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|Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. I. On the Possible Disk Nature of Bright Early-Type Dwarfs|
We present a systematic search for disk features in 476 Virgo Clusterearly-type dwarf (dE) galaxies. This is the first such study of analmost-complete, statistically significant dE sample, which includes allcertain or possible cluster members with mB<=18 that arecovered by the optical imaging data of the Sloan Digital Sky Survey DataRelease 4. Disk features (spiral arms, edge-on disks, or bars) wereidentified by applying unsharp masks to a combined image from threebands (g, r, and i), as well as by subtracting the axisymmetric lightdistribution of each galaxy from that image. Fourteen objects areunambiguous identifications of disks, 10 objects show ``probable disk''features, and 17 objects show ``possible disk'' features. The numberfraction of these galaxies, for which we introduce the term ``dEdi,''reaches more than 50% at the bright end of the dE population anddecreases to less than 5% for magnitudes mB>16. Althoughpart of this observed decline might be due to the lower signal-to-noiseratio at fainter magnitudes, we show that it cannot be caused solely bythe limitations of our detection method. The luminosity function of ourfull dE sample can be explained by a superposition of dEdis and ordinarydEs, strongly suggesting that dEdis are a distinct type of galaxy. Thisis supported by the projected spatial distribution: dEdis show basicallyno clustering and roughly follow the spatial distribution of spirals andirregulars, whereas ordinary dEs are distributed similarly to thestrongly clustered E/S0 galaxies. While the flattening distribution ofordinary dEs is typical for spheroidal objects, the distribution ofdEdis is significantly different and agrees with their being flat oblateobjects. We therefore conclude that the dEdis are not spheroidalgalaxies that just have an embedded disk component but are instead apopulation of genuine disk galaxies. Several dEdis display well-definedspiral arms with grand-design features that clearly differ from theflocculent, open arms typical for late-type spirals that have frequentlybeen proposed as progenitors of dEs. This raises the question of whatprocess is able to create such spiral arms-with pitch angles like thoseof Sab/Sb galaxies-in bulgeless dwarf galaxies.
|Stellar Populations of Dwarf Elliptical Galaxies: UBVRI Photometry of Dwarf Elliptical Galaxies in the Virgo Cluster|
We present UBVRI surface photometry for 16 dwarf elliptical galaxies inthe Virgo Cluster with previously measured kinematic properties. Theglobal optical colors are red, with median values for the sample of0.24+/-0.03 in U-B, 0.77+/-0.02 in B-V, and 1.02+/-0.03 in V-I. Werecover the well-known color-magnitude relation for cluster galaxies butfind no significant difference in dominant stellar population betweenrotating and nonrotating dwarf elliptical galaxies; the average age ofthe dominant stellar population is 5-7 Gyr in all 16 galaxies in thissample. Analysis of optical spectra confirm these age estimates andindicate Fe and Mg abundances in the range of 1/20 to one-third ofsolar, as expected for low-luminosity galaxies. Based on Lick indicesand simple stellar population models, the derived [α/Fe] ratiosare subsolar to solar, indicating a more gradual chemical enrichmenthistory for dE's as compared with giant elliptical galaxies in the VirgoCluster. These observations confirm the marked difference in stellarpopulation and stellar distribution between dwarf and giant ellipticalgalaxies and further substantiate the need for alternative evolutionaryscenarios for the lowest mass cluster galaxies. We argue that it islikely that several different physical mechanisms played a significantrole in the production of the Virgo Cluster dE galaxies including insitu formation, infall of dE's that were once part of Local Groupanalogs, and transformation of dwarf irregular galaxies by the clusterenvironment. The observations support the hypothesis that a largefraction of the Virgo Cluster dE's are formed by ram pressure strippingof gas from infalling dI's.Based on observations with the VATT: the Alice P. Lennon Telescope andthe Thomas J. Bannan Astrophysics Facility.
|Rotationally Supported Virgo Cluster Dwarf Elliptical Galaxies: Stripped Dwarf Irregular Galaxies?|
New observations of 16 dwarf elliptical galaxies (dEs) in the VirgoCluster indicate that at least seven dEs have significant velocitygradients along their optical major axis, with typical rotationamplitudes of 20-30 km s-1. Of the remaining nine galaxies inthis sample, six have velocity gradients of less than 20 kms-1 kpc-1, while the other three observations hadtoo low a signal-to-noise ratio to determine an accurate velocitygradient. Typical velocity dispersions for these galaxies are ~44+/-5 kms-1, indicating that rotation can be a significant componentof the stellar dynamics of Virgo dEs. When corrected for the limitedspatial extent of the spectral data, the rotation amplitudes of therotating dEs are comparable to those of similar-brightness dwarfirregular galaxies (dIs). Evidence of a relationship between therotation amplitude and galaxy luminosity is found and, in fact, agreeswell with the Tully-Fisher relation. The similarity in the scalingrelations of dIs and dEs implies that it is unlikely that dEs evolvefrom significantly more luminous galaxies. These observations reaffirmthe possibility that some cluster dEs may be formed when the neutralgaseous medium is stripped from dIs in the cluster environment. Wehypothesize that several different mechanisms are involved in thecreation of the overall population of dEs and that stripping ofinfalling dIs may be the dominant process in the creation of dEs inclusters like Virgo.
