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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.
|Nucleated dwarf elliptical galaxies in the Virgo cluster|
Using images from a charge-coupled device survey with the Wide FieldCamera on the Isaac Newton Telescope, we performed B- and I-bandphotometry on 156 Virgo cluster dwarf elliptical (dE) galaxies, 25candidate new cluster dwarfs, and nine candidate field dwarfs. Galaxieswere modelled with Sérsic profiles, using both 1Dχ2 and 2D cross-correlation methods, with nuclei modelledas point sources. The intensity profiles of 50 galaxies previouslyclassified as dE, dE?, or ? are more accurately fitted if a nucleus isincluded, and this results in the majority of dwarfs now beingclassified as nucleated dwarf ellipticals (dE,N). Some faint galaxieswith B magnitudes of 18-21 have particularly large relative nuclei,while a small number have apparent central dimmings. For cluster dE,Ngalaxies the nucleus magnitude is correlated with the magnitude of thehost galaxy. The profile parameters of dE and dE,N galaxies are notsignificantly different, and there is no evident discontinuity inrelative nucleus size between non-nucleated and nucleated dwarfs,suggesting that they may form a continuum. Nuclei are on average redderthan their underlying galaxies, though a spread of relative colours wasfound, and two-fifths of nuclei are bluer. Formation mechanisms ofnuclei are discussed: at least some appear to have formed in an alreadyexisting non-nucleated galaxy, though others may have formedsimultaneously with their galaxies and subsequently evolved within them.
|The dwarf low surface brightness galaxy population of the Virgo Cluster - II. Colours and HI line observations|
In order to investigate the nature of dwarf low surface brightness (LSB)galaxies we have undertaken a deep B- and I-band CCD survey of a14-deg2 strip in the Virgo Cluster and applied a Fourierconvolution technique to explore its dwarf galaxy population down to acentral surface brightness of ~26 B magarcsec-2 and a totalabsolute B mag of ~-10. In this paper we carry out an analysis of theirmorphology, (B-I) colours and atomic hydrogen content. We compare theseproperties with those of dwarf galaxies in other environments to try andassess how the cluster environment has influenced their evolution. Fielddwarfs are generally of a more irregular morphology, are bluer andcontain relatively more gas. We assess the importance that variousphysical processes have on the evolution of cluster dwarf galaxies(ram-pressure stripping, tidal interactions, supernova-driven gas loss).We suggest that enhanced star formation triggered by tidal interactionsis the major reason for the very different general properties of clusterdwarfs: they have undergone accelerated evolution.
|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.
|The luminosity function of the Virgo Cluster from MB=-22 to -11|
We measure the galaxy luminosity function (LF) for the Virgo Clusterbetween blue magnitudes MB=-22 and -11 from wide-fieldcharge-coupled device (CCD) imaging data. The LF is only graduallyrising for -22
|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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