Contents
Images
Upload your image
DSS Images Other Images
Related articles
Chandra Multiwavelength Project: Normal Galaxies at Intermediate Redshift
We have investigated 136 Chandra extragalactic sources, including 93galaxies with narrow emission lines (NELGs) and 43 with only absorptionlines (ALGs). Based on fX/fO, LX, X-rayspectral hardness, and optical emission-line diagnostics, we haveconservatively classified 36 normal galaxies and 71 AGNs. Their redshiftranges from 0.01 to 1.2, with normal galaxies in the range z=0.01-0.3.Our normal galaxies appear to share characteristics with local galaxies,as expected from the X-ray binary populations and the hot interstellarmatter (ISM). In conjunction with normal galaxies found in othersurveys, we found no statistically significant evolution inLX/LB, within the limited z range (<~0.1). Thebest-fit slope of our log(N)-log(S) relationship is -1.5 for both S(0.5-2 keV) and B (0.5-8 keV) energy bands, which is considerablysteeper than that of the AGN-dominated cosmic background sources, butslightly flatter than the previous estimate, indicating that normalgalaxies will not exceed the AGN population until fX(0.5-2.0keV)~2×10-18 ergs s-1 cm-2 (afactor of ~5 lower than the previous estimate). A group of NELGs appearto be heavily obscured in X-rays. After correcting for intrinsicabsorption, their X-ray luminosities could beLX>1044 ergs s-1, making them type 2quasar candidates. While most X-ray-luminous ALGs do not appear to besignificantly absorbed, we found two heavily obscured objects that couldbe as luminous as an unobscured broad-line quasar. Among 43 ALGs, wefound two E+A galaxy candidates. The X-ray spectra of both galaxies aresoft, and one of them has a nearby close companion galaxy, supportingthe merger/interaction scenario rather than the dusty starbursthypothesis.
| The Complex X-Ray Morphology of NGC 7618: A Major Group-Group Merger in the Local Universe?
We present results from a short Chandra ACIS-S observation of NGC 7618,the dominant central galaxy of a nearby (z=0.017309, d=74.1 Mpc) group.We detect a sharp surface brightness discontinuity 14.4 kpc north of thenucleus subtending an angle of 130° with an X-ray tail extending ~70kpc in the opposite direction. The temperature of the gas inside andoutside the discontinuity is 0.79+/-0.03 and 0.81+/-0.07 keV,respectively. There is marginal evidence for a discontinuous change inthe elemental abundance (Zinner=0.65+/-0.25,Zouter=0.17+/-0.21 at 90% confidence), suggesting that thismay be an ``abundance'' front. Fitting a two-temperature model to theASCA GIS spectrum of the NGC 7618/UGC 12491 pair shows the presence of asecond, much hotter (T=~2.3 keV) component. We consider severalscenarios for the origin of the edge and the tail, including a radiolobe/IGM interaction, nonhydrostatic ``sloshing,'' equal-mass merger andcollision, and ram pressure stripping. In the last case, we consider thepossibility that NGC 7618 is falling into UGC 12491 or that both groupsare falling into a gas-poor cluster potential. There are significantproblems with the first two models, however, and we conclude that thediscontinuity and tail are most likely the result of ram pressurestripping of the NGC 7618 group, as it falls into a larger dark matterpotential.
| A Chandra Survey of Early-Type Galaxies. I. Metal Enrichment in the Interstellar Medium
We present a Chandra study of the emission-weighted metal abundances in28 early-type galaxies, spanning ~3 orders of magnitude in X-rayluminosity (LX). We report constraints for Fe, O, Ne, Mg, Si,S, and Ni. We find no evidence of the very subsolar Fe abundance(ZFe) historically reported, confirming a trend in recentobservations of bright galaxies and groups, nor do we find anycorrelation between ZFe and luminosity. Excepting one case,the ISM is single-phase, indicating that multitemperature fits foundwith ASCA reflected temperature gradients that we resolve with Chandra.We find no evidence that ZFe (ISM) is substantially lowerthan the stellar metallicity estimated from simple stellar populationmodels. In general, these quantities are similar, which is inconsistentwith galactic wind models and recent hierarchical chemical enrichmentsimulations. Our abundance ratio constraints imply that 66%+/-11% of theISM Fe was produced in SNe Ia, similar to the solar neighborhood,indicating similar enrichment histories for elliptical galaxies and theMilky Way. Although these values are sensitive to the considerablesystematic uncertainty in the supernova yields, they are in agreementwith observations of more massive systems. This indicates considerablehomology in the enrichment process operating from cluster scales tolow-to-intermediate-LX galaxies. The data uniformly exhibitlow ZO/ZMg ratios, which have been reported insome clusters, groups, and galaxies. This is inconsistent with standardSN II metal yield calculations and may indicate an additional source ofenrichment, such as Population III hypernovae.
| Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton
We investigated the dynamical structure of 53 elliptical galaxies usingthe Chandra archival X-ray data. In X-ray-luminous galaxies, temperatureincreases with radius and gas density is systematically higher at theoptical outskirts, indicating the presence of a significant amount ofthe group-scale hot gas. In contrast, X-ray-dim galaxies show a flat ordeclining temperature profile against radius and the gas density isrelatively lower at the optical outskirts. Thus, it is found thatX-ray-bright and faint elliptical galaxies are clearly distinguished bythe temperature and gas density profile. The mass profile is well scaledby a virial radius r200 rather than an optical half-radiusre, is quite similar at (0.001-0.03)r200 betweenX-ray-luminous and dim galaxies, and smoothly connects to those profilesof clusters of galaxies. At the inner region of(0.001-0.01)r200 or (0.1-1)re, the mass profilewell traces a stellar mass with a constant mass-to-light ratio ofM/LB=3-10 Msolar/Lsolar. TheM/LB ratio of X-ray-bright galaxies rises up steeply beyond0.01r200 and thus requires a presence of massive dark matterhalo. From the deprojection analysis combined with the XMM-Newton data,we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 alsohave a high M/LB ratio of 20-30 at 20 kpc, comparable to thatof X-ray-luminous galaxies. Therefore, dark matter is indicated to becommon in elliptical galaxies; their dark matter distribution, as wellas that of galaxy clusters, almost follows the NFW profile.
| Search for high column density systems with gamma ray bursts
We investigate the possibility of searching for metal-poor high columndensity (1023~cm-2) clouds at high redshift(z) by using gamma ray burst afterglows. Such clouds could be related toprimeval galaxies that may cause a burst of star formation. We show thata large part of hydrogen is in molecular form in such a high columndensity environment. Therefore, hydrogen molecules (H2) rather thanhydrogen atoms should be searched for. Then we show that infrared H2lines are detectable for metal-poor (0.01 solar metallicity) highcolumn density (log N_H~[cm-2] 23.5) systems at high-zwithout suffering dust extinction. The optical properties of dust ininfrared could also be constrained by observations of high columndensity systems. Some possible scenarios for producing high columndensity systems are finally discussed in the context of galaxyevolution.
| 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.
| Dark matter in early-type galaxies: dynamical modelling of IC 1459, IC 3370, NGC 3379 and NGC 4105
We analyse long-slit spectra of four early-type galaxies which extendfrom ~1 to 3 effective radii: IC 1459; IC 3370; NGC 3379 and NGC 4105.We have extracted the full line-of-sight velocity distribution (in thecase of NGC 3379 we also used data from the literature), which we modelusing the two-integral approach. Using two-integral modelling, we findno strong evidence for dark haloes, but the fits suggest thatthree-integral modelling is necessary. We also find that the inferredconstant mass-to-light ratio in all the four cases is typical forearly-type galaxies. Finally, we also discuss the constraints on themass-to-light ratio, which can be obtained using X-ray haloes in thecase of IC 1459, NGC 3379 and NGC 4105, and compare the estimated valueswith the predictions from the dynamical modelling.
| A deep Chandra observation of the Centaurus cluster: bubbles, filaments and edges
X-ray images and gas temperatures taken from a deep ~200-ks Chandraobservation of the Centaurus cluster are presented. Multiple innerbubbles and outer semicircular edges are revealed, together with wispyfilaments of soft X-ray emitting gas. The frothy central structure andeastern edge are likely due to the central radio source blowing bubblesin the intracluster gas. The semicircular edges to the surfacebrightness maps 32 kpc to the east and 17.5 kpc to the west are markedby sharp temperature increases and abundance drops. The edges could bedue to sloshing motions of the central potential, or are possiblyenhanced by earlier radio activity. The high abundance of the innermostgas (about 2.5 times solar) limits the amount of diffusion and mixingtaking place.
