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X-Ray-selected Intermediate-Redshift Groups of Galaxies
We present spectroscopic confirmation of nine moderate-redshift galaxygroups and poor clusters selected from the ROSAT Deep Cluster Survey.The groups span the redshift range z~0.23-0.59 and have between 4 and 20confirmed members. The velocity dispersions of these groups range from~125 to 650 km s-1. Similar to X-ray groups at low redshift,these systems contain a significant number of early-type galaxies.Therefore, the trend for X-ray-luminous groups to have high early-typefractions is already in place by at least z~0.5. In four of the ninegroups, the X-ray emission is clearly peaked on the most luminousearly-type galaxy in the group. However, in several cases the centralgalaxy is composed of multiple luminous nuclei, suggesting that thebrightest group galaxy may still be undergoing major mergers. In atleast three (and possibly five) of the groups in our sample, a dominantearly-type galaxy is not found at the center of the group potential.This suggests that many of our groups are not dynamically evolveddespite their high X-ray luminosities. While similar systems have beenidentified at low redshift, the X-ray luminosities of theintermediate-redshift examples are 1-3 orders of magnitude higher thanthose of their low-redshift counterparts. We suggest that this may beevidence for group downsizing: while massive groups are still in theprocess of collapsing and virializing at intermediate redshifts, onlylow-mass groups are in the process of forming at the present day.
| Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton
We investigated the dynamical structure of 53 elliptical galaxies usingthe Chandra archival X-ray data. In X-ray-luminous galaxies, temperatureincreases with radius and gas density is systematically higher at theoptical outskirts, indicating the presence of a significant amount ofthe group-scale hot gas. In contrast, X-ray-dim galaxies show a flat ordeclining temperature profile against radius and the gas density isrelatively lower at the optical outskirts. Thus, it is found thatX-ray-bright and faint elliptical galaxies are clearly distinguished bythe temperature and gas density profile. The mass profile is well scaledby a virial radius r200 rather than an optical half-radiusre, is quite similar at (0.001-0.03)r200 betweenX-ray-luminous and dim galaxies, and smoothly connects to those profilesof clusters of galaxies. At the inner region of(0.001-0.01)r200 or (0.1-1)re, the mass profilewell traces a stellar mass with a constant mass-to-light ratio ofM/LB=3-10 Msolar/Lsolar. TheM/LB ratio of X-ray-bright galaxies rises up steeply beyond0.01r200 and thus requires a presence of massive dark matterhalo. From the deprojection analysis combined with the XMM-Newton data,we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 alsohave a high M/LB ratio of 20-30 at 20 kpc, comparable to thatof X-ray-luminous galaxies. Therefore, dark matter is indicated to becommon in elliptical galaxies; their dark matter distribution, as wellas that of galaxy clusters, almost follows the NFW profile.
| Evidence for radio-source heating of groups
We report evidence that the gas properties of X-ray groups containingradio galaxies differ from those of radio-quiet groups. For awell-studied sample of ROSAT-observed groups, we found that more thanhalf of the elliptical-dominated groups can be considered `radio-loud',and that radio-loud groups are likely to be hotter at a given X-rayluminosity than radio-quiet groups. We tested three different models forthe origin of the effect and conclude that radio-source heating is themost likely explanation. We found several examples of groups where thereis strong evidence from Chandra or XMM-Newton images for interactionsbetween the radio source and the group gas. A variety of radio-sourceheating processes are important, including shock-heating by youngsources and gentler heating by larger sources. The heating effects canbe longer-lasting than the radio emission. We show that the sample ofX-ray groups used in our study is not significantly biased in thefraction of radio-loud groups that it contains. This allows us toconclude that the energy per particle that low-power radio galaxies caninject over the group lifetime is comparable to the requirements ofstructure formation models.
