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A relativistic model of the radio jets in 3C296
We present new, deep 8.5-GHz VLA observations of the nearby,low-luminosity radio galaxy 3C296 at resolutions from 0.25 to 5.5arcsec.These show the intensity and polarization structures of the twin radiojets in detail. We derive the spectral-index distribution usinglower-frequency VLA observations and show that the flatter-spectrum jetsare surrounded by a sheath of steeper-spectrum diffuse emission. We alsoshow images of Faraday rotation measure and depolarization and derivethe apparent magnetic field structure. We apply our intrinsicallysymmetrical, decelerating relativistic jet model to the newobservations. An optimized model accurately fits the data in both totalintensity and linear polarization. We infer that the jets are inclinedby 58° to the line of sight. Their outer isophotes flare to ahalf-opening angle of 26° and then recollimate to form a conicalflow beyond 16kpc from the nucleus. On-axis, they decelerate from a(poorly constrained) initial velocity β = v/c ~ 0.8 to ~ 0.4 around5kpc from the nucleus, the velocity thereafter remaining constant. Thespeed at the edge of the jet is low everywhere. The longitudinal profileof proper emissivity has three principal power-law sections: an innerregion (0-1.8kpc), where the jets are faint, a bright region(1.8-8.9kpc) and an outer region with a flatter slope. The emission iscentre brightened. Our observations rule out a globally ordered, helicalmagnetic field configuration. Instead, we model the field as random onsmall scales but anisotropic, with toroidal and longitudinal componentsonly. The ratio of longitudinal to toroidal field falls with distancealong the jet, qualitatively but not quantitatively as expected fromflux freezing, so that the field is predominantly toroidal far from thenucleus. The toroidal component is relatively stronger at the edges ofthe jet. A simple adiabatic model fits the emissivity evolution only inthe outer region after the jets have decelerated and recollimated;closer to the nucleus, it predicts far too steep an emissivity declinewith distance. We also interpret the morphological differences betweenbrightness enhancements (`arcs') in the main and counter-jets as aneffect of relativistic aberration.
| The relation between accretion rate and jet power in X-ray luminous elliptical galaxies
Using Chandra X-ray observations of nine nearby, X-ray luminouselliptical galaxies with good optical velocity dispersion measurements,we show that a tight correlation exists between the Bondi accretionrates calculated from the observed gas temperature and density profilesand estimated black hole masses, and the power emerging from thesesystems in relativistic jets. The jet powers, which are inferred fromthe energies and time-scales required to inflate cavities observed inthe surrounding X-ray emitting gas, can be related to the accretionrates using a power-law model of the formlog(PBondi/1043ergs-1) = A +Blog(Pjet/1043ergs-1), with A = 0.65+/- 0.16 and B = 0.77 +/- 0.20. Our results show that a significantfraction of the energy associated with the rest mass of materialentering the Bondi accretion radius (2.2+1.0-0.7per cent, for Pjet = 1043ergs-1)eventually emerges in the relativistic jets. The data also hint thatthis fraction may rise slightly with increasing jet power. Our resultshave significant implications for studies of accretion, jet formationand galaxy formation. The observed tight correlation suggests that theBondi formulae provide a reasonable description of the accretion processin these systems, despite the likely presence of magnetic pressure andangular momentum in the accreting gas. The similarity of thePBondi and Pjet values argues that a significantfraction of the matter entering the accretion radius flows down toregions close to the black holes, where the jets are presumably formed.The tight correlation between PBondi and Pjet alsosuggests that the accretion flows are approximately stable overtime-scales of a few million years. Our results show that the black hole`engines' at the hearts of large elliptical galaxies and groups can feedback sufficient energy to stem cooling and star formation, leadingnaturally to the observed exponential cut off at the bright end of thegalaxy luminosity function.
