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|K-band Properties of Galaxy Clusters and Groups: Brightest Cluster Galaxies and Intracluster Light|
We investigate the near-infrared K-band properties of the brightestcluster galaxies (BCGs) in a sample of 93 X-ray galaxy clusters andgroups, using data from the Two Micron All Sky Survey. Our clustersample spans a factor of 70 in mass, making it sensitive to any clustermass-related trends. We derive the cumulative radial distribution forthe BCGs in the ensemble and find that 70% of the BCGs are centered inthe cluster to within 5% of the virial radius r200; thisquantifies earlier findings that BCG position coincides with the clustercenter as defined by the X-ray emission peak. We study the correlationsbetween the luminosity of the BCGs (Lb) and the mass and theluminosity of the host clusters, finding that BCGs in more massiveclusters are more luminous than their counterparts in less massivesystems and that the BCGs become less important in the overall clusterlight (L200) as cluster mass increases. By examining a largesample of optically selected groups, we find that these correlationshold for galactic systems less massive than our clusters(<3×1013 Msolar). From the differencesbetween luminosity functions in high- and low-mass clusters, we arguethat BCGs grow in luminosity mainly by merging with other luminousgalaxies as the host clusters grow hierarchically; the decreasing BCGluminosity fraction (Lb/L200) with cluster massindicates that the rate of luminosity growth in BCGs is slow compared tothe rate at which clusters acquire galaxy light from the field or othermerging clusters. Utilizing the observed correlation between the clusterluminosity and mass and a merger tree model for cluster formation, weestimate that the amount of intracluster light (ICL) increases withcluster mass; our calculations suggest that in 1015Msolar clusters more than 50% of total stellar mass is inICL, making the role of ICL very important in the evolution andthermodynamic history of clusters. The cluster baryon fractionaccounting for the ICL is in good agreement with the value derived fromcosmic microwave background observations. The inclusion of ICL reducesthe discrepancy between the observed cluster cold baryon fraction andthat found in hydrodynamical simulations. Based on the observed ironabundance in the intracluster medium, we find that the ICL predicted byour model, together with the observed galaxy light, match the ironmass-to-light ratio expected from simple stellar population models,provided that the Salpeter initial mass function is adopted. The ICLalso makes it easier to produce the ``iron excess'' found in the centralregions of cool-core clusters.
|K-Band Properties of Galaxy Clusters and Groups: Luminosity Function, Radial Distribution, and Halo Occupation Number|
We explore the near-infrared (NIR) K-band properties of galaxies within93 galaxy clusters and groups using data from the Two Micron All SkySurvey. We use X-ray properties of these clusters to pinpoint clustercenters and estimate cluster masses. By stacking all these systems, westudy the shape of the cluster luminosity function and the galaxydistribution within the clusters. We find that the galaxy profile iswell described by the Navarro, Frenk, & White (NFW) profile with aconcentration parameter c~3, with no evidence for cluster massdependence of the concentration. Using this sample, whose masses spanthe range from 3×1013 to2×1015Msolar, we confirm the existence of atight correlation between total galaxy NIR luminosity and clusterbinding mass, which indicates that NIR light can serve as a cluster massindicator. From the observed galaxy profile, together with cluster massprofile measurements from the literature, we find that the mass-to-lightratio is a weakly decreasing function of cluster radius and that itincreases with cluster mass. We also derive the mean number of galaxieswithin halos of a given mass, the halo occupation number. We find thatthe mean number scales as N~M0.84+/-0.04 for galaxiesbrighter than MK=-21, indicating that high-mass clusters havefewer galaxies per unit mass than low-mass clusters. Using publishedobservations at high redshift, we show that higher redshift clustershave higher mean occupation numbers than nearby systems of the samemass. By comparing the luminosity function and radial distribution ofgalaxies in low-mass and high-mass clusters, we show that there is amarked decrease in the number density of galaxies fainter thanM* as one moves to higher mass clusters; in addition,extremely luminous galaxies are more probable in high-mass clusters. Weexplore several processes, including tidal interactions and merging, asa way of explaining the variation in galaxy population with clustermass.
|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.
|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.
