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Globular cluster systems in low-luminosity early-type galaxies near the Fornax cluster centre
We present a photometric study of the globular cluster systems (GCSs) ofthe Fornax cluster galaxies NGC1374, NGC1379 and NGC1387. The dataconsist of images from the wide-field MOSAIC imager of the Cerro TololoInter-American Observatory (CTIO) 4-m telescope, obtained withWashington C and Kron-Cousins R filters. The images cover a field of 36× 36arcmin2, corresponding to 200 ×200kpc2 at the Fornax distance. Two of the galaxies, NGC1374and NGC1379, are low-luminosity ellipticals while NGC1387 is alow-luminosity lenticular. Their cluster systems are still embedded inthe cluster system of NGC1399. Therefore, the use of a large field iscrucial and some differences to previous work can be explained by this.The colour distributions of all GCSs are bimodal. NGC1387 presents aparticularly distinct separation between red and blue clusters and anoverproportionally large population of red clusters. The radialdistribution is different for blue and red clusters, red clusters beingmore concentrated towards the respective galaxies. The different colourand radial distributions point to the existence of two globular clustersubpopulations in these galaxies. Specific frequencies are in the rangeSN= 1.4-2.4, smaller than the typical values for ellipticalgalaxies. These galaxies might have suffered tidal stripping of blueglobular clusters by NGC1399.
| A data-driven Bayesian approach for finding young stellar populations in early-type galaxies from their ultraviolet-optical spectra
Efficient predictive models and data analysis techniques for theanalysis of photometric and spectroscopic observations of galaxies arenot only desirable, but also required, in view of the overwhelmingquantities of data becoming available. We present the results of a novelapplication of Bayesian latent variable modelling techniques, where wehave formulated a data-driven algorithm that allows one to explore thestellar populations of a large sample of galaxies from their spectra,without the application of detailed physical models. Our only assumptionis that the galaxy spectrum can be expressed as a linear superpositionof a small number of independent factors, each a spectrum of a stellarsubpopulation that cannot be individually observed. A probabilisticlatent variable architecture that explicitly encodes this assumption isthen formulated, and a rigorous Bayesian methodology is employed forsolving the inverse modelling problem from the available data. Apowerful aspect of this method is that it formulates a density model ofthe spectra, based on which we can handle observational errors. Further,we can recover missing data both from the original set of spectra whichmight have incomplete spectral coverage of each galaxy, or frompreviously unseen spectra of the same kind.We apply this method to a sample of 21 ultraviolet-optical spectra ofwell-studied early-type galaxies, for which we also derive detailedphysical models of star formation history (i.e. age, metallicity andrelative mass fraction of the component stellar populations). We alsoapply it to synthetic spectra made up of two stellar populations,spanning a large range of parameters. We apply four different datamodels, starting from a formulation of principal component analysis(PCA), which has been widely used. We explore alternative factor models,relaxing the physically unrealistic assumption of Gaussian factors, aswell as constraining the possibility of negative flux values that areallowed in PCA, and show that other models perform equally well orbetter, while yielding more physically acceptable results. Inparticular, the more physically motivated assumptions of our rectifiedfactor analysis enable it to perform better than PCA, and to recoverphysically meaningful results.We find that our data-driven Bayesian modelling allows us to identifythose early-type galaxies that contain a significant stellar populationthat is <~1-Gyr old. This experiment also concludes that our sampleof early-type spectra showed no evidence of more than two major stellarpopulations differing significantly in age and metallicity. This methodwill help us to search for such young populations in a large ensemble ofspectra of early-type galaxies, without fitting detailed models, andthereby to study the underlying physical processes governing theformation and evolution of early-type galaxies, particularly thoseleading to the suppression of star formation in dense environments. Inparticular, this method would be a very useful tool for automaticallydiscovering various interesting subclasses of galaxies, for example,post-starburst or E+A galaxies.
