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|A QSO Discovered at the Redshift of the Extended X-Ray Cluster RX J0152.7-1357|
The galaxy cluster RX J0152.7-1357 at z=0.8325 is one of the mostluminous X-ray clusters known. It is elongated toward the bright,nearby, X-ray active galaxy NGC 720 about 14' away. At the same distanceon the other side of that galaxy is an X-ray blue stellar object with aflux of 5.8 counts ks-1 in the ROSAT PSPC detector. Anintermediate-resolution optical spectrum of this blue stellar objecttaken with the Keck I telescope has resulted in the unambiguousidentification of this source with a QSO at redshift z=0.8312, the sameredshift as the galaxy cluster. We discuss the implications of thisdiscovery in the framework of the standard model for large-scalestructure formation.
|HI content in galaxies in loose groups|
Gas deficiency in cluster spirals is well known and ram-pressurestripping is considered the main gas removal mechanism. In some compactgroups too gas deficiency is reported. However, gas deficiency in loosegroups is not yet well established. Lower dispersion of the membervelocities and the lower density of the intragroup medium in small loosegroups favour tidal stripping as the main gas removal process in them.Recent releases of data from the HI Parkes All-Sky Survey (HIPASS) andcatalogues of nearby loose groups with associated diffuse X-ray emissionhave allowed us to test this notion. In this paper, we address thefollowing questions: (i) do galaxies in groups with diffuse X-rayemission statistically have lower gas content compared to the ones ingroups without diffuse X-ray emission? (ii) does HI deficiency vary withthe X-ray luminosity, LX, of the loose group in a systematicway? We find that (i) galaxies in groups with diffuse X-ray emission, onaverage, are HI deficient, and have lost more gas compared to those ingroups without X-ray emission; the latter are found not to havesignificant HI deficiency; (ii) no systematic dependence of the HIdeficiency with LX is found. Ram-pressure-assisted tidalstripping and evaporation by thermal conduction are the two possiblemechanisms to account for this excess gas loss.
|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 View of Dark Matter in Early-Type Galaxies|
We present a Chandra study of mass profiles in seven ellipticalgalaxies, of which three have galaxy-scale and four have group-scalehalos, demarcated at 1013 Msolar. These representthe best available data for nearby objects with comparable X-rayluminosities. We measure approximately flat mass-to-light (M/L) profileswithin an optical half-light radius (Reff), rising by anorder of magnitude at ~10 Reff, which confirms the presenceof dark matter (DM). The data indicate hydrostatic equilibrium, which isalso supported by agreement with studies of stellar kinematics inelliptical galaxies. The data are well fitted by a model comprising anNFW DM profile and a baryonic component following the optical light. Thedistribution of DM halo concentration parameters (c) versusMvir agrees with ΛCDM predictions and our observationsof bright groups. Concentrations are slightly higher than expected,which is most likely a selection effect. Omitting the stellar massdrastically increases c, possibly explaining large concentrations foundby some past observers. The stellar M/LK agree withpopulation synthesis models, assuming a Kroupa IMF. Allowing adiabaticcompression (AC) of the DM halo by baryons made M/L more discrepant,casting some doubt on AC. Our best-fitting models imply total baryonfractions ~0.04-0.09, consistent with models of galaxy formationincorporating strong feedback. The groups exhibit positive temperaturegradients, consistent with the ``universal'' profiles found in othergroups and clusters, whereas the galaxies have negative gradients,suggesting a change in the evolutionary history of the systems aroundMvir~=1013 Msolar.