| Multifrequency observations of the jets in the radio galaxy NGC315
We present images of the jets in the nearby radio galaxy NGC315 madewith the Very Large Array at five frequencies between 1.365 and 5GHzwith resolutions between 1.5 and 45arcsec. Within 15arcsec of thenucleus, the spectral index of the jets is α= 0.61. Further fromthe nucleus, the spectrum is flatter, with significant transversestructure. Between 15 and 70arcsec from the nucleus, the spectral indexvaries from ~0.55 on-axis to ~0.44 at the edge. This spectral structuresuggests a change of dominant particle acceleration mechanism withdistance from the nucleus and the transverse gradient may be associatedwith shear in the jet velocity field. Further from the nucleus, thespectral index has a constant value of 0.47. We derive the distributionof Faraday rotation over the inner +/-400arcsec of the radio source andshow that it has three components: a constant term, a linear gradient(both probably due to our Galaxy) and residual fluctuations at the levelof 1-2radm-2. These residual fluctuations are smaller in thebrighter (approaching) jet, consistent with the idea that they areproduced by magnetic fields in a halo of hot plasma that surrounds theradio source. We model this halo, deriving a core radius of ~225arcsecand constraining its central density and magnetic field strength. Wealso image the apparent magnetic field structure over the first+/-200arcsec from the nucleus.
| Exploring the range of black hole masses with Chandra
Efficiently accreting super-massive black holes (SMBHs) in activegalactic nuclei (AGNs, with masses in excess of 106 Mȯ)and black holes in Galactic X-ray binaries (with masses 10 Mȯ,e.g., see Tanaka & Lewin 1995) have long been studied in X-rays.AGNs and black hole X-ray binaries are luminous and fairly common X-raysources that have been successfully observed with many X-rayobservatories, since the beginning of X-ray astronomy nearly fourdecades ago. The study of black holes in X-rays has now acquired newdimensions thanks to the sub-arcsecond resolution, sensitiveobservations of the Chandra X-ray Observatory. In this paper I addresstwo new lines of investigation that have been blossoming thanks toChandra: quiescent galactic nuclei (QGNs) associated with SMBHs, and thehunt for intermediate mass black holes (IMBHs).
| Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. II. Optical Study and Interpretation
Our X-ray study of the nuclear activity in a new sample of six quiescentearly-type galaxies, as well as in a larger sample from the literature,confirmed (Paper I) that the Bondi accretion rate of diffuse hot gas isnot a good indicator of the SMBH X-ray luminosity. Here we suggest thata more reliable estimate of the accretion rate must include the gasreleased by the stellar population inside the sphere of influence of theSMBH, in addition to the Bondi inflow of hot gas across that surface. Weuse optical surface brightness profiles to estimate the mass-loss ratefrom stars in the nuclear region: we show that for our sample ofgalaxies it is an order of magnitude higher (~10-4 to10-3 Msolar yr-1) than the Bondi inflowrate of hot gas, as estimated from Chandra (Paper I). Only by takinginto account both sources of fuel can we constrain the true accretionrate, the accretion efficiency, and the power budget. Radiativelyefficient accretion is ruled out, for quiescent SMBHs. For typicalradiatively inefficient flows, the observed X-ray luminosities of theSMBHs imply accretion fractions ~1%-10% (i.e., ~90%-99% of the availablegas does not reach the SMBH) for at least five of our six targetgalaxies and most of the other galaxies with known SMBH masses. Wediscuss the conditions for mass conservation inside the sphere ofinfluence, so that the total gas injection is balanced by accretion plusoutflows. We show that a fraction of the total accretion power(mechanical plus radiative) would be sufficient to sustain aself-regulating, slow outflow that removes from the nuclear region allthe gas that does not sink into the BH (``BH feedback''). The rest ofthe accretion power may be carried out in a jet or advected. We alsodiscuss scenarios that would lead to an intermittent nuclear activity.
| Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. I. X-Ray Study
We have studied the nuclear activity in a sample of six quiescentearly-type galaxies, with new Chandra data and archival HST opticalimages. Their nuclear sources have X-ray luminosities~1038-1039 ergs s-1(LX/LEdd~10-8 to 10-7) andcolors or spectra consistent with accreting supermassive black holes(SMBHs), except for the nucleus of NGC 4486B, which is softer thantypical AGN spectra. In a few cases, the X-ray morphology of the nuclearsources shows hints of marginally extended structures, in addition tothe surrounding diffuse thermal emission from hot gas, which isdetectable on scales >~1 kpc. In one case (NGC 5845), a dusty diskmay partially obstruct our direct view of the SMBH. We have estimatedthe temperature and density of the hot interstellar medium, which is onemajor source of fuel for the accreting SMBH; typical central densitiesare ne~(0.02+/-0.01) cm-3. Assuming that the hotgas is captured by the SMBH at the Bondi rate, we show that the observedX-ray luminosities are too faint to be consistent with standard diskaccretion, but brighter than predicted by radiatively inefficientsolutions (e.g., advection-dominated accretion flows [ADAFs]). In total,there are ~20 galaxies for which SMBH mass, hot gas density, and nuclearX-ray luminosity are simultaneously known. In some cases, the nuclearsources are brighter than predicted by the ADAF model; in other cases,they are consistent or fainter. We discuss the apparent lack ofcorrelations between Bondi rate and X-ray luminosity and suggest that,in order to understand the observed distribution, we need to know twoadditional parameters: the amount of gas supplied by the stellarpopulation inside the accretion radius, and the fraction (possibly<<1) of the total gas available that is accreted by the SMBH. Weleave a detailed study of these issues to a subsequent paper.
| 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.
| The Molonglo Southern 4 Jy Sample (MS4). II. ATCA Imaging and Optical Identification
Of the 228 sources in the Molonglo Southern 4 Jy sample (MS4), the 133with angular sizes <35" have been imaged at 5 GHz at 2"-4" resolutionwith the Australia Telescope Compact Array. More than 90% of the samplehas been reliably optically identified, either on the plates of the UKSchmidt Southern Sky Survey or on R-band CCD images made with theAnglo-Australian Telescope. A subsample of 137 sources, the SMS4,defined to be a close southern equivalent of the northern 3CRR sample,was found to have global properties mostly consistent with the northernsample. Linear sizes of MS4 galaxies and quasars were found to beconsistent with galaxy-quasar unification models of orientation andevolution.
| The Molonglo Southern 4 Jy Sample (MS4). I. Definition
We have defined a complete sample of 228 southern radio sources at 408MHz with integrated flux densities S408>4.0 Jy, Galacticlatitude |b|>10deg, and declination-85deg<δ<-30deg. The main findingsurvey used was the Molonglo Reference Catalogue. We describe in detailhow the Molonglo Southern 4 Jy sample (MS4) was assembled and itscompleteness assessed. Sources in the sample were imaged at 843 MHz withthe Molonglo Observatory Synthesis Telescope to obtain positionsaccurate to about 1", as well as flux densities and angular sizes;follow-up radio and optical observations are presented in Paper II.Radio spectra for the MS4 have been compiled from the literature andused to estimate flux densities at 178 MHz. The strong-source subset ofMS4, with S178>10.9 Jy (SMS4), provides a southern sampleclosely equivalent to the well-studied northern 3CRR sample. Comparisonof SMS4 with 3CRR shows a reassuring similarity in source density andmedian flux density between the two samples.
| The host galaxy/AGN connection in nearby early-type galaxies. A new view of the origin of the radio-quiet/radio-loud dichotomy?
