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|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.
|CIRS: Cluster Infall Regions in the Sloan Digital Sky Survey. I. Infall Patterns and Mass Profiles|
We use the Fourth Data Release of the Sloan Digital Sky Survey (SDSS) totest the ubiquity of infall patterns around galaxy clusters and measurecluster mass profiles to large radii. The Cluster and Infall RegionNearby Survey (CAIRNS) found infall patterns in nine clusters, but thecluster sample was incomplete. Here we match X-ray cluster catalogs withSDSS, search for infall patterns, and compute mass profiles for acomplete sample of X-ray-selected clusters. Very clean infall patternsare apparent in most of the clusters, with the fraction decreasing withincreasing redshift due to shallower sampling. All 72 clusters in awell-defined sample limited by redshift (ensuring good sampling) andX-ray flux (excluding superpositions) show infall patterns sufficient toapply the caustic technique. This sample is by far the largest sample ofcluster mass profiles extending to large radii to date. Similar toCAIRNS, cluster infall patterns are better defined in observations thanin simulations. Further work is needed to determine the source of thisdifference. We use the infall patterns to compute mass profiles for 72clusters and compare them to model profiles. Cluster scaling relationsusing caustic masses agree well with those using X-ray or virial massestimates, confirming the reliability of the caustic technique. Weconfirm the conclusion of CAIRNS that cluster infall regions are wellfitted by Navarro-Frenk-White (NFW) and Hernquist profiles and poorlyfitted by singular isothermal spheres. This much larger sample enablesnew comparisons of cluster properties with those in simulations. Theshapes (specifically NFW concentrations) of the mass profiles agree wellwith the predictions of simulations. The mass in the infall region istypically comparable to or larger than that in the virial region.Specifically, the mass inside the turnaround radius is on average2.19+/-0.18 times that within the virial radius. This ratio agrees wellwith recent predictions from simulations of the final masses of darkmatter halos.
|The isolated elliptical NGC 4555 observed with Chandra|
We present analysis of a Chandra observation of the elliptical galaxyNGC 4555. The galaxy lies in a very low density environment, eitherisolated from all galaxies of similar mass or on the outskirts of agroup. Despite this, NGC 4555 has a large gaseous halo, extending to~60kpc. We find the mean gas temperature to be ~0.95keV and the Feabundance to be ~0.5Zsolar. We model the surface brightness,temperature and abundance distribution of the halo and use these resultsto estimate parameters such as the entropy and cooling time of the gas,and the total gravitational mass of the galaxy. In contrast to recentresults showing that moderate luminosity ellipticals contain relativelysmall quantities of dark matter, our results show that NGC 4555 has amassive dark halo and large mass-to-light ratio(56.8+34.2-35.8Msolar/LBsolarat 50kpc, 42.7+14.6-21.2 at 5re,1σ errors). We discuss this disparity and consider possiblemechanisms by which galaxies might reduce their dark matter content.
|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 old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482|
We present the first detailed X-ray observations, using Chandra, of NGC6482 - the nearest known fossil group. The group is dominated by anoptically luminous giant elliptical galaxy and all other known groupmembers are at least two magnitudes fainter. The global X-ray properties(luminosity, temperature, extent) of NGC 6482 fall within the range ofother groups, but the detailed properties show interesting differences.We derive the gas temperature and total mass profiles for the central 30h-170 kpc (~0.1 r200) using ACISspatially resolved spectroscopy. The unusually highLX/Lopt ratio is found to result from a highcentral gas density. The temperature profile shows a continuous decreaseoutward, dropping to 0.63 of its central value at 0.1r200.The derived total mass profile is strongly centrally peaked, suggestingan early formation epoch. These results support a picture in whichfossil groups are old, giving time for the most massive galaxies to havemerged (via the effects of dynamical friction) to produce a centralgiant elliptical galaxy.Although the cooling time within 0.1r200 is less than aHubble time, no decrease in central temperature is detected. The entropyof the system lies toward the low side of the distribution seen in poorgroups and drops all the way into the centre of the system, reachingvery low values. No isentropic core, such as those predicted in simplepre-heating models, is present. Given the lack of any centraltemperature drop in the system, it seems unlikely that radiative coolingcan be invoked to explain this low central entropy. The lack of anysignature of central cooling is especially striking in a system thatappears to be old and relaxed, and to have a central cooling time<=108 yr. We find that the centrally peaked temperatureprofile is consistent with a steady-state cooling-flow solution with anaccretion rate of 2 Msolar yr-1, given the large PdV work arising from the cuspy mass profile. However, solutionsinvolving distributed or non-steady heating cannot be ruled out.