| The Two-dimensional XMM-Newton Group Survey: z < 0.012 Groups
We present the results of the two-dimensional XMM-Newton Group Survey(2dXGS), an archival study of nearby galaxy groups. In this paper weconsider 11 nearby systems (z<0.012) in Mulchaey et al., which span abroad range in X-ray luminosity from 1040 to 1043ergs s-1. We measure the iron abundance and temperaturedistribution in these systems and derive pressure and entropy maps. Wefind statistically significant evidence for structure in the entropy andpressure of the gas component of seven groups on the 10%-20% level. TheXMM-Newton data for the three groups with best statistics also suggestpatchy metallicity distributions within the central 20-50 kpc of thebrightest group galaxy, probed with 2-10 kpc resolution. This providesinsights into the processes associated with thermalization of thestellar mass loss. Analysis of the global properties of the groupsreveals a subclass of X-ray-faint groups, which are characterized byboth higher entropy and lower pressure. We suggest that the mergerhistory of the central elliptical is responsible for both the source andthe observed thermodynamical properties of the hot gas of theX-ray-faint groups.
| Merging of Globular Clusters in Inner Galactic Regions. I. Do They Survive the Tidal Interaction?
The main topic of this paper is the investigation of the modes ofinteraction of globular clusters (GCs) moving in the inner part of agalaxy. This is tackled by means of high-resolution N-body simulations,whose first results are presented in this article. Our simulations dealtwith primordial, very massive (of the order of 107Msolar) GCs that were able to decay, because of dynamicalfriction, into the inner regions of triaxial galaxies on a timescalemuch shorter than their internal relaxation time. To check thedisruptive roles of both tidal forces and GC-GC collisions, theireffects were maximized by considering clusters on quasi-radial orbitsand choosing the initial conditions so as to give head-on collisions ateach passage through the center. The available CPU resources allowed usto simulate clusters with different structural parameters and to followthem on quasi-radial orbits during eight passages across the center. Themain findings are as follows: (1) clusters with an initial high-enoughKing concentration parameter (c>=1.2) preserve up to 50% of theirinitial mass, (2) the inner density distribution of the survivingclusters keeps a King model profile, (3) GC-GC collisions have anegligible effect compared with that caused by the passage through thegalactic center, (4) the orbital energy dissipation due to the tidalinteraction is of the same order as that caused by dynamical friction,(5) complex substructures like ``ripples'' and ``clumps'' are formed, asobserved around real clusters. These findings support the validity ofthe hypothesis of merging of GCs in the galactic central region, withmodes that deserve further careful investigation.
| Optical Counterparts of Ultraluminous X-Ray Sources
We present the optical identification and characterization ofcounterparts of four objects previously cataloged as ultraluminous X-ray(ULX) sources. The objects were selected from the E. Colbert & A.Ptak catalog. The optical counterparts are identified as pointlikeobjects with magnitudes in the range of ~17-19. The optical spectra ofthree of the sources (IXO 32, 37, and 40) show the presence of emissionlines typical of quasars. The position of these lines allows a preciseestimation of their redshifts (2.769, 0.567, and 0.789 for IXO 32, 37,and 40, respectively). The fourth X-ray source, IXO 35, is associatedwith a red object that has a spectrum typical of an M star in ourGalaxy. These identifications are useful for building clean samples ofULX sources, selecting suitable targets for future observations, andperforming statistical studies on the different populations of X-raysources.
| 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.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| 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).
| Dark matter in elliptical galaxies - I. Is the total mass density profile of the NFW form or even steeper?
Elliptical galaxies are modelled as Sérsic luminositydistributions with density profiles (DPs) for the total mass adoptedfrom the DPs of haloes within dissipationless ΛCDM (cold darkmatter) N-body simulations. Ellipticals turn out to be inconsistent withcuspy low-concentration NFW models representing the total massdistribution, neither are they consistent with a steeper -1.5 innerslope, nor with the shallower models proposed by Navarro et al., norwith NFW models 10 times more concentrated than predicted, as deducedfrom several X-ray observations - the mass models, extrapolated inwards,lead to local mass-to-light ratios that are smaller than the stellarvalue inside an effective radius (Re), and to centralaperture velocity dispersions that are much smaller than observed. Thisconclusion remains true as long as there is no sharp steepening (slope< -2) of the dark matter DPs just inside 0.01 virial radii.The very low total mass and velocity dispersion produced withinRe by an NFW-like total mass profile suggests that thestellar component should dominate the dark matter component out to atleast Re. It should then be difficult to kinematicallyconstrain the inner slope of the DP of ellipticals. Thehigh-concentration parameters deduced from X-ray observations appear tobe a consequence of fitting an NFW model to the total mass DP made up ofa stellar component that dominates inside and a dark matter componentthat dominates outwards.An appendix gives the virial mass dependence of the concentrationparameter, central density and total mass of the Navarro et al. model.In a second appendix are given single integral expressions for thevelocity dispersions averaged along the line of sight, in circularapertures and in thin slits, for general luminosity density and massdistributions, with isotropic orbits.
