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RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Gemini/GMOS spectra of globular clusters in the Virgo giant elliptical NGC 4649
NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We haveobtained Gemini/GMOS (Gemini North Multi-Object Spectrograph) spectrafor 38 globular clusters (GCs) associated with this galaxy. Applying themulti-index χ2 minimization technique of Proctor andSansom with the single stellar population models of Thomas, Maraston andKorn, we derive ages, metallicities and α-element abundanceratios. We find several young (2-3 Gyr old) supersolar metallicity GCs,while the majority are old (>10 Gyr), spanning a range ofmetallicities from solar to [Z/H]=-2. At least two of these young GCsare at large projected radii of 17-20 kpc. The galaxy itself shows noobvious signs of a recent starburst, interaction or merger. A trend ofdecreasing α-element ratio with increasing metallicity is found.
| Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.
| Gemini/GMOS spectra of globular clusters in the Leo group elliptical NGC 3379
The Leo group elliptical NGC 3379 is one of the few normal ellipticalgalaxies close enough to make possible observations of resolved stellarpopulations, deep globular cluster (GC) photometry and highsignal-to-noise ratio GC spectra. We have obtained Gemini/GMOS spectrafor 22 GCs associated with NGC 3379. We derive ages, metallicities andα-element abundance ratios from simple stellar population modelsusing the recent multi-index χ2 minimization method ofProctor & Sansom. All of these GCs are found to be consistent withold ages, i.e. >~10Gyr, with a wide range of metallicities. This iscomparable to the ages and metallicities that Gregg et al. found acouple of years ago for resolved stellar populations in the outerregions of this elliptical. A trend of decreasing α-elementabundance ratio with increasing metallicity is indicated.The projected velocity dispersion of the GC system is consistent withbeing constant with radius. Non-parametric, isotropic models require asignificant increase in the mass-to-light ratio at large radii. Thisresult is in contrast to that of Romanowsky et al., who recently found adecrease in the velocity dispersion profile as determined from planetarynebulae (PN). Our constant dispersion requires a normal-sized dark halo,although without anisotropic models we cannot rigorously determine thedark halo mass.A two-sided χ2 test over all radii gives a 2σdifference between the mass profile derived from our GCs compared to thePN-derived mass model of Romanowsky et al. However, if we restrict ouranalysis to radii beyond one effective radius and test if the GCvelocity dispersion is consistently higher, we determine a > 3σdifference between the mass models, and hence we favour the conclusionthat NGC 3379 does indeed have dark matter at large radii in its halo.
| The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies
The ACS Virgo Cluster Survey is a Hubble Space Telescope program toobtain high-resolution imaging in widely separated bandpasses (F475W~gand F850LP~z) for 100 early-type members of the Virgo Cluster, spanninga range of ~460 in blue luminosity. We use this large, homogenous dataset to examine the innermost structure of these galaxies and tocharacterize the properties of their compact central nuclei. We presenta sharp upward revision in the frequency of nucleation in early-typegalaxies brighter than MB~-15 (66%<~fn<~82%)and show that ground-based surveys underestimated the number of nucleidue to surface brightness selection effects, limited sensitivity andpoor spatial resolution. We speculate that previously reported claimsthat nucleated dwarfs are more concentrated toward the center of Virgothan their nonnucleated counterparts may be an artifact of theseselection effects. There is no clear evidence from the properties of thenuclei, or from the overall incidence of nucleation, for a change atMB~-17.6, the traditional dividing point between dwarf andgiant galaxies. There does, however, appear to be a fundamentaltransition at MB~-20.5, in the sense that the brighter,``core-Sérsic'' galaxies lack resolved (stellar) nuclei. A searchfor nuclei that may be offset from the photocenters of their hostgalaxies reveals only five candidates with displacements of more than0.5", all of which are in dwarf galaxies. In each case, however, theevidence suggests that these ``nuclei'' are, in fact, globular clustersprojected close to the galaxy photocenter. Working from a sample of 51galaxies with prominent nuclei, we find a median half-light radius of=4.2 pc, with the sizes of individual nucleiranging from 62 pc down to <=2 pc (i.e., unresolved in our images) inabout a half-dozen cases. Excluding these unresolved objects, the nucleisizes are found to depend on nuclear