This is the third in a series of three papers exploring the connectionbetween the multiwavelength properties of AGN in nearby early-typegalaxies and the characteristics of their hosts. Starting from aninitial sample of 332 galaxies, we selected 116 AGN candidates requiringthe detection of a radio source with a flux limit of ~1 mJy, as measuredfrom 5 GHz VLA observations. In Paper I we classified the objects withavailable archival HST images into "core" and "power-law" galaxies,discriminating on the basis of the nuclear slope of their brightnessprofiles. We used HST and Chandra data to isolate the nuclear emissionof these galaxies in the optical and X-ray bands, thus enabling us (oncecombined with the radio data) to study the multiwavelength behaviour oftheir nuclei. The properties of the nuclei hosted by the 29 coregalaxies were presented in Paper II Core galaxies invariably host aradio-loud nucleus, with a median radio-loudness of Log R = 3.6 and anX-ray based radio-loudness parameter of Log RX = -1.3. Herewe discuss the properties of the nuclei of the 22 "power-law" galaxies.They show a substantial excess of optical and X-ray emission withrespect to core galaxies at the same level of radio luminosity.Conversely, their radio-loudness parameters, Log R 1.6 and LogRX -3.3, are similar to those measured in Seyfertgalaxies. Thus the radio-loudness of AGN hosted by early-type galaxiesappears to be univocally related to the host's brightness profile:radio-loud AGN are only hosted by core galaxies, while radio-quiet AGNare found only in power-law galaxies. The brightness profile isdetermined by the galaxy's evolution, through its merger history; ourresults suggest that the same process sets the AGN flavour. In thisscenario, the black holes hosted by the merging galaxies rapidly sinktoward the centre of the newly formed object, setting its nuclearconfiguration, described by e.g. the total mass, spin, mass ratio, orseparation of the SMBHs. These parameters are most likely at the originof the different levels of the AGN radio-loudness. This connection mightopen a new path toward understanding the origin of theradio-loud/radio-quiet AGN dichotomy and provide us with a further toolfor exploring the co-evolution of galaxies and supermassive black holes.
| The host galaxy/AGN connection in nearby early-type galaxies. Is there a miniature radio-galaxy in every "core" galaxy?
This is the second of a series of three papers exploring the connectionbetween the multiwavelength properties of AGN in nearby early-typegalaxies and the characteristics of their hosts. We selected two sampleswith 5 GHz VLA radio flux measurements down to 1 mJy, reaching levels ofradio luminosity as low as 1036 erg s-1. In PaperI we presented a study of the surface brightness profiles for the 65objects with available archival HST images out of the 116 radio-detectedgalaxies. We classified early-type galaxies into "core" and "power-law"galaxies, discriminating on the basis of the slope of their nuclearbrightness profiles, following the Nukers scheme. Here we focus on the29 core galaxies (hereafter CoreG). We used HST and Chandra data toisolate their optical and X-ray nuclear emission. The CoreG invariablyhost radio-loud nuclei, with an average radio-loudness parameter of LogR = L5 {GHz} / LB 3.6. The optical and X-raynuclear luminosities correlate with the radio-core power, smoothlyextending the analogous correlations already found for low luminosityradio-galaxies (LLRG) toward even lower power, by a factor of 1000, covering a combined range of 6 orders of magnitude. This supportsthe interpretation of a common non-thermal origin of the nuclearemission also for CoreG. The luminosities of the nuclear sources, mostlikely dominated by jet emission, set firm upper limits, as low asL/L_Edd 10-9 in both the optical and X-ray band, on anyemission from the accretion process. The similarity of CoreG and LLRGwhen considering the distributions host galaxies luminosities and blackhole masses, as well as of the surface brightness profiles, indicatesthat they are drawn from the same population of early-type galaxies.LLRG represent only the tip of the iceberg associated with (relatively)high activity levels, with CoreG forming the bulk of the population. Wedo not find any relationship between radio-power and black hole mass. Aminimum black hole mass of M_BH = 108 Mȯ isapparently associated with the radio-loud nuclei in both CoreG and LLRG,but this effect must be tested on a sample of less luminous galaxies,likely to host smaller black holes. In the unifying model for BL Lacsand radio-galaxies, CoreG likely represent the counterparts of the largepopulation of low luminosity BL Lac now emerging from the surveys at lowradio flux limits. This suggests the presence of relativistic jets alsoin these quasi-quiescent early-type "core" galaxies.
