Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
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).
| Infrared Luminosity Function of the Coma Cluster
Using mid-IR and optical data, we deduce the total infrared (IR)luminosities of galaxies in the Coma Cluster and present their IRluminosity function (LF). The shape of the overall Coma IR LF does notshow significant differences from the IR LFs of the general field, whichindicates the general independence of global galaxy star formation fromenvironment up to densities ~40 times greater than in the field (wecannot test such independence above LIR~1044 ergss-1). However, a shallower faint-end slope and a smallerL*IR are found in the core region (where thedensities are still higher) compared to the outskirt region of thecluster, and most of the brightest IR galaxies are found outside thecore region. The IR LF in the NGC 4839 group region does not show anyunique characteristics. By integrating the IR LF, we find a total starformation rate in the cluster of about 97.0 Msolaryr-1. We also studied the contributions of early- andlate-type galaxies to the IR LF. The late-type galaxies dominate thebright end of the LF, and the early-type galaxies, although only makingup a small portion (~15%) of the total IR emission of the cluster,contribute greatly to the number counts of the LF atLIR<1043 ergs s-1.
| XMM-Newton observations of the Coma cluster relic 1253+275
Aims.Using XMM Newton data, we investigate the nature of the X-rayemission in the radio relic 1253+275 in the Coma cluster. We determinethe conditions of the cluster gas to check current models of relicformation, and we set constraints on the intracluster magnetic field. Methods: .Both imaging and spectral analysis are performed, andthe X-ray emission is compared with the radio emission. Results:.We found that the emission is of thermal origin and is connected to thesub-group around NGC 4839. The best-fit gas temperature in the region ofthe relic and in its vicinity is in the range 2.8-4.0 keV, comparable tothe temperature of the NGC 4839 sub-group. We do not detect any hightemperature gas, resulting from a possible shock in the region of theComa relic. We therefore suggest that the main source of energy forparticles radiating in the radio relic is likely to be turbulence. Fromthe X-ray data, we can also set a flux upper limit of 3.2×10-13 erg cm-2 s-1, in the 0.3-10 keVenergy range, to the non-thermal emission in the relic region. Thisleads to a magnetic field B > 1.05~μG.
| Gamma-ray emissions of AGN and cosmological standard candles
In this work, we compile a sample which contains 71 GeV Gamma-ray-loudActive Galactic Nuclei (AGNs) (14 BL Lacs and 57 FSRQs), 53 FR I radiogalaxies and 63 FR II radio galaxies. We make a nonlinear least-squarefit to this sample, and find that the best fit value of the Hubbleconstant is H0=71.5±3.8 kms-1Mpc-1 with a reduced χ ~= 2.46 by assumingMv = -23.0 and accepting q0 = 1.0, and thecorresponding regression line has a correlation index R ~= 0.78. Thebest fit value of H0 = 71.5±3.8 kms-1Mpc-1 is in well agreement with H0 =72±8 km s-1 obtained by the Hubble Space TelescopeKey Project. Our results show that the GeV Gamma-ray emissions of AGNscan be used as cosmological standard candles indeed.
| XMM-Newton Observation of IC 310 in the Outer Region of the Perseus Cluster of Galaxies
We present results from an XMM-Newton observation of the head-tail radiogalaxy IC 310 located in the southwest region of the Perseus cluster.The spectrum is well fitted by an absorbed power-law model with a photonindex of 2.50 ± 0.02 without significant absorption excess. TheX-ray image shows a point-like emission at IC 310 without any sign of astructure correlated with the radio halo tail. The temperature of theintracluster medium surrounding IC 310 declines as a function ofdistance from the cluster center, from kT 6 keV in the northeastcorner of the field of view to about 3keV in the southwest region.Although we do not find any sharp edges in the surface brightnessprofile, a brightness excess of about 20% over a smooth β model isseen. The temperature also rises by about 10% in the same region. Thisindicates that the IC 310 region is a subcluster probably infalling intothe Perseus cluster, and the gas in front of IC 310 towards the Perseuscluster is likely to be compressed by the large-scale motion, whichsupports the view that the IC 310 system is undergoing a merger.
| 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 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.
