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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.
| Environmental Effects on Late-Type Galaxies in Nearby Clusters
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.
| Planetary nebulae as tracers of galaxy stellar populations
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
| A Virgo high-resolution Hα kinematical survey - II. The Atlas
A catalogue of ionized gas velocity fields for a sample of 30 spiral andirregular galaxies of the Virgo cluster has been obtained by using 3Doptical data. The aim of this survey is to study the influence ofhigh-density environments on the gaseous kinematics of local clustergalaxies. Observations of the Hα line by means of Fabry-Perotinterferometry have been performed at the Canada-France-HawaiiTelescope, European Southern Observatory 3.6-m telescope, Observatoirede Haute-Provence 1.93-m telescope and Observatoire du montMégantic telescope at angular and spectral samplings from 0.4 to1.6arcsec and 7 to 16kms-1. A recently developed, automaticand adaptive spatial binning technique is used to reach a nearlyconstant signal-to-noise ratio (S/N) over the whole field of view,allowing us to keep a high spatial resolution in high-S/N regions andextend the detection of signal in low-S/N regions. This paper is part ofa series and presents the integrated emission-line and velocity maps ofthe galaxies. Both Hα morphologies and kinematics exhibit signs ofperturbations in the form of, for example, external filaments, inner andnuclear spiral- and ring-like structures, inner kinematical twists,kinematical decoupling of a nuclear spiral, streaming motions alongspiral arms and misalignment between kinematical and photometricorientation axes.
| 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.
| Chandra Observations of Gas Stripping in the Elliptical Galaxy NGC 4552 in the Virgo Cluster
We use a 54.4 ks Chandra observation to study ram pressure stripping inNGC 4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandraimages in the 0.5-2 keV band show a sharp leading edge in the surfacebrightness 3.1 kpc north of the galaxy center, a cool(kT=0.51+0.09-0.06 keV) tail with mean densityne~(5.4+/-1.7)×10-3 cm-3extending ~10 kpc to the south of the galaxy, and two 3-4 kpc horns ofemission extending southward away from the leading edge. These are allfeatures characteristic of supersonic ram pressure stripping of galaxygas, due to NGC 4552's motion through the surrounding Virgo ICM. Fittingthe surface brightness profile and spectra across the leading edge, wefind the galaxy gas inside the edge is cooler(kT=0.43+0.03-0.02 keV) and denser(ne~0.010 cm-3) than the surrounding Virgo ICM[kT=2.2+0.7-0.4 keV andne=(3.0+/-0.3)×10-4 cm-3]. Theresulting pressure ratio between the free-streaming ICM and cluster gasat the stagnation point is ~7.6+3.4-2.0 for galaxygas metallicities of 0.5+0.5-0.3Zsolar, which suggests that NGC 4552 is moving supersonicallythrough the cluster with a velocity v~1680+390-220km s-1 (Mach 2.2+0.5-0.3) at an angleξ~35deg+/-7deg toward us with respect to theplane of the sky.
| The ACS Virgo Cluster Survey. XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies
We use HST ACS imaging of 100 early-type galaxies in the ACS VirgoCluster Survey to investigate the nature of diffuse star clusters(DSCs). Compared to globular clusters (GCs), these star clusters havelow luminosities (MV>-8) and a broad distribution of sizes(320 magarcsec-2). The median colors of diffuse star cluster systems(1.1
| 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
| First Results from SAPAC: Toward a Three-dimensional Picture of the Fornax Cluster Core
A sophisticated surface brightness fluctuation (SBF) analysis packagehas been developed, designed to measure distances of early-type galaxiesby means of SBFs of unresolved stars. This suite of programs, calledSAPAC, is made readily available to the astronomical community forextensive testing, with the long-term goal of providing the necessarytools for systematic distance surveys of early-type galaxies usingmodern optical/near-IR telescopes equipped with wide-field cameras. Wediscuss the technical and scientific concepts of SAPAC and demonstrateits capabilities by analyzing deep B- and R-band CCD images of 10 dwarfelliptical galaxy candidates in the Fornax Cluster obtained with FORS1at the Very Large Telescope. All candidates are confirmed as clustermembers. We then turn our attention to the innermost region of theFornax Cluster. A total of 29 early-type galaxies closer than threecluster core radii (2deg) to the central galaxy NGC 1399 haveradial velocities and SBF distances. Their Hubble diagram exhibits apronounced S-shaped infall pattern, suggesting that Fornax is still inthe process of formation during the present epoch through a generalcollapse and possible accretion of distinct groups of galaxies. Fromfitting a model we estimate the cluster mass within 720 kpc projecteddistance of NGC 1399 to be 2.3+/-0.3×1014Msolar. The associated collapse time istcoll=2.9+1.6-0.9 Gyr. After cleansing our galaxy sample of afew kinematical outliers, the true distance of the Fornax Cluster coreis determined at 20.13+/-0.40 Mpc [(m-M)0=31.51+/-0.04 mag].Applying a bootstrap resampling technique on the distance distributionwith individual distance errors taken into account further reveals asmall intrinsic cluster depth of σint=0.74+0.52-0.74Mpc, in best agreement with the cluster's linear extension in the sky:σR.A.=σdecl.~0.5 Mpc. We conclude thatthe early-type galaxy population in the Fornax Cluster must be spatiallywell constrained, with no evidence of elongation along the line ofsight, in contrast to the Virgo Cluster. Moreover, we find marginalevidence for substructure, a result that is consistent with the youngevolutionary state of the cluster and the overall galaxy infall.Combining the kinematically defined cluster distance with the meancosmological velocity for the central cluster galaxy sample yields aHubble constant of H0=63+/-5 km s-1Mpc-1.Based on observations collected at the ESO Very Large Telescope, underprogram ESO 68.A-0176.
