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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. 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

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.

The ACS Virgo Cluster Survey. II. Data Reduction Procedures
The ACS Virgo Cluster Survey is a large program to carry out multicolorimaging of 100 early-type members of the Virgo Cluster using theAdvanced Camera for Surveys (ACS) on the Hubble Space Telescope. DeepF475W and F850LP images (~SDSS g and z) are being used to study thecentral regions of the program galaxies, their globular cluster systems,and the three-dimensional structure of Virgo itself. In this paper, wedescribe in detail the data reduction procedures used for the survey,including image registration, drizzling strategies, the computation ofweight images, object detection, the identification of globular clustercandidates, and the measurement of their photometric and structuralparameters.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 ACS Virgo Cluster Survey. I. Introduction to the Survey
The Virgo Cluster is the dominant mass concentration in the LocalSupercluster and the largest collection of elliptical and lenticulargalaxies in the nearby universe. In this paper, we present anintroduction to the ACS Virgo Cluster Survey: a program to image, in theF475W and F850LP bandpasses (~Sloan g and z), 100 early-type galaxies inthe Virgo Cluster using the Advanced Camera for Surveys on the HubbleSpace Telescope. We describe the selection of the program galaxies andtheir ensemble properties, the choice of filters, the field placementand orientation, the limiting magnitudes of the survey, coordinatedparallel observations of 100 ``intergalactic'' fields with WFPC2, andsupporting ground-based spectroscopic observations of the programgalaxies. In terms of depth, spatial resolution, sample size, andhomogeneity, this represents the most comprehensive imaging survey todate of early-type galaxies in a cluster environment. We brieflydescribe the main scientific goals of the survey, which include themeasurement of luminosities, metallicities, ages, and structuralparameters for the many thousands of globular clusters associated withthese galaxies, a high-resolution isophotal analysis of galaxiesspanning a factor of ~450 in luminosity and sharing a commonenvironment, the measurement of accurate distances for the full sampleof galaxies using the method of surface brightness fluctuations, and adetermination of the three-dimensional structure of Virgo itself.ID="FN1"> 1Based on observations with the NASA/ESA Hubble SpaceTelescope obtained at the Space Telescope Science Institute, which isoperated by the association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.

Spectrophotometry of galaxies in the Virgo cluster. II. The data
Drift-scan mode (3600-6800 Å) spectra with 500

Star Formation Histories of Early-Type Galaxies. I. Higher Order Balmer Lines as Age Indicators
We have obtained blue integrated spectra of 175 nearby early-typegalaxies, covering a wide range in galaxy velocity dispersion andemphasizing those with σ<100 km s-1. Galaxies havebeen observed both in the Virgo Cluster and in lower densityenvironments. The main goals are the evaluation of higher order Balmerlines as age indicators and differences in stellar populations as afunction of mass, environment, and morphology. In this first paper, ouremphasis is on presenting the methods used to characterize the behaviorof the Balmer lines through evolutionary population synthesis models.Lower σ galaxies exhibit a substantially greater intrinsicscatter, in a variety of line-strength indicators, than do higherσ galaxies, with the large intrinsic scatter setting in below aσ of 100 km s-1. Moreover, a greater contrast inscatter is present in the Balmer lines than in the lines of metalfeatures. Evolutionary synthesis modeling of the observed spectralindexes indicates that the strong Balmer lines found primarily among thelow-σ galaxies are caused by young age, rather than by lowmetallicity. Thus we find a trend between the population age and thecentral velocity dispersion, such that low-σ galaxies have youngerluminosity-weighted mean ages. We have repeated this analysis usingseveral different Balmer lines and find consistent results from onespectral indicator to another.

A catalogue of Mg_2 indices of galaxies and globular clusters
We present a catalogue of published absorption-line Mg_2 indices ofgalaxies and globular clusters. The catalogue is maintained up-to-datein the HYPERCAT database. The measurements are listed together with thereferences to the articles where the data were published. A codeddescription of the observations is provided. The catalogue gathers 3541measurements for 1491 objects (galaxies or globular clusters) from 55datasets. Compiled raw data for 1060 galaxies are zero-point correctedand transformed to a homogeneous system. Tables 1, 3, and 4 areavailable in electronic form only at the CDS, Strasbourg, via anonymousftp Table 2 is available both in text and electronic form.