|Redshift-Distance Survey of Early-Type Galaxies: Spectroscopic Data|
We present central velocity dispersions and Mg2 line indicesfor an all-sky sample of ~1178 elliptical and S0 galaxies, of which 984had no previous measures. This sample contains the largest set ofhomogeneous spectroscopic data for a uniform sample of ellipticalgalaxies in the nearby universe. These galaxies were observed as part ofthe ENEAR project, designed to study the peculiar motions and internalproperties of the local early-type galaxies. Using 523 repeatedobservations of 317 galaxies obtained during different runs, the dataare brought to a common zero point. These multiple observations, takenduring the many runs and different instrumental setups employed for thisproject, are used to derive statistical corrections to the data and arefound to be relatively small, typically <~5% of the velocitydispersion and 0.01 mag in the Mg2 line strength. Typicalerrors are about 8% in velocity dispersion and 0.01 mag inMg2, in good agreement with values published elsewhere.
|Redshift-Distance Survey of Early-Type Galaxies: Circular-Aperture Photometry|
We present R-band CCD photometry for 1332 early-type galaxies, observedas part of the ENEAR survey of peculiar motions using early-typegalaxies in the nearby universe. Circular apertures are used to tracethe surface brightness profiles, which are then fitted by atwo-component bulge-disk model. From the fits, we obtain the structuralparameters required to estimate galaxy distances using theDn-σ and fundamental plane relations. We find thatabout 12% of the galaxies are well represented by a pure r1/4law, while 87% are best fitted by a two-component model. There are 356repeated observations of 257 galaxies obtained during different runsthat are used to derive statistical corrections and bring the data to acommon system. We also use these repeated observations to estimate ourinternal errors. The accuracy of our measurements are tested by thecomparison of 354 galaxies in common with other authors. Typical errorsin our measurements are 0.011 dex for logDn, 0.064 dex forlogre, 0.086 mag arcsec-2 for<μe>, and 0.09 for mRC,comparable to those estimated by other authors. The photometric datareported here represent one of the largest high-quality and uniformall-sky samples currently available for early-type galaxies in thenearby universe, especially suitable for peculiar motion studies.Based on observations at Cerro Tololo Inter-American Observatory (CTIO),National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation (NSF);European Southern Observatory (ESO); Fred Lawrence Whipple Observatory(FLWO); and the MDM Observatory on Kitt Peak.
|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.
|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.
|Diffuse X-Ray Emission in Three Poor Clusters of Galaxies|
We report on ROSAT PSPC soft X-ray observations of three poor clustersof galaxies at distances above 100 Mpc (cz>8000 km s-1). Inall three cases the emission is centered on the dominant member of thecluster, i.e., NGC 4104, NGC 6269, and NGC 6329, respectively. X-rayemission was detected out to radii of 400-600 kpc. The bolometric X-rayluminosities range from 2.6 to 8.6x1042 ergs s-1.The soft X-ray emission characteristics and the physical propertiesdeduced from our observations of all three poor clusters resemble thoseof downscaled rich clusters. In each case, the soft X-ray spectrum iswell represented by a thermal model with kT~=1.1-1.3 keV and near-solarmetallicity in the center, increasing to kT~=1.4-1.6 keV toward theouter boundaries while the metallicity, Z, decreases to about 0.1 solar.Equally good fits can be achieved if the metallicity is left at thesolar value and an additional gas component with kT~=0.5 keV isintroduced. The central electron densities in all three poor clustersstudied here are enhanced with respect to a King profile by factors of2-6. This, together with the results of the spectral fits, can beinterpreted as either indicating the presence of cooling flows or of atwo-phase medium in the central areas. The spatial electron densitydistribution in the outer regions of each cluster can be fitted by Kingprofiles with core radii of 17-60 kpc and exponents of β=0.38-0.44.Using the derived radial temperature and density distributions, thetotal gravitating mass is obtained. We deriveMtot=3.7+/-0.7x1013 Msolar within aradius of 300 kpc for each of the three systems, as opposed to1014-1015 Msolar for rich clusters. Wefind that the LX versus kT relation found by A. C. Edge andG. C. Stewart (1991) for rich clusters of galaxies scales into thedomain of poor clusters and groups of galaxies. The spectral fits of thecentral regions show that none of the first-ranking galaxies of thethree poor clusters hosts a Seyfert 1 active galactic nucleus thatcontributes significantly to the emission in the ROSAT band (0.1-2.4keV).
|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.
|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.