| The unusual morphology of the intragroup medium in NGC 5171
We present the results of a 24-ks XMM-Newton observation of the NGC 5171group of galaxies. NGC 5171 is unusual in that it is an X-ray brightgroup (LX > 1042 erg s-1), withirregular contours which are not centred on a bright galaxy. The globalspectrum is adequately described by a single-temperature APEC model withTX= 0.96 +/- 0.04 keV and Z= 0.13 +/- 0.02Zsolar,in good agreement with previous ROSAT data. We find that the X-raycontours are centred on a bright ridge of emission stretching from thebrightest group galaxy to a nearby galaxy. Spectral mapping reveals thisridge to be both cool (TX~ 1.1 keV) and metallic (Z~ 0.4Zsolar) in comparison to its surroundings, suggesting it isthe result of a tidal interaction between the two galaxies. Optical datareveal the member galaxies to have a high velocity dispersion(σv= 494 +/- 99 km s-1) and a significantlynon-Gaussian velocity distribution, suggesting that the group is in theprocess of merging. A region of hot gas with TX= 1.58 +/-0.36 keV is found to the west of the bright central ridge, and weinterpret this as shock-heating resulting from the merging. A furtherregion of emission to the south-east of the bright central ridge, withTX= 1.14 +/- 0.13 keV, is probably associated with abackground group, four times more distant.
| The GEMS project: X-ray analysis and statistical properties of the group sample
The Group Evolution Multiwavelength Study (GEMS) involves amultiwavelength study of a sample of 60 galaxy groups, chosen to span awide range of group properties. Substantial ROSAT Position SensitiveProportional Counter (PSPC) observations, available for all of thesegroups, are used to characterize the state of the intergalactic mediumin each. We present the results of a uniform analysis of these ROSATdata and a statistical investigation of the relationship between X-rayand optical properties across the sample. Our analysis improves inseveral respects on previous work: (i) we distinguish between systems inwhich the hot gas is a group-scale medium and those in which it appearsto be just a hot halo associated with a central galaxy; (ii) weextrapolate X-ray luminosities to a fixed overdensity radius(r500) using fitted surface brightness models, in order toavoid biases arising from the fact that cooler systems are detectable tosmaller radii, and (iii) optical properties have been rederived in auniform manner from the NASA Extragalactic Database, rather than relyingon the data in the disparate collection of group catalogues from whichour systems are drawn.The steepening of the LX-TX relation in the groupregime reported previously is not seen in our sample, which fits well onto the cluster trend, albeit with large non-statistical scatter. Anumber of biases affect the fitting of regression lines under thesecircumstances, and until the impact of these has been thoroughlyinvestigated it seems best to regard the slope of the groupLX-TX relation as being poorly determined. Asignificant problem in comparing the properties of groups and clustersis the derivation of system radii, to allow different systems to becompared within regions having the same overdensity. We find evidencethat group velocity dispersion (σv) provides a veryunreliable measure of system mass (and hence radius), with a number ofgroups having remarkably low values of σv, given thatthey appear from their X-ray properties to be collapsed systems. Weconfirm that the surface brightness profiles of groups are significantlyflatter than those of clusters - the maximum value of theβfit parameter for our sample is 0.58, lower than thetypical value of 0.67 seen in clusters - however, we find no significanttendency within our sample for cooler groups to show flatter profiles.This result is inconsistent with simple universal pre-heating models.The morphology of the galaxies in the GEMS groups is correlated to theirX-ray properties in a number of ways: we confirm the very strongrelationship between X-ray emission and a dominant early-type centralgalaxy, which has been noted since the early X-ray studies of groups,and also find that spiral fraction is correlated with the temperature ofthe hot gas and hence the depth of the gravitational potential. A classof spiral-rich groups with little or no X-ray emission probablycorresponds to groups that have not yet fully collapsed.