luminosity according to therelation rh L0.50+/-0.03. Because the largemajority of nuclei are resolved, we can rule out low-level AGNs as anexplanation for the central luminosity excess in almost all cases. Onaverage, the nuclei are ~3.5 mag brighter than a typical globularcluster. Based on their broadband colors, the nuclei appear to have oldto intermediate age stellar populations. The colors of the nuclei ingalaxies fainter than MB~-17.6 are tightly correlated withtheir luminosities, and less so with the luminosities of their hostgalaxies, suggesting that their chemical enrichment histories weregoverned by local or internal factors. Comparing the nuclei to the``nuclear clusters'' found in late-type spiral galaxies reveals a closematch in terms of size, luminosity, and overall frequency. A formationmechanism that is rather insensitive to the detailed properties of thehost galaxy properties is required to explain this ubiquity andhomogeneity. The mean of the frequency function for thenucleus-to-galaxy luminosity ratio in our nucleated galaxies,=-2.49+/-0.09 dex (σ=0.59+/-0.10), isindistinguishable from that of the SBH-to-bulge mass ratio,=-2.61+/-0.07dex (σ=0.45+/-0.09), calculated in 23 early-type galaxies withdetected supermassive black holes (SBHs). We argue that the compactstellar nuclei found in many of our program galaxies are the low-masscounterparts of the SBHs detected in the bright galaxies. If thisinterpretation is correct, then one should think in terms of ``centralmassive objects''-either SBHs or compact stellar nuclei-that accompanythe formation of almost all early-type galaxies and contain a meanfraction ~0.3% of the total bulge mass. In this view, SBHs would be thedominant formation mode above MB~-20.5.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| The ACS Virgo Cluster Survey. VI. Isophotal Analysis and the Structure of Early-Type Galaxies
We present a detailed analysis of the morphology, isophotal parameters,and surface brightness profiles for 100 early-type members of the VirgoCluster, from dwarfs (MB=-15.1 mag) to giants(MB=-21.8 mag), imaged in the g and z passbands using theAdvanced Camera for Surveys on board the Hubble Space Telescope. Dustand complex morphological structures are common. Dust is detected in 42%of galaxies brighter than BT=12.15 mag, whilekiloparsec-scale stellar disk, bars, and nuclear stellar disks are seenin 60% of galaxies with intermediate luminosity. Isophotal parametersare derived typically within 8 kpc from the center for the brightestgalaxies, and 1.5 kpc for the faintest systems, with a resolution of 7pc. For most galaxies, the surface brightness profiles are welldescribed by a Sérsic model with index n that increases steadilywith the galaxy luminosity; only for 8 of the 10 brightest galaxies arethe inner profiles (typically within 100 pc of the center) lower thanexpected based on an extrapolation of the outer Sérsic model, andare better described by a single power-law function. Contrary toprevious claims, we find no evidence in support of a strong bimodalbehavior of the logarithmic slope of the inner surface brightnessprofile, γ in particular the γ distribution for galaxiesthat do not show evidence of multiple morphological components isunimodal across the entire magnitude range spanned by the ACSVCSgalaxies. Although the brightest galaxies have shallow inner profiles,the shallowest profiles are found in faint dwarf systems. The widelyadopted separation of early-type galaxies between ``core'' and``power-law'' types is questioned based on the present study.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theassociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| The Central Engines of 19 LINERs as Viewed by Chandra
Using archival Chandra observations of 19 LINERs, we explore the X-rayproperties of their inner kiloparsec to determine the origin of theirnuclear X-ray emission, to investigate the presence of an AGN, and toidentify the power source of the optical emission lines. The relativenumbers of LINER types in our sample are similar to those in opticalspectroscopic surveys. We find that diffuse, thermal emission is verycommon and is concentrated within the central few hundred parsecs. Theaverage spectra of the hot gas in spiral and elliptical galaxies arevery similar to those of normal galaxies. They can be fitted with athermal plasma (kT~0.5 keV) plus a power-law (photon index of 1.3-1.5)model. There are on average three detected point sources in their innerkiloparsec with 1037 ergss-1
| Spitzer IRS spectra of Virgo Early-Type Galaxies: Detection of Stellar Silicate Emission
