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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.
| Near-infrared study of CIZA J1324.7-5736, the second richest cluster of galaxies in the Great Attractor
We present the result of a deep near-infrared survey of the newlyidentified X-ray luminous cluster of galaxies CIZA J1324.7-5736 in theGreat Attractor (GA) region. In a 35 × 35 arcmin2region, 111 galaxy candidates with rKs20 >arcsec are identified. Comparison of the extinction-correctedKs-band luminosity function of CIZA J1324.7-5736 with thoseof nearby clusters indicates that the richness class of CIZAJ1324.7-5736 is almost the same as, or richer than, the Pavo, Centaurusand Hydra clusters but poorer than the Coma, Perseus and Norma clusters.CIZA J1324.7-5736 is possibly the second richest cluster in the GAregion following the Norma cluster. The position of CIZA J1324.7-5736[in the (l, b, v) space] is close to the Centaurus-Crux cluster and theagglomeration of galaxies detected by the Parkes HI survey. CIZAJ1324.7-5736, together with the Centaurus-Crux cluster and the HI galaxyagglomeration, is most likely to be one of the richest localconcentrations in the GA overdensity of galaxies.
| Chandra X-Ray Observations of Galaxies in an Off-Center Region of the Coma Cluster
We have performed a pilot Chandra survey of an off-center region of theComa Cluster to explore the X-ray properties and luminosity function ofnormal galaxies. We present results on 13 Chandra-detected galaxies withoptical photometric matches, including four spectroscopically confirmedComa-member galaxies. All seven spectroscopically confirmed giant Comagalaxies in this field have detections or limits consistent with lowX-ray to optical flux ratios[(fX/fR)<10-3]. We do not havesufficient numbers of X-ray-detected galaxies to directly measure thegalaxy X-ray luminosity function (XLF). However, since we have awell-measured optical LF, we take this low X-ray to optical flux ratiofor the seven spectroscopically confirmed galaxies to translate theoptical LF to an XLF. We find good agreement with Finoguenov et al.(2004), indicating that the X-ray emission per unit optical flux pergalaxy is suppressed in clusters of galaxies, but we extend this work toa specific off-center environment in the Coma Cluster. Finally, wereport the discovery of a region of diffuse X-ray flux that mightcorrespond to a small group interacting with the Coma intraclustermedium (ICM).
| Coma revealed as an extended hard X-rays source by INTEGRAL IBIS/ISGRI
Aims.We report the INTEGRAL/IBISobservations of the Coma Clusterin thehard X-ray/soft γ-raydomain. Methods: .Since the ComaClusterappears as an extended source, its global intensity andsignificance cannot be directly extracted with standard coded maskanalysis. We used the method of imaging the extended sources with acoded mask telescope developed by Renaud et al. (2006). Results:.The imaging capabilities and the sensitivity of the IBIS/ISGRIcodedmask instrument allows us to identify for the first time the site of theemission above ~15 keV. We have studied the Coma Clustermorphology inthe 18-30 keV band and found that it follows the prediction based onX-rayobservations. We also bring constraints on the non-thermalmechanism contribution at higher energies.
| Cluster temperatures and non-extensive thermo-statistics
We propose a novel component to the understanding of the temperaturestructure of galaxy clusters which does not rely on any heating orcooling mechanism. The new ingredient is the use of non-extensivethermo-statistics which is based on the natural generalization ofentropy for systems with long-range interactions. Such interactionsinclude gravity and attraction or repulsion due to charges. We explainthat there is growing theoretical indications for the need of thisgeneralization for large cosmological structures. The observed pseudotemperature is generally different from the true thermodynamictemperature, and we clarify the connection between the two. We explainthat this distinction is most important in the central part of thecluster where the density profile is most shallow. We show that theobserved pseudo temperature may differ up to a factor 2/5 from the truethermodynamic temperature, either larger or smaller. In general the M Tand L T relations will be affected, and the central DM slope derivedthrough hydrostatic equilibrium may be either more shallow or steeper.We show how the true temperature can be extracted correctly either fromthe spectrum or from the shape of the Doppler broadening of spectrallines.
