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X-ray spectroscopy of cooling clusters
Electronic Article Available from Elsevier Science.
| Adaptive binning of X-ray data with weighted Voronoi tessellations
We present a technique to adaptively bin sparse data using weightedVoronoi tessellations (WVTs). WVT binning is a generalization of theVoronoi binning algorithm by Cappellari & Copin, developed forintegral field spectroscopy. WVT binning is applicable to many types ofdata and creates unbiased binning structures with compact bins that donot lead the eye. We apply the algorithm to simulated data, as well asseveral X-ray data sets, to create adaptively binned intensity images,hardness ratio maps and temperature maps with constant signal-to-noiseratio per bin. We also illustrate the separation of diffuse gas emissionfrom contributions of unresolved point sources in elliptical galaxies.We compare the performance of WVT binning with other adaptive binningand adaptive smoothing techniques. We find that the csmooth tool in CIAOversions 1.1-3.1 creates serious artefacts and advise against its use tointerpret diffuse X-ray emission.
| Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.
| Globular cluster systems in low-luminosity early-type galaxies near the Fornax cluster centre
We present a photometric study of the globular cluster systems (GCSs) ofthe Fornax cluster galaxies NGC1374, NGC1379 and NGC1387. The dataconsist of images from the wide-field MOSAIC imager of the Cerro TololoInter-American Observatory (CTIO) 4-m telescope, obtained withWashington C and Kron-Cousins R filters. The images cover a field of 36× 36arcmin2, corresponding to 200 ×200kpc2 at the Fornax distance. Two of the galaxies, NGC1374and NGC1379, are low-luminosity ellipticals while NGC1387 is alow-luminosity lenticular. Their cluster systems are still embedded inthe cluster system of NGC1399. Therefore, the use of a large field iscrucial and some differences to previous work can be explained by this.The colour distributions of all GCSs are bimodal. NGC1387 presents aparticularly distinct separation between red and blue clusters and anoverproportionally large population of red clusters. The radialdistribution is different for blue and red clusters, red clusters beingmore concentrated towards the respective galaxies. The different colourand radial distributions point to the existence of two globular clustersubpopulations in these galaxies. Specific frequencies are in the rangeSN= 1.4-2.4, smaller than the typical values for ellipticalgalaxies. These galaxies might have suffered tidal stripping of blueglobular clusters by NGC1399.
| Low-Mass X-Ray Binaries in Six Elliptical Galaxies: Connection to Globular Clusters
We present a systematic study of the low-mass X-ray binary (LMXB)populations of six elliptical galaxies, aimed at investigating thedetected LMXB-globular cluster (GC) connection. We utilize Chandraarchival data to identify X-ray point sources and HST archival datasupplemented by ground observations to identify 6173 GCs. Afterscreening and cross-matching, we associate 209 LMXBs with red GC (RGCs)and 76 LMXBs with blue GCs (BGCs), while we find no optical GCcounterpart for 258 LMXBs. This is the largest GC-LMXB sample studied sofar. We confirm previous reports suggesting that the fraction of GCsassociated with LMXBs is ~3 times larger in RGCs than in BGCs,indicating that metallicity is a primary factor in the GC LMXBformation. We find that GCs located near the galaxy center have a higherprobability of harboring LMXBs than those in the outskirts, suggestingthat there must be another parameter (in addition to metallicity)governing LMXB formation in GCs. This second parameter, dependent on thegalactocentric distance, may be a distance dependent encounter rate. Wefind no significant differences in the shape of X-ray luminosityfunction, LX/LV distribution, X-ray spectra amongRGC, BGC, and field LMXBs. The similarity of the X-ray spectra isinconsistent with the irradiation-induced stellar wind model prediction.The similarity of the X-ray luminosity functions (XLFs) of GC LMXBs andfield LMXBs indicates that there is no significant difference in thefraction of black hole binaries present. We cannot either prove orreject the hypothesis that all LMXBs were formed in GCs.