| An Analytic Model of Galactic Winds and Mass Outflows
Galactic winds and mass outflows are observed both in nearby starburstgalaxies and in high-redshift star-forming galaxies. We develop a simpleanalytic model to understand the observed superwind phenomenon with adiscussion of the model uncertainties. Our model is built upon the modelof McKee & Ostriker for the interstellar medium. It allows one topredict how properties of a superwind, such as wind velocity and massoutflow rate, are related to properties of its star-forming host galaxy,such as size, gas density and star formation rate. The model predicts athreshold of star formation rate density for the generation ofobservable galactic winds. Galaxies with more concentrated starformation activities produce superwinds with higher velocities. Thepredicted mass outflow rates are comparable to (or slightly larger than)the corresponding star formation rates. We apply our model to both localstarburst galaxies and high-redshift Lyman break galaxies, and find itspredictions to be in good agreement with current observations. Our modelis simple and so can be easily incorporated into numerical simulationsand semi-analytical models of galaxy formation.
| 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.
| The Centers of Early-Type Galaxies with Hubble Space Telescope. V. New WFPC2 Photometry
We present observations of 77 early-type galaxies imaged with the PC1CCD of the Hubble Space Telescope (HST) WFPC2. ``Nuker-law'' parametricfits to the surface brightness profiles are used to classify the centralstructure into ``core'' or ``power-law'' forms. Core galaxies aretypically rounder than power-law galaxies. Nearly all power-law galaxieswith central ellipticities ɛ>=0.3 have stellar disks,implying that disks are present in power-law galaxies withɛ<0.3 but are not visible because of unfavorable geometry. Afew low-luminosity flattened core galaxies also have disks; these may betransition forms from power-law galaxies to more luminous core galaxies,which lack disks. Several core galaxies have strong isophote twistsinterior to their break radii, although power-law galaxies have interiortwists of similar physical significance when the photometricperturbations implied by the twists are evaluated. Central colorgradients are typically consistent with the envelope gradients; coregalaxies have somewhat weaker color gradients than power-law galaxies.Nuclei are found in 29% of the core galaxies and 60% of the power-lawgalaxies. Nuclei are typically bluer than the surrounding galaxy. Whilesome nuclei are associated with active galactic nuclei (AGNs), just asmany are not; conversely, not all galaxies known to have a low-level AGNexhibit detectable nuclei in the broadband filters. NGC 4073 and 4382are found to have central minima in their intrinsic starlightdistributions; NGC 4382 resembles the double nucleus of M31. In general,the peak brightness location is coincident with the photocenter of thecore to a typical physical scale of <1 pc. Five galaxies, however,have centers significantly displaced from their surrounding cores; thesemay be unresolved asymmetric double nuclei. Finally, as noted byprevious authors, central dust is visible in about half of the galaxies.The presence and strength of dust correlates with nuclear emission;thus, dust may outline gas that is falling into the central black hole.The prevalence of dust and its morphology suggest that dust clouds form,settle to the center, and disappear repeatedly on ~108 yrtimescales. We discuss the hypothesis that cores are created by thedecay of a massive black hole binary formed in a merger. Apart fromtheir brightness profiles, there are no strong differences between coregalaxies and power-law galaxies that demand this scenario; however, therounder shapes of core, their lack of disks, and their reduced colorgradients may be consistent with it.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withGO and GTO proposals 5236, 5446, 5454, 5512, 5943, 5990, 5999, 6099,6386, 6554, 6587, 6633, 7468, 8683, and 9107.
| 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.