| 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.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| Estimating galaxy cluster magnetic fields by the classical and hadronic minimum energy criterion
We wish to estimate magnetic field strengths of radio emitting galaxyclusters by minimizing the non-thermal energy density contained incosmic ray electrons (CRe), protons (CRp), and magnetic fields. Theclassical minimum energy estimate can be constructed independently ofthe origin of the radio synchrotron emitting CRe yielding thus anabsolute minimum of the non-thermal energy density. Provided theobserved synchrotron emission is generated by a CRe populationoriginating from hadronic interactions of CRp with the ambient thermalgas of the intra-cluster medium, the parameter space of the classicalscenario can be tightened by means of the hadronic minimum energycriterion. For both approaches, we derive the theoretically expectedtolerance regions for the inferred minimum energy densities. Applicationto the radio halo of the Coma cluster and the radio mini-halo of thePerseus cluster yields equipartition between cosmic rays and magneticfields within the expected tolerance regions. In the hadronic scenario,the inferred central magnetic field strength ranges from 2.4 μG(Coma) to 8.8 μG (Perseus), while the optimal CRp energy density isconstrained to 2 per cent +/- 1 per cent of the thermal energy density(Perseus). We discuss the possibility of a hadronic origin of the Comaradio halo while current observations favour such a scenario for thePerseus radio mini-halo. Combining future expected detections of radiosynchrotron, hard X-ray inverse Compton, and hadronically inducedγ-ray emission should allow an estimate of volume averaged clustermagnetic fields and provide information about their dynamical state.
| 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.
| Radio and Far-Infrared Emission as Tracers of Star Formation and Active Galactic Nuclei in Nearby Cluster Galaxies
We have studied the radio and far-infrared (FIR) emission from 114galaxies in the seven nearest clusters (<100 Mpc) with prominentX-ray emission to investigate the impact of the cluster environment onthe star formation and active galactic nucleus (AGN) activity in themember galaxies. The X-ray selection criterion is adopted to focus onthe most massive and dynamically relaxed clusters. A large majority ofcluster galaxies show an excess in radio emission over that predictedfrom the radio-FIR correlation, the fraction of sources with radioexcess increases toward cluster cores, and the radial gradient in theFIR/radio flux ratio is a result of radio enhancement. Of theradio-excess sources, 70% are early-type galaxies, and the same fractionhost an AGN. The galaxy density drops by a factor of 10 from thecomposite cluster center out to 1.5 Mpc, yet galaxies show no change inFIR properties over this region and show no indication of masssegregation. We have examined in detail the physical mechanisms thatmight impact the FIR and radio emission of cluster galaxies. Whilecollisional heating of dust may be important for galaxies in clustercenters, it appears to have a negligible effect on the observed FIRemission for our sample galaxies. The correlations between radio and FIRluminosity and radius could be explained by magnetic compression fromthermal intracluster medium pressure. We also find that simple delayedharassment cannot fully account for the observed radio, FIR, and mid-IRproperties of cluster galaxies.
| Environmental Effects in the Structural Parameters of Galaxies in the Coma Cluster
We have studied 116 bright galaxies from the Coma Cluster brighter thanmr=17 mag. From a quantitative morphological analysis we findthat the scales of the disks are smaller than those of field spiralgalaxies. There is a correlation between the scale of the disks and theposition of the galaxy in the cluster; no large disks are present nearthe center of the cluster or in high-density environments. Thestructural parameters of the bulges are not affected by the environment.We have analyzed the distribution of blue and red objects in thecluster. For spirals there is a trend between color and position in thecluster. The bluest spiral galaxies are located at larger projectedradii; they also show larger velocity dispersions than the red ones. Thedifferences in the scale of the disks between cluster galaxies and localsamples of isolated galaxies and the color distribution of the objectscan be understood in terms of the harassment scenario.
| 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.
| Hubble Space Telescope Observations of cD Galaxies and Their Globular Cluster Systems
We have used WFPC2 on the Hubble Space Telescope (HST) to obtain F450Wand F814W images of four cD galaxies (NGC 541 in Abell 194, NGC 2832 inAbell 779, NGC 4839 in Abell 1656, and NGC 7768 in Abell 2666) in therange 5400 km s-1<~cz<~8100 km s-1. For NGC541, the HST data are supplemented by ground-based B and I imagesobtained with FORS1 on the Very Large Telescope. We present surfacebrightness and color profiles for each of the four galaxies, confirmingtheir classification as cD galaxies. Isophotal analyses reveal thepresence of subarcsecond-scale dust disks in the nuclei of NGC 541 andNGC 7768. Despite the extreme nature of these galaxies in terms ofspatial extent and luminosity, our analysis of their globular cluster(GC) systems reveals no anomalies in terms of specific frequencies,metallicity gradients, average metallicities, or the metallicity offsetbetween the globular clusters and the host galaxy. We show that thelatter offset appears roughly constant at Δ[Fe/H]~0.8 dex forearly-type galaxies spanning a luminosity range of roughly 4 orders ofmagnitude. We combine the globular cluster metallicity distributionswith an empirical technique described in a series of earlier papers toinvestigate the form of the protogalactic mass spectrum in these cDgalaxies. We find that the observed GC metallicity distributions areconsistent with those expected if cD galaxies form through thecannibalism of numerous galaxies and protogalactic fragments that formedtheir stars and globular clusters before capture and disruption.However, the properties of their GC systems suggest that dynamicalfriction is not the primary mechanism by which these galaxies areassembled. We argue that cD's instead form rapidly, via hierarchicalmerging, prior to cluster virialization.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 5-26555Based in part on observations obtained at the European SouthernObservatory, for VLT program 68.D-0130(A).
