent with the envelope gradients; coregalaxies have somewhat weaker color gradients than power-law galaxies.Nuclei are found in 29% of the core galaxies and 60% of the power-lawgalaxies. Nuclei are typically bluer than the surrounding galaxy. Whilesome nuclei are associated with active galactic nuclei (AGNs), just asmany are not; conversely, not all galaxies known to have a low-level AGNexhibit detectable nuclei in the broadband filters. NGC 4073 and 4382are found to have central minima in their intrinsic starlightdistributions; NGC 4382 resembles the double nucleus of M31. In general,the peak brightness location is coincident with the photocenter of thecore to a typical physical scale of <1 pc. Five galaxies, however,have centers significantly displaced from their surrounding cores; thesemay be unresolved asymmetric double nuclei. Finally, as noted byprevious authors, central dust is visible in about half of the galaxies.The presence and strength of dust correlates with nuclear emission;thus, dust may outline gas that is falling into the central black hole.The prevalence of dust and its morphology suggest that dust clouds form,settle to the center, and disappear repeatedly on ~108 yrtimescales. We discuss the hypothesis that cores are created by thedecay of a massive black hole binary formed in a merger. Apart fromtheir brightness profiles, there are no strong differences between coregalaxies and power-law galaxies that demand this scenario; however, therounder shapes of core, their lack of disks, and their reduced colorgradients may be consistent with it.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withGO and GTO proposals 5236, 5446, 5454, 5512, 5943, 5990, 5999, 6099,6386, 6554, 6587, 6633, 7468, 8683, and 9107.
| 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.
| A wide-field photometric study of the globular cluster system of NGC 4636
Previous smaller-scale studies of the globular cluster system of NGC4636, an elliptical galaxy in the southern part of the Virgo cluster,have revealed an unusually rich globular cluster system. Were-investigate the cluster system of NGC 4636 with wide-field Washingtonphotometry. The globular cluster luminosity function can be followedroughly 1 mag beyond the turn-over magnitude found at {V} =23.31±0.13 for the blue cluster sub-population. This correspondsto a distance modulus of ({m}-{M})=31.24±0.17, 0.4 mag largerthan the distance determined from surface brightness fluctuations. Thehigh specific frequency is confirmed, yet the exact value remainsuncertain because of the uncertain distance: it varies between5.6±1.2 and 8.9±1.2. The globular cluster system has aclearly bimodal color distribution. The color peak positions show noradial dependence and are in good agreement with the values found forother galaxies studied in the same filter system. However, a luminositydependence is found: brighter clusters with an“intermediate” color exist. The clusters exhibit a shallowradial distribution within 7´, represented by a power-law with anexponent of -1.4. Within the same radial interval, the galaxy light hasa distinctly steeper profile. Because of the difference in the clusterand light distribution the specific frequency increases considerablywith radius. At 7´ and 9´ the density profiles of the redand blue clusters, respectively, change strongly: the power-law indicesdecrease to around -5 and become similar to the galaxy profile. Thissteep profile indicates that we reach the outer rim of the clustersystem at approximately 11´. This interpretation is supported bythe fact that in particular the density distribution of the blue clusterpopulation can be well fit by the projection of a truncated power-lawmodel with a core. This feature is seen for the first time in a globularcluster system. While the radial distribution of the cluster and fieldpopulations are rather different, this is not true for the ellipticityof the system: the elongation as well as the position angle of thecluster system agree well with the galaxy light. We compare the radialdistribution of globular clusters with the light profiles for a sampleof elliptical galaxies. The difference observed in NGC 4636 is typicalof an elliptical galaxy of this luminosity. The intrinsic specificfrequency of the blue population is considerably larger than that of thered one.Tables A.1 to A.6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/43
| Peculiarities and populations in elliptical galaxies. II. Visual-near IR colours as population indices
As a complement to the data collected and discussed in Paper I of thisseries, 2MASS near-IR images have been used, in connection withavailable V light aperture photometry, to derive the colours V-J, V-K,J-H and J-K within the effective aperture A_e: nearly the same completesample of 110 E-type galaxies is treated. In Paper I these wereclassified, based on morphological criteria, into the ``peculiar'' (orPec) and ``normal'' (or Nop) subsamples. For the Nop subsample, thederived colour indices are tightly related to the galaxy masses, asmeasured by the central velocity dispersion σ0,although with rather small slopes as regards J-H and J-K. For the Pecsubsample, the V-J and V-K colours behave as UBV and line-indices: partof the objects show blue residuals from the appropriatecolour-σ0 regression, which is evidence of a youngerpopulation mixed with the ``normal'' one traced by the Nop regressions;the other shows no deviations from the Nop subsample. The distinctionamong Pec objects between the YP family (NGC 2865 type), and the NP one(NGC 3923 type), is statistically supported, and generally confirmed inspecific cases.Based in part on observations collected at the Observatoire deHaute-Provence.Table 4 is only available in electonic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/819