|ROSAT HRI observations of six early-type galaxies|
High-resolution ROSAT HRI soft X-ray observations of four E/S0 galaxieswere conducted by us. The data show no signs of Seyfert activity in theX-ray regime. The central emission peaks of the four galaxies, NGC533,NGC2832, NGC4104 and NGC6329, are associated with their cooling flows.The half intensity radii of the cooling flows range from 0.8 to 3.5 kpc.We find a trend (based up to now on only five objects) of the radiopower of the cores in E/S0 galaxies to increase with the size and theaccretion rates of their cooling flows. In one galaxy, NGC4921, nocentrally peaked extended gaseous envelope was found, which is mostlikely due to the fact that it is not an E/S0 galaxy, but an early-typespiral. NGC2885, the sixth galaxy in our initial sample, shows signs ofX-ray emission from an AGN. It has also been classified as a Sy-1 AGN byBade et al. (1995). However, optical imaging suggests that this galaxyis probably not an E or S0 type system either, but rather an early-typespiral galaxy. Thus, in the context of accretion rate vs. galaxy typemodels of low-luminosity AGNs, the presence of an X-ray luminous Sy-1nucleus in NGC2885 is no surprise.
|The Intragroup Medium in Poor Groups of Galaxies|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...456...80M&db_key=AST
|Large-Scale Structure at Low Galactic Latitude|
We have extended the CfA Redshift Survey to low galactic latitudes toinvestigate the relation between the Great Wall in the North GalacticCap and the Perseus-Pisces chain in the South Galactic Cap. We presentredshifts for 2020 galaxies in the Catalogue of Galaxies and of Clustersof Galaxies (Zwicky et al. 1961-68, CGCG) in the following regions: 4^h^<= α <= 8^h^, 17^h^ <= α <= 20^h^, 0^deg^ <=δ <= 45^deg^. In these regions, the redshift catalogue includes1664 galaxies with B(0) <= 15.5 (of which 820 are newly measured) andis 97% complete. We also include redshifts for an additional 356galaxies in these regions with B(O) > 15.5; of these, 148 werepreviously unmeasured. The CGCG samples the galaxy distribution down tob_II_ = 10^deg^. In this paper, we discuss the acquisition and reductionof the spectra, and we examine the qualitative features of the redshiftdistribution. The Great Wall and the Perseus-Pisces chain are not simplyconnected across the Zone of Avoidance. These structures, which at firstappear to be coherent on scales of ~100 h^-1^ Mpc or more, actually formthe boundaries of neighboring voids of considerably smaller scale,approximately 50h^-1^ Mpc. The structures delineated by ouroptically-selected sample are qualitatively similar to those detected bythe far-infrared-selected IRAS 1.2 Jansky Survey (Fisher et al. 1995).Although the IRAS survey probes more deeply into the Zone of Avoidance,our optically-selected survey provides better sampling of structures atb_II_ >= 10^deg^.
|An image database. II. Catalogue between δ=-30deg and δ=70deg.|
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
|Spectroscopy of Zwicky galaxies identified with B3 radiosources|
Results are presented of long-slit CCD spectroscopy of 33 Zwickygalaxies identified in the B3 Survey as radiosources, includinginformation on morphological classification, photographic magnitude,line identification, measured wavelength, adopted rest wavelength,redshift computed for the line, and average redshift and rms. In thissample, 24 galaxies showed emission (and often absorption) lines, while9 galaxies showed only absorption lines. Nine of the 14 galaxiesclassified as E/SO showed the forbidden N II emission line at an EW(lim)= 1.5 A. The rest were classified as Spirals, and, out of 19 of thesegalaxies, 15 showed the forbidden N II emission line. No significantdifferences were found between the percentage of galaxies with emissionlines in this sample and the samples of optically selected galaxies ofsimilar equivalent width limit.
|A determination of the local radio luminosity function of radiogalaxies at 408 MHz|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991A&A...241...35B&db_key=AST
|Central Parts of Some Irregular Type-II Candidate Galaxies|
|An apparent increase of the Hubble constant with velocities of the determining galaxies|
A study is conducted of the variation of the Hubble constant derivedfrom the Bottinelli et al. (1983) version of the Tully-Fisher relationwith respect to kinematic distances D(v) in an infall model towardVirgo. The sample of 256 galaxies used encompasses types Sab to Sd. Itis noted that, before or after correction of the type effect, the Hubbleconstant increases from approximately log H(0) of 1.78 at D(v) of 250km/sec to 2.08 at 5000 km/sec. Attention is given to such differentpossible explanations for this as galactic extinction, the Malmquistbias, TF relation slope, and an inadequate model.
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