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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.
|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.
|Companions of Bright Barred Shapley-Ames Galaxies|
Companion galaxy environment for a subset of 78 bright and nearby barredgalaxies from the Shapley-Ames Catalog is presented. Among the spiralbarred galaxies, there are Seyfert galaxies, galaxies with circumnuclearstructures, galaxies not associated with any large-scale galaxy cloudstructure, galaxies with peculiar disk morphology (crooked arms), andgalaxies with normal disk morphology; the list includes all Hubbletypes. The companion galaxy list includes the number of companiongalaxies within 20 diameters, their Hubble type, and projectedseparation distance. In addition, the companion environment was searchedfor four known active spiral galaxies, three of them are Seyfertgalaxies, namely, NGC 1068, NGC 1097, and NGC 5548, and one is astarburst galaxy, M82. Among the results obtained, it is noted that theonly spiral barred galaxy classified as Seyfert 1 in our list has nocompanions within a projected distance of 20 diameters; six out of 10Seyfert 2 bar galaxies have no companions within 10 diameters, six outof 10 Seyfert 2 galaxies have one or more companions at projectedseparation distances between 10 and 20 diameters; six out of 12 galaxieswith circumnuclear structures have two or more companions within 20diameters.
|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.
|1.65 μm (H-band) surface photometry of galaxies. VII. Dwarf galaxies in the Virgo Cluster|
We present near-infrared H-band (1.65 μm) observations and surfacebrightness profile decompositions for 75 faint (13.5 <~ mp<~ 18.5) galaxies, primarily taken among dwarf Ellipticals members ofthe Virgo cluster, with some Centaurus Cluster members, a BCD and twopeculiar galaxies taken as fillers. We model their surface brightnessprofiles with a de Vaucouleurs (D), exponential (E), mixed (bulge+diskor M) or truncated (T) law, and we derive for each galaxy the H bandeffective surface brightness (μe) and effective radius(re), the asymptotic total magnitude HT and thelight concentration index C31, defined as the ratio betweenthe radii that enclose 75% and 25% of the total light HT. Fora subsample we compare the NIR surface photometry with similar datataken in the B and V bands, and we give the B-H and B-V color profiles.Combining the present data with those previously obtained by our group(1157 objects) we analyze the NIR properties of a nearly completesample, representative of galaxies of all morphological types, spanning4 decades in luminosity. We confirm our earlier claim that the presenceof cusps and extended haloes in the light profiles (C31>5)is a strong, non-linear function of the total luminosity. We also findthat: i) among dE and dS0 galaxies D profiles are absent; 50% of thedecompositions are of type M, the remaining being of type E or T. ii)Less than 50% of the giant elliptical galaxies have pure D profiles, themajority being represented by M profiles. iii) Most giant galaxies (fromelliptical to Sb) have M profiles. iv) Most of late type spirals (Scd toBCD) have either E or T profiles. v) The type of decomposition is astrong function of the total H band luminosity, independent of theHubble classification: the fraction of type E decompositions decreaseswith increasing luminosity, while those of type M increase withluminosity. Pure D profiles are absent in the low luminosity rangeLH<1010 Lsolar and become dominantabove 1011 Lsolar, while T profiles are presentonly among low luminosity galaxies. vi) We find that dE-peculiargalaxies have structural parameters indistinguishable from those oflate-type dwarfs, thus they might represent the missing link between dEsand dIs. Based on observations taken with the ESO/NTT (ESO program64.N-0288), with the Telescopio Nazionale Galileo (TNG) operated on theisland of La Palma by the Centro Galileo Galilei of the CNAA at theSpanish Observatorio del Roque de los Muchachos of the IAC, with the SanPedro Martir 2.1~m telescope of the Observatorio Astronomico Nacional(OAN, Mexico), and with the OHP 1.2~m telescope, operated by the FrenchCNRS.
|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.
|On the local radio luminosity function of galaxies. I. The Virgo cluster|
We cross-correlate the galaxies brighter than m_B=18 in the Virgocluster with the radio sources in the NVSS survey (1.4 GHz), resultingin 180 radio-optical identifications. We determine the radio luminosityfunction of the Virgo galaxies, separately for the early- andlate-types. Late-type galaxies develop radio sources with a probabilityproportional to their optical luminosity. In fact their radio/optical(R_B) distribution is gaussian, centered at log R_B ~ -0.5, i.e. theradio luminosity is ~ 0.3 of the optical one. The probability oflate-type galaxies to develop radio sources is almost independent oftheir detailed Hubble type, except for Sa (and S0+S0a) which are afactor of ~ 5 less frequent than later types at any R_B. Giantelliptical galaxies feed ``monster" radio sources with a probabilitystrongly increasing with mass. However the frequency of fainter radiosources is progressively less sensitive on the system mass. The faintestgiant E galaxies (M_B=-17) have a probability of feeding low power radiosources similar to that of dwarf E galaxies as faint as M_B=-13. Table~1is only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
|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.
|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.
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