| The Globular Cluster System of the Virgo Dwarf Elliptical Galaxy VCC 1087
We present an analysis of the globular cluster (GC) system of thenucleated dwarf elliptical galaxy VCC 1087 in the Virgo Cluster based onKeck LRIS spectroscopy and archival Hubble Space Telescope AdvancedCamera for Surveys imaging. We estimate that VCC 1087 hosts a totalpopulation of 77+/-19 GCs, which corresponds to a relatively high V-bandspecific frequency of 5.8+/-1.4. The g475-z850color distribution of the GCs shows a blue (metal-poor) peak with a tailof redder (metal-rich) clusters similar in color to those seen inluminous elliptical galaxies. The luminosity function of the GCs islognormal and peaks atMTOg475=-7.2+/-0.3,MTOz850=-8.1+/-0.2. These peakpositions are consistent with those found for luminous Virgo ellipticalgalaxies, suggesting either the lack of or, surprisingly similarly, thedynamical destruction processes of GCs among dwarf and giant galaxies.Spectroscopy of a subsample of 12 GCs suggests that the GC system is oldand coeval (>~10 Gyr), with a fairly broad metallicity distribution(-1.8<~[M/H]<~-0.8). In contrast, an integrated spectrum of theunderlying galaxy starlight reveals that its optical luminosity isdominated by metal-rich, intermediate-age stars. The radial velocitiesof the GCs suggest rotation close to the major axis of the galaxy, andthis rotation is dynamically significant with(vrot/σlos)*>1. A compilationof the kinematics of the GC systems of nine early-type galaxies showssurprising diversity in the (vrot/σlos)parameter for GC systems. In this context, the GC system of VCC 1087exhibits the most significant rotation-to-velocity dispersion signature.Dynamical mass modeling of the velocity dispersion profile of the GCsand galaxy stars suggests fairly constant mass-to-light ratios of ~3 outto 6.5 kpc. The present observations can entertain both baryonic andnonbaryonic solutions, and GC velocities at larger radii would be mostvaluable with regard to this issue. Finally, we discuss the evolution ofVCC 1087 in terms of the galaxy ``harassment'' scenario and concludethat this galaxy may well be the remains of a faded, tidally perturbedSc spiral.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| Peculiarities and populations in elliptical galaxies. III. Dating the last star formation event
Using 6 colours and 4 Lick line-indices we derive two-component modelsof the populations of ellipticals, involving a "primary" and a"juvenile" population. The first component is defined by the regressionsof indices against the central velocity dispersion found in Papers I andII for the {Nop} sample of non-peculiar objects. The second one isapproximated by an SSP, and the modeling derives its age A, metallicityZ and fractional V-luminosity q_V, the fractional mass qMbeing found therefrom. The model is designed for "blueish" peculiargalaxies, i.e. the {Pec} sample and NGC 2865 family in the terminologyof Paper I. The morphological peculiarities and the population anomalyare then believed to involve the same event, i.e. a merger plusstarburst. It is possible to improve the models in a few cases byintroducing diffuse dust (as suggested by far IR data), and/or by takinginto account the fact that Lick- and colour indices do not relate toidentical galaxy volumes. In most of the cases, the mass ratio of youngstars qM seems too small for the product of a recent majormerger: the events under consideration might be minor mergers bringing"the final touch" to the build-up of the structure of the E-type object.The same modeling has been successfully applied to blueish galaxies ofthe {Nop} sample, without morphological peculiarities however, tosupport the occurence of a distinct perturbing event. A few reddishobjects of the {Pec} sample (NGC 3923 family) and of the {Nop} sampleare also modeled, in terms of an excess of high metallicity stars, ordiffuse dust, or both, but the results are inconclusive.