| Environmental dependence of the structure of brightest cluster galaxies
We measure the Petrosian structural properties of 33 brightest clustergalaxies (BCGs) at redshifts z<= 0.1 in X-ray selected clusters witha wide range of X-ray luminosities. We find that some BCGs show distinctsignatures in their Petrosian profiles, likely to be due to cD haloes.We also find that BCGs in high X-ray luminosity clusters have shallowersurface brightness profiles than those in low X-ray luminosity clusters.This suggests that the BCGs in high X-ray luminosity clusters haveundergone up to twice as many equal-mass mergers in their past as thosein low X-ray luminosity clusters. This is qualitatively consistent withthe predictions of hierarchical structure formation.
| 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.
| Hot stars in old stellar populations: a continuing need for intermediate ages
We investigate the effect of a low-level contamination of hot, old,metal-poor starlight on the inferred stellar populations of early-typegalaxies in the core of the Coma Cluster. We find that the requiredcorrection to the Balmer and metal absorption-line strengths for old,metal-poor stars does not significantly affect the inferred age of thestellar population when the Hβ strength is large. Intermediate-agedpopulations are therefore still needed to explain enhanced Balmer-linestrengths in early-type galaxies. This gives us increased confidence inour age estimates for these objects. For galaxies with weak Balmer-linestrengths corresponding to very old populations (t > 10 Gyr),however, a correction for hot stars may indeed alter the inferred age,as previously suggested. Finally, the inferred metallicity [Z/H] willalways be higher after any correction for old, metal-poor starlight thanif it were not taken into account, but the enhancement ratios [E/Fe]will strengthen only slightly.
| Unification in the low radio luminosity regime: evidence from optical line emission
We address the question of whether or not the properties of alllow-luminosity flat spectrum radio sources, not just the obvious BL Lacobjects, are consistent with them being the relativistically beamedcounterparts of the low radio luminosity radio galaxies (theFanaroff-Riley type 1, FR I). We have accumulated data on a well-definedsample of low redshift, core-dominated, radio sources all of which haveone-sided core-jet structures seen with very long baselineinterferometry, just like most BL Lac objects. We first compare theemission-line luminosities of the sample of core-dominated radio sourceswith a matched sample of FR I radio galaxies. The emission lines in thecore-dominated objects are on average significantly more luminous thanthose in the comparison sample, inconsistent with the simplest unifiedmodels in which there is no orientation dependence of the line emission.We then compare the properties of our core-dominated sample with thoseof a sample of radio-emitting UGC galaxies selected without bias to corestrength. The core-dominated objects fit well on the UGC correlationbetween line emission and radio core strength found by Verdoes Kleijn etal. The results are not consistent with all the objects participating ina simple unified model in which the observed line emission isorientation independent, though they could fit a single, unified modelprovided that some FR I radio galaxies have emission line regions thatbecome more visible when viewed along the jet axis. However, they areequally consistent with a scenario in which, for the majority ofobjects, beaming has minimal effect on the observed core luminosities ofa large fraction of the FR I population and that intrinsically strongercores simply give rise to stronger emission lines. We conclude that FR Iunification is much more complex than usually portrayed, and modelscombining beaming with an intrinsic relationship between core andemission line strengths need to be explored.
| Very Small Array observations of the Sunyaev-Zel'dovich effect in nearby galaxy clusters
We present Very Small Array (VSA) observations (centred on ~34 GHz) onscales ~20 arcmin towards a complete, X-ray flux-limited sample of sevenclusters at redshift z < 0.1. Four of the clusters have significantSunyaev-Zel'dovich (SZ) detections in the presence of cosmic microwavebackground (CMB) primordial anisotropy. For all seven, we use a BayesianMarkov Chain Monte Carlo (MCMC) method for inference from the VSA data,with X-ray priors on cluster positions and temperatures, and radiopriors on sources. In this context, the CMB primordial fluctuations arean additional source of Gaussian noise, and are included in the model asa non-diagonal covariance matrix derived from the known angular powerspectrum. In addition, we make assumptions of β-model gasdistributions and of hydrostatic equilibrium, to evaluate probabilitydensities for the gas mass (Mgas) and total mass(Mr) out to r200, the radius at which the averagedensity enclosed is 200 times the critical density at the redshift ofthe cluster. This is further than has been done before and close to theclassical value for a collapsed cluster. Our combined estimate of thegas fraction (fgas=Mgas/Mr) is0.08+0.06-0.04h-1. The random errorsare poor (note, however, that the errors are higher than would have beenobtained with the usual χ2 method on the same data) butthe control of bias is good. We have described the MCMC analysis methodspecifically in terms of SZ but hope the description will be of moregeneral use. We find that the effects of primordial CMB contaminationtend to be similar in the estimates of both Mgas andMr over the narrow range of angular scales we are dealingwith, so that there is little effect of primordials on fgasdetermination. Using our Mr estimates we find a normalizationof the mass-temperature relation based on the profiles from the VSAcluster pressure maps, which is in good agreement with recent M-Tdeterminations from X-ray cluster measurements.