| Star formation history in early-type galaxies - I. The line absorption indices diagnostics
To unravel the formation mechanism and the evolutionary history ofelliptical galaxies (EGs) is one of the goals of modern astrophysics. Ina simplified picture of the issue, the question to be answered iswhether they have formed by hierarchical merging of pre-existingsubstructures (maybe disc galaxies) made of stars and gas, with eachmerging event probably accompanied by strong star formation, orconversely, whether they originated from the early aggregation of lumpsof gas turned into stars in the remote past via a burst-like episodeever since followed by quiescence so as to mimic a sort of monolithicprocess. Even if the two alternatives seem to oppose each other,actually they may both contribute to shaping the final properties of EGsas seen today. Are there distinct signatures of the underlying dominantprocess in the observational data? To this aim we have examined the lineabsorption indices on the Lick system of the normal, field EGs of Tragerand the interacting EGs (pair- and shell-objects) of Longhetti et al.The data show that both normal, field and interacting galaxies have thesame scattered but smooth distribution in the Hβ versus [MgFe]plane even if the interacting ones show a more pronounced tail towardhigh Hβ values. This may suggest that a common physical cause is atthe origin of their distribution. There are two straightforwardinterpretations of increasing complexity. (i) EGs span true large rangesof ages and metallicities. A young age is the signature of theaggregation mechanism, each event accompanied by metal enrichment. Thissimple scheme cannot, however, explain other spectro-photometricproperties of EGs and has to be discarded. (ii) The bulk population ofstars is old but subsequent episodes of star formation scatter the EGsin the diagnostic planes. However, this scheme would predict anoutstanding clump at low Hβ values, contrary to what is observed.The model can be cured by supposing that the primary star formationactivity lasted for a significant fraction of the Hubble time (5<=T<= 13 Gyr) accompanied by global metal enrichment. The`younger' galaxies are more metal-rich. The later burst of starformation should be small otherwise too many high-Hβ objects wouldbe observed. Therefore, the distribution of normal, pair- andshell-galaxies in the Hβ versus [MgFe] plane is due to global metalenrichment. Even though the above schemes provide a formal explanation,they seem to be too demanding because of the many ad hoc ingredientsthat have to be introduced. Furthermore, they neglect theobservationally grounded hint that the stellar content of EGs is likelyto be enhanced in α-elements with [α/Fe] ranging from 0.1 to0.4 dex. Here we propose a new scheme, in which the bulk dispersion ofgalaxies in the Hβ versus [MgFe] plane is caused by a differentmean degree of enhancement. In this model, neither the large age rangesnor the universal enrichment law for the old component are required andthe observed distribution along Hβ is naturally recovered.Furthermore, later bursts of stellar activity are a rare event,involving only those galaxies with very high Hβ (roughly >2.5).Finally, simulations of the scatter in broad-band colours of EGs seem toconfirm that the bulk stars have formed in the remote past, and thatmergers and companion star formation in a recent past are not likely,unless the intensity of the secondary activity is very small.
| BUDDA: A New Two-dimensional Bulge/Disk Decomposition Code for Detailed Structural Analysis of Galaxies
We present BUDDA (Bulge/Disk Decomposition Analysis), a new code devotedto perform a two-dimensional bulge/disk decomposition directly from theimages of galaxies. The bulge component is fitted with a generalizedSérsic profile, whereas disks have an exponential profile. Noother components are included. Bars and other substructures, likelenses, rings, inner bars, and inner disks, are studied with theresidual images obtained through the subtraction of bulges and disksfrom the original images. This means that a detailed structural analysisof galaxies may be performed with a small number of parameters, andsubstructures may be directly studied with no a priori assumptions. Ashas been already shown by several studies, two-dimensional fitting ismuch more reliable than one-dimensional profile fitting. Moreover, ourcode has been thoroughly tested with artificial data, and we demonstrateit to be an accurate tool for determining structural parameters ofgalaxies. We also show that our code is useful in various kinds ofstudies, including galaxies of, e.g., different morphological types, andinclinations, which also may be observed at different spatialresolutions. Thus, the code has a broader range of potentialapplications than most of the previous codes, which are developed totackle specific problems. To illustrate its usefulness, we present theresults obtained with a sample of 51 mostly early-type galaxies (butcovering the whole Hubble sequence). These results show some of theapplications in which the code may be used: the determination ofparameters for fundamental plane and structural studies, quantitativemorphological classification of galaxies, and the identification andstudy of hidden substructures. We have determined the structuralparameters of the galaxies in our sample and found many examples ofhidden inner disks in ellipticals, secondary bars, nuclear rings anddust lanes in lenticulars and spirals, and also wrong morphologicalclassification cases. We now make BUDDA generally available to theastronomical community.Based on observations made at the Pico dos Dias Observatory(PDO/LNA-CNPq), Brazil.