|XMM-Newton Observes Cl J0152.7-1357: A Massive Galaxy Cluster Forming at Merger Crossroads at z=0.83|
We present an analysis of a 50 ks XMM-Newton observation of the merginggalaxy cluster Cl J0152.7-1357 at z=0.83. In addition to the two mainsubclusters and an infalling group detected in an earlier Chandraobservation of the system, XMM-Newton detects another group of galaxiespossibly associated with the cluster. This group may be connected to thenorthern subcluster by a filament of cool(1.4+0.3-0.1 keV) X-ray-emitting gas and liesoutside the estimated virial radius of the northern subcluster. TheX-ray morphology agrees well with the projected galaxy distribution innew K-band imaging data presented herein. We use detailed spectral andimaging analysis of the X-ray data to probe the dynamics of the systemand find evidence that another subcluster or group has recently passedthrough the northern subcluster. Cl J0152.7-1357 is an extremelydynamically active system, with mergers at different stages occurringalong two perpendicular merger axes.
|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.
|Stellar Bar Evolution in Cuspy and Flat-cored Triaxial CDM Halos|
We analyze the formation and evolution of stellar bars in galactic disksembedded in mildly triaxial cold dark matter (CDM) halos that havedensity distributions ranging from large flat cores to cuspy profiles.We have applied tailored numerical simulations of analytical and livehalos that include the feedback from disk/bar system onto the halo inorder to test and extend earlier work by El-Zant and Shlosman. Thelatter employed the method of Liapunov exponents to analyze the fate ofbars in analytical asymmetric halos. We find the following: (1) The bargrowth is very similar in all rigid axisymmetric and triaxial halos. (2)Bars in live models experience vertical buckling instability and theformation of a pseudobulge with a boxy/peanut shape, while bars in rigidhalos do not buckle. (3) In live axisymmetric halos, the bar strengthvaries by a factor of <~2, in growth or decay, during the secularevolution following the buckling. The bar pattern speed evolution (i.e.,deceleration) anticorrelates with the halo core size. In such halos, thebar strength is larger for smaller disk-to-halo mass ratios (D/H) withindisk radii, the bar size correlates with the halo core sizes, and thebar pattern speeds correlate with the halo central mass concentration.In contrast, bars embedded in live triaxial halos have a starklydifferent fate: they dissolve on a timescale of ~1.5-5 Gyr due to theonset of chaos over continuous zones, sometimes leaving behind a weakoval distortion. The onset of chaos is related to the halo triaxiality,the fast-rotating bar, and the halo cuspiness. Before the bar dissolves,the region outside it develops strong spiral structures, especially inthe live triaxial halos. (4) More angular momentum is absorbed(fractionally) by the triaxial halos than in the axisymmetric models.The disk-halo angular momentum exchange is mediated by the lowerresonances in the latter models. (5) Cuspy halos are more susceptiblethan flat-core halos to having their prolateness washed out by theaction of the bar. The subsequent evolution is then similar to the caseof cuspy axisymmetric halos. We analyze the above results on disk andbar evolution in terms of the stability of trajectories and developmentof chaos in the system. We set important constraints on the triaxialityof dark matter (DM) halos by comparing our predictions to recentobservational results on the properties of bars out to intermediateredshifts z~1.
|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.
|The scaling relation of early-type galaxies in clusters. II. Spectroscopic data for galaxies in eight nearby clusters|
Aims.We present low and intermediate resolution spectroscopic datacollected for 152 early type galaxies in 8 nearby clusters with z ≤0.10. Methods: .We use low resolution data to produce the redshiftand the K-correction for each galaxy, as well as to give their overallspectral energy distribution and some spectral indicators, including the4000 Å break, the Mg2 strength and the NaD equivalent width. Wehave also obtained higher resolution data for early type galaxies inthree of the clusters, to determine their central velocity dispersion. Results: .The effect of the resolution on the measured parametersis discussed. Conclusions: .A new accurate systemic redshift andvelocity dispersion is presented for four of the surveyed clusters, A98,A3125, A3330, and DC2103-39. We have found that the K-correction valuesfor E/S0 bright galaxies in the given nearby clusters are very similar.We also find that the distribution of the line indicators significantlydiffers from cluster to cluster.
|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.
|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.