|The Chandra Large Area Synoptic X-Ray Survey (CLASXS) of the Lockman Hole-Northwest: The X-Ray Catalog|
We present the X-ray catalog and basic results from our Chandra LargeArea Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest field.Our nine ACIS-I fields cover a contiguous solid angle of ~0.4deg2 and reach fluxes of 5×10-16 ergscm-2 s-1 (0.4-2 keV) and 3×10-15ergs cm-2 s-1 (2-8 keV). Our survey bridges thegap between ultradeep pencil-beam surveys, such as the Chandra DeepFields (CDFs), and shallower, large-area surveys, allowing a betterprobe of the X-ray sources that contribute most of the 2-10 keV cosmicX-ray background (CXB). We find a total of 525 X-ray point sources andfour extended sources. At ~10-14 ergs cm-2s-1 (2-8 keV), our number counts are significantly higherthan those of several noncontiguous, large-area surveys. Such a largedifference is an indication of clustering in the X-ray sources. On theother hand, the integrated flux from the CLASXS field, combined withASCA and Chandra ultradeep surveys, is consistent with results fromother large-area surveys, within the variance of the CXB. We seespectral evolution in the hardening of the sources at fluxes below10-14 ergs cm-2 s-1, which agrees withprevious observations from Chandra and XMM-Newton. About one third ofthe sources in the CLASXS field have multiple observations, which allowvariability tests. Above 4×10-14 ergs cm-2s-1 (0.4-8 keV), ~60% of the sources are variable. We alsoinvestigated the spectral variability of the variable sources. Whilemost show spectral softening with increasing flux, or no significantspectral change, there are a few sources that show a different trend. Weobserved four extended sources in CLASXS, which is consistent with thepreviously measured logN-logS relation for galaxy clusters. Using X-rayspectra and optical colors, we argue that three of the four extendedsources are galaxy clusters or galaxy groups. We report the discovery ofa gravitational lensing arc associated with one of these sources. Usingred-sequence and brightest cluster galaxy methods, we find that theredshifts of the extended sources are in the range z~0.5-1. The inferredmasses within the Einstein radii are consistent with the mass profilesof local groups scaled to the same virial radii.
|The Birmingham-CfA cluster scaling project - II. Mass composition and distribution|
We investigate the spatial distribution of the baryonic and non-baryonicmass components in a sample of 66 virialized systems. We have used X-raymeasurements to determine the deprojected temperature and densitystructure of the intergalactic medium and have employed these to map theunderlying gravitational potential. In addition, we have measured thedeprojected spatial distribution of galaxy luminosity for a subset ofthis sample, spanning over two decades in mass. With this combinedX-ray/optical study, we examine the scaling properties of the baryonsand address the issue of mass-to-light (M/L) ratio in groups andclusters of galaxies.We measure a median mass-to-light ratio of 249h70(M/L)solar in the rest frame BJband, in good agreement with other measurements based on X-raydetermined masses. There is no trend in M/L with X-ray temperature andno significant trend for mass to increase faster than luminosity:M~L1.08 +/- 0.12BJ. This implied lackof significant variation in star formation efficiency suggests that gascooling cannot be greatly enhanced in groups, unless it drops out toform baryonic dark matter. Correspondingly, our results indicate thatnon-gravitational heating must have played a significant role inestablishing the observed departure from self-similarity in low-masssystems. The median baryon fraction for our sample is 0.162h-3/270, which allows us to place an upper limiton the cosmological matter density, Ωm<= 0.27h-170, in good agreement with the latest resultsfrom WMAP.We find evidence of a systematic trend towards higher central densityconcentration in the coolest haloes, indicative of an early formationepoch and consistent with hierarchical formation models.
