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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.
| Spectrophotometry of Galaxies in the Virgo Cluster. I. The Star Formation History
As a result of an extensive observational campaign targeting the VirgoCluster, we obtained integrated (drift-scan mode) optical spectra andmultiwavelength (UV, U, B, V, H) photometry for 124 and 330 galaxies,respectively, spanning the whole Hubble sequence, and withmp<=16(Mp<=-15). These data were combined toobtain galaxy Spectral Energy Distributions (SEDs) extending from 2000to 22000 Å. By fitting these SEDs with synthetic ones derivedusing Bruzual & Charlot population synthesis models we try toconstrain observationally the star formation history (SFH) of galaxiesin the rich cluster of galaxies nearest to us. Assuming a Salpeter IMFand an analytical form for the SFH, the fit free parameters are the age(T) of the star formation event, its characteristic timescale (τ),and the initial metallicity (Z). In this work we test the (simplistic)case in which all galaxies have a common age T=13 Gyr, exploring a SFHwith ``delayed'' exponential form (which we call ``a la Sandage''), thusallowing for an increasing SFR with time. This SFH is consistent withthe full range of observed SEDs, provided that the characteristictimescale τ is let free to vary between 0.1 (quasi-instantaneousburst) and 25 Gyr (increasing SFR) and Z between 1/50 and 2.5 Zsolar.Elliptical galaxies (including dEs) are best fitted with shorttimescales (τ~3 Gyr) and metallicity varying between 1/5 and Zsolar.The model metallicity is found to increase as a function of H-bandluminosity. Spiral galaxies require that both τ and metallicitycorrelate with H-band luminosity: low-mass Im+BCD have subsolar Z andτ>=10 Gyr, whereas giant spirals have solar metallicities andτ~3 Gyr, consistent with elliptical galaxies. Moreover, we find thatthe SFH of spiral galaxies in the Virgo Cluster depends upon thepresence at their interior of fresh gas capable of sustaining the starformation. In fact, the residuals of the τ vs. LHrelation depend significantly on the H I content. H I deficient galaxieshave shorter (up to a factor of 4) τ (truncated SFH) than spiralswith normal H I content. Based on observations collected at theObservatoire de Haute Provence (OHP) (France), operated by the CNRS,France, and at the European Southern Observatory (Chile) (programme66.B-0026).
| 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.
| 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.
| 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.
| Near Infrared Imaging of Dwarf Ellipticals Irregulars and Blue Compact Galaxies in the Virgo Cluster
New near-IR images are presented for 13 dwarf galaxies in the Virgocluster. Together with previous data these provide a data base of JHKimaging for 26 dwarf ellipticals (dEs), dwarf irregulars (dIs) and bluecompact dwarfs (BCDs). These images show the dIs to be highly asymmetricand unrelaxed, implying that they are dynamically young and unevolved.This is consistent with their blue near-IR and optical-IR colours whichare most easily explained by young stellar populations. The dEs aresymmetrical and apparently relaxed, with very uniform colours indicatingthat they are dominated by old stars. They generally have exponentiallight profiles, but the brighter galaxies tend to exhibit more cuspedlight distributions, similar to the de Vaucouleurs profiles of brightellipticals. The BCDs have moderately asymmetric light profiles, andparadoxically red colours, possibly indicating an intermediate-agestellar population. They are probably dEs which have undergone bursts ofstar formation in the last few X 10^9^ years, whilst the dIs are afundamentally distinct population. Colour gradients are present in manyof the galaxies, invariably in the sense that the nuclei are redder thanthe surrounding galaxy light.
| 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.
| The surface brightness test for the expansion of the universe. II - Radii, surface brightness, and absolute magnitude correlations for nearby E galaxies
Data for elliptical galaxies in the Virgo, Fornax, and Coma clusters andin the general field are analyzed in order to determine the dispersionin average surface brightness. The data are discussed using measures ofboth the effective radius and the Petrosian r(eta) radii. The dispersionis found to be about 0.5 mag after reducing the data to absolutemagnitude M(B) = -22. This value is smaller than the 1.8 mag Tolman (1 +z) exp 4 factor, even at the modest redshift of z = 0.5, showing thatthe Tolman test is feasible in practice as well as in principle.
| Surface photometry of dwarf elliptical galaxies in the Virgo cluster
Photographic surface photometry is carried out for 69 dwarf ellipticalsin the central region of the Virgo Cluster, using two plates in the Bband taken with the 2.5 m du Pont telescope at Las Campanas Observatory.Luminosity profiles and various photometric parameters are obtained, andthe following three results are derived: (1) All the dwarf ellipticalsin the sample have luminosity profiles consistent with exponential lawsexcept for six galaxies which are probably giant ellipticals in thebackground. If they are physcial members of the Virgo Cluster, they maybe identified with 'classical' dwarf ellipticals recognized by Wirth andGallagher. (2) The distribution of apparent flattenings of dwarfellipticals suggests that the population of dwarf ellipticals is notdominated by flat disk systems like spiral galaxies, in spite of theirexponential profiles. (3) Two important diagrams, a diagram ofconcentration index versus absolute magnitude and a diameter versussurface brightness diagram (DSBD), show a hint of structuraldiscontinuity in the sequence of spheroidal stellar systems consistingof giant ellilpticals, dwarf ellipticals, and globular clusters.
| 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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