|Tidal streams in a MOND potential: constraints from Sagittarius|
We compare orbits in a thin axisymmetric disc potential in ModifiedNewtonian Dynamics (MOND) with those in a thin disc plus near-sphericaldark matter halo predicted by a ΛCDM cosmology. Remarkably, theamount of orbital precession in MOND is nearly identical to that whichoccurs in a mildly oblate CDM Galactic halo (potential flattening q=0.9), consistent with recent constraints from the Sagittarius stream.Since very flattened mass distributions in MOND produce rounderpotentials than in standard Newtonian mechanics, we show that it will bevery difficult to use the tidal debris from streams to distinguishbetween a MOND galaxy and a standard CDM galaxy with a mildly oblatehalo.If a galaxy can be found with either a prolate halo or one that is moreoblate than q~ 0.9 this would rule out MOND as a viable theory. Improveddata from the leading arm of the Sagittarius dwarf - which samples theGalactic potential at large radii - could rule out MOND if the orbitalpole precession can be determined to an accuracy of the order of+/-1°.
|Group, field and isolated early-type galaxies - II. Global trends from nuclear data|
We have derived ages, metallicities and enhanced-element ratios[α/Fe] for a sample of 83 early-type galaxies essentially ingroups, the field or isolated objects. The stellar-population propertiesderived for each galaxy correspond to the nuclear re/8aperture extraction. The median age found for Es is 5.8+/-0.6 Gyr andthe average metallicity is +0.37+/-0.03 dex. For S0s, the median age is3.0+/-0.6 Gyr and [Z/H]= 0.53+/-0.04 dex. We compare the distribution ofour galaxies in the Hβ-[MgFe] diagram with Fornax galaxies. Ourelliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster.We find that the galaxies lie in a plane defined by [Z/H]= 0.99logσ0- 0.46 log(age) - 1.60, or in linear terms Z~σ0× (age) -0.5. More massive (largerσ0) and older galaxies present, on average, large[α/Fe] values, and therefore must have undergone shorterstar-formation time-scales. Comparing group against field/isolatedgalaxies, it is not clear that environment plays an important role indetermining their stellar-population history. In particular, ourisolated galaxies show ages differing by more than 8 Gyr. Finally weexplore our large spectral coverage to derive log(O/H) metallicity fromthe Hα and NIIλ6584 and compare it with model-dependent[Z/H]. We find that the O/H abundances are similar for all galaxies, andwe can interpret it as if most chemical evolution has already finishedin these galaxies.
|Group, field and isolated early-type galaxies - I. Observations and nuclear data|
This is the first paper of a series on the investigation of stellarpopulation properties and galaxy evolution of an observationallyhomogeneous sample of early-type galaxies in groups, field and isolatedgalaxies.Here we present high signal-to-noise ratio (S/N) long-slit spectroscopyof 86 nearby elliptical and S0 galaxies. Eight of them are isolated,selected according to a rigorous criterion, which guarantees a genuinelow-density subsample. The present survey has the advantage of coveringa larger wavelength range than normally found in the literature, whichincludes [OIII]λ5007 and Hα, both lines important foremission correction. Among the 86 galaxies with S/N >= 15 (perresolution element, for re/8 central aperture), 57 have theirHβ-index corrected for emission (the average correction is 0.190Åin Hβ) and 42 galaxies reveal [OIII]λ5007 emission,of which 16 also show obvious Hα emission. Most of the galaxies inthe sample do not show obvious signs of disturbances nor tidal featuresin the morphologies, although 11 belong to the Arp catalogue of peculiargalaxies; only three of them (NGC 750, 751 and 3226) seem to be stronglyinteracting. We present the measurement of 25 central line-strengthindices calibrated to the Lick/IDS system. Kinematic information isobtained for the sample. We analyse the line-strength index versusvelocity dispersion relations for our sample of mainly low-densityenvironment galaxies, and compare the slope of the relations withcluster galaxies from the literature. Our main findings are that theindex-σ0 relations presented for low-density regionsare not significantly different from those of cluster E/S0s. The slopeof the index-σ0 relations does not seem to change forearly-type galaxies of different environmental densities, but thescatter of the relations seems larger for group, field and isolatedgalaxies than for cluster galaxies.