| Spherical models for early-type galaxies with cuspy mass densities
Spherical mass density models are used to fit the central surfacebrightness profiles of early-type galaxies which are generated fromNuker law parameters obtained from the literature. The mass density andthe corresponding potential are in an analytical form. It is shown thatonly a few mass density components are necessary to obtain a good fitand that for all power-law galaxies and for the core galaxies that weconsider, most or all of the mass density components must have cusps toprovide good fits. The applied quadratic programming fitting allows fora method of deprojection, which is reliable and convenient. The resultscan be used directly for further dynamical modelling.
| A Small X-Ray Corona of the Narrow-Angle Tail Radio Galaxy NGC 1265 Soaring through the Perseus Cluster
A deep Chandra observation of NGC 1265 (3C 83.1B), the prototype for thenarrow-angle tail (NAT) radio galaxy, reveals a small cool X-ray thermalcorona (~0.6 keV) embedded in the hot ICM of the Perseus cluster (~6.7keV). The corona is asymmetric with a sharp edge (~2.2", or 0.8 kpc fromthe nucleus) to the south and an extension to the north (at least~8'' from the nucleus), which are interpreted as the resultof ram pressure, as it cannot be explained solely by the static ICMconfinement. We estimate that the corona is moving with a velocity of~2.4-4.2 times the local sound speed to the south. The presence of thesharp edge on this small corona indicates that the transport processesare largely suppressed by the magnetic field there. The magnetic fieldaround the corona also suppresses heat conduction by at least a factorof ~60 across the corona boundary. We conclude that it is unrealistic tostudy the interaction between the small X-ray coronae and the hot ICMwithout considering the roles played by the magnetic field, a factor notincluded in current simulations. An absorbed(NH=1.5-3×1022 cm-2) nucleus isalso detected, which is not usual for FR I radio galaxies. Weak X-rayemission from three inner radio knots in the jets is also detected.Indentations at the east and west of the corona indicate interactionbetween the jets and the X-ray corona. Narrow jets carry great amountsof energy out of the central AGN and release the energy outside thecorona, preserving the tiny and vulnerable corona. This case revealsthat the inner kiloparsec core of the corona of massive galaxies cansurvive both high-speed stripping and powerful AGN feedback. Thus, thecooling of the X-ray coronae potentially provides fuel to the centralSMBH in rich environments in which the amount of galactic cold gas is ata minimum.
| Why Are Radio Galaxies Prolific Producers of Type Ia Supernovae?
An analysis of SN Ia events in early-type galaxies from the database ofCappellaro and coworkers provides conclusive evidence that the rate ofSNe Ia in radio-loud galaxies is about 4 times higher than the ratemeasured in radio-quiet galaxies, i.e., SN Ia rate(radio-loudgalaxies)=0.43+0.19-0.14h275 SNu as compared to SN Ia rate(radio-quietgalaxies)=0.11+0.06-0.03h275 SNu. The actual value of the enhancement islikely to be in the range ~2-7 (P~10-4). This finding puts onrobust empirical grounds the results obtained by Della Valle &Panagia on the basis of a smaller sample of SNe. We analyze the possiblecauses of this result and conclude that the enhancement of the SN Iaexplosion rate in radio-loud galaxies has the same origin as their beingstrong radio sources, but there is no causal link between the twophenomena. We argue that repeated episodes of interaction and/or mergersof early-type galaxies with dwarf companions, on timescales of about 1Gyr, are responsible for both inducing strong radio activity observed in~14% of early-type galaxies and supplying an adequate number of SN Iaprogenitors to the stellar population of elliptical galaxies.
