Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the Nucleus
We analyze the idea that nuclear activity, either active galactic nuclei(AGNs) or star formation, can be triggered by interactions by studyingthe percentage of active, H II, and quiescent galaxies with companions.Our sample was selected from the Palomar survey and avoids selectionbiases faced by previous studies. This sample was split into fivedifferent groups, Seyfert galaxies, LINERs, transition galaxies, H IIgalaxies, and absorption-line galaxies. The comparison between the localgalaxy density distributions of the different groups showed that in mostcases there is no statistically significant difference among galaxies ofdifferent activity types, with the exception that absorption-linegalaxies are seen in higher density environments, since most of them arein the Virgo Cluster. The comparison of the percentage of galaxies withnearby companions showed that there is a higher percentage of LINERs,transition galaxies, and absorption-line galaxies with companions thanSeyfert and H II galaxies. However, we find that when we consider onlygalaxies of similar morphological types (elliptical or spiral), there isno difference in the percentage of galaxies with companions amongdifferent activity types, indicating that the former result was due tothe morphology-density effect. In addition, only small differences arefound when we consider galaxies with similar Hα luminosities. Thecomparison between H II galaxies of different Hα luminositiesshows that there is a significantly higher percentage of galaxies withcompanions among H II galaxies with L(Hα)>1039 ergss-1 than among those with L(Hα)<=1039ergs s-1, indicating that interactions increase the amount ofcircumnuclear star formation, in agreement with previous results. Thefact that we find that galaxies of different activity types have thesame percentage of companions suggests that interactions betweengalaxies is not a necessary condition to trigger the nuclear activity inAGNs. We compare our results with previous ones and discuss theirimplications.
| The peculiar motions of early-type galaxies in two distant regions - II. The spectroscopic data
We present the spectroscopic data for the galaxies studied in the EFARproject, which is designed to measure the properties and peculiarmotions of early-type galaxies in two distant regions. We have obtained1319 spectra of 714 early-type galaxies over 33 observing runs on 10different telescopes. We describe the observations and data reductionsused to measure redshifts, velocity dispersions and the Mgb and Mg_2Lick linestrength indices. Detailed simulations and intercomparison ofthe large number of repeat observations lead to reliable error estimatesfor all quantities. The measurements from different observing runs arecalibrated to a common zero-point or scale before being combined,yielding a total of 706 redshifts, 676 velocity dispersions, 676 Mgblinestrengths and 582 Mg_2 linestrengths. The median estimated errors inthe combined measurements are Delta cz=20 km s^-1, Delta sigma sigma=9.1 per cent, Delta Mgb Mgb=7.2 per cent and Delta Mg_2=0.015 mag.Comparison of our measurements with published data sets shows nosystematic errors in the redshifts or velocity dispersions, and onlysmall zero-point corrections to bring our linestrengths on to thestandard Lick system. We have assigned galaxies to physical clusters byexamining the line-of-sight velocity distributions based on EFAR andZCAT redshifts, together with the projected distributions on the sky. Wederive mean redshifts and velocity dispersions for these clusters, whichwill be used in estimating distances and peculiar velocities and to testfor trends in the galaxy population with cluster mass. The spectroscopicparameters presented here for 706 galaxies combine high-quality data,uniform reduction and measurement procedures, and detailed erroranalysis. They form the largest single set of velocity dispersions andlinestrengths for early-type galaxies published to date.
| Galaxy coordinates. II. Accurate equatorial coordinates for 17298 galaxies
Using images of the Digitized Sky Survey we measured coodinates for17298 galaxies having poorly defined coordinates. As a control, wemeasured with the same method 1522 galaxies having accurate coordinates.The comparison with our own measurements shows that the accuracy of themethod is about 6 arcsec on each axis (RA and DEC).