| Self-regulated active galactic nuclei heating in elliptical galaxies
We study the effect of active galactic nuclei (AGN) heating on thechemodynamical evolution of elliptical galaxies and their X-ray andoptical properties using high-resolution Λ-dominated cold darkmatter cosmological simulations. Our model considers an AGN as being`active' when a convergent gas inflow condition exists within thenucleus of a galaxy; otherwise, the AGN is assumed to remain dormant.This induces a self-regulated activity for the AGN, the result of whichleads to a stable hot corona and the suppression of significantlate-time star formation - characteristics not encountered intraditional chemodynamical models of ellipticals. These properties ofour AGN heating model lead to a system consistent with both the X-rayand optical properties of comparable elliptical galaxies.
| 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.
| Evidence for radio-source heating of groups
We report evidence that the gas properties of X-ray groups containingradio galaxies differ from those of radio-quiet groups. For awell-studied sample of ROSAT-observed groups, we found that more thanhalf of the elliptical-dominated groups can be considered `radio-loud',and that radio-loud groups are likely to be hotter at a given X-rayluminosity than radio-quiet groups. We tested three different models forthe origin of the effect and conclude that radio-source heating is themost likely explanation. We found several examples of groups where thereis strong evidence from Chandra or XMM-Newton images for interactionsbetween the radio source and the group gas. A variety of radio-sourceheating processes are important, including shock-heating by youngsources and gentler heating by larger sources. The heating effects canbe longer-lasting than the radio emission. We show that the sample ofX-ray groups used in our study is not significantly biased in thefraction of radio-loud groups that it contains. This allows us toconclude that the energy per particle that low-power radio galaxies caninject over the group lifetime is comparable to the requirements ofstructure formation models.
| 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.
| Mid-Infrared Emission from Elliptical Galaxies: Sensitivity to Stellar Age
Mid-infrared observations (3.6-24 μm) of normal giant ellipticalgalaxies with the Spitzer Space Telescope are consistent with purepopulations of very old stars with no evidence of younger stars. Most ofthe stars in giant elliptical galaxies are old, but the mean stellar agedetermined from Balmer absorption in optical spectra can appear muchyounger due to a small admixture of younger stars. The mean stellar agecan also be determined from the spectral energy distribution in themid-infrared, which decreases with time relative to the optical emissionand shifts to shorter wavelengths. The observed flux ratiosF8μm/F3.6μm andF24μm/F3.6μm for elliptical galaxies withthe oldest Balmer line ages are lower than predicted by recent models ofsingle stellar populations. For elliptical galaxies with the youngestBalmer line ages in our sample, 3-5 Gyr, the flux ratiosF24μm/F3.6μm are identical to those of theoldest stars. When theoretical mid-IR spectra of old (12 Gyr) and youngstellar populations are combined, errors in theF24μm/F3.6μm observations are formallyinconsistent with a mass fraction of young stars that exceeds ~1%. Thisis less than the fraction of young stars expected in discussions ofrecent surveys of elliptical galaxies at higher redshifts. However, thisinconsistency between Balmer line ages and those inferred from mid-IRobservations must be regarded as provisional until more accurateobservations and theoretical spectra become available. Finally, there isno evidence to date that central disks or patches of dust commonlyvisible in optical images of elliptical galaxies contribute sensibly tothe mid-IR spectrum.
| O VI in Elliptical Galaxies: Indicators of Cooling Flows
Early-type galaxies often contain a hot X-ray-emitting interstellarmedium [(3-8)×106 K] with an apparent radiative coolingtime much less than a Hubble time. If unopposed by a heating mechanism,the gas will radiatively cool to temperatures <~104 K at arate proportional to LX/TX, typically 0.03-1Msolar yr-1. We can test whether gas is coolingthrough the 3×105 K range by observing the O VIdoublet, whose luminosity is proportional to the cooling rate. Here wereport on a study of an unbiased sample of 24 galaxies, obtaining FarUltraviolet Spectroscopic Explorer spectra to complement the X-ray dataof ROSAT and Chandra. The O VI line emission was detected in about 40%of the galaxies and at a luminosity level similar to the prediction fromthe cooling flow model. There is a correlation betweenM˙OVI and M˙X, although there issignificant dispersion about the relationship, where the O VI isbrighter or dimmer than expected by a factor of 3 or more. If thecooling flow picture is to be retained, then this dispersion requiresthat cooling flows be time-dependent, as might occur by the activity ofan AGN. However, of detected objects, those with the highest or lowestvalues of M˙OVI/M˙X are not systematicallyhot or cool, as one might predict from AGN heating.
