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|Environmental Effects on Late-Type Galaxies in Nearby Clusters|
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.
|Chandra Observations of A 2670 and A 2107: A Comet Galaxy and cDs with Large Peculiar Velocities|
We present an analysis of Chandra observations of the galaxy clusters A2670 and A 2107. Their cD galaxies have large peculiar velocities(>200 km s-1) and thus the clusters appear to beundergoing mergers. In A 2670, we find a comet-like structure around oneof the brightest galaxies. At the leading edge of the structure, thereis a cold front. The mass of the X-ray gas in the comet-like structuresuggests that the galaxy was in a small cluster or group, and itsintracluster medium is being stripped by ram-pressure. The regions ofcool interstellar medium of the cD galaxies in A 2670 and A 2107 arevery compact. This is similar to the brightest galaxies in the Comacluster, which is also a merging cluster. In each galaxy, the shortcooling time of the ISM requires a heating source; the compact nature ofthe ISM makes it unlikely that the heating source is a central activegalactic nucleus.
|The association between gas and galaxies - I. CFHT spectroscopy and pair analysis|
We investigate the relative distribution of the gaseous contents of theUniverse (as traced by a sample of Lyα absorbers), and theluminous baryonic matter (as traced by a redshift survey of galaxies inthe same volume searched for Lyα absorbers), along 16 lines ofsight (LOS) between redshifts 0 and 1. Our galaxy redshift survey wasmade with the multi-object spectrograph on the Canada-France-HawaiiTelescope and, when combined with galaxies from the literature in thesame LOS, gives us a galaxy sample of 636 objects. By combining thiswith an absorption-line sample of 406 absorbing systems drawn frompublished works, we are able to study the relationship between gas andgalaxies over the latter half of the age of the Universe. A correlationbetween absorbers and galaxies is detected out to separation of 1.5Mpc.This correlation is weaker than the galaxy-galaxy correlation. There isalso some evidence that the absorbing systems seen in CIV are moreclosely related to galaxies, although this correlation could be withcolumn density rather than metallicity. The above results are allconsistent with the absorbing gas and the galaxies coexisting in darkmatter filaments and knots as predicted by current models where thecolumn density of the absorbing gas is correlated with the underlyingmatter density.
|The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies|
The Hubble Space Telescope has revealed a dichotomy in the centralsurface brightness profiles of early-type galaxies, which havesubsequently been grouped into two families: core, boxy, anisotropicsystems; and cuspy (`power-law'), discy, rotating ones. Here weinvestigate whether a dichotomy is also present in the X-ray propertiesof the two families. We consider both their total soft emission(LSX,tot), which is a measure of the galactic hot gascontent, and their nuclear hard emission (LHX,nuc), mostlycoming from Chandra observations, which is a measure of the nuclearactivity. At any optical luminosity, the highest LSX,totvalues are reached by core galaxies; this is explained by their beingthe central dominant galaxies of groups, subclusters or clusters, inmany of the logLSX,tot (ergs-1) >~ 41.5 cases.The highest LHX,nuc values, similar to those of classicalactive galactic nuclei (AGNs), in this sample are hosted only by core orintermediate galaxies; at low luminosity AGN levels, LHX,nucis independent of the central stellar profile shape. The presence ofoptical nuclei (also found by HST) is unrelated to the level ofLHX,nuc, even though the highest LHX,nuc are allassociated with optical nuclei. The implications of these findings forgalaxy evolution and accretion modalities at the present epoch arediscussed.