| High-frequency radio observations of the Kühr sample and the epoch-dependent luminosity function of flat-spectrum quasars
We discuss our ATCA 18.5 and 22 GHz flux density measurements ofSouthern extragalactic sources in the complete 5 GHz sample of Kühret al. (1981, A&AS, 45, 367). The high frequency (5-18.5 GHz)spectral indices of steep-spectrum sources for which we have 18.5 GHzdata (66% of the complete sample) are systematically steeper than thelow frequency (2.7-5 GHz) ones, with median α^52.7 =0.76, median α18.55 = 1.18(Sν∝ ν-α), and median steepeningΔα = 0.32, and there is evidence of an anti-correlation ofΔα18.55 with luminosity. Thecompleteness of 18.5 GHz data is much higher (89%) for flat-spectrumsources (mostly quasars), which also exhibit a spectral steepening:median α^52.7=-0.14, medianα18.55=0.16 (Sν∝ν-α), and median Δα = 0.19. Takingadvantage of the almost complete redshift information on flat-spectrumquasars, we have estimated their 5 GHz luminosity function in severalredshift bins. The results confirm that their radio luminosity densitypeaks at z_peak ≃ 2.5 but do not provide evidence for deviationsfrom pure luminosity evolution as hinted at by other data sets. Acomparison of our 22 GHz flux densities with WMAP K-band data forflat-spectrum sources suggests that WMAP flux densities may be low by amedian factor of ≃1.2. The extrapolations of 5 GHz counts andluminosity functions of flat-spectrum radio quasars using the observeddistribution of the 5-18.5 GHz spectral indices match those deriveddirectly from WMAP data, indicating that the high frequency WMAP surveydoes not detect any large population of FSRQs with anomalous spectra.
| 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.
| 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 Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies
We report on the discovery of a new correlation between globalparameters of the hot interstellar gas in elliptical galaxies. Wereanalyze archival Chandra data for 30 normal early-type systems,removing the contributions of resolved and unresolved point sources toreveal the X-ray morphology of the hot gas. We determine the half-lightradius, RX, and the mean surface brightness, IX,from the gas surface brightness profiles. A spectral analysis determinesthe temperature, TX, of the gas within 3 optical effectiveradii. We find that the galaxies lie on an X-ray gas fundamental plane(XGFP) of the formTX~R0.28XI0.22X.This is close to, but distinct from, a simple luminosity-temperaturerelation. The intrinsic width of the XGFP is only 0.07 dex, nearlyidentical to that of the stellar (optical) fundamental plane (SFP). Thisis surprising since X-ray gas masses are typically ~10-2 ofthe stellar masses. We show that the XGFP is not a simple consequence ofthe virial theorem or hydrostatic equilibrium and that it is essentiallyindependent of the SFP. The XGFP thus represents a genuinely newconstraint on the hydrodynamical evolution of elliptical galaxies.
| A Possible Detection of M31* with Chandra
Two independent sets of Chandra and HST images of the nuclear region ofM31 allow registration of X-ray and optical images to ~0.1". Thisregistration shows that none of the bright (~1037 ergss-1) X-ray sources near the nucleus is coincident with thecentral supermassive black hole, M31*. A 50 ks Chandra HRC image shows2.5 σ evidence for a faint (~1036 ergs s-1)discrete source that is consistent with the position of M31*. The Bondiradius of M31* is 0.9", making it one of the few supermassive blackholes with a resolvable accretion flow. This large radius and theprevious detections of diffuse X-ray-emitting gas in the nuclear regionmake M31* one of the most secure cases for a radiatively inefficientaccretion flow and place some of the most severe constraints on theradiative processes in such a flow.
| A CCD Photometric and Morphological Study of the Extended Halo and Filaments of ESO 383-45: A Galaxy Undergoing Ram Pressure Stripping, or a Tidal Merger Remnant?
