目录
图像
上传图像
DSS Images Other Images
相关文章
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.
| Interstellar Gas in the NGC 4410 Galaxy Group
We present new radio continuum, 21 cm H I, and 2.6 mm CO data for thepeculiar radio galaxy NGC 4410A and its companion NGC 4410B and comparewith available optical and X-ray maps. Our radio continuum maps show anasymmetric double-lobed structure, with a high surface brightness lobeextending 3.6′ (~100 kpc) to the southeast and a 6.2′(~180 kpc) low surface brightness feature in the northwest. Moleculargas is abundant in NGC 4410A, withMH2~4×109 Msolar(using the standard Galactic conversion factor) but is undetected in NGC4410B. H I is less abundant, with MHI~109Msolar for the pair. Our H I map shows a3×108 Msolar H I tail extending 1.7′(50 kpc) to the southeast of the pair, coincident with a faint opticaltail and partially overlapping with the southeastern radio lobe. The H Itail is anticoincident with a 2' (56 kpc) long X-ray structure alignedwith a stellar bridge that connects the pair to a third galaxy. If thisX-ray emission is associated with the group, we infer(3-8)×108 Msolar of hot gas in this feature.This may be either intracluster gas or shocked gas associated with thebridge. Our detection of abundant interstellar gas in this pair,including an H I-rich tidal tail near the southeastern radio lobe,suggests that the observed distortions in this lobe may have been causedby the interstellar medium in this system. The gravitational interactionof the two galaxies and the subsequent motion of the interstellar mediumin the system relative to the jet may have produced sufficient rampressure to bend and distort the radio jet. An alternative hypothesis isthat the jet was distorted by ram pressure due to an intraclustermedium, although the small radial velocity of NGC 4410A relative to thegroup and the lack of diffuse X-ray emission in the group makes thisless likely unless the group is not virialized or is in the process ofmerging with another group. Using our VLA data, we also searched for H Icounterparts to the other 10 known members of the NGC 4410 group and COfrom three other galaxies in the inner group. In our velocity range of6690-7850 km s-1, we detected six other galaxies above our HI sensitivity limits of 2×108 Msolar for theinner group and 4×108 Msolar for the outergroup. The total H I in the group is 1.4×1010Msolar, 80% of which arises from four galaxies in the outergroup. Three of these galaxies (VCC 822, VCC 831, and VCC 847) arespirals with MHI/LB ratios typical of fieldgalaxies, while FGC 170A appears to be a gas-rich dwarf galaxy(MB~-18, MHI~3×109Msolar). In the inner group, the SBa galaxy NGC 4410D (VCC934) was detected in H I and CO (MHI~5×108Msolar and MH2~8×108Msolar) and has a 1' (28 kpc) long H I tail that pointstoward the nearby disk galaxy NGC 4410F. NGC 4410F was also detected inH I (MHI~4×108 Msolar). Thegalaxies in the inner group appear to be somewhat deficient in H Icompared to their blue luminosities, suggesting phase changes driven bygalaxy-galaxy or galaxy-intracluster medium encounters.
| 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.
| 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.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