| Radio sources in low-luminosity active galactic nuclei. IV. Radio luminosity function, importance of jet power, and radio properties of the complete Palomar sample
We present the completed results of a high resolution radio imagingsurvey of all ( 200) low-luminosity active galactic nuclei (LLAGNs) andAGNs in the Palomar Spectroscopic Sample of all ( 488) bright northerngalaxies. The high incidences of pc-scale radio nuclei, with impliedbrightness temperatures ≳107 K, and sub-parsec jetsargue for accreting black holes in ≳50% of all LINERs andlow-luminosity Seyferts; there is no evidence against all LLAGNs beingmini-AGNs. The detected parsec-scale radio nuclei are preferentiallyfound in massive ellipticals and in type 1 nuclei (i.e. nuclei withbroad Hα emission). The radio luminosity function (RLF) of PalomarSample LLAGNs and AGNs extends three orders of magnitude below, and iscontinuous with, that of “classical” AGNs. We find marginalevidence for a low-luminosity turnover in the RLF; nevertheless LLAGNsare responsible for a significant fraction of present day massaccretion. Adopting a model of a relativistic jet from Falcke &Biermann, we show that the accretion power output in LLAGNs is dominatedby the kinetic power in the observed jets rather than the radiatedbolometric luminosity. The Palomar LLAGNs and AGNs follow the samescaling between jet kinetic power and narrow line region (NLR)luminosity as the parsec to kilo-parsec jets in powerful radio galaxies.Eddington ratios {l_Edd} (=L_Emitted/L_Eddington) of≤10-1{-}10-5 are implied in jet models of theradio emission. We find evidence that, in analogy to Galactic black holecandidates, LINERs are in a “low/hard” state (gas poornuclei, low Eddington ratio, ability to launch collimated jets) whilelow-luminosity Seyferts are in a “high” state (gas richnuclei, higher Eddington ratio, less likely to launch collimated jets).In addition to dominating the radiated bolometric luminosity of thenucleus, the radio jets are energetically more significant thansupernovae in the host galaxies, and are potentially able to depositsufficient energy into the innermost parsecs to significantly slow thegas supply to the accretion disk.
| Massive elliptical galaxies in X-rays: The role of late gas accretion
We present a new chemical evolution model meant to be a first step inthe self-consistent study of both optical and X-ray properties ofelliptical galaxies. Detailed cooling and heating processes in theinterstellar medium (ISM) are taken into account using a mono-phaseone-zone treatment which allows a more reliable modelling of thegalactic wind regime with respect to previous work. The modelsuccessfully reproduces simultaneously the mass-metallicity, thecolour-magnitude, the LX - LB and theLX - T relations, as well as the observed trend of the[Mg/Fe] ratio as a function of σ, by adopting the prescriptions ofPipino & Matteucci (2004) for the gas infall and star formationtimescales. We found that a late secondary accretion of gas from theenvironment plays a fundamental role in driving the LX -LB and LX - T relations and can explain theirlarge observational scatter. The iron discrepancy, namely the too highpredicted iron abundance in X-ray haloes of ellipticals compared toobservations, still persists. On the other hand, we predict [O/Fe] inthe ISM which is in good agreement with the most recent observations. Wesuggest possible mechanisms acting on a galactic scale which may solvethe iron discrepancy. In particular, mixing of gas driven by AGNs maypreserve the gas mass (and thus the X-ray luminosity) while diluting theiron abundance. New predictions for the amounts of iron, oxygen andenergy ejected into the intracluster medium (ICM) are presented and weconclude that type Ia supernovae (SNe Ia) play a fundamental role in theICM enrichment. SNe Ia activity, in fact, may power a galactic windlasting for a considerable amount of the galactic lifetime, even in thecase for which the efficiency of energy transfer into the ISM per SN Iaevent is less than unity.
| The isolated elliptical NGC 4555 observed with Chandra
We present analysis of a Chandra observation of the elliptical galaxyNGC 4555. The galaxy lies in a very low density environment, eitherisolated from all galaxies of similar mass or on the outskirts of agroup. Despite this, NGC 4555 has a large gaseous halo, extending to~60kpc. We find the mean gas temperature to be ~0.95keV and the Feabundance to be ~0.5Zsolar. We model the surface brightness,temperature and abundance distribution of the halo and use these resultsto estimate parameters such as the entropy and cooling time of the gas,and the total gravitational mass of the galaxy. In contrast to recentresults showing that moderate luminosity ellipticals contain relativelysmall quantities of dark matter, our results show that NGC 4555 has amassive dark halo and large mass-to-light ratio(56.8+34.2-35.8Msolar/LBsolarat 50kpc, 42.7+14.6-21.2 at 5re,1σ errors). We discuss this disparity and consider possiblemechanisms by which galaxies might reduce their dark matter content.
| 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.
| 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.
| 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.