|The Birmingham-CfA cluster scaling project - I. Gas fraction and the M-TX relation|
We have assembled a large sample of virialized systems, comprising 66galaxy clusters, groups and elliptical galaxies with high-quality X-raydata. To each system we have fitted analytical profiles describing thegas density and temperature variation with radius, corrected for theeffects of central gas cooling. We present an analysis of the scalingproperties of these systems and focus in this paper on the gasdistribution and M-TX relation. In addition to clusters andgroups, our sample includes two early-type galaxies, carefully selectedto avoid contamination from group or cluster X-ray emission. We comparethe properties of these objects with those of more massive systems andfind evidence for a systematic difference between galaxy-sized haloesand groups of a similar temperature.We derive a mean logarithmic slope of the M-TX relationwithin R200 of 1.84 +/- 0.06, although there is some evidenceof a gradual steepening in the M-TX relation, with decreasingmass. We recover a similar slope using two additional methods ofcalculating the mean temperature. Repeating the analysis with theassumption of isothermality, we find the slope changes only slightly, to1.89 +/- 0.04, but the normalization is increased by 30 per cent.Correspondingly, the mean gas fraction within R200 changesfrom (0.13 +/- 0.01) h-3/270 to (0.11+/- 0.01) h-3/270, for the isothermalcase, with the smaller fractional change reflecting different behaviourbetween hot and cool systems. There is a strong correlation between thegas fraction within 0.3R200 and temperature. This reflectsthe strong (5.8σ) trend between the gas density slope parameter,β, and temperature, which has been found in previous work.These findings are interpreted as evidence for self-similarity breakingfrom galaxy feedback processes, active galactic nuclei heating orpossibly gas cooling. We discuss the implications of our results in thecontext of a hierarchical structure formation scenario.
|X-ray bright groups and their galaxies|
Combining X-ray data from the ROSAT PSPC and optical data drawn from theliterature, we examine in detail the relationship between the X-ray andoptical properties of X-ray bright galaxy groups. We find a relationshipbetween optical luminosity and X-ray temperature consistent with thatexpected from self-similar scaling of galaxy systems,LB~T1.6+/-0.2. The self-similar form andcontinuity of the LB : T relation from clusters to groups andthe limited scatter seen in this relation, implies that the starformation efficiency is rather similar in all of these systems. We findthat the bright extended X-ray components associated with many centralgalaxies in groups appear to be more closely related to the group thanthe galaxy itself, and we suggest that these are group cooling flowsrather than galaxy haloes. In addition we find that the optical light inthese groups appears to be more centrally concentrated than the light inclusters.We also use the optical and X-ray data to investigate whether early- orlate-type galaxies are primarily responsible for pre-heating in groups.Using three different methods, we conclude that spiral galaxies appearto play a comparable role to early types in the pre-heating of theintragroup medium. This tends to favour models in which the pre-heatingarises primarily from galaxy winds rather than active galactic nucleiand implies that spirals have played a significant role in the metalenrichment of the intragroup medium.
|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.
|Chandra Observations of the NGC 1550 Galaxy Group: Implication for the Temperature and Entropy Profiles of 1 keV Galaxy Groups|
We present a detailed Chandra study of the galaxy group NGC 1550. Forits temperature (1.37+/-0.01 keV) and velocity dispersion (~300 kms-1), the NGC 1550 group is one of the most luminous knowngalaxy groups (Lbol=1.65×1043 ergss-1 within 200 kpc, or 0.2rvir). We find thatwithin ~60 kpc, where the gas cooling time is less than a Hubble time,the gas temperature decreases continuously toward the center, implyingthe existence of a cooling core. The temperature also declines beyond~100 kpc (or 0.1rvir). The temperature profile of NGC 1550 isremarkably similar to those of two other 1 keV groups with accuratetemperature determination. The temperature begins to decline at0.07rvir-0.1rvir, while in hot clusters thedecline begins at or beyond 0.2rvir. Thus, there are at leastsome 1 keV groups that have temperature profiles significantly differentfrom those of hot clusters, which may reflect the role ofnongravitational processes in intracluster medium/intergalactic mediumevolution. NGC 1550 has no isentropic core in its entropy profile, incontrast to the predictions of ``entropy floor'' simulations. We comparethe scaled entropy profiles of three 1 keV groups (including NGC 1550)and three 2-3 keV groups. The scaled entropy profiles of 1 keV groupsshow much larger scatter than those of hotter systems, which impliesvaried preheating levels. We also discuss the mass content of the NGC1550 group and the abundance profile of heavy elements.
