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|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.
|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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