We present high signal-to-noise ratio Spitzer Infrared Spectrographobservations of 17 Virgo early-type galaxies. The galaxies were selectedfrom those that define the color-magnitude relation of the cluster, withthe aim of detecting the silicate emission of their dusty, mass-losingevolved stars. To flux calibrate these extended sources, we have deviseda new procedure that allows us to obtain the intrinsic spectral energydistribution and to disentangle resolved and unresolved emission withinthe same object. We have found that 13 objects of the sample (76%) arepassively evolving galaxies with a pronounced broad silicate featurethat is spatially extended and likely of stellar origin, in agreementwith model predictions. The other four objects (24%) are characterizedby different levels of activity. In NGC 4486 (M87), the line emissionand the broad silicate emission are evidently unresolved, and, givenalso the typical shape of the continuum, they likely originate in thenuclear torus. NGC 4636 shows emission lines superposed on extended(i.e., stellar) silicate emission, thus pushing the percentage ofgalaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435are characterized by polycyclic aromatic hydrocarbon (PAH) and lineemission, arising from a central unresolved region. A more detailedanalysis of our sample, with updated models, will be presented in aforthcoming paper.
| 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.
| The ACS Virgo Cluster Survey. IX. The Color Distributions of Globular Cluster Systems in Early-Type Galaxies
We present the color distributions of globular cluster (GC) systems for100 early-type galaxies observed in the ACS Virgo Cluster Survey, thedeepest and most homogeneous survey of this kind to date. On average,galaxies at all luminosities in our study (-22
| Stellar Populations of Elliptical Galaxies in Virgo Cluster. I. The Data and Stellar Population Analysis
We have determined spectroscopic ages of elliptical galaxies in theVirgo Cluster using spectra of very high signal-to-noise ratio(S/N>100 Å-1). We observed eight galaxies with theSubaru Telescope and have combined this sample with six galaxiespreviously observed with the WHT. To determine their ages, we have useda new method based on the Hγσ age indicator,which is virtually independent of the effects of metallicity. Apart fromages we have estimated abundances of various elements. In this paper wepresent the observations, the data reduction, and the reliability of theHγσ method. The results of this investigation arepresented in a companion paper.
| 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.
| The photometric evolution of dissolving star clusters. I. First predictions
The broad-band photometric evolution of unresolved star clusters iscalculated in a simplified way, including the preferential loss oflow-mass stars due to mass segregation. The stellar mass function of acluster evolves due to three effects: (a) the evolution of the massivestars reduces their number; (b) tidal effects before cluster-wide masssegregation reduce the mass function homogeneously, i.e. independentlyof the stellar mass; (c) after mass segregation has finished, tidaleffects preferentially remove the lowest-mass stars from the cluster.These effects result in a narrowing of the stellar mass range. Theseeffects are described quantitatively, following the results of N-bodysimulations, and are taken into account in the calculation of thephotometric history, based on the galev cluster evolution models forsolar metallicity and a Salpeter mass function. We find the followingresults: (1) During the first ~40% of the lifetime of a cluster itscolour evolution is adequately described by the standard galev models(without mass segregation) but the cluster becomes fainter due to theloss of stars by tidal effects. (2) Between ~40 and ~80% of its lifetime(independent of the total lifetime), the cluster becomes bluer due tothe loss of low-mass stars. This will result in an underestimate of theage of clusters if standard cluster evolution models are used. (3) After~80% of the total lifetime of a cluster it will rapidly become redder.This will result in an overestimate of the age of clusters if standardcluster evolution models are used. (4) Clusters with mass segregationand the preferential loss of low-mass stars evolve along almost the sametracks in colour-colour diagrams as clusters without mass segregation.Only if the total lifetime of clusters can be estimated can the coloursbe used to give reliable age estimates. (5) The changes in the colourevolution of unresolved clusters due to the preferential loss oflow-mass stars will affect the determination of the star formationhistories of galaxies if they are derived from clusters that have atotal lifetime of less than about 30 Gyr. (6) The preferential loss oflow-mass stars might explain the presence of old (~13 Gyr) clusters inNGC 4365 which are photometrically disguised as intermediate-ageclusters (2-5 Gyr), if the expected total lifetime of these clusters isbetween 16 and 32 Gyr.