|The Epochs of Early-Type Galaxy Formation as a Function of Environment|
The aim of this paper is to set constraints on the epochs of early-typegalaxy formation through the ``archaeology'' of the stellar populationsin local galaxies. Using our models of absorption-line indices thataccount for variable abundance ratios, we derive ages, totalmetallicities, and element ratios of 124 early-type galaxies in high-and low-density environments. The data are analyzed by comparison withmock galaxy samples created through Monte Carlo simulations taking thetypical average observational errors into account, in order to eliminateartifacts caused by correlated errors. We find that all threeparameters, age, metallicity, and α/Fe ratio, are correlated withvelocity dispersion. We show that these results are robust againstrecent revisions of the local abundance pattern at high metallicities.To recover the observed scatter we need to assume an intrinsic scatterof about 20% in age, 0.08 dex in [Z/H], and 0.05 dex in [α/Fe].All low-mass objects withM*<~1010Msolar (σ<~130kms-1) show evidence for the presence of intermediate-agestellar populations with low α/Fe ratios. About 20% of theintermediate-mass objects with1010<~M*/Msolar<~1011[110<~σ/(kms-1)<~230 both elliptical andlenticular galaxies] must have either a young subpopulation or a bluehorizontal branch. On the basis of the above relationships, valid forthe bulk of the sample, we show that the Mg-σ relation is mainlydriven by metallicity, with similar contributions from the α/Feratio (23%) and age (17%). We further find evidence for an influence ofthe environment on the stellar population properties. Massive early-typegalaxies in low-density environments seem on average ~2 Gyr younger andslightly (~0.05-0.1 dex) more metal-rich than their counterparts inhigh-density environments. No offsets in the α/Fe ratios areinstead detected. With the aid of a simple chemical evolution model, wetranslate the derived ages and α/Fe ratios into star formationhistories. We show that most star formation activity in early-typegalaxies is expected to have happened between redshifts ~3 and 5 inhigh-density environments and between redshifts 1 and 2 in low-densityenvironments. We conclude that at least 50% of the total stellar massdensity must have already formed at z~1, in good agreement withobservational estimates of the total stellar mass density as a functionof redshift. Our results suggest that significant mass growth in theearly-type galaxy population below z~1 must be restricted to lessmassive objects, and a significant increase of the stellar mass densitybetween redshifts 1 and 2 should be present, caused mainly by the fieldgalaxy population. The results of this paper further imply the presenceof vigorous star formation episodes in massive objects at z~2-5 andevolved elliptical galaxies around z~1, both observationally identifiedas SCUBA galaxies and extremely red objects, respectively.
|The K-Band Luminosities of Galaxies: Do S0s Come from Spiral Galaxies?|
There are two main theories for the formation of S0 galaxies: that S0and elliptical galaxies are formed in the same manner ab initio, or thatS0s are mainly spiral galaxies from which the gas has been stripped byinteractions with either hot cluster gas or other galaxies. Perhaps thebest way to test these formation ideas is to calculate the luminositiesof S0 and spiral galaxies in the K band, where dust effects for thespiral galaxies are minimal. If S0s are stripped spirals whose starformation has been quenched, we expect from existing models that S0sshould be ~0.75 mag less luminous in K absolute magnitudes thanearly-type spirals. Using the nearly 25,000 galaxies in the Two MicronAll Sky Survey that have total K magnitudes <=11.25, we are able toselect over 19,000 of these galaxies for analysis. Of these, 10,977 arein the Third Reference Catalogue of Bright Galaxies and 8230 are not. Weseparate galaxies by both Hubble type and whether they are in or out ofnearby clusters and near or far from us. K-band absolute magnitudes arepredicted using the velocity field model of Faber & Burstein forgalaxies with a velocity of 6000 km s-1 or less with respectto the cosmic background radiation. We present histograms, medians, andmeans for galaxies of all Hubble types in absolute K magnitudes. In allcases, we find that giant ellipticals are more luminous than S0s, andthat S0s are at least as luminous, if not more luminous, in the K bandthan are spiral galaxies. The K absolute magnitudes of the S0s comparedto those of the spirals show that S0s are not gas-stripped spiralgalaxies. While a few S0s can be gas-stripped spirals, as a class theyare not. We suggest that the E+A galaxies seen in clusters at moderateredshift are S0s onto which hot gas has cooled, producing a temporarysprinkling of star formation.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center, California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
|The Survival and Destruction of X-Ray Coronae of Early-Type Galaxies in Rich Cluster Environments: A Case Study of A1367|
A new Chandra observation of the northwest region of the galaxy clusterA1367 reveals four cool galaxy coronae (0.4-1.0 keV) embedded in the hot(5-6 keV) intracluster medium (ICM). While the large coronae of NGC 3842and NGC 3837 appear symmetric and relaxed, the galaxy coronae of the<~L* galaxies (NGC 3841 and CGCG 97090) are disturbed and beingstripped. Massive galaxies, generally with dense cooling cores, arebetter able to resist ram pressure stripping and survive in richenvironments than <~L* galaxies, whose galactic coronae are generallymuch less dense. The survival of these cool coronae implies that thermalconduction from the hot surrounding ICM has to be suppressed by a factorof at least 60 at the corona boundary. Within the galaxy coronae of NGC3842 and NGC 3837, stellar mass loss or heat conduction with the Spitzervalue may be sufficient to balance radiative cooling. Energy depositionat the ends of collimated jets may heat the outer coronae but allow thesurvival of a small, dense gas core (e.g., NGC 3842 in A1367 and NGC4874 in the Coma Cluster). The surviving X-ray coronae becomesignificantly smaller and fainter with the increasing ambient pressure.