|Spherical models for early-type galaxies with cuspy mass densities|
Spherical mass density models are used to fit the central surfacebrightness profiles of early-type galaxies which are generated fromNuker law parameters obtained from the literature. The mass density andthe corresponding potential are in an analytical form. It is shown thatonly a few mass density components are necessary to obtain a good fitand that for all power-law galaxies and for the core galaxies that weconsider, most or all of the mass density components must have cusps toprovide good fits. The applied quadratic programming fitting allows fora method of deprojection, which is reliable and convenient. The resultscan be used directly for further dynamical modelling.
|Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis|
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
|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.
|Chandra Study of X-Ray Point Sources in the Early-Type Galaxy NGC 4552 (M89)|
We present a Chandra ACIS study of the early-type galaxy NGC 4552. Wedetect 47 X-ray point sources, most of which are likely low-mass X-raybinaries (LMXBs), within four effective radii (Re). Thebrightest X-ray source coincides with the optical, UV, and radio centerof the galaxy and shows variability on >1 hr timescales, indicatingthe possible existence of a low-luminosity active galactic nucleus(AGN). The 46 off-center sources and the unresolved point sourcescontribute about 29% and 20% to the total luminosity of the galaxy,respectively. We find that after correcting for the incompleteness atthe low-luminosity end, the observed cumulative X-ray luminosityfunction (XLF) of the off-center sources is best fitted by a brokenpower-law model with a break atLb=4.4+2.0-1.4×1038ergs s-1. We identified 210 globular cluster (GC) candidatesin a HST WFPC2 optical image of the galaxy's central region. Of the 25off-center LMXBs that fall within the WFPC2 field of view, 10 sourcesare coincident with a GC. Thus, the fraction of the GCs hosting brightLMXBs and the fraction of the LMXBs associated with GCs are 4.8% and40%, respectively. In the V and I bands, the GCs hosting bright LMXBsare typically 1-2 mag brighter than the GCs with no detected LMXBs.There are about 1.9+/-0.4 times as many LMXBs in the red, metal-rich GCsas there are in the blue, metal-poor ones. We find no obvious differencebetween the luminosity distributions of LMXBs in GCs and in the field,but the cumulative spectrum of the LMXBs in GCs tends to be softer thanthat of the LMXBs in field. We detected three X-ray sources that haveisotropic luminosities larger than 1039 ergs s-1.Only one of these is located in the joint Chandra-HST field and is foundto be associated with a GC. By studying its ACIS spectra we infer thatthe this may be a candidate black hole system with a mass of 15-135Msolar. One of the other sources with a luminosity brighterthan 1039 ergs s-1 reveals temporal variations inbrightness on timescales greater than 1 hr.
|Thermal Evolution of Supernova Iron in Elliptical Galaxies|
Interpretations of the spatial distribution, abundance ratios, andglobal masses of metals in the hot gas of galaxy clusters in terms ofsupernova enrichment have been problematical. For example, the abundanceof iron and other elements occasionally declines toward the center justwhere the stellar and supernova densities are highest. Also, the mass ofgas-phase iron per unit stellar mass or light is lower in ellipticalgalaxies and groups than in rich galaxy clusters. We discusshypothetical scenarios in which these abundance anomalies can resultfrom the preferential buoyant separation of metals. However, in this andall previous attempts to explain these metallicity observations it hasbeen assumed that all metals created by supernovae are present in eithervisible stars or the hot gas. We discuss here the possibility that someof the iron expelled into the hot gas by Type Ia supernovae may haveradiatively cooled, avoiding detection by X-ray and optical observers.Hydrodynamic models of Type Ia explosions in the hot gas insideelliptical galaxies create a gas of nearly pure iron that is severaltimes hotter than the local interstellar gas. We describe the subsequentthermal evolution of the iron-rich gas as it radiates and thermallymixes with the surrounding gas. There is a critical time by which theiron ions must mix into the ambient gas to avoid rapid radiativecooling. We find that successful mixing is possible if the iron ionsdiffuse with large mean free paths, as in an unmagnetized plasma.However, the Larmor radii of the iron ions are exceptionally small inmicrogauss fields, so the field geometry must be highly tangled orradial to allow the iron to mix by diffusion faster than it cools byradiative losses. The possibility that some of the supernova iron coolscannot be easily discounted.