| Detection of Intracluster Planetary Nebulae in the Coma Cluster
[O III] λ5007 emission lines of 16 intracluster planetary nebula(PN) candidates in the Coma Cluster were detected with a multislitimaging spectroscopy technique using the Faint Object Camera andSpectrograph on the Subaru telescope. The identification of these faintemission sources as PNs is supported by (1) their pointlike fluxdistribution in both space and wavelength, with tight limits on thecontinuum flux; (2) the identification of the second [O III]λ4959 line in the only object at high enough velocity that thisline too falls into the filter bandpass; (3) emission-line fluxesconsistent with PNs at 100 Mpc distance, in the range2.8×10-19 to 1.7×10-18 ergss-1 cm-2 and (4) a narrow velocity distributionapproximately centered on the systemic velocity of the Coma Cluster.Comparing with the velocities of galaxies in our field, we conclude thatthe great majority of these candidates would be intracluster PNs, freefloating in the Coma Cluster core. Their velocity dispersion is ~760 kms-1, and their mean velocity is lower than that of thegalaxies. The velocity distribution suggests that the intraclusterstellar population has different dynamics from the galaxies in the ComaCluster core.Based on data collected with the Faint Object Camera and Spectrograph atthe Subaru telescope, which is operated by the National AstronomicalObservatory of Japan, during observing run S04A-024.
| 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 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.
| The Bologna Complete Sample of Nearby Radio Sources
We present a new, complete sample of 95 radio sources selected from theB2 Catolog of Radio Sources and the Third Cambridge Revised Catalog(3CR), with z<0.1. Since no selection effect on the core radio power,jet velocity, or source orientation is present, this sample is wellsuited for statistical studies. In this first paper we present theobservational status of all sources on the parsec (milliarcsecond) andkiloparsec (arcsecond) scale; we give new parsec-scale data for 28sources and discuss their parsec-scale properties. By combining thesedata with those in the literature, information on the parsec-scalemorphology is available for a total of 53 radio sources with differentradio power and kiloparsec-scale morphologies. We investigate theirproperties. We find a dramatically higher fraction of two-sided sourcesin comparison with that of previous flux-limited VLBI surveys.
| 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.
| Active and Star-forming Galaxies and Their Supernovae
To investigate the extent to which nuclear starbursts or other nuclearactivity may be connected with enhanced star formation activity in thehost galaxy, we perform a statistical investigation of supernovae (SNe)discovered in host galaxies from four samples: the Markarian galaxiessample, the Second Byurakan Survey (SBS) sample, the north Galactic pole(NGP) sample of active or star-forming galaxies, and the NGP sample ofnormal galaxies. Forty-seven SNe in 41 Mrk galaxies, 10 SNe in six SBSgalaxies, 29 SNe in 26 NGP active or star-forming galaxies, and 29 SNein 26 NGP normal galaxies have been studied. We find that the rate ofSNe, particularly core-collapse (Types Ib/c and II) SNe, is higher inactive or star-forming galaxies in comparison with normal galaxies.Active or star-forming host galaxies of SNe are generally of latermorphological type and have lower luminosity and smaller linear sizethan normal host galaxies of SNe. The radial distribution of SNe inactive and star-forming galaxies shows a higher concentration toward thecenter of the active host galaxy than is the case for normal hostgalaxies, and this effect is more pronounced for core-collapse SNe.Ib/c-type SNe have been discovered only in active and star-forminggalaxies of our samples. About 78% of these SNe are associated with H IIregions or are located very close to the nuclear regions of these activegalaxies, which are in turn hosting AGNs or starburst nuclei. Besidesthese new results, our study also supports the conclusions of severalother earlier papers. We find that Type Ia SNe occur in all galaxytypes, whereas core-collapse SNe of Types Ib/c and II are found only inspiral and irregular galaxies. The radial distribution of Type Ib SNe intheir host galaxies is more centrally concentrated than that of Type IIand Ia SNe. The radial distances of Types Ib/c and II SNe, from thenuclei of their host galaxies, is larger for barred spiral hosts.Core-collapse SNe are concentrated in spiral arms and are often close toor in the H II regions, whereas Type Ia SNe show only a looseassociation with spiral arms and no clear association with H II regions.
| 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.