| Hot Gas Structure in the Elliptical Galaxy NGC 4472
We present X-ray spectroscopic and morphological analyses using ChandraACIS and ROSAT observations of the giant elliptical galaxy NGC 4472 inthe Virgo Cluster. We discuss previously unobserved X-ray structureswithin the extended galactic corona. In the inner 2' of the galaxy, wefind X-ray holes or cavities with radii of ~2 kpc, corresponding to theposition of radio lobes. These holes were produced during a period ofnuclear activity that began 1.2×107 yr ago and may beongoing. We also find an asymmetrical edge in the galaxy X-ray emission3' (14 kpc) northeast of the core and an ~8' tail (36 kpc) extendingsouthwest of the galaxy. These two features probably result from theinteraction of NGC 4472 gas with the Virgo gas, which producescompression in the direction of NGC 4472's infall and an extended tailfrom ram pressure stripping. Assuming that the tail is in pressureequilibrium with the surrounding gas, we compute its angle to our lineof sight and estimate that its true extent exceeds 100 kpc. Finally, inaddition to emission from the nucleus (first detected by Soldatenkov,Vikhlinin, and Pavlinsky), we detect two small extended sources within10" of the nucleus of the galaxy, both of which have luminosities of~7×1038 ergs s-1.
| A Correlation between Light Profile and [Mg/Fe] Abundance Ratio in Early-Type Galaxies
We explore possible correlations between light profile shapes, asparameterized by the Sérsic index n or the concentration indexCre(1/3), and relevant stellar populationparameters in early-type galaxies. Mean luminosity-weighted ages,metallicities, and abundance ratios were obtained from spectra of veryhigh signal-to-noise ratio and stellar population models that synthesizegalaxy spectra at the resolution given by their velocity dispersions,σ, in combination with an age indicator(Hγσ) that is virtually free of the effects ofmetallicity. We do not find any significant correlation between n [orCre(1/3)] and mean age or metallicity, but we dofind a strong positive correlation of the shape parameters with Mg/Feabundance ratio. This dependence is as strong as the Mg/Fe-σ andCre(1/3)-σ relations. We speculate thatearly-type galaxies settle up their structure on timescales in agreementwith those imposed by their Mg/Fe ratios. This suggests that the globalstructure of larger galaxies, with larger Mg/Fe ratios and shortertimescales, was already in place at high z, without experiencing asignificant time evolution.
| 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.
| Evidence for a New Elliptical-Galaxy Paradigm: Sérsic and Core Galaxies
We fitted the surface-brightness profiles of 21 elliptical galaxiesusing both the Sérsic function and a new empirical model thatcombines an inner power law with an outer Sérsic function. Theprofiles are combinations of deconvolved Hubble Space Telescope (HST)profiles from the literature and ellipse fits to the full WFPC2 mosaicimages and thus span a radial range from ~0.02" to about twice thehalf-light radius. We are able to accurately fit the entire profilesusing either the Sérsic function or our new model. In doing so,we demonstrate that most, if not all, so-called ``power-law'' galaxiesare better described as ``Sérsic galaxies''-they are well modeledby the three-parameter Sérsic profile into the limits of HSTresolution-and that ``core'' galaxies are best understood as consistingof an outer Sérsic profile with an inner power-law cusp, which isa downward deviation from the inward extrapolation of the Sérsicprofile. This definition of cores resolves ambiguities that result whenthe popular ``Nuker law'' is fitted to the profiles of ellipticals andbulges, particularly at lower luminosities. We also find that using theNuker law to model core-galaxy nuclear profiles systematicallyoverestimates the core radii by factors of 1.5-4.5 and underestimatesthe inner power-law slope by ~20%-40% or more.
| 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
| Near-infrared imaging of ellipticals: surface brightness profiles and photometry
We present near-infrared K-band imaging of a large sample of candidatemerger remnant galaxies and Hickson Compact Group ellipticals. We derivelight profile indices, effective radii and surface brightnesses, as wellas total K-band magnitudes. We find that the light distributions of themerger remnant candidates are consistent with those of `normal'ellipticals, and scatter around a mean profile index of (1/n) = 0.20.Many of our sample galaxies have surface brightness profiles that arenot well described by a de Vaucouleurs law (1/n= 0.25), and we discussthe implications of this on the derived total magnitudes. Comparing thetotal K magnitudes calculated by extrapolating a de Vaucouleurs profileand those derived using a generalized Sérsic form, we find that asignificant bias is introduced if the de Vaucouleurs law is not a gooddescription of the actual light profile.
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