| The Central Engines of 19 LINERs as Viewed by Chandra
Using archival Chandra observations of 19 LINERs, we explore the X-rayproperties of their inner kiloparsec to determine the origin of theirnuclear X-ray emission, to investigate the presence of an AGN, and toidentify the power source of the optical emission lines. The relativenumbers of LINER types in our sample are similar to those in opticalspectroscopic surveys. We find that diffuse, thermal emission is verycommon and is concentrated within the central few hundred parsecs. Theaverage spectra of the hot gas in spiral and elliptical galaxies arevery similar to those of normal galaxies. They can be fitted with athermal plasma (kT~0.5 keV) plus a power-law (photon index of 1.3-1.5)model. There are on average three detected point sources in their innerkiloparsec with 1037 ergss-1
| A Chandra View of Dark Matter in Early-Type Galaxies
We present a Chandra study of mass profiles in seven ellipticalgalaxies, of which three have galaxy-scale and four have group-scalehalos, demarcated at 1013 Msolar. These representthe best available data for nearby objects with comparable X-rayluminosities. We measure approximately flat mass-to-light (M/L) profileswithin an optical half-light radius (Reff), rising by anorder of magnitude at ~10 Reff, which confirms the presenceof dark matter (DM). The data indicate hydrostatic equilibrium, which isalso supported by agreement with studies of stellar kinematics inelliptical galaxies. The data are well fitted by a model comprising anNFW DM profile and a baryonic component following the optical light. Thedistribution of DM halo concentration parameters (c) versusMvir agrees with ΛCDM predictions and our observationsof bright groups. Concentrations are slightly higher than expected,which is most likely a selection effect. Omitting the stellar massdrastically increases c, possibly explaining large concentrations foundby some past observers. The stellar M/LK agree withpopulation synthesis models, assuming a Kroupa IMF. Allowing adiabaticcompression (AC) of the DM halo by baryons made M/L more discrepant,casting some doubt on AC. Our best-fitting models imply total baryonfractions ~0.04-0.09, consistent with models of galaxy formationincorporating strong feedback. The groups exhibit positive temperaturegradients, consistent with the ``universal'' profiles found in othergroups and clusters, whereas the galaxies have negative gradients,suggesting a change in the evolutionary history of the systems aroundMvir~=1013 Msolar.
| The Two-dimensional XMM-Newton Group Survey: z < 0.012 Groups
We present the results of the two-dimensional XMM-Newton Group Survey(2dXGS), an archival study of nearby galaxy groups. In this paper weconsider 11 nearby systems (z<0.012) in Mulchaey et al., which span abroad range in X-ray luminosity from 1040 to 1043ergs s-1. We measure the iron abundance and temperaturedistribution in these systems and derive pressure and entropy maps. Wefind statistically significant evidence for structure in the entropy andpressure of the gas component of seven groups on the 10%-20% level. TheXMM-Newton data for the three groups with best statistics also suggestpatchy metallicity distributions within the central 20-50 kpc of thebrightest group galaxy, probed with 2-10 kpc resolution. This providesinsights into the processes associated with thermalization of thestellar mass loss. Analysis of the global properties of the groupsreveals a subclass of X-ray-faint groups, which are characterized byboth higher entropy and lower pressure. We suggest that the mergerhistory of the central elliptical is responsible for both the source andthe observed thermodynamical properties of the hot gas of theX-ray-faint groups.
| Determining Distances to Clusters of Galaxies Using Resonant X-Ray Emission Lines
Bright clusters of galaxies can be seen out to cosmological distances,and thus they can be used to derive cosmological parameters. Althoughthe continuum X-ray emission from the intracluster gas is opticallythin, the optical depth of resonant lines of ions of heavy elements canbe larger than unity. In this Letter, we study the feasibility ofderiving distances to clusters of galaxies by determining the spatialdistribution of the intracluster gas from X-ray imaging and the opticaldepth from resonant emission lines (the XREL method). We solve theradiative transfer problem for line scattering in the hot intraclustergas using Monte Carlo simulations. We discuss the spatial and spectralresolutions needed to use the XREL method for accurate determination ofdistances, and hence cosmological parameters, and show that accuratedistances will be obtained by applying this technique with the nextgeneration of high-resolution X-ray spectrometers.