| The Properties of Poor Groups of Galaxies. I. Spectroscopic Survey and Results
We use multifiber spectroscopy of 12 poor groups of galaxies to address(1) whether the groups are bound systems or chance projections ofgalaxies along the line of sight; (2) why the members of each group havenot already merged to form a single galaxy, despite the groups' highgalaxy densities, short crossing times, and likely environments forgalaxy-galaxy mergers; and (3) how galaxies might evolve in thesegroups, where the collisional effects of the intragroup gas and thetidal influences of the global potential are weaker than in richclusters. Each of the 12 groups has fewer than about five catalogedmembers in the literature. Our sample consists of 1002 galaxyvelocities, 280 of which are group members. The groups have meanrecessional velocities between 1600 and 7600 km s-1. Nine groups,including three Hickson compact groups, have the extended X-ray emissioncharacteristic of an intragroup medium (see Paper II). We conclude thefollowing: 1. The nine poor groups with diffuse X-ray emission are boundsystems with at least ~20-50 group members with absolute magnitudes asfaint as MB ~ -14 + 5 log10 h to -16 + 5 log10 h. The large number ofgroup members, the significant early-type population (up to ~55% of themembership) and its concentration in the group center, and thecorrespondence of the central, giant elliptical with the optical andX-ray group centroids argue that the X-ray groups are not radialsuperpositions of unbound galaxies. The velocity dispersions of theX-ray groups range from 190 to 460 km s-1. We are unable to determine ifthe three non-X-ray groups, which have lower velocity dispersions(<130 km s-1) and early-type fractions (=0%), are also bound. 2.Galaxies in each X-ray-detected group have not all merged togetherbecause a significant fraction of the group mass lies outside of thegalaxies and in a common halo. The velocity dispersion of the combinedgroup sample is constant as a function of radius out to the virialradius of the system (typically ~0.5 h-1 Mpc). The virial mass of eachgroup (~0.5-1 x 1014 h-1 Mȯ) is large compared with the mass in theX-ray gas and in the galaxies (e.g., ~1 x 1012 h-5/2 Mȯ and ~1 x1013 h-1 Mȯ, respectively, in NGC 533). These results imply thatmost of the group mass is in a common, extended halo. The small fraction(~10%-20%) of group mass associated with individual galaxies suggeststhat the rate of galaxy-galaxy interactions is lower than for agalaxy-dominated system, allowing these groups to virialize before allof their galaxies merge and to survive for more than a few crossingtimes. 3. The position of the giant, brightest elliptical in each X-raygroup is indistinguishable from the center of the group potential, asdefined by the mean velocity and the projected spatial centroid of thegroup galaxies. This result suggests that dominant cluster ellipticals,such as cD galaxies, may form via the merging of galaxies in the centersof poor group-like environments. Groups with a central, dominantelliptical may then fall into richer clusters. This scenario explainswhy cD galaxies do not always lie in the spatial and kinematic center ofrich clusters but instead occupy the centers of subclusters innonvirialized clusters. 4. The fraction of early-type galaxies in thepoor groups varies significantly, ranging from that characteristic ofthe field (<~25%) to that of rich clusters (~55%). The highearly-type fractions are particularly surprising because all of thegroups in this sample have substantially lower velocity dispersions (bya factor of ~2-5) and galaxy number densities (by a factor of ~5-20)than are typical of rich clusters. Hence, the effects of disruptivemechanisms like galaxy harassment on the morphology of poor groupgalaxies are weaker than in cluster environments. In contrast, thekinematics of poor groups make them preferred sites for galaxy-galaxymergers, which may alter the morphologies and star formation historiesof some group members. If galaxy-galaxy interactions are not responsiblefor the high early-type fractions, it is possible that the effects ofenvironment are relatively unimportant at the current epoch and that thesimilarity of the galaxy populations of rich clusters and some poorgroups reflects conditions at the time of galaxy formation. 5. Thefraction of early-type group members that have experienced starformation within the last ~2 h-1 Gyr is consistent with that in richclusters with significant substructure (~15%). If some of thesubclusters in these rich, complex clusters are groups that haverecently fallen into the cluster environment, the similarity between thestar formation histories of the early types in the subclusters and ofthose in our sample of field groups indicates that the clusterenvironment and associated mechanisms like ram pressure stripping arenot required to enhance and/or quench star formation in these particulargalaxies. If the recent star formation is tied to the externalenvironment of the galaxies and not to internal instabilities, it ismore likely that galaxy-galaxy encounters have altered the starformation histories of some early-type galaxies in groups and insubclusters.