| Low power compact radio galaxies at high angular resolution
We present sub-arcsecond resolution multi-frequency (8 and 22 GHz) VLAimages of five low power compact (LPC) radio sources, and phasereferenced VLBA images at 1.6 GHz of their nuclear regions. At the VLAresolution we resolve the structure and identify component positions andflux densities. The phase referenced VLBA data at 1.6 GHz revealsflat-spectrum, compact cores (down to a few milliJansky) in four of thefive sources. The absolute astrometry provided by the phase referencingallows us to identify the center of activity on the VLA images.Moreover, these data reveal rich structures, including two-sided jetsand secondary components. On the basis of the arcsecond scale structuresand of the nuclear properties, we rule out the presence of strongrelativistic effects in our LPCs, which must be intrinsically small(deprojected linear sizes 10 kpc). Fits of continuous injectionmodels reveal break frequencies in the GHz domain, and ages in the range10 5{-}107 yrs. In LPCs, the outermost edge may beadvancing more slowly than in more powerful sources or could even bestationary; some LPCs might also have ceased their activity. In general,the properties of LPCs can be related to a number of reasons, including,but not limited to: youth, frustration, low kinematic power jets, andshort-lived activity in the radio.
| 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.
| 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.
| A deep near-infrared survey around the giant radio galaxy PKS 1343-601
We present the results of a deep near-infrared survey of a 36 × 36arcmin2 region centred on the giant elliptical radio galaxyPKS 1343-601, suggested to be the core of an unknown rich clusterlocated at the low Galactic latitude of b= 1.°73 in the GreatAttractor (GA) region. 19 obvious galaxies and 38 galaxy candidates havebeen detected; only three of them were previously identified as agalaxy. The total Galactic extinction AK towards our surveyarea is estimated to be 0.6-0.8 mag from the J-K colour of foregroundgiants. This is systematically lower by about 0.4 mag than AKtaken from the IRAS/DIRBE extinction map. The number density of galaxiesbrighter than an extinction-corrected Ks band magnitude of 13 is 42galaxies deg-2, five times as high as the overall average inthe GA region. However, the number of galaxies within the central270-kpc radius is less than that of the Norma, Pavo and Centaurusclusters in the GA region. We found no evidence that a rich cluster isassociated with PKS 1343-601.
| A transition in the accretion properties of radio-loud active nuclei
We present evidence for the presence of a transition in the accretionproperties of radio-loud sources. For a sample of radio galaxies andradio-loud quasars, selected based on their extended radio properties,the accretion rate is estimated from the black hole mass and nuclearluminosity. The inferred distribution is bimodal, with a paucity ofsources at accretion rates, in Eddington units, of the order of~10-2- assuming a radiative efficiency of 10 per cent - andpossibly spanning 1-2 orders of magnitude. Selection biases are unlikelyto be responsible for such behaviour. We discuss possible physicalexplanations, including a fast transition to low accretion rates, achange in the accretion mode/actual accretion rate/radiative efficiency,the lack of stable disc solutions at intermediate accretion rates or theinefficiency of the jet formation processes in geometrically thin flows.This transition might be analogous to spectral states (and jet)transitions in black hole binary systems.
| The K-band galaxy luminosity functions of three massive high-redshift clusters of galaxies
K-band observations of the galaxy populations of three high-redshift (z=0.8-1.0), X-ray-selected, massive clusters are presented. Theobservations reach a depth of K~= 21.5, corresponding to K*+ 3.5 mag.The evolution of the galaxy properties is discussed in terms of theirK-band luminosity functions and the K-band Hubble diagram of brightestcluster galaxies (BCGs).The bulk of the galaxy luminosities, as characterized by the parameterK* from the Schechter function, are found to be consistent with passiveevolution with a redshift of formation of zf~ 1.5-2. This isconsistent with observations of other high-redshift clusters, but may bein disagreement with galaxies in the field at similar redshifts. A goodmatch to the shape of the Coma cluster luminosity function is found bysimply dimming the high-redshift luminosity function by an amountconsistent with passive evolution. The evolution of the cumulativefraction of K-band light as a function of luminosity shows no evidenceof merger activity in the brighter galaxies.The evolution of the BCGs is tested by their K-band Hubble diagram andby the fraction of K-band cluster light in the BCGs. The evolutionobserved is consistent with recent previous observations although thescatter in the Hubble diagram allows for a range of evolutionaryhistories. The fraction of cluster light contained in the BCGs is notsmaller than that in Coma, suggesting that they are already very massivewith no need to hypothesize significant mergers in their futures.