| Time-dependent Circulation Flows: Iron Enrichment in Cooling Flows with Heated Return Flows
We describe a new type of dynamical model for hot gas in galaxy groupsand clusters in which gas moves simultaneously in both radialdirections. The observational motivations for this type of flow arecompelling. X-ray spectra indicate that little or no gas is cooling tolow temperatures. Bubbles of hot gas typically appear in Chandra X-rayimages and XMM-Newton X-ray spectra within ~50 kpc of the centralelliptical galaxy. These bubbles must be buoyant. Furthermore, theelemental composition and total mass of gas-phase iron observed within~100 kpc of the center can be understood as the accumulated outflow ofmost or all of the iron produced by Type Ia supernovae in the centralgalaxy over time. This gaseous iron has been circulating for manygigayears, unable to cool. As dense inflowing gas cools, it produces apositive central temperature gradient, a characteristic feature ofnormal cooling flows. This gas dominates the local X-ray spectrum butshares the total available volume with centrally heated, outflowing gas.Circulating flows eventually cool catastrophically if the outflowing gastransports mass but no heat; to maintain the circulation both mass andenergy must be supplied to the inflowing gas over a large volume,extending to the cooling radius. The rapid radial recirculation of gaswithin ~50 kpc results in a flat core in the gas iron abundance, similarto many group and cluster observations. We believe the circulation flowsdescribed here are the first gasdynamic, long-term evolutionary modelsthat are in good agreement with all essential features observed in thehot gas: little or no gas cools as required by XMM spectra, the gastemperature increases outward near the center, and the gaseous ironabundance is about solar near the center and decreases outward.
| XMM-Newton Observations of NGC 507: Supersolar Metal Abundances in the Hot Interstellar Medium
We present the results of the X-ray XMM-Newton observations of NGC 507,a dominant elliptical galaxy in a small group of galaxies, and reportsupersolar metal abundances of both Fe and α-elements in the hotinterstellar medium (ISM) of this galaxy. These results are robust inthat we considered all possible systematic effects in our analysis. Wefind ZFe=2-3 times solar inside the D25 ellipse ofNGC 507. This is the highest ZFe reported so far for the hothalo of an elliptical galaxy; this high iron abundance is fullyconsistent with the predictions of stellar evolution models, whichinclude the yield of both Type II and Type Ia supernovae (SNe). Ouranalysis shows that abundance measurements are critically dependent onthe selection of the proper emission model. The spatially resolved,high-quality XMM-Newton spectra provide enough statistics to formallyrequire at least three emission components in each of four circumnuclearconcentric shells (within 5' or 100 kpc): two soft thermal componentsindicating a range of temperatures in the hot ISM plus a hardercomponent, consistent with the integrated output of low-mass X-raybinaries (LMXBs) in NGC 507. The two-component (thermal+LMXB) modelcustomarily used in past studies yields a much lower ZFe,consistent with previous reports of subsolar metal abundances. Thismodel, however, gives a significantly worse fit to the data (F-testprobability<0.0001). The abundance of α-elements (mostaccurately determined by Si) is also found to be supersolar. Theα-element-to-Fe abundance ratio is close to the solar ratio,suggesting that ~70% of the iron mass in the hot ISM originated fromType Ia SNe. The α-element-to-Fe abundance ratio remains constantout to at least 100 kpc, indicating that Types II and Ia SN ejecta arewell mixed on a scale much larger than the extent of the stellar body.
| K-Band Properties of Galaxy Clusters and Groups: Luminosity Function, Radial Distribution, and Halo Occupation Number
We explore the near-infrared (NIR) K-band properties of galaxies within93 galaxy clusters and groups using data from the Two Micron All SkySurvey. We use X-ray properties of these clusters to pinpoint clustercenters and estimate cluster masses. By stacking all these systems, westudy the shape of the cluster luminosity function and the galaxydistribution within the clusters. We find that the galaxy profile iswell described by the Navarro, Frenk, & White (NFW) profile with aconcentration parameter c~3, with no evidence for cluster massdependence of the concentration. Using this sample, whose masses spanthe range from 3×1013 to2×1015Msolar, we confirm the existence of atight correlation between total galaxy NIR luminosity and clusterbinding mass, which indicates that NIR light can serve as a cluster massindicator. From the observed galaxy profile, together with cluster massprofile measurements from the literature, we find that the mass-to-lightratio is a weakly decreasing function of cluster radius and that itincreases with cluster mass. We also derive the mean number of galaxieswithin halos of a given mass, the halo occupation number. We find thatthe mean number scales as N~M0.84+/-0.04 for galaxiesbrighter than MK=-21, indicating that high-mass clusters havefewer galaxies per unit mass than low-mass clusters. Using publishedobservations at high redshift, we show that higher redshift clustershave higher mean occupation numbers than nearby systems of the samemass. By comparing the luminosity function and radial distribution ofgalaxies in low-mass and high-mass clusters, we show that there is amarked decrease in the number density of galaxies fainter thanM* as one moves to higher mass clusters; in addition,extremely luminous galaxies are more probable in high-mass clusters. Weexplore several processes, including tidal interactions and merging, asa way of explaining the variation in galaxy population with clustermass.