|The Heated Core of the Radio-quiet Galaxy Cluster A644|
We present an analysis of a Chandra ACIS-I observation of the massivegalaxy cluster A644. This cluster was previously classified as a coolingflow, but no radio emission has been detected from its cD galaxy.Outside the core (R~75 kpc~0.03rvir) the hot ICM hasproperties consistent with a (relaxed) cool-core cluster out to thelargest radii investigated (R~415 kpc~0.14rvir). Over thisregion the gravitating mass profile is described well by aNavarro-Frenk-White (NFW) profile with concentration parameterc=6.1+/-1.2 and virial radius rvir=2.9+/-0.4 Mpc. However,inside the core the temperature and entropy profiles reverse theirinward radial decline and rise at the center; the inner temperatureprofile is inconsistent with a constant at the 2.3 σ level.Although the core region does not display X-ray cavities or filamentarystructures characteristic of radio-loud, cool-core clusters, the peak ofthe X-ray emission is offset from that of the centroid of the globalX-ray halo by ~60 kpc. The position of the cD galaxy lies approximatelybetween the X-ray peak and centroid, further testifying to a mergerorigin for the properties of the X-ray emission in the core. We discussthe implications of A644 and the small number of radio-quiet, cool-coreclusters for the AGN feedback paradigm to suppress cooling flows inclusters.
|Nuclear Accretion in Galaxies of the Local Universe: Clues from Chandra Observations|
In order to find an explanation for the radiative quiescence ofsupermassive black holes in the local universe, the most accurateestimates for a sample of nearby galaxies are collected for the mass ofa central black hole (MBH), the nuclear X-ray luminosityLX,nuc, and the circumnuclear hot gas density andtemperature, by using Chandra data. The nuclear X-ray luminosityLX,nuc varies by ~3 orders of magnitude and does not show arelationship with MBH or with the Bondi mass accretion rateM˙B LX,nuc is always much lower than expectedif M˙B ends in a standard accretion disk with highradiative efficiency (this instead can be the case of the active nucleusof Cen A). Radiatively inefficient accretion as in the standardadvection-dominated accretion flow (ADAF) modeling may explain the lowluminosities of a few cases; for others, the predicted luminosity isstill too high, and, in terms of Eddington-scaled quantities, it isincreasingly higher than that observed for increasingM˙B. Variants of the simple radiatively inefficientscenario including outflow and convection may reproduce the low emissionlevels observed, since the amount of matter actually accreted is reducedconsiderably. However, the most promising scenario includes feedbackfrom accretion on the surrounding gas; this has the important advantagesof naturally explaining the observed lack of relationship amongLX,nuc, MBH, and M˙B, and evadingthe problem of the fate of the material accumulating in the centralgalactic regions over cosmological times.
|The Nature of Ultraluminous X-Ray Sources|
We present spectroscopic observations of six optical counterparts ofultraluminous X-ray sources (ULXs) around nearby galaxies. The spectraof the six objects show the presence of broad emission features. Theidentification of these allow us to classify all of the objects asquasars at higher redshift than their assigned parent galaxy. This isone of the first and largest identifications of such objects usingunambigous optical spectral features. These results, in conjuction withprevious similar identifications of other sources, indicate thathigh-redshift quasars represent an important fraction of cataloged ULXsources. We estimate the density of such sources and compare this withexpectations for a population of randomly distributed backgroundquasars.