| A counter-rotating core in the dwarf elliptical galaxy VCC 510
Context: . Aims: .We present optical long-slit spectra of theVirgo dwarf elliptical galaxy VCC 510 at high spectral (σ 30km s-1) and spatial resolution. The principal aim is tounravel its kinematical and stellar population properties.Methods: .Heliocentric velocities and velocity dispersions as functionsof galaxy radius are derived by deconvolving line-of-sight velocitydistributions. The luminosity-weighted stellar population parameters ageand element abundances are obtained by comparison of Lickabsorption-line indices with stellar population models. Results:.A maximum rotation v_rot=8± 2.5 km s-1 inside halfthe effective radius (re≈ 20 arcsec) and a mean, radiallyflat velocity dispersion σ=44± 5 km s-1 aremeasured. The core extending over the inner 2´´ ( 140pc) is found to rotate in the opposite sense with v_rot^core≈ -1/2v_rot. VCC 510 (M_B -15.7) is therefore by far the faintest andsmallest galaxy with a counter-rotating core known. From the main bodyrotation and the velocity dispersion profile we deduce that VCC 510 isanisotropic and clearly not entirely supported by rotation. We derive anold luminosity-weighted age (10± 3 Gyr) and sub-solar metallicity([Z/H]=-0.4± 0.1) inside the effective radius. There is tentativeevidence that the counter-rotating core might be younger and lessα/Fe enhanced. From the stellar population parameters we obtain atotal stellar mass-to-light ratio of 3.6 (M/L_B)ȯwhich is significantly lower than a rough dynamical estimate obtainedfrom the kinematics through the virial theorem ( 15). Thisdiscrepancy hints toward the possible presence of dark matter in thecentre of VCC 510. Conclusions: .We discuss the origin of thecounter-rotating core and exclude fly-by encounters as a viablepossibility. Gas accretion or galaxy merging provide more likelyexplanations. VCC 510 is therefore the direct observational evidencethat such processes do occur in cluster satellite galaxies on dwarfgalaxy scales.
| Modelling the Mg[b] Spectroscopic Index with the Tunable Filters of OSIRIS
We present our first attempt to model a new metallicity indicator forold and intermediate aged stellar populations on the basis of the Mg[b]absorption index at λ 5175 Å. Model spectra ofstellar populations are convolved with the responses of the TunableFilters of OSIRIS-GTC to provide a photometric index matching thespectroscopic Mg[b]. The new index allows us to obtain galaxy images ofthis feature on the basis of a photometric approach, without the veryhigh signal-to-noise requirements of the spectroscopic methods.
| Some astronomical niches with 3D spectroscopy
An overview of some of the most interesting results obtained with theuse of 3D spectrometers working in 4m-class telescopes is given with thepurpose of taking advantage of those experiences in the definition ofscientific programs for telescopes of larger diameter as the GTC.
| The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies
The Hubble Space Telescope has revealed a dichotomy in the centralsurface brightness profiles of early-type galaxies, which havesubsequently been grouped into two families: core, boxy, anisotropicsystems; and cuspy (`power-law'), discy, rotating ones. Here weinvestigate whether a dichotomy is also present in the X-ray propertiesof the two families. We consider both their total soft emission(LSX,tot), which is a measure of the galactic hot gascontent, and their nuclear hard emission (LHX,nuc), mostlycoming from Chandra observations, which is a measure of the nuclearactivity. At any optical luminosity, the highest LSX,totvalues are reached by core galaxies; this is explained by their beingthe central dominant galaxies of groups, subclusters or clusters, inmany of the logLSX,tot (ergs-1) >~ 41.5 cases.The highest LHX,nuc values, similar to those of classicalactive galactic nuclei (AGNs), in this sample are hosted only by core orintermediate galaxies; at low luminosity AGN levels, LHX,nucis independent of the central stellar profile shape. The presence ofoptical nuclei (also found by HST) is unrelated to the level ofLHX,nuc, even though the highest LHX,nuc are allassociated with optical nuclei. The implications of these findings forgalaxy evolution and accretion modalities at the present epoch arediscussed.