|Galaxy Populations and Evolution in Clusters. IV. Deep H I Observations of Dwarf Elliptical Galaxies in the Virgo Cluster|
In this paper we present deep Arecibo H I and WIYN optical observationsof Virgo Cluster dwarf elliptical galaxies. Based on this data we arguethat a significant fraction of low-mass galaxies in the Virgo Clusterrecently underwent evolution. Our new observations consist of H I 21 cmline observations for 22 classified dE galaxies with optical radialvelocities consistent with membership in the Virgo Cluster. Clustermembers VCC 390 and VCC 1713 are detected with H I massesMHI=6×107 and 8×107Msolar, respectively, while MHI values in theremaining 20 dE galaxies have upper limits as low as~5×105 Msolar. We combine our results withthose for 26 other Virgo Cluster dE galaxies with H I observations inthe literature, seven of which have H I detection claims. New opticalimages from the WIYN telescope of five of these H I-detected dEgalaxies, along with archival data, suggest that seven of the claimeddetections are true H I detections, yielding a ~15% detection rate.These H I-detected, classified dE galaxies are preferentially locatednear the periphery of the Virgo Cluster. Three Virgo dE galaxies haveobserved H I velocity widths greater than 200 km s-1,possibly indicating the presence of a large dark matter content ortransient extended H I. We discuss the possible origins of these objectsand argue that they originate from field galaxies accreted onto highangular momentum orbits by Virgo in the last few Gyr. As a result ofthis, we argue, these galaxies are slowly transformed within the clusterby gradual gas-stripping processes, associated truncation of starformation, and passive fading of stellar populations. Low-mass,early-type cluster galaxies are therefore currently being produced asthe product of cluster environmental effects. We utilize our results ina simple model to estimate the recent (past 1-3 Gyr) average massaccretion rate into the Virgo Cluster, deriving a value of M~50Msolar yr-1.
|The UZC-SSRS2 Group Catalog|
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
|Galaxy Populations and Evolution in Clusters. I. Dynamics and the Origin of Low-Mass Galaxies in the Virgo Cluster|
Early-type dwarfs are the most common galaxy in the local universe, yettheir origin and evolution remain a mystery. Various cosmologicalscenarios predict that dwarf-like galaxies in dense areas are the firstto form and hence should be the oldest stellar systems in clusters. Byusing radial velocities of early-type dwarfs in the Virgo cluster wedemonstrate that these galaxies are not an old cluster population buthave signatures of production from the infall of field galaxies.Evidence of this includes the combined large dispersions andsubstructure in spatial and kinematic distributions for Virgo early-typedwarfs and a velocity dispersion ratio with giant ellipticals expectedfor virialized and accreted populations. We also argue that thesegalaxies cannot originate from accreted field dwarfs, but must havephysically evolved from a precursor population, of different morphology,that fell into Virgo some time in the past.
|Detailed Surface Photometry of Dwarf Elliptical and Dwarf S0 Galaxies in the Virgo Cluster|
We analyze new V-band images of 14 dwarf S0 galaxies and 10 dwarfelliptical galaxies in the Virgo Cluster, in combination with R-bandimages of 70 dwarf elliptical galaxies from an earlier paper. We computethe intensity-weighted mean ellipticity, the mean deviations fromelliptical isophotes, and a newly defined parameter to measure isophotaltwists. We also fit each major-axis profile to a power lawSigma(a)~exp[-(a/a_s)^n], where n is allowed to vary. Consistent withother studies of the Virgo dwarf ellipticals, we find that the profileshapes for the entire sample is strongly peaked near n=1 (exponentialprofiles) and that no galaxies have n=1/4 (de Vaucouleurs profile). Thefaintest galaxies all have nearly exponential profiles, while thebrighter ones on average have n<1. The correlation betweenellipticity and the boxy/disky parameter is similar to that of largeelliptical galaxies, suggesting that dwarfs may also be divided into twogroups with differing internal dynamics. The Virgo dEs also show agreater degree of isophotal twisting than more luminous ellipticalgalaxies. There does not seem to be any combination of parameters fromthe surface photometry that statistically correlates with the dE/dS0designation: in particular, the dS0 galaxies do not, on average, havemore pointed (disky) isophotes than the dEs.
|An image database. II. Catalogue between δ=-30deg and δ=70deg.|
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
|HI-observations of galaxies in the Virgo cluster of galaxies. I - The data|
New H I-data for a large number of bright galaxies inside the 10 degradius area of the Virgo cluster of galaxies have been obtained with the100 m radiotelescope at Effelsberg. A total of 234 galaxies was observedfor the first time. Among them, 53 have been detected providing newaccurate radial velocities. Data from the literature have been compiled.Together with the new data, they form a (nearly homogeneous) set of H Iobservations for more than 450 galaxies.
|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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