|Infall, the Butcher-Oemler Effect, and the Descendants of Blue Cluster Galaxies at z~0.6|
Using wide-field Hubble Space Telescope WFPC2 imaging and extensive KeckLRIS spectroscopy, we present a detailed study of the galaxy populationsin MS 2053-04, a massive, X-ray-luminous cluster at z=0.5866+/-0.0011.Analysis of 149 confirmed cluster members shows that MS 2053 is composedof two structures that are gravitationally bound to each other; theirrespective velocity dispersions are 865+/-71 km s-1 (113members) and 282+/-51 km s-1 (36 members). MS 2053's totaldynamical mass is 1.2×1015Msolar. MS 2053 isa classic Butcher-Oemler cluster with a high fraction of blue members(24%+/-5%) and an even higher fraction of star-forming members(44%+/-7%), as determined from their [O II] λ3727 emission. Thenumber fraction of blue/star-forming galaxies is much higher in theinfalling structure than in the main cluster. This result is the mostdirect evidence to date that the Butcher-Oemler effect is linked togalaxy infall. In terms of their colors, luminosities, estimatedinternal velocity dispersions, and [O II] λ3727 equivalentwidths, the infalling galaxies are indistinguishable from the fieldpopulation. MS 2053's deficit of S0 galaxies combined with itsoverabundance of blue spirals implies that many of these late-typegalaxies will evolve into S0 members. The properties of the blue clustermembers in both the main cluster and infalling structure indicate thatthey will evolve into low-mass, L3.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the California Institute of Technology and theUniversity of California.
|Study of the Structure of the Coma Cluster Based on a Hierarchical Force Clustering Method|
Six subclusters in the Coma cluster have been selected on the basis of ahierarchical clustering method that takes the gravitational interactionamong galaxies into account. Of these, 3 central subclusters around thegalaxies NGC 4889, NGC 4874, and NGC 4839 have been singled out. We haveused the objective statistical criterion applied by Vennik and Anosovain studies of close groups of galaxies to evaluate each member includedin a subcluster with a high probability. Galaxies with a significantdeficit of hydrogen HI, including objects from the Bravo-Alfaro list,have been identified with members of the subclusters, with the greatestnumber of them in the subclusters around NGC 4874 and NGC 4839. Aquantitative estimate of the hydrogen deficit using the HI index in theRCG3 catalog reveals a statistically significant excess value for thosegalaxies that are members of the subclusters compared to galaxies with ahydrogen deficit in the overall Coma cluster field. A substantial numberof the spiral galaxies with a hydrogen deficit in the subclusters turnedout to be radio galaxies as well.
|EDisCS - the ESO distant cluster survey. Sample definition and optical photometry|
We present the ESO Distant Cluster Survey (EDisCS), a survey of 20fields containing distant galaxy clusters with redshifts ranging from0.4 to almost 1.0. Candidate clusters were chosen from among thebrightest objects identified in the Las Campanas Distant Cluster Survey,half with estimated redshift z_est0.5 and half with z_est0.8.They were confirmed by identifying red sequences in moderately deep twocolour data from VLT/FORS2. For confirmed candidates we have assembleddeep three-band optical photometry using VLT/FORS2, deep near-infraredphotometry in one or two bands using NTT/SOFI, deep optical spectroscopyusing VLT/FORS2, wide field imaging in two or three bands using the ESOWide Field Imager, and HST/ACS mosaic images for 10 of the most distantclusters. This first paper presents our sample and the VLT photometry wehave obtained. We present images, colour-magnitude diagrams and richnessestimates for our clusters, as well as giving redshifts and positionsfor the brightest cluster members. Subsequent papers will present ourinfrared photometry, spectroscopy, HST and wide-field imaging, as wellas a wealth of further analysis and science results. Our reduced databecome publicly available as these papers are accepted.
|The build-up of the Coma cluster by infalling substructures|
We present a new multiwavelength analysis of the Coma clustersubclustering based on recent X-ray data and on a compilation of nearly900 redshifts. We characterize subclustering using the Serna &Gerbal (1996, A&A, 309, 65) hierarchical method, which makes use ofgalaxy positions, redshifts, and magnitudes, and identify 17 groups. Oneof these groups corresponds to the main cluster, one is the well knowngroup associated with the infalling galaxy NGC 4839, and one isassociated with NGC 4911/NGC 4926. About one third of the 17 groups havevelocity distributions centered on the velocities of the very brightcluster galaxies they contain (magnitudes R < 13). In order to searchfor additional substructures, we made use of the isophotes of X-raybrightness residuals left after the subtraction of the best-fitβ-model from the overall X-ray gas distribution (Neumann et al.2003, A&A, 400, 811). We selected galaxies within each of theseisophotes and compared their velocity distributions with that of thewhole cluster. We confirm in this way the two groups associated,respectively, with NGC 4839, and with the southern part of the extendedwestern substructure visible in X-rays. We discuss the group propertiesin the context of a scenario in which Coma is built by the accretion ofgroups infalling from the surrounding large-scale structure. We estimatethe recent mass accretion rate of Coma and compare it with hierarchicalmodels of cluster evolution.
|Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles|
We present a study of the optical brightness profiles of early typegalaxies, using a number of samples of radio galaxies and opticallyselected elliptical galaxies. For the radio galaxy samples - B2 ofFanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined anumber of parameters that describe a "Nuker-law" profile, which werecompared with those already known for the optically selected objects. Wefind that radio active galaxies are always of the "core" type (i.e. aninner Nuker law slope γ < 0.3). However, there are core-typegalaxies which harbor no significant radio source and which areindistinguishable from the radio active galaxies. We do not find anyradio detected galaxy with a power law profile (γ > 0.5). Thisdifference is not due to any effect with absolute magnitude, since in aregion of overlap in magnitude the dichotomy between radio active andradio quiescent galaxies remains. We speculate that core-type objectsrepresent the galaxies that have been, are, or may become, radio activeat some stage in their lives; active and non-active core-type galaxiesare therefore identical in all respects except their eventualradio-activity: on HST scales we do not find any relationship betweenboxiness and radio-activity. There is a fundamental plane, defined bythe parameters of the core (break radius rb and breakbrightness μ_b), which is seen in the strong correlation betweenrb and μ_b. The break radius is also linearly proportionalto the optical Luminosity in the I band. Moreover, for the few galaxieswith an independently measured black hole mass, the break radius turnsout to be tightly correlated with MBH. The black hole masscorrelates even better with the combination of fundamental planeparameters rb and μ_b, which represents the centralvelocity dispersion.
|A dichotomy in the orientation of dust and radio jets in nearby low-power radio galaxies|
We examine the properties of central dust in nearby quiescent and activeearly-type galaxies. The active galaxies are low-power radio galaxieswith Fanaroff & Riley type I or I/II radio jets. We focus on (a) thecomparison of the dust distributions in the active and quiescent galaxysamples; and (b) the relation between the radio jet and dustorientations. Our main observational conclusions are: (i) in line withprevious studies, the dust detection rate is higher in radio-jetgalaxies than in non radio-jet galaxies; (ii) radio galaxies contain ahigher fraction of regular dust “ellipses” compared toquiescent galaxies which contain more often irregular dustdistributions; (iii) the morphology, size and orientation of dustellipses and lanes in quiescent early-types and active early-types withkpc-scale radio jets is very similar; (iv) dust ellipses are alignedwith the major axis of the galaxy, dust lanes do not show a preferredalignment except for large (>kpc) dust lanes which are aligned withthe minor axis of the galaxy; and (v) as projected on the sky, jets donot show a preferred orientation relative to the galaxy major axis (andhence dust ellipses), but jets are preferentially perpendicular to dustlanes. We show that the dust ellipses are consistent with being nearlycircular thin disks viewed at random viewing angles. The lanes arelikely warped dust structures, which may be in the process of settlingdown to become regular disks or are being perturbed by anon-gravitational force. We use the observed dust-jet orientations toconstrain the three-dimensional angle θDJ between jetand dust. For dust-lane galaxies, the jet is approximately perpendicularto the dust structure, while for dust-ellipse galaxies there is a muchwider distribution of θDJ. We discuss two scenariosthat could explain the dust/jet/galaxy orientation dichotomy. If lanesare indeed settling, then the jet orientation apparently is roughlyaligned with the angular momentum of the dust before it settles. Iflanes are perturbed by a jet-related force, it appears that it causesthe dust to move out of its equilibrium plane in the galaxy into a planewhich is perpendicular to the jet.
|Large scale diffuse light in the Coma cluster: A multi-scale approach|
We have obtained wide field images of the Coma cluster in the B, V, Rand I bands with the CFH12K camera at CFHT. To search for large scalediffuse emission, we have applied to these images an iterativemultiscale wavelet analysis and reconstruction technique which made itpossible to model all the sources (stars and galaxies) and subtract themfrom the original images. We found various concentrations of diffuseemission present in the central zone around the central galaxies NGC4874 and NGC 4889. We characterize the positions, sizes and colors ofthese concentrations. Some sources do not seem to have strong starformation, while one probably exhibits spiral-like colors. One possibleorigin for the star forming diffuse emission sources is that in theregion of the two main galaxies NGC 4874 and NGC 4889 spiral galaxieshave recently been disrupted and star formation is still active in thedispersed material. We also use the characteristics of the sources ofdiffuse emission to trace the cluster dynamics. A scenario in which thegroup around NGC 4874 is moving north is consistent with our data.