| Group, field and isolated early-type galaxies - II. Global trends from nuclear data
We have derived ages, metallicities and enhanced-element ratios[α/Fe] for a sample of 83 early-type galaxies essentially ingroups, the field or isolated objects. The stellar-population propertiesderived for each galaxy correspond to the nuclear re/8aperture extraction. The median age found for Es is 5.8+/-0.6 Gyr andthe average metallicity is +0.37+/-0.03 dex. For S0s, the median age is3.0+/-0.6 Gyr and [Z/H]= 0.53+/-0.04 dex. We compare the distribution ofour galaxies in the Hβ-[MgFe] diagram with Fornax galaxies. Ourelliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster.We find that the galaxies lie in a plane defined by [Z/H]= 0.99logσ0- 0.46 log(age) - 1.60, or in linear terms Z~σ0× (age) -0.5. More massive (largerσ0) and older galaxies present, on average, large[α/Fe] values, and therefore must have undergone shorterstar-formation time-scales. Comparing group against field/isolatedgalaxies, it is not clear that environment plays an important role indetermining their stellar-population history. In particular, ourisolated galaxies show ages differing by more than 8 Gyr. Finally weexplore our large spectral coverage to derive log(O/H) metallicity fromthe Hα and NIIλ6584 and compare it with model-dependent[Z/H]. We find that the O/H abundances are similar for all galaxies, andwe can interpret it as if most chemical evolution has already finishedin these galaxies.
| Group, field and isolated early-type galaxies - I. Observations and nuclear data
This is the first paper of a series on the investigation of stellarpopulation properties and galaxy evolution of an observationallyhomogeneous sample of early-type galaxies in groups, field and isolatedgalaxies.Here we present high signal-to-noise ratio (S/N) long-slit spectroscopyof 86 nearby elliptical and S0 galaxies. Eight of them are isolated,selected according to a rigorous criterion, which guarantees a genuinelow-density subsample. The present survey has the advantage of coveringa larger wavelength range than normally found in the literature, whichincludes [OIII]λ5007 and Hα, both lines important foremission correction. Among the 86 galaxies with S/N >= 15 (perresolution element, for re/8 central aperture), 57 have theirHβ-index corrected for emission (the average correction is 0.190Åin Hβ) and 42 galaxies reveal [OIII]λ5007 emission,of which 16 also show obvious Hα emission. Most of the galaxies inthe sample do not show obvious signs of disturbances nor tidal featuresin the morphologies, although 11 belong to the Arp catalogue of peculiargalaxies; only three of them (NGC 750, 751 and 3226) seem to be stronglyinteracting. We present the measurement of 25 central line-strengthindices calibrated to the Lick/IDS system. Kinematic information isobtained for the sample. We analyse the line-strength index versusvelocity dispersion relations for our sample of mainly low-densityenvironment galaxies, and compare the slope of the relations withcluster galaxies from the literature. Our main findings are that theindex-σ0 relations presented for low-density regionsare not significantly different from those of cluster E/S0s. The slopeof the index-σ0 relations does not seem to change forearly-type galaxies of different environmental densities, but thescatter of the relations seems larger for group, field and isolatedgalaxies than for cluster galaxies.
| To Be or Not to Be: Very Young Globular Clusters in M31
We present observations made with the newly commissioned Keck laserguide star adaptive optics (LGSAO) system of six objects in M31 that arealleged in multiple recent studies to be young globular clusters (GCs);all are supposed to have ages <=5 Gyr. The resulting FWHM of the PSFcore in our images is ~70 mas. The four youngest of these objects areasterisms; they are with certainty not young GCs in M31. Based on theirmorphology, the two oldest are GCs in M31. While the M31 GCs with ages5-8 Gyr appear to be mostly genuine, it appears that many of the allegedvery young GCs in M31 are spurious identifications. This problem will beeven more severe in studies now underway of the GC systems of moredistant spiral galaxies, for which imaging at the spatial resolution ofour observations in M31 may not be adequate to detect samplecontamination by asterisms.Based in part on observations obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration.
