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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.
| Distances, Metallicities, and Ages of Dwarf Elliptical Galaxies in the Virgo Cluster from Surface Brightness Fluctuations
We have employed FORS1 and 2 at the Very Large Telescope at the EuropeanSouthern Observatory (ESO) to acquire deep B and R-band CCD images of 16dwarf elliptical (dE) galaxies in the direction of the Virgo cluster.For each dwarf, we measure the apparent R-band surface brightnessfluctuation (SBF) magnitude mR and the (B-R)0color in a number of fields at different galactocentric distances. Fromthe field-to-field variation of the two quantities, we determine the SBFdistance by means of the (B-R)0-MR relation. Thederived distances of the dwarfs range from 14.9 to 21.3 Mpc, with a mean1 σ uncertainty of 1.4 Mpc or 8% of the distance, which confirmsthat there is considerable depth in the distance distribution ofearly-type cluster members. For VCC 1104 (IC 3388), our SBF distancemodulus of (m-M)SBF=31.15+/-0.19 (17.0+/-1.5 Mpc) is in goodagreement with the Harris et al result of(m-M)TRGB=30.98+/-0.19 mag (15.7+/-1.5 Mpc) based on HubbleSpace Telescope (HST) observations and the tip magnitude of the redgiant branch. Combining our results with existing distances for giantVirgo ellipticals, we identify two major galaxy concentrations in thedistance distribution: a broad primary clump around (M-m)=31.0 mag (15.8Mpc) and a narrow secondary clump around 31.33 mag (18.5 Mpc). Anadaptive kernel analysis finds the two concentrations to be significantat the 99% (2.5 σ) and 89% (1.6 σ) levels, respectively.While the near-side clump of Virgo early-type galaxies can be associatedwith the subcluster centered on M87, the second clump is believed to bemainly due to the far side infalling group of galaxies around M86. Theages and metallicities of the dE stellar populations are estimated bycombining the observed (B-R)0 colors with Worthey's stellarpopulation synthesis models. It appears that the Virgo dE galaxies covera wider range in metallicity, from [Fe/H]~-1.4 (VCC 0815) to -0.5 (NGC4415), than Fornax cluster dEs. The derived metallicities place theVirgo dEs on the extension of the metallicity-luminosity relationdefined by the low-luminosity Local Group dEs. The data further suggestan age range from genuinely old (~17 Gyr) stellar systems such as IC3019 and IC 0783 to intermediate-age (8-12 Gyr) dwarfs such as NGC 4431and IC 3468.
| VLT surface photometry and isophotal analysis of early-type dwarf galaxies in the Virgo cluster
We have carried out surface photometry and an isophotal analysis for asample of 25 early-type dwarf (dE and dS0) galaxies in the Virgo clusterbased on CCD images taken at the VLT with FORS1 and FORS2. For eachgalaxy we present B and R-band surface brightness profiles, as well asthe radial colour (B-R) profile. We give total apparent BR magnitudes,effective radii, effective surface brightnesses and total colourindices. The light profiles have been fitted with Sérsic modelsand the corresponding parameters are compared to the ones for otherclasses of objects. In general, dEs and dS0s bridge the gap in parameterspace between the giant ellipticals and the low-luminosity dwarfspheroidals in the Local Group, in accordance with previous findings.However, the observed profiles of the brightest cluster dwarfs showsignificant deviations from a simple Sérsic model, indicatingthat there is more inner structure than just a nucleus. This picture isreinforced by our isophotal analysis where complex radial dependenciesof ellipticity, position angle, and isophotal shape parameter a_4 areexhibited not only by objects like IC 3328, for which the presence of adisk component has been confirmed, but by many apparently normal dEs aswell. In addition, we find a relation between the effective surfacebrightness, at a given luminosity, and the strength of the offset of thegalaxy's nucleus with respect to the center of the isophotes. Dwarfswith large nuclear offsets also tend to have stronger isophotal twists.However, such twists are preferentially found in apparently round(epsilon < 0.3) galaxies and are always accompanied by significantradial changes of the ellipticity, which clearly points to a projectioneffect. In sum, our findings suggest the presence of substructure inmost, and preferentially in the less compact, bright early-type dwarfs.The physical (dynamical) meaning of this has yet to be explored.Based on observations collected at the European Southern Observatory(ESO 63.O-0055 and 65.N-0062).Figure \ref{fig1} is only available in electronic form athttp://www.edpsciences.org
| 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
| Hα surface photometry of galaxies in the Virgo cluster III. Observations with INT and NOT 2.5 m telescopes
We present Hα line imaging observations of 30 galaxies obtained atthe 2.5 m INT and NOT telescopes. The observed galaxies are mostly BCDVirgo cluster galaxies. Hα +[NII] fluxes and equivalent widths, aswell as images of all the detected targets are presented. With theseobservations, Hα data are available for =~ 50% of the BCD galaxieslisted in the VCC. Based on observations taken at the INT and NOTtelescopes, operated on the island of La Palma by the ING and NOT teamsin the Spanish Observatorio del Roque de Los Muchachos of the Institutode AstrofÃsica de Canarias. Figure 1 is only available inelectronic form at http://www.edpsciences.org
| 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.
| 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).
| 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.
| 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.
| 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.
| 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.
| 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.
| On the Largescale Distribution of Faint Blue Galaxies
Not Available
| Faint blue galaxies in the Virgo cluster region
Finding charts, coordinates, B magnitudes, U-B color estimates, andsurface brightnesses are given for 110 faint blue galaxies located nearthe center of the Virgo cluster of galaxies. The galaxies were found byblinking on plates taken with the Tautenburg 52-in. Schmidt telescope.
| 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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