|The Ages of Elliptical Galaxies from Mid-Infrared Emission|
The mid-infrared (10-20 μm) luminosity of elliptical galaxies isdominated by the integrated emission from circumstellar dust in redgiant stars. As a single stellar population evolves, the rate of dustymass loss from red giant stars decreases with time, so the mid-infraredluminosity should also decline with stellar age. To seek such acorrelation, we have used archival Infrared Space Observatory (ISO)observations to determine surface brightness profiles and central fluxesat 15 μm in 17 early-type galaxies for which stellar ages have beendetermined from optical spectral indices. The radial surface brightnessdistributions at 15 μm generally follow the stellar de Vaucouleursprofile, as expected. We find that the surface brightness ratioμ15μm/μIband is systematically higher inelliptical galaxies with ages <~5 Gyr and in galaxies that exhibitevidence of recent mergers. Within the accuracy of our observations,μ15μm/μIband shows no age dependence forages >~5 Gyr. The corresponding flux ratiosF15μm/FIband within apertures scaled to theeffective radius (Re/8) are proportional to theμ15μm/μIband ratios at larger galacticradii, indicating that no 15 μm emission is detected from centraldust clouds visible in optical images in some of our sample galaxies.Emission at 15 μm is observed in noncentral massive clouds of dustand cold gas in NGC 1316, an elliptical galaxy that is thought to havehad a recent merger. Recent Spitzer Space Telescope data also indicatethe presence of polycyclic aromatic hydrocarbon (PAH) emission at 8μm. Several ellipticals have extended regions of 15 μm emissionthat have no obvious counterparts at other frequencies.
|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.
|Star Formation Histories of Nearby Elliptical Galaxies. I. Volume-Limited Sample|
This work presents high signal-to-noise ratio spectroscopic observationsof a representative sample of nearby elliptical galaxies. Theseobservations provide a strong test of models for the formation ofelliptical galaxies and their star formation histories. Combining thesedata with the González data set, a volume-limited sample of 45galaxies has been defined. Results are in agreement with previousstudies: the existence of the metallicity hyperplane and the Z-plane ofTrager and coworkers is confirmed, and the distribution is clearly dueto physical variations in stellar population parameters and notmeasurement uncertainty. Trends between stellar population parametersand galaxy structural parameters suggest that angular momentum maydetermine the chemical abundance of a galaxy at a given mass.
|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.
|The elliptical galaxy NGC 720: An unequal-mass galaxy merger remnant|
The stellar population of the central region of the galaxy NGC720 has been investigated through longslit opticalspectroscopy. The velocity dispersion and 13 Lick indices were obtainedas a function of the radius along the semimajor axis of the galaxy. TheMg2 index presents a gradient of d Mg2 / d log r=-0.079 whichbehaves similarly to the Fe I lines, indicating no enhancement of Mg2 inrelation to Fe I. The stellar population ages and metallicities werederived by a population synthesis method using available evolutivespectrophotometric models. The synthesis indicates a strong age gradientalong the semimajor axis of NGC 720. In the centralregion a 13 Gyr and solar metallicity stellar population dominates theflux at 5870 Å; the contribution of this component vanishes at aradius of 0.73 kpc, where the total flux is accounted for by a younger,5 Gyr and solar metallicity stellar population. For distances largerthan 1 kpc a 2.5 Gyr component becomes very important. Moreover,NGC 720 is probably overabundant in CN in the centerwith respect to the solar abundance. The estimated total mass(3.29×1011~M_ȯ) and the Mg2 gradient valuessuggest that this galaxy might have undergone a merger event. Thecorrespondence between the J-band brightness profile decomposition andthe result of the spectral synthesis shows that this galaxy is formed byan old (13-5 Gyr), bulge-like, small-scale and massive spheroid and ayounger (5-2.5 Gyr), large-scale disk component. We discuss our resultsin comparison with available numerical simulations and propose that themerger event must have occurred about 4 Gyr ago.
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