| A Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies
We report on the discovery of a new correlation between globalparameters of the hot interstellar gas in elliptical galaxies. Wereanalyze archival Chandra data for 30 normal early-type systems,removing the contributions of resolved and unresolved point sources toreveal the X-ray morphology of the hot gas. We determine the half-lightradius, RX, and the mean surface brightness, IX,from the gas surface brightness profiles. A spectral analysis determinesthe temperature, TX, of the gas within 3 optical effectiveradii. We find that the galaxies lie on an X-ray gas fundamental plane(XGFP) of the formTX~R0.28XI0.22X.This is close to, but distinct from, a simple luminosity-temperaturerelation. The intrinsic width of the XGFP is only 0.07 dex, nearlyidentical to that of the stellar (optical) fundamental plane (SFP). Thisis surprising since X-ray gas masses are typically ~10-2 ofthe stellar masses. We show that the XGFP is not a simple consequence ofthe virial theorem or hydrostatic equilibrium and that it is essentiallyindependent of the SFP. The XGFP thus represents a genuinely newconstraint on the hydrodynamical evolution of elliptical galaxies.
| 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.
| Integral Field Spectroscopy of the Central Regions of 3C 120: Evidence of a Past Merging Event
Optical integral field spectroscopy (IFS), combined with Hubble SpaceTelescope (HST) WFPC imaging, was used to characterize the centralregions of the Seyfert 1 radio galaxy 3C 120. We carried out theanalysis of the data, deriving intensity maps of different emissionlines and the continua at different wavelengths from the observedspectra. Applying a two-dimensional modeling to the HST images, wedecoupled the nucleus and the host galaxy and analyzed the hostmorphology. The host is a highly distorted bulge-dominated galaxy, richin substructures. We developed a new technique to model the IFS dataextending the two-dimensional modeling (hereafter three-dimensionalmodeling). Using this technique, we separated the Seyfert nucleus andthe host galaxy spectra and derived a residual data cube with spectraland spatial information of the different structures in 3C 120. Threecontinuum-dominated structures (named A, B, and C) and three otherextended emission-line regions (EELRs, named E1,E2, and E3) are found in 3C 120, which does notfollow the general behavior of a bulge-dominated galaxy. We also foundshells in the central kiloparsec that may be remnants of a past mergingevent in this galaxy. The origin of E1 is most probably dueto the interaction of the radio jet of 3C 120 with the intergalacticmedium (Axon et al. 1989; Sánchez et al. 2004a). Structures A, B,and the shell at the southeast of the nucleus seem to correspond to alarger morphological clumpy structure that may be a tidal tail, aconsequence of the past merging event. We found a bright EELR(E2) in the innermost part of this tidal tail, nearby thenucleus, which shows a high ionization level. The kinematics of theE2 region and its connection to the tidal tail suggest thatthe tail has channeled gas from the outer regions to the center.
| A Tiny Galactic Dust Cloud Projected onto NGC 3269?
We report on new observations obtained with the Magellan Clay telescopeof a tiny dust patch in the Sa galaxy NGC 3269, in the Antlia galaxycluster. It has already been suspected to be a projected Galacticforeground cloud. In this case, a size of about 4" and a maximumabsorption of ~1 mag in the B band would make it unique so far. Wepresent further evidence for its Galactic nature from imaging underexcellent seeing conditions (0.3"-0.6"). This dust cloud could be thefirst optically identified counterpart of a new type of molecular cloudrecently discovered by Heithausen.
| 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.