|The Halo Mass Distribution of Field and Cluster Early-Type Galaxies|
We describe an ongoing program to study the halo kinematics of a broadsample of early-type galaxies using integrated light measurementsobtained with the Hobby-Eberly and Gemini Telescopes.The Hobby-Eberly Telescope is operated by McDonald Observatory on behalfof the University of Texas at Austin, The Pennsylvania State University,Stanford University, Ludwid-Maximilians-Universit at Munchen, andGeorg-August-Universitat Gottingen.Part of this project is based on observations obtained at the GeminiObservatory, which is operated by AURA, INC., under a cooperativeagreement with the NSF on behalf of the Gemini Partnership: The NSF(USA), PPARC (UK), NRC (Canada), CONICYT (Chile), ARC (Australia), CNPq(Brazil) and CONICET (Argentina ).
|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.
|Early-Type Galaxies in the Coma Cluster: A New Piece in the Calcium Puzzle|
We present measurements of the Ca II triplet and the Ca4227 Lick indexfor a sample of early-type galaxies in the Coma Cluster, deriving, forthe first time, their corresponding relationships with velocitydispersion. Compared with a similar subsample of elliptical galaxies inthe field, Coma galaxies with velocity dispersions in the range ~180-270km s-1 exhibit significant differences in the strengths ofthe Ca features, suggesting an influence of the environment on the starformation histories of these galaxies. We argue that the main scenariospreviously proposed to explain the relatively low Ca II triplet ofgalaxies are not able by themselves to simultaneously reconcile thestrengths of the two Ca indices in both environments.
|The Extreme-Ultraviolet Emission in the Coma Cluster of Galaxies and the Underlying Source of this Radiation|
Observations with the Extreme Ultraviolet Explorer (EUVE) have shown theComa Cluster to be a source of EUV emission in excess of that producedby X-ray gas in the cluster. We have reexamined the EUVE data on thiscluster in an attempt to obtain clues as to the origin of this emission.We find two important new results. First, the ratio between theazimuthally averaged EUV excess emission and the ROSAT hard X-ray fluxis constant as a function of distance from the cluster center outward.Second, a correlation analysis between the EUV excess emission and theX-ray emission shows that on a detailed level the EUV excess isspatially closely related to the X-ray emission. These findingscontradict previous suggestions as to the underlying source of thediffuse EUV emission in Coma and provide important information in regardto the true source of this emission. We propose a new explanation forthe source of this emission: inverse Compton scattering of microwavebackground photons by secondary electrons and positrons. We explore thispossibility in some detail and show that it is consistent with all ofthe available observational evidence. The parent cosmic-ray protons mayhave been produced by any of a number of sources, including supernovae,active galaxies, galactic winds, and cluster formation shocks, but webelieve that the most likely source is cluster formation shocks. If theEUV emission in the Coma Cluster is, in fact, the result of secondaryelectrons, this may be the only direct evidence for secondary electronsin the intracluster medium of a cluster of galaxies, since recent worksuggests that secondary electrons may not be the cause of radio halos.
|Quantitative Morphology of Galaxies in the Core of the Coma Cluster|
We present a quantitative morphological analysis of 187 galaxies in aregion covering the central 0.28 deg2 of the Coma Cluster.Structural parameters from the best-fitting Sérsicr1/n bulge plus, where appropriate, exponential disk model,are tabulated here. This sample is complete down to a magnitude of R=17mag. By examining the recent compilation by Edwards et al. of galaxyredshifts in the direction of Coma, we find that 163 of the 187 galaxiesare Coma Cluster members and that the rest are foreground and backgroundobjects. For the Coma Cluster members, we have studied differences inthe structural and kinematic properties between early- and late-typegalaxies and between the dwarf and giant galaxies. Analysis of theelliptical galaxies reveals correlations among the structural parameterssimilar to those previously found in the Virgo and Fornax Clusters.Comparing the structural properties of the Coma Cluster disk galaxieswith disk galaxies in the field, we find evidence for an environmentaldependence: the scale lengths of the disk galaxies in Coma are 30%smaller. An analysis of the kinematics shows marginal differencesbetween the velocity distributions of elliptical galaxies withSérsic index n<2 (dwarfs) and those with n>2 (giants), thedwarf galaxies having a greater (cluster) velocity dispersion. Finally,our analysis of all 421 background galaxies in the catalog of Edwards etal. reveals a nonuniform distribution in redshift with contrasts indensity of ~3, characterized by a void extending from ~10,000 to ~20,000km s-1, and two dense and extended structures centered at~20,000 and ~47,000 km s-1.
