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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. 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 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
| 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.
| 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.
| Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles
We present a study of the optical brightness profiles of early typegalaxies, using a number of samples of radio galaxies and opticallyselected elliptical galaxies. For the radio galaxy samples - B2 ofFanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined anumber of parameters that describe a "Nuker-law" profile, which werecompared with those already known for the optically selected objects. Wefind that radio active galaxies are always of the "core" type (i.e. aninner Nuker law slope γ < 0.3). However, there are core-typegalaxies which harbor no significant radio source and which areindistinguishable from the radio active galaxies. We do not find anyradio detected galaxy with a power law profile (γ > 0.5). Thisdifference is not due to any effect with absolute magnitude, since in aregion of overlap in magnitude the dichotomy between radio active andradio quiescent galaxies remains. We speculate that core-type objectsrepresent the galaxies that have been, are, or may become, radio activeat some stage in their lives; active and non-active core-type galaxiesare therefore identical in all respects except their eventualradio-activity: on HST scales we do not find any relationship betweenboxiness and radio-activity. There is a fundamental plane, defined bythe parameters of the core (break radius rb and breakbrightness μ_b), which is seen in the strong correlation betweenrb and μ_b. The break radius is also linearly proportionalto the optical Luminosity in the I band. Moreover, for the few galaxieswith an independently measured black hole mass, the break radius turnsout to be tightly correlated with MBH. The black hole masscorrelates even better with the combination of fundamental planeparameters rb and μ_b, which represents the centralvelocity dispersion.
| The 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.
| Companions of Bright Barred Shapley-Ames Galaxies
Companion galaxy environment for a subset of 78 bright and nearby barredgalaxies from the Shapley-Ames Catalog is presented. Among the spiralbarred galaxies, there are Seyfert galaxies, galaxies with circumnuclearstructures, galaxies not associated with any large-scale galaxy cloudstructure, galaxies with peculiar disk morphology (crooked arms), andgalaxies with normal disk morphology; the list includes all Hubbletypes. The companion galaxy list includes the number of companiongalaxies within 20 diameters, their Hubble type, and projectedseparation distance. In addition, the companion environment was searchedfor four known active spiral galaxies, three of them are Seyfertgalaxies, namely, NGC 1068, NGC 1097, and NGC 5548, and one is astarburst galaxy, M82. Among the results obtained, it is noted that theonly spiral barred galaxy classified as Seyfert 1 in our list has nocompanions within a projected distance of 20 diameters; six out of 10Seyfert 2 bar galaxies have no companions within 10 diameters, six outof 10 Seyfert 2 galaxies have one or more companions at projectedseparation distances between 10 and 20 diameters; six out of 12 galaxieswith circumnuclear structures have two or more companions within 20diameters.
| 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.
| Ellipticals with Kinematically Distinct Cores: V-I Color Images with WFPC2
We have analyzed Hubble Space Telescope WFPC2 F555W and F814W (i.e., Vand I) images for 15 elliptical galaxies with kinematically distinctcores. For each of them, we have derived surface brightness andisophotal parameter profiles in the two bands, color maps, and radialprofiles in V - I. Most galaxies show patchy dust absorption close totheir nuclei. However, there are generally no indications ofhomogeneous, diffuse dust components close to the nuclei. The nuclearcolors in the unobscured regions are most likely representative of thecentral stellar populations. We have detected photometric evidence forfaint stellar disks, on scales of a few tens to a few arcseconds, inseven galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494, and 5322.In NGC 1700, the isophotes are slightly boxy at the scale of thecounterrotating component and disky at larger radii. We find nodifference in V - I color greater than 0.02 mag between these disks andthe surrounding galactic regions. Hence, the stellar populations in thekinematically distinct cores are not strongly deviant from thepopulation of the main body. Specifically, there is no evidence for adominating population of blue, very metal weak stars as predicted bysome of the formation scenarios. This argues against models in whichsmall galaxies fall in and survive in the nuclei, unless supermassiveblack holes are present. These would likely disrupt the accreted smallsystems. For one galaxy, NGC 4365, the innermost region is bluer thanthe surrounding regions. This area extends to ~15 pc and contains aluminosity of ~2.5 x 106 Lȯ. If interpreted as a stellar populationeffect, an age difference of ~3--4 Gyr, or an [Fe/H] variation of about0.2 dex, is derived. The nuclear intensity profiles show a largevariety: some galaxies have steep cusp profiles, while others haveshallow cusps and a "break radius." The nuclear cusps of galaxies withkinematically distinct cores follow the same trends as the nuclei ofnormal galaxies. We have not been able to identify a unique, qualifyingfeature in the WFPC2 images that distinguishes the galaxies withkinematically distinct cores from the kinematically normal cores. It ispossible that statistical differences exist: possibly, the kinematicallydistinct cores have a higher fraction of nuclear disks. The similarityof both types of cores puts strong constraints on the formationscenarios. Simulations of galaxy mergers, with the inclusion of starformation and nuclear black holes, are needed to resolve the question ofhow these structures may have formed. Spectra with high spatialresolution are needed to study the nuclear structure of the distinctcomponent in detail.