| An X-Ray Atlas of Groups of Galaxies
A search was conducted for a hot intragroup medium in 109 low-redshiftgalaxy groups observed with the ROSAT PSPC. Evidence for diffuse,extended X-ray emission is found in at least 61 groups. Approximatelyone-third of these detections have not been previously reported in theliterature. Most of the groups are detected out to less than half of thevirial radius with ROSAT. Although some spiral-rich groups do contain anintragroup medium, diffuse emission is restricted to groups that containat least one early-type galaxy.
| 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.
| Compact groups in the UZC galaxy sample
Applying an automatic neighbour search algorithm to the 3D UZC galaxycatalogue (Falco et al. \cite{Falco}) we have identified 291 compactgroups (CGs) with radial velocity between 1000 and 10 000 kms-1. The sample is analysed to investigate whether Tripletsdisplay kinematical and morphological characteristics similar to higherorder CGs (Multiplets). It is found that Triplets constitute lowvelocity dispersion structures, have a gas-rich galaxy population andare typically retrieved in sparse environments. Conversely Multipletsshow higher velocity dispersion, include few gas-rich members and aregenerally embedded structures. Evidence hence emerges indicating thatTriplets and Multiplets, though sharing a common scale, correspond todifferent galaxy systems. Triplets are typically field structures whilstMultiplets are mainly subclumps (either temporarily projected orcollapsing) within larger structures. Simulations show that selectioneffects can only partially account for differences, but significantcontamination of Triplets by field galaxy interlopers could eventuallyinduce the observed dependences on multiplicity. Tables 1 and 2 are onlyavailable in electronic at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/35
| X-ray luminosities of galaxies in groups
We have derived the X-ray luminosities of a sample of galaxies ingroups, making careful allowance for contaminating intragroup emission.The LX:LB and LX:LFIRrelations of spiral galaxies in groups appear to be indistinguishablefrom those in other environments, however the elliptical galaxies fallinto two distinct classes. The first class is central-dominant groupgalaxies, which are very X-ray luminous and may be the focus of groupcooling flows. All other early-type galaxies in groups belong to thesecond class, which populates an almost constant band ofLX/LB over the range9.8
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| The Dynamics of Poor Systems of Galaxies
We assemble and observe a sample of poor galaxy systems that is suitablefor testing N-body simulations of hierarchical clustering and otherdynamical halo models. We (1) determine the parameters of the densityprofile rho(r) and the velocity dispersion profile sigma_p(R), (2)separate emission-line galaxies from absorption-line galaxies, examiningthe model parameters and as a function of spectroscopic type, and (3)for the best-behaved subsample, constrain the velocity anisotropyparameter, beta, which determines the shapes of the galaxy orbits. Oursample consists of 20 systems, 12 of which have extended X-ray emissionin the ROSAT All-Sky Survey. We measure the 877 optical spectra ofgalaxies brighter than m_R~15.4 within 1.5 h^-1 Mpc of the systemcenters (we take H_0=100 h km s^-1 Mpc^-1). Thus, we sample the systemmembership to a radius typically three times larger than other recentoptical group surveys. The average system population is 30 galaxies, andthe average line-of-sight velocity dispersion is ~300 km s^-1. TheNavarro, Frenk, & White universal profile and the Hernquist modelboth provide good descriptions of the spatial data. In most cases anisothermal sphere is ruled out. Systems with declining sigma_p(R) arewell-matched by theoretical profiles in which the star-forming galaxieshave predominantly radial orbits (beta>0) many of these galaxies areprobably falling in for the first time. There is significant evidencefor spatial segregation of the spectroscopic classes regardless ofsigma_p(R).
| 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).