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
| 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
| The isolated elliptical NGC 4555 observed with Chandra
We present analysis of a Chandra observation of the elliptical galaxyNGC 4555. The galaxy lies in a very low density environment, eitherisolated from all galaxies of similar mass or on the outskirts of agroup. Despite this, NGC 4555 has a large gaseous halo, extending to~60kpc. We find the mean gas temperature to be ~0.95keV and the Feabundance to be ~0.5Zsolar. We model the surface brightness,temperature and abundance distribution of the halo and use these resultsto estimate parameters such as the entropy and cooling time of the gas,and the total gravitational mass of the galaxy. In contrast to recentresults showing that moderate luminosity ellipticals contain relativelysmall quantities of dark matter, our results show that NGC 4555 has amassive dark halo and large mass-to-light ratio(56.8+34.2-35.8Msolar/LBsolarat 50kpc, 42.7+14.6-21.2 at 5re,1σ errors). We discuss this disparity and consider possiblemechanisms by which galaxies might reduce their dark matter content.
| The GEMS project: X-ray analysis and statistical properties of the group sample
The Group Evolution Multiwavelength Study (GEMS) involves amultiwavelength study of a sample of 60 galaxy groups, chosen to span awide range of group properties. Substantial ROSAT Position SensitiveProportional Counter (PSPC) observations, available for all of thesegroups, are used to characterize the state of the intergalactic mediumin each. We present the results of a uniform analysis of these ROSATdata and a statistical investigation of the relationship between X-rayand optical properties across the sample. Our analysis improves inseveral respects on previous work: (i) we distinguish between systems inwhich the hot gas is a group-scale medium and those in which it appearsto be just a hot halo associated with a central galaxy; (ii) weextrapolate X-ray luminosities to a fixed overdensity radius(r500) using fitted surface brightness models, in order toavoid biases arising from the fact that cooler systems are detectable tosmaller radii, and (iii) optical properties have been rederived in auniform manner from the NASA Extragalactic Database, rather than relyingon the data in the disparate collection of group catalogues from whichour systems are drawn.The steepening of the LX-TX relation in the groupregime reported previously is not seen in our sample, which fits well onto the cluster trend, albeit with large non-statistical scatter. Anumber of biases affect the fitting of regression lines under thesecircumstances, and until the impact of these has been thoroughlyinvestigated it seems best to regard the slope of the groupLX-TX relation as being poorly determined. Asignificant problem in comparing the properties of groups and clustersis the derivation of system radii, to allow different systems to becompared within regions having the same overdensity. We find evidencethat group velocity dispersion (σv) provides a veryunreliable measure of system mass (and hence radius), with a number ofgroups having remarkably low values of σv, given thatthey appear from their X-ray properties to be collapsed systems. Weconfirm that the surface brightness profiles of groups are significantlyflatter than those of clusters - the maximum value of theβfit parameter for our sample is 0.58, lower than thetypical value of 0.67 seen in clusters - however, we find no significanttendency within our sample for cooler groups to show flatter profiles.This result is inconsistent with simple universal pre-heating models.The morphology of the galaxies in the GEMS groups is correlated to theirX-ray properties in a number of ways: we confirm the very strongrelationship between X-ray emission and a dominant early-type centralgalaxy, which has been noted since the early X-ray studies of groups,and also find that spiral fraction is correlated with the temperature ofthe hot gas and hence the depth of the gravitational potential. A classof spiral-rich groups with little or no X-ray emission probablycorresponds to groups that have not yet fully collapsed.
| Dynamical friction in flattened systems: a numerical test of Binney's approach
We carry out a set of self-consistent N-body calculations to investigatehow important the velocity anisotropy in non-spherical dark matterhaloes is for dynamical friction. For this purpose, we allow satellitegalaxies to orbit within flattened and live dark matter haloes (DMHs)and compare the resulting orbit evolution with a semi-analytic code.This code solves the equation of motion of the same satellite orbitswith mass loss and assumes the same DMH, but either employsChandrasekhar's dynamical friction formula, which does not incorporatethe velocity anisotropy, or Binney's description of dynamical frictionin anisotropic systems. In the numerical and the two semi-analyticmodels, the satellites are given different initial orbital inclinationsand orbital eccentricities, whereas the parent galaxy is composed of aDMH with aspect ratio qh= 0.6.We find that Binney's approach successfully describes the overallsatellite decay and orbital inclination decrease for the whole set oforbits, with an averaged discrepancy of less than 4 per cent in orbitalradius during the first three orbits. If Chandrasekhar's expression isused instead, the discrepancy increases to 20 per cent. Binney'streatment therefore appears to provide a significantly improvedtreatment of dynamical friction in anisotropic systems.The velocity anisotropy of the DMH velocity distribution function leadsto a significant decrease with time of the inclination of non-polarsatellite orbits. But, at the same time, it reduces the difference indecay times between polar and coplanar orbits evident in a flattened DMHwhen the anisotropic DMH velocity distribution function is not takeninto account explicitly. Our N-body calculations furthermore indicatethat polar orbits survive about 1.6 times longer than coplanar orbitsand that the orbital eccentricity e remains close to its initial valueif satellites decay slowly towards the galaxy centre. However, orbits ofrapidly decaying satellites modelled with the semi-analytic code show astrong orbital circularization () not present in the N-bodycomputations.