| An Unusual Discontinuity in the X-Ray Surface Brightness Profile of NGC 507: Evidence of an Abundance Gradient?
We present results from a 45 ks Chandra ACIS-I observation of the nearby(z=0.01646) NGC 507 group. The X-ray surface brightness profile of theouter region of the group is well described by an isothermalβ-profile consistent with earlier ROSAT observations. We find asharp edge or discontinuity in the radial surface brightness profile 55kpc east and southeast of NGC 507 covering an ~125° arc. At thesharpest part of the discontinuity, the surface brightness declines by afactor of ~2 over a distance of 6.9 kpc (~15"). The inner and outer gastemperatures across the discontinuity differ by only about 0.2 keV(interior and exterior gas temperatures are 1.2 and 1.4 keV,respectively). Spectral analysis indicates that there is a gradient inthe elemental abundance across the discontinuity, and comparison withthe low-resolution NRAO VLA Sky Survey radio maps suggests that thediscontinuity is aligned with a low surface brightness radio lobe. Weconclude that the appearance of this discontinuity may be the result oftwo phenomena: the transonic (M~0.75-1) expansion/inflation of the radiolobe close to our line of sight and the transport of high-abundancematerial from the center of the galaxy due to this expansion.
| M/L, Hα Rotation Curves, and H I Gas Measurements for 329 Nearby Cluster and Field Spirals. II. Evidence for Galaxy Infall
We have conducted a study of optical and H I properties of spiralgalaxies (size, luminosity, Hα flux distribution, circularvelocity, and H I gas mass) to explore the role of gas stripping as adriver of morphological evolution in clusters. We find a strongcorrelation between the spiral and S0 fractions within clusters, and thespiral fraction scales tightly with cluster X-ray gas luminosity. Weexplore young star formation and identify spirals that are (1)asymmetric, with truncated Hα emission and H I gas reservoirs onthe leading edge of the disk, on a first pass through the denseintracluster medium in the cores of rich clusters; (2) strongly H Ideficient and stripped, with star formation confined to the inner 5h-1 kpc and 3 disk scale lengths; or (3) reddened, extremelyH I deficient, and quenched, where star formation has been halted acrossthe entire disk. We propose that these spirals are in successive stagesof morphological transformation, between infalling field spirals andcluster S0's, and that the process that acts to remove the H I gasreservoir suppresses new star formation on a similarly fast timescale.These data suggest that gas stripping plays a significant role inmorphological transformation and rapid truncation of star formationacross the disk.
| M/L, Hα Rotation Curves, and H I Measurements for 329 Nearby Cluster and Field Spirals. I. Data
A survey of 329 nearby galaxies (redshift z<0.045) has been conductedto study the distribution of mass and light within spiral galaxies overa range of environments. The 18 observed clusters and groups span arange of richness, density, and X-ray temperature and are supplementedby a set of 30 isolated field galaxies. Optical spectroscopy taken withthe 200 inch (5 m) Hale Telescope provides separately resolved Hαand [N II] major-axis rotation curves for the complete set of galaxies,which are analyzed to yield velocity widths and profile shapes, extents,and gradients. H I line profiles provide an independent velocity widthmeasurement and a measure of H I gas mass and distribution. I-bandimages are used to deconvolve profiles into disk and bulge components,to determine global luminosities and ellipticities, and to checkmorphological classification. These data are combined to form a unifieddata set ideal for the study of the effects of environment upon galaxyevolution.