| An Arecibo Search for Broad 21 Centimeter Lines of Atomic Hydrogen in Clusters of Galaxies
We present the results of sensitive observations of the inner3.3′ (~100 kpc) of 21 clusters of galaxies with the Arecibotelescope utilizing the autocorrelation spectrometer in complex-samplingmode to obtain a total bandwidth of 40 MHz, which corresponds to avelocity coverage of ~7000-8000 km s^-1. These are the most sensitiveobservations capable of detecting H i with line widths expected for gasin the intracluster medium (ICM). We did not detect any significantemission features with 500 km s^-1 <= FWHM <= 4000 km s^-1 throughthe centers of these clusters. Limits on optically thin H i from``narrow'' galaxy-type features are typically ~5 x 10^8 M_ȯ, whilefrom broad cluster-type features they are about an order of magnitudehigher. We derive constraints on the covering factor of any opticallythick H i within the Arecibo beam. We obtain upper limits c_f ~ 3 x10^-3 assuming that the temperature of the H i is 10 K. This correspondsto a limited range of parameter space for the clouds in which the cloudtemperature is typically less than a few tens of kelvins and the numberof clouds along the line of sight is less than a few tens of clouds.Cloud temperatures this low are probably inconsistent with cloud heatingdue to X-rays from the ICM; however, this is uncertain. The very smallrange of parameter space that is allowed by the observations makes itunlikely that there is very much atomic hydrogen in the ICM of theseclusters. The Arecibo Observatory is part of the National Astronomy andIonosphere Center, which is operated by Cornell University undercooperative agreement with the National Science Foundation.
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
| Kinematics and dynamics of the MKW/AWM poor clusters
We report 472 new redshifts for 416 galaxies in the regions of the 23poor clusters of galaxies originally identified by Morgan, Kayser, andWhite (MKW), and Albert, White, and Morgan (AWM). Eighteen of the poorclusters now have 10 or more available redshifts within 1.5/h Mpc of thecentral galaxy; 11 clusters have at least 20 available redshifts. Basedon the 21 clusters for which we have sufficient velocity information,the median velocity scale is 336 km/s, a factor of 2 smaller than foundfor rich clusters. Several of the poor clusters exhibit complex velocitydistributions due to the presence of nearby clumps of galaxies. We checkon the velocity of the dominant galaxy in each poor cluster relative tothe remaining cluster members. Significantly high relative velocities ofthe dominant galaxy are found in only 4 of 21 poor clusters, 3 of whichwe suspect are due to contamination of the parent velocity distribution.Several statistical tests indicate that the D/cD galaxies are at thekinematic centers of the parent poor cluster velocity distributions.Mass-to-light ratios for 13 of the 15 poor clusters for which we havethe required data are in the range 50 less than or = M/LB(0)less than or = 200 solar mass/solar luminosity. The complex nature ofthe regions surrounding many of the poor clusters suggests that thesegroupings may represent an early epoch of cluster formation. Forexample, the poor clusters MKW7 and MKWS are shown to be gravitationallybound and likely to merge to form a richer cluster within the nextseveral Gyrs. Eight of the nine other poor clusters for which simpletwo-body dynamical models can be carried out are consistent with beingbound to other clumps in their vicinity. Additional complex systems withmore than two gravitationally bound clumps are observed among the poorclusters.
| Integrated photoelectric magnitudes and color indices of bright galaxies in the Johnson UBV system
The photoelectric total magnitudes and color indices published in theThird Reference Catalogue of Bright Galaxies (RC3) are based on ananalysis of approximately equals 26,000 B, 25,000 B-V, and 17,000 U-Bmultiaperture measurements available up to mid 1987 from nearly 350sources. This paper provides the full details of the analysis andestimates of internal and external errors in the parameters. Thederivation of the parameters is based on techniques described by theVaucouleurs & Corwin (1977) whereby photoelectric multiaperture dataare fitted by mean Hubble-type-dependent curves which describe theintegral of the B-band flux and the typical B-V and U-B integrated colorgradients. A sophisticated analysis of the residuals of thesemeasurements from the curves was made to allow for the random andsystematic errors that effect such data. The result is a homogeneous setof total magnitudes BTA total colors(B-V)T and (U-B)T, and effective colors(B-V)e and (U-B)e for more than 3000 brightgalaxies in RC3.
