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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.
|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.
|The Nuclear Cusp Slopes of Dwarf Elliptical Galaxies|
We derive the light profiles for a sample of 25 dwarf ellipticalgalaxies observed by us with Hubble Space Telescope's Wide Field andPlanetary Camera 2 in F555W and F814W. These profiles are fitted withNuker, R1/4, exponential, and Sersic laws and are also usedto derive the nuclear cusp slopes γ. We discuss the correlation ofnuclear cusp slope with galactic luminosity, the presence of a nucleus,and the type of light profile. The results are compared with those foundin the literature for elliptical galaxies and the bulges of spiralgalaxies. We find that, as a class, the nuclear regions of dwarfellipticals are very similar to those of the exponential bulges ofspiral galaxies and have nuclear cusp slopes shallower than those ofbulges with the same luminosity that were well fitted by a deVaucouleurs R1/4 profile. For the 14 nucleated galaxies inour sample, this conclusion is less certain than for the 11 nonnucleatedobjects, since it relies on an extrapolation of galaxy light under thenucleus. In terms of their light profiles and nuclear properties, mostspheroidal stellar systems can be broadly divided into two subclasses:the exponential shallow cusp objects and the R1/4 steep cuspobjects. Membership of a class does not appear to correlate with thepresence of a massive stellar disk. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.
|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.
|The Specific Globular Cluster Frequencies of Dwarf Elliptical Galaxiesfrom the Hubble Space Telescope|
The specific globular cluster frequencies (S_N) for 24 dwarf elliptical(dE) galaxies in the Virgo and Fornax Clusters and the Leo Group thatwere imaged with the Hubble Space Telescope are presented. Combining allavailable data, we find that for nucleated dE (dE, N) galaxies, whichare spatially distributed like giant elliptical galaxies in galaxyclusters, S_N(dE, N)=6.5+/-1.2 and S_N increases with M_V, while fornonnucleated dE (dE, noN) galaxies, which are distributed like late-typegalaxies, S_N(dE, noN)=3.1+/-0.5 and there is little or no trend withM_V. Thus, the S_N values for dE galaxies are, on average, significantlyhigher than those for late-type galaxies, which have S_N<~1. Thissuggests that dE galaxies are more akin to giant elliptical galaxiesthan to late-type galaxies. If there are dormant or stripped irregulargalaxies hiding among the dE population, they are likely to be among thenonnucleated dE galaxies. Furthermore, the similarities in theproperties of the globular clusters (GCs) and in the spatialdistributions of dE, N galaxies and giant elliptical galaxies suggestthat neither galaxy mass nor galaxy metallicity is responsible for thehigh values of S_N. Instead, most metal-poor GCs may have formed indwarf-sized fragments that merged into larger galaxies.
|The flattening distribution of dwarf elliptical galaxies in the Virgo Cluster|
We have obtained R-band surface photometry of 70 dwarf ellipticalgalaxies in the Virgo Cluster. We find, in contrast to the results ofearlier studies, that the dwarfs have a markedly flatter distribution ofellipticities than either 'normal' elliptical galaxies or brightestcluster ellipticals. The ensemble of nucleated dwarfs is rounder thanthe non-nucleated galaxies. Neither the nucleated nor the nonnucleateddwarfs, however, have distributions as round as giant 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.
|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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