| Nearby early-type galaxies with ionized gas. II. Line-strength indices for 18 additional galaxies
We previously presented a data-set of line-strength indices for 50early-type galaxies in the nearby Universe. The galaxy sample is biasedtoward galaxies showing emission lines, located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. The present addendum enlargesthe above data-set of line-strength indices by analyzing 18 additionalearly-type galaxies (three galaxies, NGC 3607, NGC 5077 and NGC 5898were presented in the previous set). We measured 25 line-strengthindices, defined by the Lick IDS "standard" system (Trager et al. 1998,ApJS, 116, 1; Worthey & Ottaviani 1997, ApJS, 111, 377), for 7luminosity weighted apertures and 4 gradients of each galaxy. Thisaddendum presents the line-strength data-set and compares it with theavailable data in the literature.
| 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.
| The cool wake around 4C 34.16 as seen by XMM-Newton
We present XMM-Newton observations of the wake-radio galaxy system 4C34.16, which shows a cool and dense wake trailing behind the host galaxyof 4C 34.16. A comparison with numerical simulations is enlightening, asthey demonstrate that the wake is produced mainly by ram pressurestripping during the galactic motion through the surrounding cluster.The mass of the wake is a substantial fraction of the mass of the X-rayhalo of an elliptical galaxy. This observational fact supports a wakeformation scenario similar to that recently demonstrated numerically byAcreman et al.: the host galaxy of 4C 34.16 has fallen into its cluster,and is currently crossing its central regions. A substantial fraction ofits X-ray halo has been stripped by ram pressure, and remains behind toform the galaxy wake.
| Mass-to-light ratio gradients in early-type galaxy haloes
Owing to the fact that the near future should see a rapidly expandingset of probes of the halo masses of individual early-type galaxies, weintroduce a convenient parameter for characterizing the halo masses fromboth observational and theoretical results:∇lΥ, the logarithmic radial gradient of themass-to-light ratio. Using halo density profiles from Λ-cold darkmatter (CDM) simulations, we derive predictions for this gradient forvarious galaxy luminosities and star formation efficienciesɛSF. As a pilot study, we assemble the available∇lΥ data from kinematics in early-type galaxies- representing the first unbiased study of halo masses in a wide rangeof early-type galaxy luminosities - and find a correlation betweenluminosity and ∇lΥ, such that the brightestgalaxies appear the most dark-matter dominated. We find that thegradients in most of the brightest galaxies may fit in well with theΛCDM predictions, but that there is also a population of faintergalaxies whose gradients are so low as to imply an unreasonably highstar formation efficiency ɛSF > 1. This difficultyis eased if dark haloes are not assumed to have the standard ΛCDMprofiles, but lower central concentrations.
| 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.
| Star Formation Rates in Cooling Flow Clusters: A UV Pilot Study with Archival XMM-Newton Optical Monitor Data
We have analyzed XMM-Newton Optical Monitor UV (180-400 nm) data for asample of 33 galaxies. Thirty are cluster member galaxies, and nine arecentral cluster galaxies (CCGs) in cooling flow clusters having massdeposition rates between 8 and 525 Msolar yr-1. Bycomparing the ratio of UV to 2MASS J-band fluxes, we find a significantUV excess in many, but not all, cooling flow CCGs, consistent withseveral previous studies based on optical imaging data (McNamara &O'Connell Cardiel et al.; Crawford et al.). This UV excess is a directindication of the presence of young massive stars and, therefore, recentstar formation. Using the Starburst99 model of continuous star formationover a 900 Myr period, we derive star formation rates of 0.2-219Msolar yr-1 for the cooling flow sample. Fortwo-thirds of this sample, it is possible to equate Chandra/XMM coolingflow mass deposition rates with UV-inferred star formation rates, for acombination of starburst lifetime and IMF slope. This is a pilot studyof the well-populated XMM UV cluster archive, and a more extensivefollow-up study is currently underway.