|Uncovering Additional Clues to Galaxy Evolution. II. The Environmental Impact of the Virgo Cluster on the Evolution of Dwarf Irregular Galaxies|
The impact of the cluster environment on the evolution of dwarf galaxiesis investigated by comparing the properties of a sample of dwarfirregular galaxies (dI's) in the Virgo Cluster with a control sample ofnearby (``field'') dI's having oxygen abundances derived from [O III]λ4363 measurements and measured distances from resolved stellarconstituents. Spectroscopic data are obtained for H II regions in 11Virgo dI's distributed in the central and outer regions of the cluster.To ensure that oxygen abundances are derived in a homogeneous manner,oxygen abundances for field and Virgo dI's are computed using thebright-line method and compared with abundances directly obtained from[O III] λ4363, where available. They are found to agree to withinabout 0.2 dex, with no systematic offset. At a given optical luminosity,there is no systematic difference in oxygen abundance between the sampleof Virgo dI's and the sample of nearby dI's. However, five of the 11Virgo dI's exhibit much lower baryonic gas fractions than field dI's atcomparable oxygen abundances. Using field dI's as a reference, agas-deficiency index for dI's is constructed, making it possiblequantitatively to identify which galaxies have lost gas. For the Virgosample, some of the dwarfs are gas-deficient by a factor of 30. The gasdeficiency correlates roughly with the X-ray surface brightness of theintracluster gas. Ram pressure stripping can best explain the observedgas-poor dI's in the cluster sample. Together with the lack ofsignificant fading and reddening of the gas-poor dI's compared withgas-normal dI's, these observations suggest that the gas-poor dI's inVirgo have recently encountered the intracluster medium for the firsttime. Faded remnants of gas-poor dI's in Virgo will resemble brightdwarf elliptical galaxies currently seen in the cluster core.
|Entropy scaling in galaxy clusters: Insights from an XMM-Newton observation of the poor cluster A1983|
An XMM-Newton observation of the cool (kT =2.1 keV ) cluster A1983, atz=0.044, is presented. Gas density and temperature profiles arecalculated over the radial range up to 500 h50-1kpc, corresponding to ~ 0.35 rv . The outer regions of the surfacebrightness profile are well described with a beta -model with beta=0.74, but the central regions require the introduction of a secondcomponent. The temperature profile is flat at the exterior with a slightdip towards the centre. The total mass profile, calculated from thetemperature and density information assuming hydrostatic equilibrium, isconsistent with an NFW profile, but with a low concentration parameterc=3.75 +/- 0.74, which may be due to the cluster not being totallyrelaxed. Published optical data are used to calculate the mlb ratioprofile and the overall iron mass over luminosity ratio. The mlb ratioprofile shows that, at large scale, light traces mass to a reasonableextent, and the mlb ratio at 0.35 r200 (M/LB= 135+/- 45 h50 Msun/Lsun) is consistentwith the trends with mass observed in the optical. The iron mass overluminosity ratio is about two times less than that observed for acluster at 5 keV. The gas mass fraction rises rapidly in the centralregions to level off quickly at ~ 200 h50-1 kpc;the value at 0.35 rv is ~ 8%. The scaling properties of the emissionmeasure profile are consistent with the empirical relationMgas, ~ Tx 1.94; use of the standard self-similarrelation Mgas, ~ Tx 1.5 results in a scaledprofile that is a factor of about two too low as compared to thereference mean profile for hot clusters. Comparison of the entropyprofile of this cool cluster with that of the hot cluster A1413 showsthat the two profiles are extremely well scaled using the empiricallydetermined relation S ~ Tx 0.65, suggesting that the slope ofthe S-Tx relation is shallower than expected in the standardself-similar model. The form of the two entropy profiles is remarkablysimilar, and there is no sign of a larger isentropic core in the coolercluster. These data provide powerful agruments against preheatingmodels. In turn, there is now increasing observational support for atrend of fgas with system mass, which may go some way towardsexplaining the observed scaling behaviour.