|A Comparison of the Galaxy Populations in the Coma and Distant Clusters: The Evolution of k+a Galaxies and the Role of the Intracluster Medium|
The spectroscopic properties of galaxies in the Coma Cluster arecompared with those of galaxies in rich clusters at z~0.5, toinvestigate the evolution of the star formation history in clusters.Luminous galaxies with MV<=-20 andpoststarburst/post-star-forming (k+a) spectra that constitute asignificant fraction of galaxies in distant cluster samples are absentin Coma, where spectacular cases of k+a spectra are found instead atMV>-18.5 and represent a significant proportion of thecluster dwarf galaxy population. A simple inspection of their positionson the sky indicates that this type of galaxy does not show apreferential location within the cluster, but the bluest and strongestlined group of k+a galaxies lie in projection toward the central 1.4 Mpcof Coma and have radial velocities significantly higher than the clustermean. We find a striking correlation between the positions of theseyoung and strong poststarburst galaxies and substructure in the hotintracluster medium (ICM) identified from XMM-Newton data, with thesegalaxies lying close to the edges of two infalling substructures. Thisresult strongly suggests that the interaction with the dense ICM couldbe responsible for the quenching of the star formation (thus creatingthe k+a spectrum) and, possibly, for any previous starburst. Theevolution with redshift of the luminosity distribution of k+a galaxiescan be explained by a ``downsizing effect,'' with the maximumluminosity/mass of actively star-forming galaxies infalling ontoclusters decreasing at lower redshift. We discuss the possible physicalorigin of this downsizing effect and the implications of our results forcurrent scenarios of environmental effects on the star formation ingalaxies.Based on observations made with the William Herschel Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
|Revised Rates of Stellar Disruption in Galactic Nuclei|
We compute rates of tidal disruption of stars by supermassive blackholes in galactic nuclei, using downwardly revised black hole massesfrom the MBH-σ relation. In galaxies with steep nucleardensity profiles, which dominate the overall event rate, the disruptionfrequency varies inversely with assumed black hole mass. We compute atotal rate for nondwarf galaxies of ~10-5 yr-1Mpc-3, about a factor of 10 higher than in earlier studies.Disruption rates are predicted to be highest in nucleated dwarfgalaxies, assuming that such galaxies contain black holes. Monitoring ofa rich galaxy cluster for a few years could rule out the existence ofintermediate-mass black holes in dwarf galaxies.
|Improved Models for the Evolution of the Coma Cluster of Galaxies|
The analysis by Fitchett & Webster of the observations of the Comacluster of galaxies has demonstrated that the center of the Coma Clusterconsists of two subclusters. Therefore, it is important to constructrealistic dynamical models of a galaxy cluster with two mass centers.Our previous N-body models for the Coma Cluster consisted of pointmasses or particles with simple interaction properties. In the currentpaper, we employ a more sophisticated N-body code, which includesdynamical friction, mass exchange, and mergers between galaxies. Ourstarting point is a model where the two subclusters form a binarysystem. The rest of the cluster galaxies are in nearly radial, boundorbits around the center of mass of the binary. The initial galaxydensities and velocities are chosen according to a particularcosmological model. At the end of the N-body simulation of 250 galaxies,we extract the projected galaxy surface density and radial velocitydispersion profiles as a function of the distance from the center of themass of the cluster. With certain initial parameters, excellentagreement with observations is obtained. In such models, the use of thevirial theorem in the standard way gives an overestimate of the clustermass by a factor of about 3. Therefore, the true mass of the ComaCluster should be smaller than the usually quoted value by the samefactor. The mass-to-light ratio of the Coma Cluster should be about 100in solar units, in agreement with the analysis of the X-ray data byCowie et al.