| O VI in Elliptical Galaxies: Indicators of Cooling Flows
Early-type galaxies often contain a hot X-ray-emitting interstellarmedium [(3-8)×106 K] with an apparent radiative coolingtime much less than a Hubble time. If unopposed by a heating mechanism,the gas will radiatively cool to temperatures <~104 K at arate proportional to LX/TX, typically 0.03-1Msolar yr-1. We can test whether gas is coolingthrough the 3×105 K range by observing the O VIdoublet, whose luminosity is proportional to the cooling rate. Here wereport on a study of an unbiased sample of 24 galaxies, obtaining FarUltraviolet Spectroscopic Explorer spectra to complement the X-ray dataof ROSAT and Chandra. The O VI line emission was detected in about 40%of the galaxies and at a luminosity level similar to the prediction fromthe cooling flow model. There is a correlation betweenM˙OVI and M˙X, although there issignificant dispersion about the relationship, where the O VI isbrighter or dimmer than expected by a factor of 3 or more. If thecooling flow picture is to be retained, then this dispersion requiresthat cooling flows be time-dependent, as might occur by the activity ofan AGN. However, of detected objects, those with the highest or lowestvalues of M˙OVI/M˙X are not systematicallyhot or cool, as one might predict from AGN heating.
| 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.
| Diffuse Light in the Virgo Cluster
We present deep optical imaging of the inner ~1.5d×1.5d of theVirgo Cluster to search for diffuse intracluster light (ICL). Our imagereaches a 1 σ depth of μV=28.5 magarcsec-2, which is 1.5 mag arcsec-2 deeper thanprevious surveys, and reveals an intricate web of diffuse ICL. We seeseveral long (>100 kpc) tidal streamers, as well as a myriad ofsmaller scale tidal tails and bridges between galaxies. The diffuse haloof M87 is traced out to nearly 200 kpc, appearing very irregular onthese scales, while significant diffuse light is also detected aroundthe M84/M86 pair. Several galaxies in the core are embedded in commonenvelopes, suggesting they are true physical subgroups. The complexsubstructure of Virgo's diffuse ICL reflects the hierarchical nature ofcluster assembly, rather than being the product of smooth accretionaround a central galaxy.
| 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.
| Metal-poor Globular Clusters and the Formation of Their Host Galaxies
We have determined the total numbers and specific frequencies of blue,metal-poor globular clusters (GCs) in eight spiral and early-typegalaxies. These data, along with five measurements from the literature,show a trend of increasing blue GC specific frequency with increasingmass of the host galaxy. The increase is not accounted for in a simplegalaxy formation model in which elliptical galaxies and their GC systemsare formed by the merger of typical spiral galaxies. The data appearbroadly consistent with hierarchical formation scenarios in whichmetal-poor GCs are formed over a finite period in the early universeduring the initial stages of galaxy assembly. In this picture, theobserved trend is related to biasing, in the sense that the more massivegalaxies of today began assembling earlier and therefore formedrelatively more GCs during this early epoch of metal-poor GC formation.We discuss how comparisons of the observed specific frequency ofmetal-poor GCs with model calculations can constrain the formationredshift of these objects.
| XMM-Newton Observation of an X-Ray Trail between the Spiral Galaxy NGC 6872 and the Central Elliptical Galaxy NGC 6876 in the Pavo Group
We present XMM-Newton observations of a trail of enhanced X-ray emissionextending along the full 8.7 arcmin ×4' region betweenthe large spiral galaxy NGC 6872 and the dominant elliptical galaxy NGC6876 in the Pavo group, the first known X-ray trail associated with aspiral galaxy in a poor galaxy group and, with a projected length of 90kpc, one of the longest known X-ray trails. The X-ray surface brightnessin the trail region is roughly constant beyond ~20 kpc of NGC 6876 inthe direction of the spiral galaxy. The trail is hotter (~1 keV) thanthe undisturbed Pavo IGM (~0.5 keV) and has low metal abundances (0.2Zsolar). The 0.5-2 keV luminosity of the trail, measuredusing a 67×90 kpc rectangular region, is 6.6×1040ergs s-1. We compare the properties of gas in the trail tothe spectral properties of gas in the spiral galaxy NGC 6872 and in theelliptical galaxy NGC 6876 to constrain its origin. We suggest that theX-ray trail is either IGM gas gravitationally focused into a Bondi-Hoylewake, a thermal mixture of ~60% Pavo IGM gas with ~40% galaxy gas thathas been removed from the spiral galaxy NGC 6872 by turbulent viscousstripping, or both, due to the spiral galaxy's supersonic motion atangle ξ~40deg with respect to the plane of the sky,through the densest region of the Pavo IGM. Assumingξ=40deg and a filling factor η in a cylindrical volumewith radius 33 kpc and projected length 90 kpc, the mean electrondensity and total hot gas mass in the trail are1×10-3η-1/2 cm-3 and1.1×1010η1/2 Msolar,respectively.