| The Size of the Cooling Region of Hot Gas in Two Elliptical Galaxies
Some early-type galaxies show O VI emission, a tracer of gas at105.5 K, and a predicted product of gas cooling from theX-ray-emitting temperatures. We studied the spatial extent and velocitystructure of this cooling gas by obtaining spectra of the O VI doubletin NGC 4636 and NGC 5846 with the Far Ultraviolet SpectroscopicExplorer. For NGC 4636, the central LWRS pointing shows that the O VI lines are double-peaked and symmetrical about the systemic velocity ofthe galaxy, with a separation of 210 km s-1. An LWRSobservation 30'' from the center failed to show additional OVI emission. For NGC 5846, three spectra were obtained with4''×20'' apertures (MDRS) at the center and4'' to the east and west of the center. The O VI line fluxseen in the previous LWRS is contained in the sum of the smallerapertures, with most of the flux in a single noncentral MDRS aperture,suggesting a size for the emission <=0.5 kpc; the emission consistsof a blue and red peak. For both galaxies, the O VI velocity structureis similar to that of the optical [N II] emission and is consistent withrotation. The compactness and velocity structure of the O VI emissionrules out cooling flow models with broadly distributed mass dropout butis consistent with cooling flow models in which the cooling occursprimarily in the central region. The 104 K gas may be the endstate of the O VI emitting gas.
| 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.
| Spitzer IRS spectra of Virgo Early-Type Galaxies: Detection of Stellar Silicate Emission
We present high signal-to-noise ratio Spitzer Infrared Spectrographobservations of 17 Virgo early-type galaxies. The galaxies were selectedfrom those that define the color-magnitude relation of the cluster, withthe aim of detecting the silicate emission of their dusty, mass-losingevolved stars. To flux calibrate these extended sources, we have deviseda new procedure that allows us to obtain the intrinsic spectral energydistribution and to disentangle resolved and unresolved emission withinthe same object. We have found that 13 objects of the sample (76%) arepassively evolving galaxies with a pronounced broad silicate featurethat is spatially extended and likely of stellar origin, in agreementwith model predictions. The other four objects (24%) are characterizedby different levels of activity. In NGC 4486 (M87), the line emissionand the broad silicate emission are evidently unresolved, and, givenalso the typical shape of the continuum, they likely originate in thenuclear torus. NGC 4636 shows emission lines superposed on extended(i.e., stellar) silicate emission, thus pushing the percentage ofgalaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435are characterized by polycyclic aromatic hydrocarbon (PAH) and lineemission, arising from a central unresolved region. A more detailedanalysis of our sample, with updated models, will be presented in aforthcoming paper.
| The Hot Gas Environment of the Radio Galaxy 3C 388: Quenching the Accumulation of Cool Gas in a Cluster Core by a Nuclear Outburst
We present results from a 35 ks Chandra ACIS-I observation of the hotintracluster medium (ICM) around the FR II radio galaxy 3C 388. 3C 388resides in a cluster environment with an ICM temperature of ~3.5 keV. Wedetect cavities in the ICM coincident with the radio lobes. The enthalpyof these cavities is ~1.2×1060 ergs. The work done onthe gas by the inflation of the lobes is ~3×1059 ergs,or ~0.87 keV per particle out to the radius of the lobes. The radiativetimescale for gas at the center of the cluster at the currenttemperature is a few Gyr. The gas in the core was probably cooler anddenser before the outburst, so the cooling time was considerablyshorter. We are therefore likely to be witnessing the quenching of acluster cooling flow by a radio galaxy outburst. The mechanical power ofthe lobes is at least 20 times larger than the radiative losses out tothe cooling radius. Outbursts of similar power with a ~5% duty cyclewould be more than sufficient to continually reheat the cluster coreover the Hubble time and prevent the cooling of any significant amountof gas. The mechanical power of the outburst is also roughly 2 orders ofmagnitude larger than either the X-ray luminosity of the active nucleusor the radio luminosity of the lobes. The equipartition pressure of theradio lobes is more than an order of magnitude lower than that of theambient medium, indicating that the pressure of the lobe is dominated bysomething other than the relativistic electrons radiating at GHzfrequencies.
| 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.