We present BV CCD surface photometry, profiles, and images of the galaxyESO 383-45, together with other galaxies in the same CCD field. We alsopresent a B-V color map of the field and images of the galaxies enhancedby self-correlation of pixel values and by digital ``unsharp masking.''The extended halo and system of filaments of ESO 383-45 are seenclearly. We suggest that the evidence (radio jets and their curvatureand areas of diffuse optical emission) of a dense intergalactic medium(IGM) in this field toward the center of the IC 4296 cluster mayindicate that the galaxy ESO 383-45 is still undergoing ram pressurestripping of its gas, forming stars in the filaments, while the centralgalaxy has evolved to have a lenticular morphology. On the other hand,there are ``knots'' in the filaments that look like tidal dwarf galaxiesin formation, and previous simulations of the tidal interaction of twodisk galaxies have produced galaxies that can resemble ESO 383-45 fromcertain viewing angles. Other galaxies in the field appear to lie beyondthe IC 4296 cluster and may be part of sheets of galaxies previouslyidentified as connecting the Abell clusters of the Shapley supercluster.We identify many uncataloged faint extended objects that may representbackground clusters of galaxies or knots (possibly of star formation)associated with the filaments and diffuse IGM of ESO 383-45. The presentwork represents the first multicolor surface photometric study for allof these galaxies, and only ESO 383-45 has previously been studiedmorphologically, using digitally co-added Schmidt plates obtained bysome of the authors.
| Nuclear Accretion in Galaxies of the Local Universe: Clues from Chandra Observations
In order to find an explanation for the radiative quiescence ofsupermassive black holes in the local universe, the most accurateestimates for a sample of nearby galaxies are collected for the mass ofa central black hole (MBH), the nuclear X-ray luminosityLX,nuc, and the circumnuclear hot gas density andtemperature, by using Chandra data. The nuclear X-ray luminosityLX,nuc varies by ~3 orders of magnitude and does not show arelationship with MBH or with the Bondi mass accretion rateM˙B LX,nuc is always much lower than expectedif M˙B ends in a standard accretion disk with highradiative efficiency (this instead can be the case of the active nucleusof Cen A). Radiatively inefficient accretion as in the standardadvection-dominated accretion flow (ADAF) modeling may explain the lowluminosities of a few cases; for others, the predicted luminosity isstill too high, and, in terms of Eddington-scaled quantities, it isincreasingly higher than that observed for increasingM˙B. Variants of the simple radiatively inefficientscenario including outflow and convection may reproduce the low emissionlevels observed, since the amount of matter actually accreted is reducedconsiderably. However, the most promising scenario includes feedbackfrom accretion on the surrounding gas; this has the important advantagesof naturally explaining the observed lack of relationship amongLX,nuc, MBH, and M˙B, and evadingthe problem of the fate of the material accumulating in the centralgalactic regions over cosmological times.
| An X-Ray View of Weak-Line Radio Galaxies/LINERs
We present X-ray observations of nine weak-line radio galaxies (WLRGs),optically classified as confirmed or possible LINERs. The data weretaken from the Chandra, XMM-Newton, and BeppoSAX archives. The Chandraimages typically show complex X-ray morphologies, with hard (2-10 keV)point sources embedded in diffuse soft (0.3-2.0 keV) emission in allcases except those of 1246-41 (NGC 4696), in which only diffuse emissionis detected on the scale of the cluster, and 0334-01 (3C 15), in whichonly a point source is detected. The nuclear X-ray spectra are wellfitted at hard energies by an absorbed power law, with a wide range ofphoton indices, Γ=1.5-2.7. Excess absorption over the Galacticvalue is detected in six of the nine sources, with column densitiesNH~1021-1022 cm-2. A thermalcomponent is required at softer energies, in agreement with the resultsof the spatial analysis. We find that there is no correlation betweenthe core X-ray luminosity and the radio core dominance parameter,suggesting that the bulk of the core X-ray emission is not beamed butrather is isotropic and thus likely related to the accretion flow. In anattempt to constrain the nature of the accretion flow, we calculate theratios of bolometric to Eddington luminosities,Lbol/LEdd, and the radiative efficiency ηbased on the Bondi accretion rates. We find thatLbol/LEdd~10-4-10-6 andη~10-2-10-6 for all the objects in our sample,suggesting radiatively inefficient accretion flows.
| 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.