| The Ghosts of Galaxies: Tidal Debris in Clusters
Gravitational interactions in rich clusters can strip material from theouter parts of galaxies or even completely disrupt entire systems,giving rise to large scale, low surface brightness ghostly featuresstretching across intergalactic space. The nearby Coma and Centaurusclusters both have striking examples of galaxy ghosts, in the form of100 kpc-long plumes of intergalactic debris. By searching HST archivalimages, we have found numerous other examples of galaxy ghosts in richclusters at low redshift, evidence that galaxy destruction and recyclingare ubiquitous, important in cluster formation and evolution, andcontinue to mold clusters at the present epoch. Many ghosts appear inX-ray bright clusters, perhaps signaling a connection with energeticsubcluster mergers. The fate of such material has importantramifications for cluster evolution. Our new HST WFPC2 V & I imagesof a portion of the Centaurus plume reveal that it contains an excess ofdiscrete objects with -12 < MV < -6, consistent withbeing globular clusters or smaller dwarf galaxies. This tidallyliberated material is being recycled directly into the intraclusterpopulation of stars, dwarf galaxies, globular clusters, and gas, whichmay have been built largely from a multitude of similar events over thelife of the cluster.
| Obscuration and Origin of Nuclear X-Ray Emission in FR I Radio Galaxies
We present X-ray observations of the nuclear region of 25 Fanaroff-Rileytype I (FR I) radio galaxies from the 3CRR and B2 catalogs, using datafrom the Chandra and XMM-Newton archives. We find the presence of aX-ray central compact core (CCCX) in 13/25 sources; in 3/25 sources thedetection of a CCCX is uncertain, while in the remaining 9/25 sources noCCCX is found. All the sources are embedded in a diffuse soft X-raycomponent, generally on kiloparsec scales, which is in agreement withthe halo of the host galaxy and/or with the intracluster medium. TheX-ray spectra of the cores are described by a power law with photonindices Γ=1.1-2.6. In eight sources excess absorption over theGalactic value is detected, with rest-frame column densitiesNzH~1020-1021cm-2 thus, we confirm the previous claim, based on opticaldata, that most FR I radio galaxies lack a standard optically thicktorus. We find significant correlations between the X-ray coreluminosity and the radio and optical luminosities, suggesting that atleast a fraction of the X-ray emission originates in a jet; however, theorigin of the X-rays remains ambiguous. If the X-ray emission isentirely attributed to an isotropic, accretion-related component, wefind very small Eddington ratios,Lbol/LEdd~10-3to10-8, and wecalculate the radiative efficiency to beη~10-2to10-6 on the basis of the Bondiaccretion rates from the spatial analysis. This suggests thatradiatively inefficient accretion flows are present in the cores oflow-power radio galaxies.
| ESO 3060170: A Massive Fossil Galaxy Group with a Heated Gas Core?
We present a detailed study of the ESO 3060170 galaxy group, combiningChandra, XMM-Newton, and optical observations. The system is found to bea fossil galaxy group. The group X-ray emission is composed of acentral, dense, cool core (10 kpc in radius) and an isothermal mediumbeyond the central 10 kpc. The region between 10 and 50 kpc (the coolingradius) has the same temperature as the gas from 50 to 400 kpc, althoughthe gas cooling time between 10 and 50 kpc (2-6 Gyr) is shorter than theHubble time. Thus, the ESO 3060170 group does not have a group-sizedcooling core. We suggest that the group cooling core may have beenheated by a central active galactic nucleus (AGN) outburst in the pastand that the small, dense, cool core is the truncated relic of aprevious cooling core. The Chandra observations also reveal a variety ofX-ray features in the central region, including a ``finger,'' anedgelike feature, and a small ``tail,'' all aligned along a north-southaxis, as are the galaxy light and group galaxy distribution. Theproposed AGN outburst may cause gas to ``slosh'' around the center andproduce these asymmetric features. The observed flat temperature profileto 1/3rvir is not consistent with the predicted temperatureprofile in recent numerical simulations. We compare the entropy profileof the ESO 3060170 group with those of three other groups and find aflatter relation than that predicted by simulations involving only shockheating, S~r~0.85. This is direct evidence of the importanceof nongravitational processes in group centers. We derive the massprofiles within 1/3rvir and find that the ESO 3060170 groupis the most massive fossil group known [(1-2)×1014Msolar]. The M/L ratio of the system, ~150 at0.3rvir, is normal.
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