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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 structure of elliptical galaxies in the Virgo cluster. Results from the INT Wide Field Survey
We report on a complete CCD imaging survey of 226 elliptical galaxies inthe North-East quadrant of the Virgo cluster, representative of theproperties of giant and dwarf elliptical galaxies in this cluster. Wefit their radial light profiles with the Sersic r1/n model oflight distribution. We confirm the result of Graham & Guzman(\cite{Graham03}, AJ, 125, 2936) that the apparent dichotomy between Eand dE galaxies in the luminosity-< μ>e plane nolonger appears when other structural parameters are considered and canbe entirely attributed to the onset of ``core'' galaxies atBT -20.5 mag. When ``core'' galaxies are notconsidered, E and dE form a unique family with n linearly increasingwith the luminosity. For 90 galaxies we analyze the B-I color indices,both in the nuclear and in the outer regions. Both indices are bluertoward fainter luminosities. We find also that the outer color gradientsdo not show any significant correlation with the luminosity. The scatterin all color indicators increases significantly toward lowerluminosities, e.g. galaxies fainter than BT -15 have aB-I spread > 0.5 mag.Table 2, Figs. 13 and 14 are only available in electronic form athttp://www.edpsciences.org
| 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.
| The faint end of the galaxy luminosity function
We present and discuss optical measurements of the faint end of thegalaxy luminosity function down to MR=-10 in five differentlocal environments of varying galaxy density and morphological content.The environments we studied, in order of decreasing galaxy density, arethe Virgo Cluster, the NGC 1407 Group, the Coma I Group, the Leo Groupand the NGC 1023 Group. Our results come from a deep wide-angle surveywith the National Astronomical Observatories of Japan Subaru 8-mTelescope on Mauna Kea and are sensitive down to very faintsurface-brightness levels. Galaxies were identified as group or clustermembers on the basis of their surface brightness and morphology. Thefaintest galaxies in our sample have R~ 22.5. There were thousands offainter galaxies but we cannot distinguish cluster members frombackground galaxies at these faint limits so do not attempt to determinea luminosity function fainter than MR=-1010. In all cases,there are far fewer dwarfs than the numbers of low-mass haloesanticipated by cold dark matter theory. The mean logarithmic slope ofthe luminosity function between MR=-1018 andMR=-1010 is α~=-1.2, far shallower than the cold darkmatter mass function slope of α~=-1.8. We would therefore need tobe missing about 90 per cent of the dwarfs at the faint end of oursample in all the environments we study to achieve consistency with CDMtheory. It is unlikely that such large numbers of dwarfs are missedbecause (i) the data are deep enough that we are sensitive to very lowsurface brightness galaxies, and (ii) the seeing is good enough that wecan have some confidence in our ability to distinguish high surfacebrightness dwarfs from background galaxies brighter than R= 22.5. Onecaveat is that we miss compact members taken to be background galaxies,but such objects (like M32) are thought to be rare.
| 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
| 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.
| Distances to 64 Virgo dwarf-elliptical galaxies and the depth in their spatial distribution
We derive distances for 64 dwarf ellipticals (dEs) in the direction ofthe Virgo cluster's (VC) core, by means of the luminosity-profilecurvature (L-n) relationship and by means of their global scalelengths,which we find to be correlated with the shapes of theirsurface-brightness profiles. The great depth we find in the spatialdistribution of Virgo dEs is not consistent with a unimodal distributiondue to a single spherically symmetric concentration of galaxies. Thisdepth is also sufficient to explain much of the disagreement over theVC's distance, and thereby much of the Hubble-constant (H_0)controversy.
| The kinematics of the Virgo cluster revisited
The paper updates the velocity data of Virgo cluster galaxies andreconsiders the kinematic structure of the Virgo cluster. New velocitiesare given for 144 galaxies listed in the Virgo Cluster Catalog (VCC).Improved velocities are given for another 131 VCC galaxies. The Virgocluster is disentangled from its surrounding clouds of galaxies, and thelikely members of each of these clouds are listed. The velocitydistribution of dwarf elliptical cluster members is found to be highlyasymmetric. This phenomenon is interpreted as evidence for the imminentmerging of two subclusters in the core region, which points to thedynamical youth of the Virgo cluster. The mean heliocentric velocity ofthe Virgo cluster is estimated at 1050 +/- 35 km/s.
| Extreme blue compact dwarf galaxies in the Virgo Cluster
CCD images and surface photometry are presented for six extreme(especially compact) blue compact dwarf (BCD) galaxies in the VirgoCluster. The photometric parameters of the galaxies are used to studytheir possible fate once star formation has ended. It is found that theymay fade to form dwarf elliptical galaxies, as has previously beensuggested, but the dE galaxies formed from these BCDs would lie amongthe most compact in existing samples. Two of the BCDs studied have nosurrounding low-surface-brightness envelopes detected to a limit ofmu(v) = 27 mag/sq arcsec.
| Medium-resolution spectroscopy of nucleated dwarf elliptical galaxies
The results of medium-resolution spectroscopy of a small sample ofnucleated dwarf elliptical (dE) galaxies in the Virgo Cluster arepresented. The velocity dispersion of the dEs is as large as it is forthe dwarf irregulars and blue compact dwarfs. The velocity distributionis not similar to that which holds for the luminous Es and SOs in Virgo.A significant percentage of dEs are at quite low velocity. The Balmerline strength in the dE nuclei never exceeds 2.5 A. Overall, the spectramost closely resemble the spectra of metal-rich Galactic globularclusters with possibly an extended main sequence due to a younger meanage.
| 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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