| AGN and starburst radio activity in the A3558 cluster complex
We present Very Large Array (VLA) 1.4 GHz (21 cm) observations of theregion between the centres of A3558 and A3562, in the major clustermerger complex of the Shapley Concentration. Our final catalogueincludes a total of 174 radio sources above the flux density limit of0.25 mJy b-1. By cross-correlation with optical andspectroscopic catalogues we found 33 optical counterparts belonging tothe Shapley Concentration. We investigated the effects of clustermerger on the radio emission properties of the galaxy population bymeans of the radio source counts and the radio luminosity functions(RLF). We found that the radio source counts are consistent with thefield source counts. The RLF of elliptical and S0 galaxies in the regionsurveyed, is consistent with the ``universal'' RLF for early-typegalaxies. This result suggests that the deficit in radio galaxies foundin our previous work over the whole A3558 chain is entirely due to thecluster A3558. A population of faint radio galaxies (log P1.4GHz (W Hz-1) 22) is also found. Half of theseobjects are also blue, suggesting that starburst is the main mechanismdriving the radio emission. Finally, we detected 14 spiral radiogalaxies, whose ratio between radio and optical emission is similar tothose found in galaxies located in rich and dynamically evolvedclusters. Our results are briefly discussed in the light of the ageand stage of the merger in the A3558 cluster complex.Full Tables \ref{tab:cat} and \ref{tab:optic} are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/419/71
| 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.
| Investigation of Galaxy Alignment in X-ray Subclusters of the Coma Cluster
Not Available
| Dark matter distribution in the Coma cluster from galaxy kinematics: breaking the mass-anisotropy degeneracy
We study velocity moments of elliptical galaxies in the Coma clusterusing Jeans equations. The dark matter distribution in the cluster ismodelled by a generalized formula based upon the results of cosmologicalN-body simulations. Its inner slope (cuspy or flat), concentration andmass within the virial radius are kept as free parameters, as well asthe velocity anisotropy, assumed independent of position. We show thatthe study of line-of-sight velocity dispersion alone does not allow usto constrain the parameters. By a joint analysis of the observedprofiles of velocity dispersion and kurtosis, we are able to break thedegeneracy between the mass distribution and velocity anisotropy. Wedetermine the dark matter distribution at radial distances larger than 3per cent of the virial radius and we find that the galaxy orbits areclose to isotropic. Due to limited resolution, different inner slopesare found to be consistent with the data and we observe a strongdegeneracy between the inner slope α and concentration c; thebest-fitting profiles have the two parameters related with c=19-9.6α. Our best-fitting Navarro-Frenk-White profile hasconcentration c= 9, which is 50 per cent higher than standard valuesfound in cosmological simulations for objects of similar mass. The totalmass within the virial radius of 2.9h-170 Mpc is1.4 × 1015h-170 Msolar(with 30 per cent accuracy), 85 per cent of which is dark. At thisdistance from the cluster centre, the mass-to-light ratio in the blueband is 351h70 solar units. The total mass within the virialradius leads to estimates of the density parameter of the Universe,assuming that clusters trace the mass-to-light ratio and baryonicfraction of the Universe, with Ω0= 0.29 +/- 0.1.
| Simulations of the effects of stripping and accretion on galaxy haloes in clusters
We present results from a series of hydrodynamic simulationsinvestigating ram pressure stripping of galactic haloes as the hostgalaxy falls radially into a cluster. We perform a parameter studycomprising variations in initial gas content, gas injection rate (viastellar mass loss processes), galaxy mass and amplitude of infall. Fromthe simulation results we track variations in both physical quantities(e.g. gas mass) and directly observable quantities (e.g. X-rayluminosities). The luminosity of the X-ray halo of the galaxy is foundto compare favourably with the observationally determined correlationwith the optical blue-band luminosity (LX:LB)relation. Factors affecting the X-ray luminosity are explored and it isfound that the gas injection rate is a dominant factor in determiningthe integrated luminosity. Observational properties of the materialstripped from the galaxy, which forms an X-ray wake, are investigatedand it is found that wakes are most visible around galaxies with asubstantial initial gas content, during their first passage though thecluster. We define a statistical skewness measure that may be used todetermine the direction of motion of a galaxy using X-ray observations.Structures formed in these simulations are similar to the cold frontsseen in observations of cluster mergers where a sharp increase insurface brightness is accompanied by a transition to a cooler region.