| The Centers of Early-Type Galaxies with HST. IV. Central Parameter Relations.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1771F&db_key=AST
| The Centers of Early-Type Galaxies With HST. III. Non-Parametric Recovery of Stellar Luminosity Distribution
We have non-parametrically determined the luminosity density profilesand their logarithmic slopes for 42 early-type galaxies observed withHST. Assuming that the isodensity contours are spheroidal, then theluminosity density is uniquely determined from the surface brightnessdata through the Abel equation. For nearly all the galaxies in oursample, the logarithmic slope of the luminosity density (S = d log v/dlog r) measured at 0.1" (the innermost reliable measurement with theuncorrected HST) is significantly different from zero; i.e., mostelliptical galaxies have cusps. There are only two galaxies for which ananalytic core (S approaches 0) cannot be excluded. The distribution oflogarithmic slopes at 0.1" appears to be bimodal, confirming theconclusion of Lauer et al. [AJ, 110,2622(1995)] that early-type galaxiescan be divided into two types based on their surface brightnessprofiles; i.e., those with cuspy cores and those whose steep power-lawprofiles continue essentially unchanged in to the resolution limit. Thepeaks in the slope distribution occur at S = -0.8 and - 1.9. More thanhalf of the galaxies have slopes steeper than - 1.0. Taken together withthe recent theoretical work of Merritt and Fridman, these resultssuggest that many (and maybe most) elliptical galaxies are either nearlyaxisymmetric or spherical near the center, or slowly evolve due to theinfluence of stochastic orbits.
| The Centers of Early-Type Galaxies With HST. II. Empirical Models and Structural Parameters
We present a set of structural parameters for the central parts of 57early-type galaxies observed with the Planetary Camera of the HubbleSpace Telescope. These parameters are based on a new empirical law thatsuccessfully characterizes the centers of early-type galaxies. Thisempirical law assumes that the surface brightness profile is acombination of two power laws with different slopes γ and βfor the inner and outer regions. Conventional structural parameters suchas core radius and central surface brightness are replaced by breakradius r_b_, where the transition between power-law slopes takes place,and surface brightness μ_b_ at that radius. An additional parameterα describes the sharpness of the break. The structural parametersare derived using a X^2^ minimization process applied to the meansurface brightness profiles. The resulting model profiles generally givevery good agreement to the observed profiles out to the radius of ~10"imaged by the Planetary Camera. Exceptions include galaxies which departfrom pure power laws at large radius, those with strong nuclearcomponents, and galaxies partly obscured by dust. The uncertainties inthe derived parameters are estimated using Monte Carlo simulations whichtest the stability of solutions in the face of photon noise and theeffects of the deconvolution process. The covariance of the structuralparameters is examined by computing contours of constant X^2^ inmulti-dimensional parameter space.
| The Centers of Early-Type Galaxies with HST.I.An Observational Survey
We have obtained V-band images of 45 nearby elliptical galaxies andbulges using the original Planetary Camera of the Hubble SpaceTelescope. The majority of the sample is at distances of 10-20 Mpc. Thisrepresents a substantial increase in the number of galaxies that havebeen studied at a resolution of a few parsecs. At this resolution, manygalaxies reveal previously unknown central disks, dust clouds, andnuclear components. We find that galaxies have two types of brightnessprofiles. The first type consists of galaxies that have cores. Thesegalaxies have brightness profiles that "break" from steep outer powerlaws to shallow inner cusps. The core class includes many galaxies thathad cores apparently resolved from the ground. The second type consistsof galaxies that have profiles that continue into the resolution limitas steep power laws, showing no evidence of cores of any sort. We thusfind that all galaxies studied so far have singular brightness profilesin the sense that I(r) ~ r^-γ^ as r - 0.1", with 0 < γ< 0.3 at the few parsec scale for galaxies with cores, and γ ~1 for power-law galaxies. No galaxies in our sample have a centralregion that is constant in surface brightness. This implies that thestellar density in these systems is still increasing steeply at the HSTresolution limit. Many galaxies reach stellar mass densities of ~5 x10^4^ M_sun_ pc^-3^ at the resolution limit, appearing similar in formto M32 at radii of a few parsecs. The core and power-law profile classescorrespond to the Jaffe et al. (AJ, 108, 1567 (1994)] Type I and IIprofiles; however, we disagree with their suggestion that the presenceof a central stellar disk is closely related to, or even determines,profile type. Power-law galaxies are seen at all ellipticities, and themajority of them show no evidence for central disks.
| 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.
| Are nucleated dwarf galaxies genuine ellipticals?
There are now six known dwarf elliptical galaxies outside the LocalGroup (mostly in the Virgo and Fornax clusters) that appear to containglobular-cluster systems of their own; all of them were discoveredaccidentally during surveys for other purposes. All six of these arenucleated (dE, N) dwarfs. This evidence, although quite preliminary,supports the suggestion of Ferguson and Sandage that nucleated dwarfsare 'genuine' E galaxies, generically more strongly related to largeE/S0 galaxies than are nonnucleated dwarfs of the same luminosity level.A more systematic search for globular cluster in both types of dE's isrecommended.
| Spectroscopy of globular clusters in M87 and M49
Optical multislit spectra of globular clusters in two giant ellipticalgalaxies in the Virgo Cluster indicate average metal abundances ofone-sixth and one-tenth solar composition for M49 and M87, respectively.These estimates are based on the strength of Mg lines. Taken togetherwith observations of globular clusters in dwarf ellipticals thissuggests a direct relation between mean metallicity of the clustersystem and the luminosity of the parent galaxy which is an interestingconstraint on theories of cluster formation during the protogalacticepoch. Rotation is evident in both globular cluster systems. Theline-of-sight velocity dispersions of clusters in the two galaxies areestimated to be 340 + or - 50 km/s (M49) and 386 + or - 42 km/s (M87).Although sparsely sampled, the velocity dispersion profiles of thecluster systems are flat, suggesting the existence of an isothermal haloof dark matter in these ellipticals.
| 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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