| The Ages of Globular Clusters in NGC 4365 Revisited with Deep HST Observations
We present new Hubble Space Telescope (HST) NIC3, near-infrared H-bandphotometry of globular clusters (GCs) around NGC 4365 and NGC 1399 incombination with archival HST WFPC2 and ACS optical data. We find thatNGC 4365 has a number of globular clusters with bluer optical colorsthan expected for their red optical-to-near-infrared colors and an oldage. The only known way to explain these colors is with a significantpopulation of intermediate-age (2-8 Gyr) clusters in this ellipticalgalaxy. On the other hand, our result for NGC 1399 is in agreement withprevious spectroscopic work that suggests that its clusters have a largemetallicity spread and are nearly all old. In the literature, there arevarious results from spectroscopic studies of modest samples of NGC 4365globular clusters. The spectroscopic data allow for either the presenceor absence of a significant population of intermediate-age clusters,given the index uncertainties indicated by comparing objects in commonbetween these studies and the few spectroscopic candidates withoptical-to-near-IR colors indicative of intermediate ages. Our newnear-IR data of the NGC 4365 GC system with a much highersignal-to-noise ratio agree well with earlier published photometry, andboth give strong evidence of a significant intermediate-age component.The agreement between the photometric and spectroscopic results for NGC1399 and other systems lends further confidence to this conclusion andto the effectiveness of the near-IR technique.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| The ACS Virgo Cluster Survey. X. Half-Light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation
We have measured half-light radii, rh, for thousands ofglobular clusters (GCs) belonging to the 100 early-type galaxiesobserved in the ACS Virgo Cluster Survey and the elliptical galaxy NGC4697. An analysis of the dependencies of the measured half-light radiion both the properties of the GCs themselves and their host galaxiesreveals that, in analogy with GCs in the Galaxy but in a milder fashion,the average half-light radius increases with increasing galactocentricdistance or, alternatively, with decreasing galaxy surface brightness.For the first time, we find that the average half-light radius decreaseswith the host galaxy color. We also show that there is no evidence for avariation of rh with the luminosity of the GCs. Finally, wefind in agreement with previous observations that the averagerh depends on the color of GCs, with red GCs being ~17%smaller than their blue counterparts. We show that this difference isprobably a consequence of an intrinsic mechanism, rather than projectioneffects, and that it is in good agreement with the mechanism proposed byJordán. We discuss these findings in light of two simple picturesfor the origin of the rh of GCs and show that both lead to abehavior in rough agreement with the observations. After accounting forthe dependencies on galaxy color, galactocentric radius, and underlyingsurface brightness, we show that the average GC half-light radii can be successfully used as a standard ruler fordistance estimation. We outline the methodology, provide a calibrationfor its use, and discuss the prospects for this distance estimator withfuture observing facilities. We find =2.7+/-0.35 pcfor GCs with (g-z)=1.2 mag in a galaxy with color(g-z)gal=1.5 mag and at an underlying surface z-bandbrightness of μz=21 mag arcsec-2. Using thistechnique, we place an upper limit of 3.4 Mpc on the 1 σline-of-sight depth of the Virgo Cluster. Finally, we examine the formof the rh distribution for our sample galaxies and provide ananalytic expression that successfully describes this distribution.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| 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.
| The Birthplace of Low-Mass X-Ray Binaries: Field Versus Globular Cluster Populations
Recent Chandra studies of low-mass X-ray binaries (LMXBs) withinearly-type galaxies have found that LMXBs are commonly located withinglobular clusters of the galaxies. However, whether all LMXBs are formedwithin globular clusters has remained an open question. If all LMXBsformed within globular clusters, the summed X-ray luminosity of theLMXBs in a galaxy should be directly proportional to the number ofglobular clusters in the galaxy regardless of where the LMXBs currentlyreside. We have compared these two quantities over the same angular areafor a sample of 12 elliptical and S0 galaxies observed with Chandra andfound that the correlation between the two quantities is weaker thanexpected if all LMXBs formed within globular clusters. This indicatesthat a significant number of the LMXBs were formed in the field andnaturally accounts for the spread in field-to-cluster fractions of LMXBsfrom galaxy to galaxy. We also find that the ``pollution'' of globularcluster LMXBs into the field has been minimal within ellipticalgalaxies, but there is evidence that roughly half of the LMXBsoriginally in the globular clusters of S0 galaxies in our sample haveescaped into the field. This is likely due to higher globular clusterdisruption rates in S0s, resulting from stronger gravitational shockscaused by the passage of globular clusters through the disks of S0galaxies that are absent in elliptical galaxies.