|The X-ray luminosity function of galaxies in the Coma cluster|
The XMM-Newton survey of the Coma cluster of galaxies covers an area of1.86 square degrees with a mosaic of 16 pointings and has a total usefulintegration time of 400 ks. Detected X-ray sources with extent less than10'' were correlated with cataloged galaxies in the Comacluster region. The redshift information, which is abundant in thisregion of the sky, allowed us to separate cluster members frombackground and foreground galaxies. For the background sources, werecover a typical Log N-Log S in the flux range10-15-10-13 ergs s-1 cm-2 inthe 0.5-2.0 keV band. The X-ray emission from the cluster galaxiesexhibits X-ray colors typical of thermal emission. The luminosities ofComa galaxies lie in the 1039-1041 ergss-1 interval in the 0.5-2.0 keV band. The luminosity functionof Coma galaxies reveals that their X-ray activity is suppressed withrespect to the field by a factor of 5.6, indicating a lower level ofX-ray emission for a given stellar mass.
|The impact of high pressure cluster environment on the X-ray luminosity of Coma early-type galaxies|
We present an observational study of the LX vs. LBσ2 relation for early-type galaxies in the Coma clusterbased on the XMM-Newton survey data. Compared to a similar relation fora sample dominated by field early-type galaxies, the Coma clustergalaxies show a flatter slope. Our calculations show that adiabaticcompression produces a flattening in the LX vs.LBσ2 relation that is in remarkableagreement with the observed effect. Our scenario is further supported bythe observed compactness of the X-ray emission of Coma galaxies.
|Chemically consistent evolution of galaxies. II. Spectrophotometric evolution from zero to high redshift|
The composite stellar populations of galaxies comprise stars of a widerange of metallicities. Subsolar metallicities become increasinglyimportant, both in the local universe when going from early towardslater galaxy types as well as for dwarf galaxies and for all types ofgalaxies towards higher redshifts.We present a new generation of chemically consistent evolutionarysynthesis models for galaxies of various spectral types from E throughSd. The models follow the chemical enrichment of the ISM and take intoaccount the increasing initial metallicity of successive stellargenerations using recently published metallicity dependent stellarevolutionary isochrones, spectra and yields.Our first set of closed-box 1-zone models does not include any spatialresolution or dynamics. For a Salpeter initial mass function (IMF) thestar formation rate (SFR) and its time evolution are shown tosuccessfully parameterise spectral galaxy types E, ..., Sd. We show howthe stellar metallicity distribution in various galaxy types build upwith time to yield after 12 Gyr agreement with stellar metallicitydistributions observed in our and other local galaxies.The models give integrated galaxy spectra over a wide wavelength range(90.9 Å-160 μm), which for ages of 12 Gyr are in goodagreement not only with observed broad band colours but also withtemplate spectra for the respective galaxy types.Using filter functions for Johnson-Cousins U, B, V, RC,IC, as well as for HST broad band filters in the optical andBessel & Brett's NIR J, H, K filter system, we calculate theluminosity and colour evolution of model galaxies over a Hubble time.Including a standard cosmological model (H0 = 65,Ω0 = 0.1) and the attenuation by intergalactic hydrogenwe present evolutionary and cosmological corrections as well as apparentluminosities in various filters over the redshift range from z 5to the present for our galaxy types and compare to earlier models usingsingle (=solar) metallicity input physics only. We also resent a firstcomparison of our cc models to HDF data. A more detailed comparison withHubble Deep Field (HDF) and other deep field data and an analysis andinterpretation of high redshift galaxies in terms of ages,metallicities, star formation histories and, galaxy types will be thesubject of a forthcoming paper.
|Investigation of Galaxy Alignment in X-ray Subclusters of the Coma Cluster|
|Stellar collisions in galactic centres: black hole growth and colour gradients|
We study the effects of stellar collisions, particularly on feedingmassive black holes (BHs) and colour gradients, in realistic galacticcentres. We find that the mass released by stellar collisions is notsufficient to account for the present BH mass in galactic centres,especially in bright galaxies. This study, together with the study byMagorrian & Tremaine on tidal disruption of stars by massive BHs,implies that the material for BH growth (especially in galaxies brighterthan ~109 Lsolar) can only come from othersources, for example, the mass released by stellar evolution in theinitial ~1 Gyr of the lifetime of the galaxy, or the gas that sinks tothe galactic centre in a galaxy merger. We also analyse how the colourof a stellar system is affected by collisions of stars. We find thatcollisions between main-sequence stars cannot cause observable colourgradients in the visible bands at projected radius R>~ 0.1 arcsec inM31, M32 and other nearby galactic centres. This result is consistentwith the lack of an observable colour gradient in M32 at R >~ 0.1arcsec. At even smaller radii, the colour differences caused bycollisions between main-sequence stars are at most 0.08 mag at R = 0.02arcsec. The averaged bluing caused by stellar collisions in the region R< 0.1 arcsec of M32 should not be larger than 0.06 mag in colourindex U - V and 0.02 mag in V - I. The observed bluing in the centre ofthe galaxy M31 (in a 0.14 × 0.14 arcsec2 box) must becaused by some mechanism other than collisions between main-sequencestars.