| Close Binaries as the Progenitors of the Brightest Planetary Nebulae
We investigate the possible progenitors of the planetary nebulae (PNs)that populate the top 0.5 mag of the [O III] λ5007 planetarynebula luminosity function (PNLF). We show that the absolute luminosityof the PNLF cutoff demands that the central stars of these most luminousPNs be >~0.6 Msolar and that such high-mass PN cores mustexist in every galaxy. We also use the bolometric luminosity-specific PNnumber density to show that in early-type galaxies, [O III]-bright PNsare relatively rare, with only ~10% of stars evolving to these brightmagnitudes. We demonstrate that the combination of these two factsimplies that either all early-type systems contain a small, smoothlydistributed component of young (<~1 Gyr old) stars or that anothermechanism exists for creating high core mass PNs. We argue that binarystar evolution is this second mechanism and demonstrate that bluestragglers have the appropriate core properties and number density toexplain the observations. We discuss the implications of thisalternative mode of stellar evolution and speculate on how coalescedbinaries might affect the use of PNs for measuring a galaxy's starformation history and chemical evolution.
| A Dark Hydrogen Cloud in the Virgo Cluster
VIRGOHI 21 is an H I source detected in the Virgo Cluster survey ofDavies et al. that has a neutral hydrogen mass of 108Msolar and a velocity width of ΔV20=220 kms-1. From the Tully-Fisher relation, a galaxy with thisvelocity width would be expected to be 12 mag or brighter; however, deepCCD imaging has failed to turn up a counterpart down to a surfacebrightness level of 27.5 B mag arcsec-2. The H I observationsshow that it is extended over at least 16 kpc, which, if the system isbound, gives it a minimum dynamical mass of ~1011Msolar and a mass-to-light ratio ofMdyn/LB>500Msolar/Lsolar. If it is tidal debris, then theputative parents have vanished; the remaining viable explanation is thatVIRGOHI 21 is a dark halo that does not contain the expected brightgalaxy. This object was found because of the low column density limit ofour survey, a limit much lower than that achieved by all sky H I surveyssuch as those carried out at Parkes and Jodrell Bank. Further suchsensitive surveys might turn up a significant number of the dark matterhalos predicted by galaxy formation models.
| Infall of the Elliptical Galaxy NGC 1404 into the Fornax Cluster
We use three Chandra observations, totaling 134.3 ks, to constrain thedynamical motion of NGC 1404 falling toward the dominant elliptical NGC1399 through the Fornax Cluster gas. The surface brightness profile ofNGC 1404 shows a sharp edge at ~8 kpc from its center in the directionof NGC 1399, characteristic of jumplike temperature and densitydiscontinuities from ram pressure stripping of the galaxy gas, caused byits motion through the surrounding intracluster medium (ICM). We findthat the temperature of the galaxy gas inside the edge is ~2.8 timescooler (kT=0.55+0.01-0.02 keV with abundanceA=0.73+0.65-0.16Zsolar) than thecluster gas (kT=1.53+0.10-0.13 keV,A=0.42+0.2-0.13Zsolar). We use theshape of the surface brightness profile across the edge to fit theposition of the edge, the power-law behavior of NGC 1404's densitydistribution in the leading direction, and the density discontinuity atthe edge. The electron density inside the edge(3.9-4.3)×10-3 cm-3 depends strongly on thegas abundance, while the density of the ICM (7-8)×10-4cm-3 depends strongly on the assumed geometry (relativedistance) between NGC 1404 and NGC 1399. The corresponding pressure jumpof 1.7-2.1 across the leading edge of the galaxy and the clusterfree-stream region implies near sonic motion (Mach number 0.83-1.03) forNGC 1404 with a velocity 531-657 km s-1 relative to thesurrounding cluster gas. The inclination angle of the motion, inferredusing the relative radial velocity between NGC 1404 and 1399 asrepresentative of that between NGC 1404 and the cluster ICM, isuncomfortably large (>~40°) given the sharpness of the surfacebrightness edge, suggesting either a nonzero impact parameter betweenNGC 1404 and 1399 or that NGC 1399 is also moving radially with respectto the cluster ICM.
| Dynamical interaction of M 31 and M 32.
Not Available
| 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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