| A VLA 20 CM survey of poor groups of galaxies
The paper reports on VLA 20 cm observations of an extensive sample ofgalaxies in 139 poor groups. These groups, composed of galaxies down tothe limit of the Zwicky et al. (CGCG) catalog, were chosen using apercolation algorithm set at a high surface-density threshold.Approximately 50 percent of the groups have measured redshifts. Thesegroups were surveyed using a 'snapshot' mode of the VLA with aresolution of about 13 arcsec. Analysis of the resulting radio andoptical properties reveals that the presence of a nearby companiongalaxy has an important role in generating radio emission in a galaxy.CCD observations of two radio-loud, disturbed galaxies with companionsare presented and are used to discuss models of radio-source production.Nine tailed radio galaxies are found in the poor groups, which is muchmore than had been expected from previous work on rich clusters and fromtheoretical models. The paper discusses previous statistical biases andproposes a method for bending head-tail sources in poor groups. From theconfinement of extended radio features associated with tailed sources,the presence of a substantial intracluster medium that should radiatesignificantly at soft-X-ray energies is predicted.
| Photometric properties of poor clusters of galaxies
Photographic pahotometry was made for sixteen poor clusters of galaxieslisted by Morgan et al. (1975) and Albert et al. (1977). Data were fromthe V-band plates taken with the 105-cm Schmidt telescope of the KisoObservatory. Luminosity functions of individual clusters are obtained byapplying a detailed statistical field correction. The compositeluminosity function of poor clusters is found to be similar to those ofrich clusters and small groups of galaxies. The luminosity of the firstranked galaxies of the poor clusters is closely related to the amount ofhot intracluster gas and total visible mass in the systems.
| Photometry of 18 clusters of galaxies
Data accumulated in the study of the poor clusters of galaxiesidentified by Morgan, Kayser, and White (1975) and Albert, White, andMorgan (1977) as well as two rich clusters are presented. The reductionsystem developed for the present study (the Kiso Image Detection System)is described. Positions, apparent magnitudes, ellipticities, and majoraxis position angles are provided.
| Supplement to the detailed bibliography on the surface photometry of galaxies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985A&AS...60..517P&db_key=AST
| Brightest members of rich and poor clusters of galaxies. I Surface brightness profiles of cD-type galaxies
Surface photometry of a sample of the brightest members of rich and poorclusters of galaxies has been carried out on plates obtained at theprime focus of the CTIO 4 m telescope. Some of the galaxies resembleMorgan's cD types, and most of them have surface brightness profileswhich are well represented by de Vaucouleur's empirical formula atsurface brightnesses brighter than 24 mag/sq arcsec. The de Vaucouleurseffective radius and the surface brightness at the radius for all thegalaxies in the sample are tightly correlated in a manner much like thatrecently discussed by Kormendy. Two inferences follow from thiscorrelation: (1) the central parts of brightest cluster members appearto be indistinguishable from normal giant elliptical galaxies; (2) thearguments for cannibalism suggested by Ostriker and Hausman (1977) andby White (1978) are qualitatively justified.
| The Trivariate / Radio Optical X-Ray / Luminosity Function CD Galaxies - Part Two - the Fuelling of Radio Sources
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1983A&A...125..223V&db_key=AST
| 21 CM line observations of cD galaxies
A small 21 cm line survey of five cD galaxies, all first ranked clustermembers, and all except one known to be centered on an extended X-raysource, is reported. Rms noise values for each object as obtained aftersmoothing the signals are listed. Estimated upper limits of the H I fluxand an average flux upper limit of twice the rms over the whole widthare used to estimate an upper limit of the neutral hydrogen mass.Hydrogen mass is computed to photographic luminosity ratios, with nocorrection applied to the optical luminosity of the galaxies except theone that allows for galactic extinction. Dressler's (1979) threecomponent cD galaxy model is scaled to the sample galaxies and, assumingthat the H I velocity distribution follows that of the stars, an upperlimit to the hydrogen mass to total mass ratio in
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