| The host galaxy/AGN connection in nearby early-type galaxies. Sample selection and hosts brightness profiles
This is the first of a series of three papers exploring the connectionbetween the multiwavelength properties of AGNs in nearby early-typegalaxies and the characteristics of their hosts. We selected twosamples, both with high resolution 5 GHz VLA observations available andproviding measurements down to 1 mJy level, reaching radio-luminositiesas low as 1019 W Hz-1. We focus on the 116radio-detected galaxies as to boost the fraction of AGN with respect toa purely optically selected sample. Here we present the analysis of theoptical brightness profiles based on archival HST images, available for65 objects. We separate early-type galaxies on the basis of the slope oftheir nuclear brightness profiles, into core and power-law galaxiesfollowing the Nuker's scheme, rather than on the traditionalmorphological classification (i.e. into E and S0 galaxies). Our sampleof AGN candidates is indistinguishable, when their brightness profilesare concerned, from galaxies of similar optical luminosity but hostingweaker (or no) radio-sources. We confirm previous findings thatrelatively bright radio-sources (Lr > 1021.5 WHz-1) are uniquely associated to core galaxies. However,below this threshold in radio-luminosity core and power-law galaxiescoexist and they do not show any apparent difference in theirradio-properties. Not surprisingly, since our sample is deliberatelybiased to favour the inclusion of active galaxies, we found a higherfraction of optically nucleated galaxies. Addressing the multiwavelengthproperties of these nuclei will be the aim of the two forthcomingpapers.
| Nearby early-type galaxies with ionized gas. I. Line-strength indices of the underlying stellar population
With the aim of building a data-set of spectral properties of wellstudied early-type galaxies showing emission lines, we presentintermediate resolution spectra of 50 galaxies in the nearby Universe.The sample, which covers several of the E and S0 morphologicalsub-classes, is biased toward objects that might be expected to haveongoing and recent star formation, at least in small amounts, because ofthe presence of the emission lines. The emission is expected to comefrom the combination of active galactic nuclei and star formationregions within the galaxies. Sample galaxies are located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. Our long-slit spectra coverthe 3700-7250 Å wavelength range with a spectral resolution of≈7.6 Å at 5550 Å. The specific aim of this paper, and ourfirst step in the investigation, is to map the underlying galaxy stellarpopulation by measuring, along the slit positioned along the galaxymajor axis, line-strength indices at several, homogeneousgalacto-centric distances. For each object we extracted 7luminosity-weighted apertures (with radii 1.5´´,2.5´´, 10´´, r_e/10, r_e/8, r_e/4 and r_e/2)corrected for the galaxy ellipticity and 4 gradients (0 ≤ r ≤r_e/16, r_e/16 ≤ r ≤ r_e/8, r_e/8 ≤ r ≤ r_e/4 and r_e/4≤ r ≤ r_e/2). For each aperture and gradient we measured 25line-strength indices: 21 of the set defined by the Lick-IDS“standard” system (Trager et al. [CITE], ApJS, 116, 1) and 4introduced by Worthey & Ottaviani ([CITE], ApJS, 111, 377).Line-strength indices have been transformed to the Lick-IDS system.Indices derived then include Hβ, Mg1, Mg2, Mgb, MgFe, Fe5270,Fe5335 commonly used in classic index-index diagrams. The paperintroduces the sample, presents the observations, describes the datareduction procedures, the extraction of apertures and gradients, thedetermination and correction of the line-strength indices, the procedureadopted to transform them into the Lick-IDS System and the proceduresadopted for the emission correction. We finally discuss the comparisonsbetween our dataset and line-strength indices available in theliterature. A significant fraction, about 60%, of galaxies in thepresent sample has one previous measurement in the Lick-IDS system butbasically restricted within the r_e/8 region. Line-strength measuresobtained both from apertures and gradients outside this area and withinthe r_e/8 region, with the present radial mapping, are completely new.Full appendix and Figs. 8 to 13 are only available in electronic form athttp://www.edpsciences.org Full Tables 6, 7, 9 and 10 are only availableat the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/497 Based onobservations obtained at the European Southern Observatory, La Silla,Chile (Programs Nr. 60.A-0647 and 61.A-0406).
| 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.