Extended LY alpha -absorbing Halos around Nearby Galaxies
In order to establish the Lyα absorption cross section ofpresent-day galaxies, we have identified 38 galaxies with z = 0-0.08that lie within 40-500 h^-1^ kpc of the line of sight to a QSO observedwith the Faint Object Spectrograph aboard the Hubble Space Telescope(HST). Including three galaxies in the field of 3C 273 investigated byprevious authors, we find that nine of 41 galaxies have associatedLyα absorption. If the identified Lyα absorption systems aregenuinely associated with the galaxies, then the covering factor of gasaround galaxies remains roughly constant at ~40% between 100 and 300h^-1^ kpc. Beyond 300 h^-1^ kpc, the incidence of absorption dropssharply. We conclude that (1) nearby galaxies do not possessLyα-absorbing halos beyond 300 h^-1^ kpc in radius and (2) thecovering factor of present-day galaxies between 50 and 300 h^-1^ kpc is44% at an equivalent width limit of W >= 0.3 A. For the nine galaxieswith associated Lyα absorption lines, differences in the galaxiessystemic velocities and the velocity of the absorption line, {DELTA}v,range over +/- 300 km s^-1^, consistent with the distribution found atredshifts > 0.1 by Lanzetta et al. and Le Brun, Bergeron, &Boisse. Values of {DELTA}v spanning several hundred km s^-1^ areprobably real for some of the QSO-galaxy pairs, however, and do notsimply arise from errors in measuring cz_gal_ or cz_abs_. This suggeststhat the absorbing clouds are kinematically tied to the galaxy disks andthat the distribution of {DELTA}v may arise because of the effects ofgalaxy inclination. We find no evidence for a correlation betweenLyα equivalent width and galaxy line-of-sight separation, whichweakens the argument that the identified galaxies are directlyassociated with the Lyα lines. Also, we find that absorption doesnot arise only from bright galaxies, since there are several examples inwhich low-luminosity galaxies apparently cause absorption. Yet we showthat the absorbing halos around galaxies cannot be independent of galaxyluminosity because if all low- redshift galaxies were surrounded byextended halos, the number of Lyα absorption systems found in HSTspectra would be much larger than has recently been determined. Thisresult leads us to question whether the galaxies are actuallyresponsible for the Lyα absorption lines or whether theassociation in redshift is fortuitous. Our results support the picturesuggested by others that Lyα lines arise in filaments or sheetsthat connect and contain the overdense regions that galaxies inhabit.This conclusion, however, remains at odds with the results of Lanzettaet al., who find that Lyα systems are more intimately linked withgalaxies. It may be, therefore, that we are seeing an evolution of theway in which Ly& lines associate with galaxies over the last fewbillion years. We suggest that at least some of the Lyα cloudsthat are not associated with galaxies at z ~ 0.5 have been slowlymerging with galaxies over time, creating larger but less uniform gasdistributions around the galaxies we see today.

The kinematics of the Virgo cluster revisited
The paper updates the velocity data of Virgo cluster galaxies andreconsiders the kinematic structure of the Virgo cluster. New velocitiesare given for 144 galaxies listed in the Virgo Cluster Catalog (VCC).Improved velocities are given for another 131 VCC galaxies. The Virgocluster is disentangled from its surrounding clouds of galaxies, and thelikely members of each of these clouds are listed. The velocitydistribution of dwarf elliptical cluster members is found to be highlyasymmetric. This phenomenon is interpreted as evidence for the imminentmerging of two subclusters in the core region, which points to thedynamical youth of the Virgo cluster. The mean heliocentric velocity ofthe Virgo cluster is estimated at 1050 +/- 35 km/s.

Studies of the Virgo Cluster. II - A catalog of 2096 galaxies in the Virgo Cluster area.
The present catalog of 2096 galaxies within an area of about 140 sq degapproximately centered on the Virgo cluster should be an essentiallycomplete listing of all certain and possible cluster members,independent of morphological type. Cluster membership is essentiallydecided by galaxy morphology; for giants and the rare class of highsurface brightness dwarfs, membership rests on velocity data. While 1277of the catalog entries are considered members of the Virgo cluster, 574are possible members and 245 appear to be background Zwicky galaxies.Major-to-minor axis ratios are given for all galaxies brighter than B(T)= 18, as well as for many fainter ones.

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J/AJ/90/1681VCC 538

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