| Optical nuclei of radio-loud AGN and the Fanaroff-Riley divide
We investigate the nature of the point-like optical nuclei that havebeen found in the centres of the host galaxies of a majority of radiogalaxies by the Hubble Space Telescope. We examine the evidence thatthese optical nuclei are relativistically beamed, and look fordifferences in the behaviour of the nuclei found in radio galaxies ofthe two Fanaroff-Riley types. We also attempt to relate this behaviourto the properties of the optical nuclei in their highly beamedcounterparts (the BL Lac objects and radio-loud quasars) as hypothesizedby the simple Unified Scheme. Simple model-fitting of the data suggeststhat the emission may be coming from a non-thermal relativistic jet. Itis also suggestive that the contribution from an accretion disk is notsignificant for the FRI objects and for the narrow-line radio galaxiesof FRII type, while it may be significant for the Broad-line objects,and consistent with the idea that the FRII optical nuclei seem to sufferfrom extinction due to an obscuring torus while the FRI optical nucleido not. These results are broadly in agreement with the Unified Schemefor radio-loud AGNs.Appendix C is only available in electronic form athttp://www.edpsciences.org
| Infant Galaxy Clusters at Low Redshifts?
The population and population composition of galaxies in galaxy clustersat present reflect the mass of the clusters and the mass growth of thegalaxies in the past. We investigated them for six clusters. We showedthat galaxies in massive clusters stopped growing at redshifts of 4. Moreover, we found that some small galaxy clusters (groups) have toomany massive galaxies for their apparent masses. One possibility is thatthese groups are much more massive and in a phase just beforevirialization. If this is the case, they should be called `infant galaxyclusters' that will mature in the dynamical time-scale ( 109yr).
| X-ray scaling properties of early-type galaxies
We present an analysis of 39 X-ray luminous early-type galaxies observedwith the ROSAT PSPC. Using multicomponent spectral and spatial fits tothese data, we have measured halo abundance, temperature, luminosity andsurface brightness profile. We compare these measurements to similarresults from galaxy groups and clusters, fitting a number of relationscommonly used in the study of these larger objects. In particular, wefind that the σ-TX relation for our sample is similarto that reported for clusters, consistent with βspec= 1,and that the LX-TX relation has a steep slope(gradient 4.8 +/- 0.7) comparable with that found for galaxy groups.Assuming isothermality, we construct three-dimensional models of ourgalaxies, allowing us to measure gas entropy. We find no correlationbetween gas entropy and system mass, but do find a trend forlow-temperature systems to have reduced gas fractions. We conclude thatthe galaxies in our sample are likely to have developed their haloesthrough galaxy winds, influenced by their surrounding environment.
| The Birmingham-CfA cluster scaling project - I. Gas fraction and the M-TX relation
We have assembled a large sample of virialized systems, comprising 66galaxy clusters, groups and elliptical galaxies with high-quality X-raydata. To each system we have fitted analytical profiles describing thegas density and temperature variation with radius, corrected for theeffects of central gas cooling. We present an analysis of the scalingproperties of these systems and focus in this paper on the gasdistribution and M-TX relation. In addition to clusters andgroups, our sample includes two early-type galaxies, carefully selectedto avoid contamination from group or cluster X-ray emission. We comparethe properties of these objects with those of more massive systems andfind evidence for a systematic difference between galaxy-sized haloesand groups of a similar temperature.We derive a mean logarithmic slope of the M-TX relationwithin R200 of 1.84 +/- 0.06, although there is some evidenceof a gradual steepening in the M-TX relation, with decreasingmass. We recover a similar slope using two additional methods ofcalculating the mean temperature. Repeating the analysis with theassumption of isothermality, we find the slope changes only slightly, to1.89 +/- 0.04, but the normalization is increased by 30 per cent.Correspondingly, the mean gas fraction within R200 changesfrom (0.13 +/- 0.01) h-3/270 to (0.11+/- 0.01) h-3/270, for the isothermalcase, with the smaller fractional change reflecting different behaviourbetween hot and cool systems. There is a strong correlation between thegas fraction within 0.3R200 and temperature. This reflectsthe strong (5.8σ) trend between the gas density slope parameter,β, and temperature, which has been found in previous work.These findings are interpreted as evidence for self-similarity breakingfrom galaxy feedback processes, active galactic nuclei heating orpossibly gas cooling. We discuss the implications of our results in thecontext of a hierarchical structure formation scenario.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