| CIRS: Cluster Infall Regions in the Sloan Digital Sky Survey. I. Infall Patterns and Mass Profiles
We use the Fourth Data Release of the Sloan Digital Sky Survey (SDSS) totest the ubiquity of infall patterns around galaxy clusters and measurecluster mass profiles to large radii. The Cluster and Infall RegionNearby Survey (CAIRNS) found infall patterns in nine clusters, but thecluster sample was incomplete. Here we match X-ray cluster catalogs withSDSS, search for infall patterns, and compute mass profiles for acomplete sample of X-ray-selected clusters. Very clean infall patternsare apparent in most of the clusters, with the fraction decreasing withincreasing redshift due to shallower sampling. All 72 clusters in awell-defined sample limited by redshift (ensuring good sampling) andX-ray flux (excluding superpositions) show infall patterns sufficient toapply the caustic technique. This sample is by far the largest sample ofcluster mass profiles extending to large radii to date. Similar toCAIRNS, cluster infall patterns are better defined in observations thanin simulations. Further work is needed to determine the source of thisdifference. We use the infall patterns to compute mass profiles for 72clusters and compare them to model profiles. Cluster scaling relationsusing caustic masses agree well with those using X-ray or virial massestimates, confirming the reliability of the caustic technique. Weconfirm the conclusion of CAIRNS that cluster infall regions are wellfitted by Navarro-Frenk-White (NFW) and Hernquist profiles and poorlyfitted by singular isothermal spheres. This much larger sample enablesnew comparisons of cluster properties with those in simulations. Theshapes (specifically NFW concentrations) of the mass profiles agree wellwith the predictions of simulations. The mass in the infall region istypically comparable to or larger than that in the virial region.Specifically, the mass inside the turnaround radius is on average2.19+/-0.18 times that within the virial radius. This ratio agrees wellwith recent predictions from simulations of the final masses of darkmatter halos.
| Uncovering Planetary Nebulae in Early-Type Galaxies using the Rutgers Fabry-Pérot
We report on observations of four early-type galaxies performed with theRutgers Fabry-Pérot in order to search for planetary nebulae(PNe) in these systems. The aim is to use the PNe as kinematic tracersof the galaxy potential. We describe our data reduction and analysisprocedure and show that the proper calibration of our detectionstatistic is crucial in getting down to our limiting magnitude ofm5007=26.1. In the case of the two Leo galaxies, we findmoderately sized samples: 54 PNe in NGC 3379 and 50 PNe in NGC 3384; NGC4636 (two PNe) and NGC 1549 (six PNe) are included for completeness. Wepresent our samples in tabular form, as well as the spectrum for eachPN. We constructed simple nonparametric spherical mass models for NGC3379 using a Monte Carlo Markov chain method to explore the space oflikely mass models. We find a remarkably constant mass-to-light ratiowithin five half-light radii with an overall B-band mass-to-light ratioof ~5. A simple mass-to-light ratio estimate for NGC 3384 yieldsΥB~11, but it is likely an overestimate.
| Large-scale study of the NGC 1399 globular cluster system in Fornax
We present a Washington C and Kron-Cousins R photometric study of theglobular cluster system of NGC 1399, the central galaxy of the Fornaxcluster. A large areal coverage of 1 square degree around NGC 1399 isachieved with three adjoining fields of the MOSAIC II Imager at the CTIO4-m telescope. Working on such a large field, we can perform the firstindicative determination of the total size of the NGC 1399 globularcluster system. The estimated angular extent, measured from the NGC 1399centre and up to a limiting radius where the areal density of blueglobular clusters falls to 30 per cent of the background level, is 45± 5 arcmin, which corresponds to 220-275 kpc at the Fornaxdistance. The bimodal colour distribution of this globular clustersystem, as well as the different radial distribution of blue and redclusters, up to these large distances from the parent galaxy, areconfirmed. The azimuthal globular cluster distribution exhibitsasymmetries that might be understood in terms of tidal stripping ofglobulars from NGC 1387, a nearby galaxy. The good agreement between theareal density profile of blue clusters and a projected dark-matter NFWdensity profile is emphasized.
