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|Discovery of Ultracompact Dwarf Galaxies in the Virgo Cluster|
We have discovered nine ultracompact dwarf galaxies (UCDs) in the VirgoCluster, extending samples of these objects outside the Fornax Cluster.Using the Two Degree Field (2dF) multifiber spectrograph on theAnglo-Australian Telescope, the new Virgo members were found among 1500color-selected, starlike targets with 16.0
|The Colors of Dwarf Elliptical Galaxy Globular Cluster Systems, Nuclei, and Stellar Halos|
We present the results of a Hubble Space Telescope WFPC2 F555W and F814Wsurvey of 69 dwarf elliptical galaxies (dEs) in the Virgo and FornaxClusters and Leo Group. The V-I colors of the dE globular clusters,nuclei, and underlying field-star populations are used to trace the dEstar formation histories. We find that the dE globular clustercandidates are as blue as the metal-poor globular clusters of the MilkyWay. The observed correlation of the dE globular cluster systems' V-Icolor with the luminosity of the host dE is strong evidence that theglobular clusters were formed within the halos of dEs and do not have apregalactic origin. Assuming that the majority of dE clusters are old,the mean globular cluster color-host galaxy luminosity correlationimplies a cluster metallicity-galaxy luminosity relation of~L0.22+/-0.05B, which issignificantly shallower than the field-star metallicity-host galaxyluminosity relationship observed in Local Group dwarfs(~L0.4). The dE stellar envelopes are0.1-0.2 mag redder in V-I than their globular clusters and nuclei. Thiscolor offset implies separate star formation episodes within the dEs forthe clusters and field stars, while the very blue colors of two dEnuclei trace a third star formation event in those dEs less than 1 Gyrago.
|Dynamical Friction in DE Globular Cluster Systems|
The dynamical friction timescale for globular clusters to sink to thecenter of a dwarf elliptical galaxy (dE) is significantly less than aHubble time if the halos have King-model or isothermal profiles and theglobular clusters formed with the same radial density profile as theunderlying stellar population. We examine the summed radial distributionof the entire globular cluster systems and the bright globular clustercandidates in 51 Virgo and Fornax Cluster dE's for evidence of dynamicalfriction processes. We find that the summed distribution of the entireglobular cluster population closely follows the exponential profile ofthe underlying stellar population. However, there is a deficit of brightclusters within the central regions of dE's (excluding the nuclei),perhaps due to the orbital decay of these massive clusters into the dEcores. We also predict the nuclear magnitude of each dE assuming thatthe nuclei form via dynamical friction. The observed trend of decreasingnuclear luminosity with decreasing dE luminosity is much stronger thanpredicted if the nuclei formed via simple dynamical friction processes.We find that the bright dE nuclei could have been formed from the mergerof orbitally decayed massive clusters, but the faint nuclei are severalmagnitudes fainter than expected. These faint nuclei are found primarilyin MV>-14 dE's, which have high globular cluster specificfrequencies and extended globular cluster systems. In these galaxies,supernova-driven winds, high central dark matter densities, extendeddark matter halos, the formation of new star clusters, or tidalinteractions may act to prevent dynamical friction from collapsing theentire globular cluster population into a single bright nucleus.
|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.
|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.
|A catalog of dwarf galaxies in Virgo|
A catalog listing the location, apparent angular diameter, type,estimated central light concentration, and estimated brightness of 846dwarf galaxies in a 200-deg-sq region in Virgo is presented. Thegalaxies comprise 634 ellipticals, 137 IC-3475-type galaxies, 73 dwarfspirals and irregulars, and two objects which are jets of normalgalaxies, and were found on nine long-exposure IIIa-J-emulsion platesmade with the 1.2-m-Schmidt telescope at Palomar Observatory from 1971to 1976. Concordances to other catalogs, tables of additionalparameters, maps, graphs, and photographs are provided. The projecteddistributions of normal and dwarf galaxies and the dependence ofapparent luminosity on central light concentration are discussed. It isfound that dwarf ellipticals and IC-3475-type galaxies are probablemembers of the Virgo cluster, while dwarf spirals and possibly dwarfirregulars are not.
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