| The peculiar motions of early-type galaxies in two distant regions. III - The photometric data
We present R-band CCD photometry for 776 galaxies observed in the EFARproject. The photometry is compared with photoelectric data, showingthat a common zero-point good to better than 1 per cent and a precisionof 0.03 mag per zero-point have been achieved. We give the circularlyaveraged surface brightness profiles and the photometric parameters ofthe 762 program galaxies, D(n) diameters, half-luminosity radii, totalmagnitudes, and average effective surface brightnesses. More than 80percent of the profiles have a global S/N ratio larger than 300. Theextrapolation needed to derive total magnitudes is less than 10 percentfor 80 percent of the fits. More than 80 percent of the galaxies havemean effective surface brightness larger than the observed skybrightness. In 90 percent of the profiles the estimate of thecontamination of the sky by the galaxy light is less than 1 percent. Wederive total magnitudes and half-luminosity radii to better than 0.15mag and 25 percent, respectively, for 90 percent of our sample. Incontrast, external comparisons show that data in the literature can bestrongly affected by systematic errors due to large extrapolations,small radial range, sky subtraction errors, seeing effects, and the useof a simple R exp 1/4 fit. The resulting errors can easily amount tomore than 0.5 mag in the total magnitudes and 50 percent in thehalf-luminosity radii.
| The Peculiar Motions of Early-Type Galaxies in Two Distant Regions. I. Cluster and Galaxy Selection
The EFAR project is a study of 736 candidate elliptical galaxies in 84clusters lying in two regions, toward Hercules-Corona Borealis andPerseus-Pisces-Cetus, at distances cz ~ 6000-15,000 km s^-1^. In thispaper (the first of a series), we present an introduction to the EFARproject and describe in detail the selection of the clusters andgalaxies in our sample. Fundamental data for the galaxies and clustersare given, including accurate new positions for each galaxy andredshifts for each cluster. The galaxy selection functions aredetermined by using diameters measured from Schmidt sky survey imagesfor 2185 galaxies in the cluster fields. Future papers in this serieswill present the spectroscopic and photometric observations of thissample, investigate the properties of the fundamental plane forelliptical galaxies, and determine the large- scale peculiar velocityfields in these two regions of the universe.
| Arm structure in normal spiral galaxies, 1: Multivariate data for 492 galaxies
Multivariate data have been collected as part of an effort to develop anew classification system for spiral galaxies, one which is notnecessarily based on subjective morphological properties. A sample of492 moderately bright northern Sa and Sc spirals was chosen for futurestatistical analysis. New observations were made at 20 and 21 cm; thelatter data are described in detail here. Infrared Astronomy Satellite(IRAS) fluxes were obtained from archival data. Finally, new estimatesof arm pattern radomness and of local environmental harshness werecompiled for most sample objects.
| The small scale environment of low surface brightness disk galaxies
We use a sample of about 340 low surface brightness (LSB) disk galaxieswith measured redshifts in combination with the Center for Astrophysicsredshift survey to test the hypothesis that LSB galaxies have a deficitof nearby companion galaxies compared to high surface brightness (HSB)disk galaxies. We find a very strong statistical deficit of galaxieslocated within a projected radius of 0.5 Mpc and within a velocity of500 km/s around LSB disk galaxies compared to HSB ones. Further,comparing LSB and HSB disk galaxies which are located in the sameportion of the sky indicates that the average distance to the nearestneighbor is 1.7 times farther for LSB disk galaxies. AKomologorov-Smirnoff test rules out, at greater than the 99 percentconfidence level, the hypothesis that the distribution ofnearest-neighbor distances is the same for HSB and LSB disk galaxies. Wespeculate that LSB disk galaxies have relatively long formation timescales and therefore must form in relative isolation. In addition, thelack of tidal interactions over a Hubble time serves to suppress theoverall star-formation rate as no external trigger is available to helpclump the gas. The observed low surface densities of H I in combinationwith the low probability of tidal interactions effectively preventsthese disk galaxies from evolving very rapidly.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
Catálogos y designaciones:
|