|The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources|
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
|Maximum feedback and dark matter profiles of dwarf galaxies|
The observed rotation curves of dark matter-dominated dwarf galaxiesindicate low-density cores, contrary to the predictions of CDM models. Apossible solution of this problem involves stellar feedback. A strongbaryonic wind driven by vigorous star formation can remove a largefraction of the gas, causing the dark matter to expand. Using bothnumerical and analytical techniques, we explore the maximum effect ofthe feedback with an instantaneous removal of the gaseous disc. Theenergy input depends on the compactness of the disc, hence the specificangular momentum of the disc. For the plausible cosmological parametersand a wide range of the disc angular momenta, the feedback isinsufficient to destroy the central halo cusp, while the inner densityis lowered only by a modest factor of 2 to 6. Any realistic modelling ofthe feedback would have even lesser impact on dark matter. We find thatno star formation effect can resolve the problems of CDM cusps.
|ASCA Observations of Groups at Radii of Low Overdensity: Implications for the Cosmic Preheating|
Through a three-dimensional modeling of ASCA observations, we performeda spatially resolved X-ray spectroscopic study, extending to radiiexceeding 150 kpc, for a sample of nine groups of galaxies. Combinedwith published ROSAT results, we conclude that these systems generallyexhibit a strong temperature decline at outer radii. In our best case,NGC 3268, this corresponds to a flattening of the entropy profile at alevel of ~400 keV cm2. This value is high compared both tothe observed entropy floor of ~100 keV cm2 and to theexpected value from gravitational heating. We suggest that the observedentropy profile in most groups at densities exceeding 500 times thecritical is driven purely by nongravitational heating processes. Aftercomparison with a larger sample of groups and clusters, we conclude thatthere is a variation in the level of nongravitational heating between~100 and ~400 keV cm2 within every system. Using models ofcluster formation as a reference frame, we established that the accretedgas reaches an entropy level of 400 keV cm2 by redshift2.0-2.5, while such high entropies where not present at redshifts higherthan 2.8-3.5, favoring nearly instantaneous preheating. Adoptinggalactic winds as a source of preheating and scaling the released energyby the observed metal abundance, the variation in the preheating couldbe ascribed mostly to variation in the typical overdensity of the energyinjection, from ~30 for an entropy floor of 100 keV cm2 to ~5for an entropy of 400 keV cm2.
|Observational Mass-to-Light Ratio of Galaxy Systems from Poor Groups to Rich Clusters|
We study the mass-to-light ratio of galaxy systems from poor groups torich clusters and present for the first time a large database for usefulcomparisons with theoretical predictions. We extend a previous work,where Bj band luminosities and optical virial masses wereanalyzed for a sample of 89 clusters. Here we also consider a sample of52 more clusters, 36 poor clusters, seven rich groups, and two catalogs,of ~500 groups each, recently identified in the Nearby Optical Galaxysample by using two different algorithms. We obtain the blue luminosityand virial mass for all systems considered. We devote a large effort toestablishing the homogeneity of the resulting values, as well as toconsidering comparable physical regions, i.e., those included within thevirial radius. By analyzing a fiducial, combined sample of 294 systemswe find that the mass increases faster than the luminosity: the linearfit gives M~L1.34+/-0.03B, with a tendency for asteeper increase in the low-mass range. In agreement with the previouswork, our present results are superior owing to the much higherstatistical significance and the wider dynamical range covered(~1012-1015 Msolar). We present acomparison between our results and the theoretical predictions on therelation between M/LB and halo mass, obtained by combiningcosmological numerical simulations and semianalytic modeling of galaxyformation.