| Companions to Isolated Elliptical Galaxies: Revisiting the Bothun-Sullivan Sample
We investigate the number of physical companion galaxies for a sample ofrelatively isolated elliptical galaxies. The NASA/IPAC ExtragalacticDatabase (NED) has been used to reinvestigate the incidence of satellitegalaxies for a sample of 34 elliptical galaxies, first investigated byBothun & Sullivan using a visual inspection of Palomar Sky Surveyprints out to a projected search radius of 75 kpc. We have repeatedtheir original investigation using data cataloged in NED. Nine of theseelliptical galaxies appear to be members of galaxy clusters; theremaining sample of 25 galaxies reveals an average of +1.0+/-0.5apparent companions per galaxy within a projected search radius of 75kpc, in excess of two equal-area comparison regions displaced by 150-300kpc. This is significantly larger than the +0.12+/-0.42companions/galaxy found by Bothun & Sullivan for the identicalsample. Making use of published radial velocities, mostly availablesince the completion of the Bothun-Sullivan study, identifies thephysical companions and gives a somewhat lower estimate of +0.4companions per elliptical galaxy. This is still 3 times larger than theoriginal statistical study, but given the incomplete and heterogeneousnature of the survey redshifts in NED, it still yields a firm lowerlimit on the number (and identity) of physical companions. An expansionof the search radius out to 300 kpc, again restricted to sampling onlythose objects with known redshifts in NED, gives another lower limit of4.5 physical companions per galaxy. (Excluding five elliptical galaxiesin the Fornax Cluster, this average drops to 3.5 companions perelliptical.) These physical companions are individually identified andlisted, and the ensemble-averaged radial density distribution of theseassociated galaxies is presented. For the ensemble, the radial densitydistribution is found to have a falloff consistent withρ~R-0.5 out to approximately 150 kpc. For non-FornaxCluster companions the falloff continues out to the 300 kpc limit of thesurvey. The velocity dispersion of these companions is found to reach amaximum of 350 km s-1 at around 120 kpc, after which theyfall at a rate consistent with Keplerian falloff. This falloff may thenindicate the detection of a cut-off in the mass-density distribution inthe elliptical galaxies' dark matter halo at ~100 kpc.
| The Lack of Very Ultraluminous X-Ray Sources in Early-Type Galaxies
We have searched for ultraluminous X-ray sources (ULXs) in a sample of28 elliptical and S0 galaxies observed with Chandra. We find that thenumber of X-ray sources detected at a flux level that would correspondto a 0.3-10 keV X-ray luminosity of ~2×1039 ergss-1 or greater (for which we have used the designation veryultraluminous X-ray sources [VULXs]) at the distance of each galaxy isequal to the number of expected foreground/background objects. Inaddition, the VULXs are uniformly distributed over the Chandra field ofview rather than distributed like the optical light of the galaxies,strengthening the argument that the high-flux sources are unassociatedwith the galaxies. We have also taken the VULX candidate list of Colbertand Ptak and determined the spatial distribution of VULXs in early-typegalaxies and late-type galaxies separately. While the spiral galaxyVULXs are clearly concentrated toward the centers of the galaxies, theearly-type galaxy VULXs are distributed randomly over the ROSAT HRIfield of view, again indicating that they are not associated with thegalaxies themselves. We conclude that with the exception of two rarehigh-luminosity objects within globular clusters of the ellipticalgalaxy NGC 1399, VULXs are generally not found within old stellarsystems. However, we do find a significant population of sources withluminosities of (1-2)×1039 ergs s-1 thatreside within the sample galaxies that can be explained by accretiononto 10-20 Msolar black holes. Given our results, we proposethat ULXs be defined as X-ray sources with LX(0.3-10keV)>2×1039 ergs s-1.
| 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
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