| The Advanced Camera for Surveys Virgo Cluster Survey. V. Surface Brightness Fluctuation Calibration for Giant and Dwarf Early-Type Galaxies
As part of the Advanced Camera for Surveys (ACS) Virgo Cluster Survey,we have measured surface brightness fluctuations (SBFs) in a sample of100 early-type Virgo galaxies. Distances derived from these measurementsare needed to explore the three-dimensional structure of the VirgoCluster, study the intrinsic parameters of globular clusters associatedwith the program galaxies, and compare with the galaxy distances derivedfrom globular cluster luminosity functions. Our SBF measurements havebeen performed in the F850LP bandpass of the Wide Field Channel of theACS on the Hubble Space Telescope. These are the first measurements ofthis kind, and we present the first SBF calibration for this bandpass.The measured fluctuations depend on galaxy stellar populationvariations, which we quantify by galaxy color(g475-z850)0, where g475 andz850 are the galaxy magnitudes in the F475W and F850LP ACSfilters, respectively. We derive the calibration for the absolute SBFmagnitudeM850=-2.06+/-0.04+(2.0+/-0.2)[(g475-z850)0-1.3]in the range1.3<(g475-z850)0<=1.6, andM850=-2.06+/-0.04+(0.9+/-0.2)[(g475-z850)0-1.3]in the range1.0<=(g475-z850)0<=1.3. Thequoted zero-point uncertainty here includes all sources of internalerror; there is an additional systematic uncertainty of ~0.15 mag, dueto the uncertainty in the distance scale calibration. Physically, thetwo different color regimes correspond to different galaxy types: giantellipticals and S0s at the red end, and early-type dwarfs at the blueend. For the first time in SBF studies, we are able to provide a firmempirical calibration of SBF in early-type dwarf galaxies. Our resultsagree with stellar population model predictions from Bruzual &Charlot in the range1.3<(g475-z850)0<=1.6, while ourempirical slope is somewhat steeper than the theoretical prediction inthe range 0.9<=(g475-z850)0<=1.3.
| 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.
| On the Nature of X-Ray Sources in Early-Type Galaxies
We show that the observed relationship between the fraction of low-massX-ray binaries (LMXBs) found in globular clusters (GCs) and theGC-specific frequency for early-type galaxies is consistent with an LMXBformation model in which the field population of LMXBs is formed in situvia primordial binary formation. The suggestion that a significantfraction of the field LMXB population in early-type galaxies was formedin GCs is not required by the data. Finally, we discuss observationalstudies that will test this model more thoroughly.
| Simulating the Spitzer Mid-Infrared Color-Color Diagrams
We use a simple parameterization of the mid-IR spectra of a wide rangeof galaxy types in order to predict their distribution in the InfraredArray Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 μm and MultibandPhotometer for Spitzer 24 μm color-color diagrams. We distinguishthree basic spectral types by the energetically dominant component inthe 3-12 μm regime: stellar-dominated, polycyclic aromatichydrocarbon (PAH)-dominated, and continuum-dominated. We use a Markovchain Monte Carlo approach to arrive at a more systematic and robustrepresentation of the mid-IR spectra of galaxies than do moretraditional approaches. We find that IRAC color-color plots are wellsuited to distinguishing the above spectral types, while the addition of24 μm data allows us to suggest practical three-color cuts thatpreferentially select higher redshift sources of a specific type. Wecompare our simulations with the color-color plot obtained by theSpitzer First Look Survey and find reasonable agreement. Lastly, wediscuss other applications as well as future directions for this work.
| 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.
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