|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 far-ultraviolet emission of early-type galaxies|
We have assembled a UV-flux selected sample of 82 early-type galaxiesand collected additional information at other wavelengths. These dataconfirm a large spread of the UV-V color in the range 2 to 5. The spreadin UV-V is accompanied by a spread in B-V that is mainly attributed tothe range of morphological types and luminosities. A large fraction ofthe objects have red colors, UV-V = 4 +/- 0.4, corresponding to a weakUV-upturn as observed with IUE. If the current interpretation for the UVemission from early-type galaxies is applicable to our sample, the PAGB(Post-Asymptotic Giant Branch) tracks are the most common evolution pathfor the low-mass stars responsible for the UV emission. A small numberof very blue (UV-V < 1.4) objects have been found that can bereasonably interpreted as harbouring some low level of star formation.In contrast to a previous sample based on IUE observations, nocorrelation is found between the UV-V color and the Mg2spectral line index; possible explanations are reviewed. The potentialof a more extended UV survey like GALEX is briefly presented.
|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
|The Optical and Near-Infrared Morphologies of Isolated Early-Type Galaxies|
To study early-type galaxies in their simplest environments, we haveconstructed a well-defined sample of 30 isolated galaxies. The samplecontains all early-type galaxies listed in the Third Reference Catalogueof Bright Galaxies (RC3) with no other cataloged galaxy with a knownredshift lying within a projected radius of 1h-1100 Mpc and +/-1000 km s-1 (where weuse the recession velocities in the RC3). We have obtained optical andnear-infrared images of 23 of the galaxies and of a comparison sample of13 early-type galaxies in X-ray-detected poor groups of galaxies. Wehave applied the techniques of unsharp-masking, galaxy model division,and color maps to search for morphological features that might provideclues to the evolution of these galaxies. Evidence for dust features isfound in approximately 75% of both the isolated and group galaxies (17of 22 and 9 of 12, respectively). However, shells or tidal features aremuch more prevalent in our isolated sample than in our group sample (9of 22=41% vs. 1 of 12=8%, respectively). The isolation and colors ofthese shell galaxies make it unlikely that tidal interactions orasymmetric star formation are the causes of such features. One modelthat is not ruled out is that mergers produce the shells. If shells anddust are both merger signatures, the absence of shells in groupelliptical galaxies implies that shells (1) form more easily, (2) areyounger, and/or (3) are longer lived in isolated environments.
|Details of the mass-temperature relation for clusters of galaxies|
We present results on the total mass and temperature determination usingtwo samples of clusters of galaxies. One sample is constructed withemphasis on the completeness of the sample, while the advantage of theother is the use of the temperature profiles, derived with ASCA. Weobtain remarkably similar fits to the M-T relation for both samples,with the normalization and the slope significantly different from bothprediction of self-similar collapse and hydrodynamical simulations. Wediscuss the origin of these discrepancies and also combine the X-raymass with velocity dispersion measurements to provide a comparison withhigh-resolution dark matter simulations. Finally, we discuss theimportance of a cluster formation epoch in the observed M-T relation.
|The intragroup medium in loose groups of galaxies|
We have used the ROSAT PSPC to study the properties of a sample of 24X-ray-bright galaxy groups, representing the largest sample examined indetail to date. Hot plasma models are fitted to the spectral data toderive temperatures, and modified King models are used to characterizethe surface brightness profiles. In agreement with previous work, wefind evidence for the presence of two components in the surfacebrightness profiles. The extended component is generally found to bemuch flatter than that observed in galaxy clusters, and there isevidence that the profiles follow a trend with system mass. We deriverelationships between X-ray luminosity, temperature and optical velocitydispersion. The relation between X-ray luminosity and temperature isfound to be LX~T4.9, which is significantlysteeper than the same relation in galaxy clusters. These results are ingood agreement with pre-heating models, in which galaxy winds raise theinternal energy of the gas, inhibiting its collapse into the shallowpotential wells of poor systems.