| Unifying B2 radio galaxies with BL Lacertae objects
In an earlier paper we presented nuclear X-ray flux densities, measuredwith ROSAT, for the B2 bright sample of nearby low-luminosity radiogalaxies. In this paper we construct a nuclear X-ray luminosity functionfor the B2 radio galaxies, and discuss the consequences of our resultsfor models in which such radio galaxies are the parent population of BLLacertae (BL Lac) objects. Based on our observations of the B2 sample,we use Monte Carlo techniques to simulate samples of beamed radiogalaxies, and use the selection criteria of existing samples of BL Lacobjects to compare our simulated results to what is observed. We findthat previous analytical results are not applicable since the BL Lacsamples are selected on beamed flux density. A simple model in which BLLacs are the moderately beamed (γ~ 3) counterparts of radiogalaxies, with some random dispersion (~0.4 decades) in the intrinsicradio-X-ray relationship, can reproduce many of the features of theradio-selected and X-ray-selected BL Lac samples, including their radioand X-ray luminosity functions and the distributions of theirradio-to-X-ray spectral indices. In contrast, models in which the X-rayand radio emission have systematically different beaming parameterscannot reproduce important features of the radio-galaxy and BL Lacpopulations, and recently proposed models in which the radio-to-X-rayspectral index is a function of source luminosity cannot in themselvesaccount for the differences in the slopes of the radio- andX-ray-selected BL Lac luminosity functions. The redshift distributionand number counts of the X-ray-selected Einstein Medium SensitivitySurvey (EMSS) sample are well reproduced by our best models, supportinga picture in which these objects are beamed Fanaroff-Riley type I radiogalaxies with intrinsic luminosities similar to those of the B2 sample.However, we cannot match the redshift distribution of the radio-selected1-Jy sample, and it is likely that a population of Fanaroff-Riley typeII radio galaxies is responsible for the high-redshift objects in thissample, in agreement with previously reported results on the sample'sradio and optical emission-line properties.
| Radio-selected Galaxies in Very Rich Clusters at z <= 0.25. I. Multiwavelength Observations and Data Reduction Techniques
Radio observations were used to detect the ``active'' galaxy populationwithin rich clusters of galaxies in a nonbiased manner that is notplagued by dust extinction or the K-correction. We present wide-fieldradio, optical (imaging and spectroscopy), and ROSAT All-Sky Survey(RASS) X-ray data for a sample of 30 very rich Abell (R>=2) clusterswith z<=0.25. The VLA radio data samples the ultrafaint radio(L1.4>=2×1022 W Hz-1) galaxypopulation within these extremely rich clusters for galaxies withMR<=-21. This is the largest sample of low-luminosity 20cm radio galaxies within rich Abell clusters collected to date.The radio-selected galaxy sample represents the starburst (starformation rate >=5 Msolar yr-1) and activegalactic nuclei populations contained within each cluster. Archival andnewly acquired redshifts were used to verify cluster membership for most(~95%) of the optical identifications. Thus, we can identify all thestarbursting galaxies within these clusters, regardless of the level ofdust obscuration that would affect these galaxies being identified fromtheir optical signature. Cluster sample selection, observations, anddata reduction techniques for all wavelengths are discussed.
| Detection of Nonrandom Galaxy Orientations in X-Ray Subclusters of the Coma Cluster
This study on the Coma Cluster suggests that there are deviations from acompletely random galaxy orientation on small scales. Since we found asignificant coincidence of hot-gas features identified in the latestX-ray observations of Coma with these local anisotropies, they mayindicate regions of recent mutual interaction of member galaxies withinsubclusters that are currently falling in on the main cluster.
| 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.
| A Comprehensive Radio and Optical Study of Abell 2256: Activity from an Infalling Group
Abell 2256 is a nearby (z~0.06), rich cluster of galaxies withfascinating observed properties across a range of wavelengths. Longbelieved to represent a cluster merger, recent X-ray and optical resultshave suggested that in addition to the primary cluster and subclusterthere is evidence for a third, poorer system. We present wide-fieldhigh-sensitivity 1.4 GHz VLA radio observations of Abell 2256 inconjunction with optical imaging and additional spectroscopy. Over 40cluster radio galaxies are identified, with optical spectroscopyindicating the emission source (star formation or active galacticnucleus) for most of them. While the overall fraction of galaxiesexhibiting radio emission is consistent with a large sample of othernearby clusters, we find an increase in the activity level of galaxiesbelonging to the third system (hereafter ``the Group''). Specifically,the Group has relatively more star formation than both the primarycluster and main subcluster. The position of the Group is alsocoincident with the observed cluster radio relic. We suggest that theGroup recently (~0.3 Gyr) merged with the primary cluster and that thismerger, not the ongoing merger of the primary and the main subcluster,might be responsible for many of the unusual radio properties of Abell2256. Furthermore, the greater star formation activity of the Groupsuggests that the infall of groups is an important driver of galaxyevolution in clusters.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
