Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
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 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
| Total Magnitudes of Virgo Galaxies. II. An Investigation into the mp Scale of Volume I of Zwicky et al.'s Catalogue of Galaxies and of Clusters of Galaxies
We investigate the photographic-magnitude (mp) scale ofVolume I of the Catalogue of Galaxies and of Clusters of Galaxies forgalaxies in the Virgo direction. It is found that for 11.5 <= Bt <14 5, the mp values listed correspond very closely (with ascatter of only 0.16 magnitude) to blue magnitudes measured to theμB = 24 4 isophote. If mp values need to beused as estimates of total blue magnitude the scatter is 0.27 magnitudeover the same range in Bt and the mean offset ismp-Bt=0 28 magnitude. However, a serious scaleerror at the bright end causes both the isophotal and total luminositiesof galaxies brighter than Bt~11.5 to be severelyunderestimated. At the faint end there also appears to be a significantscale error. This causes galaxy luminosities to be seriouslyoverestimated faintward of Bt ~ 14.5. We demonstrate thatthis is a real effect based on a detailed galaxy-by-galaxy study of thecatalogue's completeness down to Bt= 17.0. The catalogue isfound only to be complete to Bt ~ 14.7 whilst its degree ofincompleteness is found to be relatively constant at the 30% level overthe range 14.75 <= Bt < 15.75. Most of the missingobjects are found to be elliptical galaxies and so late-type objects areover-represented at the faint end.
| 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.
| 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.
| Virgo dwarfs - New light on faint galaxies
Photographically amplified UK Schmidt plates have been used to define anew sample of low surface brightness (LSB) galaxies in the VirgoCluster. One of these galaxies is the largest, most gas-rich spiralgalaxy known, located well beyond the Virgo Cluster. The rest of the 137galaxies appear to be dwarf ellipticals. CCD photometry shows that thefaint galaxies are well modeled by an exponential radial profile. Thecolors of very LSB galaxies in Virgo are unusually blue in B-V, bluerthan the metal-poor Galactic globular clusters. If these galaxies do notviolate the mass-metallicity relation, then they must have smaller meanages than Galactic globulars. It is confirmed that any faint galaxysurvey tends to choose objects of a luminosity and surface brightnessthat give them the maximum angular size on the discovery material.
| 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.
| The distribution of Sculptor-type dwarf galaxies.
Not Available
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
Каталог и designations:
|