| Emission lines and optical continuum in low-luminosity radio galaxies
We present spectroscopic observations of a complete subsample of 13low-luminosity radio galaxies selected from the 2-Jy sample of Tadhunteret al. The underlying continuum in these sources was carefully modelledin order to make a much-needed comparison between the emission-line andcontinuum properties of Fanaroff-Riley type Is (FRIs) and those of otherclasses of radio sources. We find that five galaxies in the sample showa measurable ultraviolet (UV) excess: two of these sources are BL Lacs,but in the remaining three galaxies we argue that the most likelycontributor to the UV excess is a young stellar component. Therefore,excluding the BL Lacs, we find that ~30 per cent of the sample showevidence for young stars, which is similar to the results obtained forhigher luminosity samples. We compare our results with far-infraredmeasurements in order to investigate the far-infrared-starburst link.The nature of the optical-radio correlations is investigated in light ofthese new available data and, in contrast to previous studies, we findthat the FRI sources follow the correlations with similar slopes tothose found for the Fanaroff-Riley type IIs. Finally, we compare theluminosities of the emission lines in the FRI and BL Lac sources andfind a significant difference between the [OIII] line luminosities ofthe two groups. Our results are discussed in the context of the unifiedschemes for low-powered radio sources.
| 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.
| Chandra Observations of the Quiescent Nuclear Black Hole of NGC 821: Evidence of Nuclear Activity?
We report the results of the Chandra ACIS-S observations of theelliptical galaxy NGC 821, which harbors a supermassive nuclear blackhole (3.5×107 Msolar) but does not showsigns of active galactic nucleus activity. A small, 8.5" long (~1 kpc atthe galaxy's distance of 23 Mpc), possibly S-shaped, jetlike featurecentered on the nucleus is detected in the 38 ks ACIS-S integratedexposure of this region. The luminosity of this feature isLX~2.6×1039 ergs s-1 (0.3-10keV), and its spectrum is hard (described by a power law ofΓ=1.8+0.7-0.6 or by thermal emission withkT>2 keV). We discuss two possibilities for the origin of thisfeature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. Ineither case, it is a clear indication of nuclear activity, detectableonly in the X-ray band. Steady spherical accretion of the mass lossesfrom the central stellar cusp within the accretion radius, when coupledwith a high radiative efficiency, already provides a power sourceexceeding the observed radiative losses from the nuclear region. A thirdpossibility, that this feature may arise from a fortuitous distributionof luminous X-ray binaries in NGC 821, is also discussed.
| 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.
| Stacking Searches for Gamma-Ray Emission above 100 MeV from Radio and Seyfert Galaxies
The EGRET telescope on board Compton Gamma Ray Observatory detected morethan 60 sources of high-energy gamma radiation associated with activegalactic nuclei (AGNs). All but one of those belong to the blazarsubclass; the only exception is the nearby radio galaxy Centaurus A.Since there is no obvious reason other than proximity to expect Cen A tobe the only nonblazar AGN emitting in high-energy gamma rays, we haveutilized the ``stacking'' technique to search for emission above 100 MeVfrom two nonblazar AGN subclasses, radio galaxies and Seyfert galaxies.Maps of gamma-ray counts, exposure, and diffuse background have beencreated, then co-added in varying numbers based on sorts by redshift, 5GHz flux density, and optical brightness, and finally tested forgamma-ray emission. No detection significance greater than 2 σ hasbeen found for any subclass, sorting parameter, or number of objectsco-added. Monte Carlo simulations have also been performed to validatethe technique and estimate the significance of the results.
| Test predictions on the X-ray jets in radio-loud AGNs via X-ray observation for FR II radio galaxies
In an earlier paper, based on the knowledge of blazars we predictedcommon existence of X-ray jets within ~10 kpc from the nucleus inradio-loud AGNs. Recent Chandra X-ray observations for low-power radiogalaxies are consistent with our predictions. In this paper, we studythe electron accelerations in the kpc scale jets, and propose to studythe importance of Compton cooling and the environmental differencesbetween the inner compact jets and the larger scale extended jets in redblazars through Chandra X-ray observation for the predicted X-ray jetsin powerful FR II radio galaxies, to further test our model for kpcscale X-ray jets in radio-loud AGNs.
| Quasars and active galaxies.
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