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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.
| Off-center nuclei in dwarf elliptical galaxies
We have searched for off-center nuclei in 78 ``nucleated'' dwarfelliptical (dE,N) galaxies, drawing on digitized photographic imagesfrom a previous study of Virgo cluster dwarfs. The search is based on asimple algorithm which compares the center coordinates of a series ofouter elliptical isophotes with the position of the galaxy's nucleus.Monte Carlo simulations of the measuring procedure are used to assessrandom and systematic errors. Roughly 20% of all dwarf nuclei in thesample (neglecting uncertain cases) are found to be significantlyoff-centered. The typical displacement is 1arcsec , or 100 pc (assuminga Virgo cluster distance of 20 Mpc), corresponding to 0.5 to 1 effectiveradii of the dwarf galaxy. There is a tendency of the nuclear off-centerdisplacement to increase with decreasing surface brightness of theunderlying galaxy. A similar trend was found with normal ellipticalgalaxies before. If real, the effect could mean that a nucleus canoscillate about the galaxy center with larger amplitude in a shallower(less cuspy) gravitational potential. In an appendix we present evidencefor the existence of a strong, unambiguous relation between the nuclearmagnitude and the ellipticity of dE,N galaxies. If a nucleus iscomprising 4% or more of the total light of the underlying galaxy, thatgalaxy is nearly always round, i.e. ellipticity less than 0.15 (dE0,dE1). This effect was predicted qualitatively long ago as the result ofbox orbit disruption caused by a central massive compact object (blackhole).
| Far infrared and Ultraviolet emissions of individual galaxies at z=0: selection effects on the estimate of the dust extinction
We have cross-correlated Far Infrared (IRAS) and UV (FOCA) observationsof galaxies to construct a sample of FIR selected galaxies with a UVobservation at 0.2 mu m. The FIR and UV properties of this sample arecompared to the mean properties of the local Universe deduced from theluminosity distributions at both wavelengths. Almost all the galaxies ofour sample have a FIR to UV flux ratio larger than the ratio of the FIRand UV luminosity densities, this effect becoming worse as the galaxiesbecome brighter: the increase of the UV (0.2 mu m) extinction is about0.5 mag per decade of FIR (60 mu m) luminosity. Quantitative starformation rates are estimated by adding the contribution of the FIR andUV emissions. They are found consistent with the corrections forextinction deduced from the FIR to UV flux ratio. A total localvolume-average star formation rate is calculated by summing thecontribution of the FIR and UV wavelengths bands. Each band contributesfor an almost similar amount to the total star formation rate with rhoSFR = 0.03 +/- 0.01 h * Msun/yr/Mpc3 at z=0. Thisis equivalent to a global extinction of 0.75 mag to apply to the localluminosity density at 0.2 mu m. The trend of a larger FIR to UV fluxratio for a larger FIR luminosity found for our sample of nearbygalaxies is extended and amplified toward the very large FIRluminosities when we consider the galaxies detected by ISOCAM in a CFRSfield and the Ultra Luminous Infrared Galaxies at low and high redshift.A UV extinction is tentatively estimated for these objects.
| Is the shape of the luminosity profile of dwarf elliptical galaxies an useful distance indicator?
The shape of the surface brightness profile of dE galaxies, quantifiedby parameter n of Sersic's generalized profile law, has recently beenput forward as new extragalactic distance indicator (Young & Currie1994). Its application to the Virgo cluster has subsequently led to theclaim that the Virgo dEs are not lying in the cluster core but aredistributed in a prolate structure stretching from 8 to 20 Mpc distance(Young & Currie 1995). This claim is refuted here. We have fitted aSersic law to the surface brightness profiles of 128 Virgo cluster dEsand dS0s from the photometry of Binggeli & Cameron (1991). Thedispersion of the n - M relation is indeed large (sigma_rms ~ 0.9 mag).However, we argue that this scatter is not due to the depth of the Virgocluster, but is essentially intrinsic. Contrary to what one would expectfrom the cluster depth hypothesis, there is no clearvelocity-``distance'' relation for a sample of 43 Virgo dEs and dS0swith known redshifts. The analysis of Young & Currie (1995) ishampered by the use of low-resolution photometry and flawed by theassumption that the n - M and n - R relations can be used independently.By combining different Sersic law parameters, the scatter of the scalingrelations can be reduced somewhat, but never below sigma_rms ~ 0.7 mag,at least for the Virgo cluster. For the purpose of distancemeasurements, this falls short of the well-established Tully-Fisher andD_n - sigma methods, and it is comparable to what one can get alreadyfrom the < mu >_eff - M relation for dEs, which does not requireany profile modelling.
| Optical identifications of the IRAS faint sources in the Virgo cluster area.
This paper presents a study on the deep optical identifications of 369IRAS faint sources in a 102 square degree area centered on the Virgocluster. We obtained the positions and magnitudes of candidates fromfour UK Schmidt Telescope IIIa-J direct plates. 89 (24%) are identifiedas stars and 276 (75%) as galaxies. There are 4 (1%) empty fields to theplate limit of B=~22. The infrared-optical database for 193 FSC-onlysources is given. Most of the IRAS galaxy sources are spirals. There are83 IRAS galaxy sources being regarded as the cluster members by Binggeliet al. (1985). In our sample, a correlation between the luminosity ratioL_IR_/L_B_ and the infrared flux ratio F(100μm)/F(60μm) isevident. For the member galaxies, a weak correlation between L_IR_/L_B_and L_IR_ is also found.
| Dwarf galaxies in the Virgo cluster. II - Photometric techniques and basic data
Results are presented of photographic surface photometry carried out for305 (mostly dwarf) galaxies in the Virgo cluster, in which the galaxyimages were digitized on 14 of the 67 du Pont plates used for the Virgocluster survey. Azimuthally averaged surface brightness profiles areshown for all galaxies. The following model-free photometric parametersare derived and listed for each galaxy: total apparent blue magnitude,mean effective radius and surface brightness, and various isophotalradii, ellipticity, and position angle. Most galaxies were fitted by anexponential form and/or a King model profile. The best-fittingparameters, including the 'nuclear' (central residual) magnitudes fordE+dS0 galaxies, are listed.
| Dwarf galaxies in the Virgo cluster. I - The systematic photometric properties of early-type dwarfs
The azimuthally averaged surface brightness profiles of 200 faintearly-type Virgo cluster galaxies have been analyzed. Faint dwarfs arevery well described by an exponential or a King model. The magnitudes ofthe nuclei vary greatly at a given galaxian magnitude, but the maximumnuclear luminosity is a strong function of M(T). In the 0.1-1 kpc radiusrange, the logarithmically plotted profiles of all early-type galaxiescome in two well-defined classes identified with classical types versusdwarf types. The former are all classified E or S0, while the lattercomprise all galaxies classified dE or dS0, all morphologically'intermediate' types, and even two classified 'E'. The mean SB profilesof dS0 galaxies are indistinguishable from bright dE profiles. In 2D,the dS0s appear highly flattened and/or show asymmetric and irregularfeatures which may indicate their disk nature.
| 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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