|The Extinction and Distance of Maffei 1|
We have obtained low- and high-resolution spectra of the core of thehighly reddened elliptical galaxy Maffei 1. From these data, we haveobtained the first measurement of the Mg2 index and havemeasured the velocity dispersion and radial velocity with improvedaccuracy. To evaluate the extinction, a correlation between theMg2 index and effective V-I color has been established forelliptical galaxies. Using a new method for correcting for effectivewavelength shifts, the V-I color excess reveals that the optical depthof Galactic dust at 1 μm is 1.69+/-0.07. Thus,AV=4.67+/-0.19 mag, which is lower by 0.4 mag than previouslythought. To establish the distance, the fundamental plane for ellipticalgalaxies has been constructed in I. The velocity dispersion of Maffei 1,measured to be 186.8+/-7.4 km s-1, in combination with modernwide-field photometry in I, leads to a distance of 2.92+/-0.37 Mpc. TheDn-σ relation, which is independently calibrated, gives3.08+/-0.85 and 3.23+/-0.67 Mpc from photometry in B and K',respectively. The weighted mean of the three estimates is 3.01+/-0.30Mpc, which is lower than distances judged with reference to M32 and thebulge of M31 from the brightest stars seen at K'. Since the luminosityof asymptotic giant branch stars at K' is strongly dependent on age, thelower distance suggests that the last epoch of star formation in Maffei1 occurred farther in the past than in these other systems. The distanceand luminosity make Maffei 1 the nearest giant elliptical galaxy. In theabsence of extinction, the galaxy would be among the brightest in thesky and would have an apparent size 2/3 that of the full Moon. Theradial velocity of Maffei 1 is +66.4+/-5.0 km s-1,significantly higher than the accepted value of -10 km s-1.The Hubble distance corresponding to the mean velocity of Maffei 1,Maffei 2, and IC 342 is 3.5 Mpc. Thus, it is unlikely that Maffei 1 hashad any influence on Local Group dynamics.
|Intergalactic Globular Clusters and the Faint End of the Galaxy Number Counts in A1656 (Coma)|
The existence of an intergalactic globular cluster population in theComa cluster of galaxies has been tested using surface brightnessfluctuations. The main result is that the intergalactic globular clustersurface density (NIGC) does not correlate with the distanceto the center of Coma and hence with the environment. Furthermore,comparing these results with different Coma mass distribution modelpredictions, it is suggested that NIGC must in fact be zeroall over Coma. On the other hand, the results for NIGC andthe faint end of the galaxy number counts (beyond mR=23.5)are connected. So NIGC=0 settles the slope of this function,which turns out to be γ=0.36+/-0.01 down to mR=26.5.The fact that NIGC=0 all over Coma suggests that globularclusters were formed only, or almost only, from protogalactic clouds.None, or perhaps very few, could have formed in isolated regions. Italso seems inappropriate to advocate a relationship betweenintergalactic globular clusters and dark matter distributions, althoughit is true that the relationship could still exist but not be strongenough to have been detected. Finally, since our conclusion is thatintergalactic globular clusters do not exist in Coma, accretion ofintergalactic globular clusters might not be significant in galaxyformation and evolutionary processes in the Coma galaxies.
|Lensing and the Centers of Distant Early-Type Galaxies|
Gravitational lensing provides a unique probe of the inner 10-1000 pc ofdistant galaxies (z~0.2-1). Theoretical studies have predicted that eachstrong lens system should have a faint image near the center of the lensgalaxy, which should, in principle, be visible in radio lenses but hasnever been detected. We study the predicted ``core'' images using modelsderived from the stellar distributions in nearby early-type galaxies. Wefind that realistic lens galaxies produce a remarkably wide range ofcore images, with magnifications spanning some 6 orders of magnitude.More concentrated galaxies produce fainter core images, although notwith any model-independent relation between the galaxy properties andthe core images. Some real galaxies have diffuse cores that should yieldbright core images (magnification μcore>~0.1), but morecommon are galaxies that yield faint core images(μcore<~0.001). Thus, stellar mass distributions aloneare probably concentrated enough to explain the lack of observed coreimages. Observational sensitivity may need to improve by an order ofmagnitude before detections of core images become common. Two-imagelenses should tend to have brighter core images than four-image lenses,so they will be the better targets for finding core images andexploiting these tools for studying the central mass distributions ofdistant galaxies.
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