|X-ray evidence for multiphase hot gas with nearly solar Fe abundances in the brightest groups of galaxies|
We analyse the ASCA spectra accumulated within ~100kpc radii of 12 ofthe brightest groups of galaxies. Upon fitting isothermal models (1T)jointly to the ASCA SIS and GIS spectra we obtain fits for most groupsthat are of poor or at best marginal quality and give very subsolarmetallicities similar to previous studies,=0.29+/-0.12Zsolar. Two-temperature models (2T)provide significantly better fits for 11 out of the 12 groups, and inevery case have metallicities that are substantially larger thanobtained for the 1T models, =0.75+/-0.24Zsolar.Though not very well constrained, for most of the groups absorption inexcess of the Galactic value is indicated for the cooler temperaturecomponent of the 2T models. A simple multiphase cooling flow model givesresults analogous to the 2T models including large metallicities,=0.65+/-0.17Zsolar. The nearly solar Fe abundancesand also solar α/Fe ratios indicated by the 2T and cooling flowmodels are consistent with models of the chemical enrichment ofellipticals, groups, and clusters which assume ratios of Type Ia to TypeII supernovae and an initial mass function (IMF) similar to those of theMilky Way. Thus we have shown that the very subsolar Fe abundances andSi/Fe enhancements obtained from most previous studies within r~100kpcof galaxy groups are an artefact of fitting isothermal models to theX-ray spectra, which also has been recently demonstrated for thebrightest elliptical galaxies. Owing to the importance of these resultsfor interpreting X-ray spectra, in an appendix we use simulated ASCAobservations to examine in detail the `Fe bias' and `Si bias' associatedwith the spectral fitting of ellipticals, groups and clusters ofgalaxies.
|The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey. I. X-Ray Properties of Clusters Detected as Extended X-Ray Sources|
In the construction of an X-ray-selected sample of galaxy clusters forcosmological studies, we have assembled a sample of 495 X-ray sourcesfound to show extended X-ray emission in the first processing of theROSAT All-Sky Survey. The sample covers the celestial region withdeclination δ>=0deg and Galactic latitude|bII|>=20deg and comprises sources with a countrate >=0.06 counts s-1 and a source extent likelihood ofL>=7. In an optical follow-up identification program we find 378(76%) of these sources to be clusters of galaxies. It was necessary toreanalyze the sources in this sample with a new X-ray sourcecharacterization technique to provide more precise values for the X-rayflux and source extent than obtained from the standard processing. Thisnew method, termed growth curve analysis (GCA), has the advantage overprevious methods in its ability to be robust, to be easy to model and tointegrate into simulations, to provide diagnostic plots for visualinspection, and to make extensive use of the X-ray data. The sourceparameters obtained assist the source identification and provide moreprecise X-ray fluxes. This reanalysis is based on data from the morerecent second processing of the ROSAT Survey. We present a catalog ofthe cluster sources with the X-ray properties obtained as well as a listof the previously flagged extended sources that are found to have anoncluster counterpart. We discuss the process of source identificationfrom the combination of optical and X-ray data. To investigate theoverall completeness of the cluster sample as a function of the X-rayflux limit, we extend the search for X-ray cluster sources to the dataof the second processing of the ROSAT Survey for the northern sky regionbetween 9h and 14h in right ascension. We includethe search for X-ray emission of known clusters as well as a newinvestigation of extended X-ray sources. In the course of this search wefind X-ray emission from 85 additional Abell clusters and 56 veryprobable cluster candidates among the newly found extended sources. Acomparison of the X-ray cluster number counts of the NORAS sample withthe ROSAT-ESO Flux-limited X-Ray (REFLEX) Cluster Survey results leadsto an estimate of the completeness of the NORAS sample of ROSAT All-SkySurvey (RASS) I extended clusters of about 50% at an X-ray flux ofFX(0.1-2.4 keV)=3×10-12 ergs s-1cm-2. The estimated completeness achieved by adding thesupplementary sample in the study area amounts to about 82% incomparison to REFLEX. The low completeness introduces an uncertainty inthe use of the sample for cosmological statistical studies that will becured with the completion of the continuing Northern ROSAT All-Sky(NORAS) Cluster Survey project. Results reported here are based onobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
|The Properties of Poor Groups of Galaxies. III. The Galaxy Luminosity Function|
The form of the galaxy luminosity function (GLF) in poor groups-regionsof intermediate galaxy density that are common environments forgalaxies-is not well understood. Multiobject spectroscopy and wide-fieldCCD imaging now allow us to measure the GLF of bound group membersdirectly (i.e., without statistical background subtraction) and tocompare the group GLF with the GLFs of the field and of rich clusters.We use R-band images in 1.5×1.5 degree2 mosaics toobtain photometry for galaxies in the fields of six nearby(2800=MR>-19+5logh) to giants(MR<=-19+5logh) is significantly larger for the fivegroups with luminous X-ray halos than for the one marginallyX-ray-detected group; (2) the composite GLF for the luminous X-raygroups is consistent in shape with two measures of the composite R-bandGLF for rich clusters (Trentham; Driver et al.) and flatter at the faintend than another (α~-1.5 Smith et al.); (3) the composite groupGLF rises more steeply at the faint end than the R-band GLF of the LasCampanas Redshift Survey (LCRS; α=-0.7 from Lin et al.), a largevolume survey dominated by galaxies in environments more rarefied thanluminous X-ray groups; (4) the shape difference between the LCRS fieldand composite group GLFs results mostly from the population ofnon-emission line galaxies (EW [O II]<5 Å), whosedwarf-to-giant ratio is larger in the denser group environment than inthe field (cf. Ferguson & Sandage; Bromley et al.); and (5) thenon-emission line dwarfs are more concentrated about the group centerthan the non-emission line giants, except for the central, brightest(MR
|Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with ASCA|
Based on mapping observations with ASCA, an unusual hot region with aspatial extent of 1 deg2 was discovered between M87 and M49at a center coordinate ofR.A.=12h27m36s anddecl.=9deg18' (J2000). The X-ray emission from theregion has a 2-10 keV flux of 1x10-11 ergs s-1cm-2 and a temperature of kT>~4 keV, which issignificantly higher than that in the surrounding medium of ~2 keV. Theinternal thermal energy in the hot region is estimated to beVnkT~1060 ergs with a gas density of ~10-4cm-3. A power-law spectrum with a photon index of 1.7-2.3 isalso allowed by the data. The hot region suggests there is an energyinput due to a shock that is probably caused by the motion of the gasassociated with M49, infalling toward the M87 cluster with a velocity>~1000 km s-1.
|The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1|
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
|The Asiago-ESO/RASS QSO Survey. I. The Catalog and the Local QSO Luminosity Function|
This paper presents the first results of a survey for bright quasars(V<14.5 and R<15.4) covering the northern hemisphere at Galacticlatitudes |b|>30°. The photometric database is derived from theGuide Star and USNO catalogs. Quasars are identified on the basis oftheir X-ray emission measured in the ROSAT All-Sky Survey. The surfacedensity of quasars brighter than 15.5 mag turns out to be(10+/-2)×10-3 deg-2, about 3 times higherthan that estimated by the PG survey. The quasar optical luminosityfunction (LF) at 0.04
|IC3328: A ``dwarf elliptical galaxy'' with spiral structure|
We present the 2-D photometric decomposition of the Virgo galaxyIC3328. The analysis of the global light distributionof this morphologically classified nucleated dwarf elliptical galaxy(dE1,N) reveals a tightly wound, bi-symmetric spiral structure with adiameter of 4.5 kpc, precisely centered on the nucleus of the dwarf. Theamplitude of the spiral is only three percent of the dwarf's surfacebrightness making it the faintest and smallest spiral ever found in agalaxy. In terms of pitch angle and arm winding the spiral is similar tothe intermediate-type galaxy M51, but it lacks the dust and prominent HIi regions which signal the presence of gas. The visual evidence of aspiral pattern in an early-type dwarf galaxy reopens the question onwhether these dwarfs are genuine rotationally supported or anisotropicstellar systems. In the case of IC3328, we argue for a nearly face-ondisk (dS0) galaxy with an estimated maximum rotation velocity ofvc,max~ 55 km s-1 . The faintness of the spiraland the small motions within it, suggests that we could be seeingswing-amplified noise. The other possibility is a tidal origin, causedby the near passage of a small companion. Based on observationscollected at the European Southern Observatory (ESO 63.O-0055)
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