| A radio census of nuclear activity in nearby galaxies
In order to determine the incidence of black hole accretion-drivennuclear activity in nearby galaxies, as manifested by their radioemission, we have carried out a high-resolution Multi-ElementRadio-Linked Interferometer Network (MERLIN) survey of LINERs andcomposite LINER/Hii galaxies from a complete magnitude-limited sample ofbright nearby galaxies (Palomar sample) with unknown arcsecond-scaleradio properties. There are fifteen radio detections, of which three arenew subarcsecond-scale radio core detections, all being candidate AGN.The detected galaxies supplement the already known low-luminosity AGN -low-luminosity Seyferts, LINERs and composite LINER/Hii galaxies - inthe Palomar sample. Combining all radio-detected Seyferts, LINERs andcomposite LINER/Hii galaxies (LTS sources), we obtain an overall radiodetection rate of 54% (22% of all bright nearby galaxies) and weestimate that at least ~50% (~20% of all bright nearby galaxies) aretrue AGN. The radio powers of the LTS galaxies allow the construction ofa local radio luminosity function. By comparing the luminosity functionwith those of selected moderate-redshift AGN, selected from the 2dF/NVSSsurvey, we find that LTS sources naturally extend the RLF of powerfulAGN down to powers of about 10 times that of Sgr A*.
| 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.
| Nearby early-type galaxies with ionized gas. II. Line-strength indices for 18 additional galaxies
We previously presented a data-set of line-strength indices for 50early-type galaxies in the nearby Universe. The galaxy sample is biasedtoward galaxies showing emission lines, located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. The present addendum enlargesthe above data-set of line-strength indices by analyzing 18 additionalearly-type galaxies (three galaxies, NGC 3607, NGC 5077 and NGC 5898were presented in the previous set). We measured 25 line-strengthindices, defined by the Lick IDS "standard" system (Trager et al. 1998,ApJS, 116, 1; Worthey & Ottaviani 1997, ApJS, 111, 377), for 7luminosity weighted apertures and 4 gradients of each galaxy. Thisaddendum presents the line-strength data-set and compares it with theavailable data in the literature.
| Radio bubbles in clusters of galaxies
We extend our earlier work on cluster cores with distinct radio bubbles,adding more active bubbles, i.e. those with GHz radio emission, to oursample and also investigating `ghost bubbles', i.e. those without GHzradio emission. We have determined k, which is the ratio of the totalparticle energy to that of the electrons radiating between 10 MHz and 10GHz. Constraints on the ages of the active bubbles confirm that theratio of the energy factor, k, to the volume filling factor, f, lieswithin the range 1 <~k/f<~ 1000. On the assumption that there ispressure equilibrium between the radio-emitting plasma and thesurrounding thermal X-ray gas, none of the radio lobes has equipartitionbetween the relativistic particles and the magnetic field. A Monte Carlosimulation of the data led to the conclusion that there are not enoughbubbles present in the current sample to be able to determine the shapeof the population. An analysis of the ghost bubbles in our sample showedthat on the whole they have higher upper limits on k/f than the activebubbles, especially when compared with those in the same cluster. Astudy of the Brightest 55 (B55) cluster sample shows that 17, possibly20, clusters required some form of heating as they have a short centralcooling time, tcool<= 3Gyr, and a large centraltemperature drop, Tcentre/Touter < 1/2. Ofthese, between 12 (70 per cent) and 15 (75 per cent) contain bubbles.This indicates that the duty cycle of bubbles is large in such clustersand that they can play a major role in the heating process.
| 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.
| Star-forming accretion flows and the low-luminosity nuclei of giant elliptical galaxies
The luminosities of the centres of nearby elliptical galaxies are verylow compared to models of thin disc accretion on to their black holes atthe Bondi rate, typically a few hundredths to a few tenths of a solarmass per year. This has motivated models of inefficiently radiatedaccretion that invoke weak electron-ion thermal coupling, and/orinhibited accretion rates due to convection or outflows. Here we pointout that, even if such processes are operating, a significant fractionof the accreting gas is prevented from reaching the central black holebecause it condenses into stars in a gravitationally unstable disc. Starformation occurs inside the Bondi radius (typically ~100 pc in giantellipticals), but still relatively far from the black hole in terms ofSchwarzschild radii. Star formation depletes and heats the gas disc,eventually leading to a marginally stable, but much reduced, accretionflow to the black hole. We predict the presence of cold (~100 K), dustygas discs, containing clustered Hα emission and occasional Type IIsupernovae, both resulting from the presence of massive stars. Starformation accounts for several features of the M87 system: a thin disc,traced by Hα emission, is observed on scales of about 100 pc, withfeatures reminiscent of spiral arms and dust lanes; the star formationrate inferred from the intensity of Hα emission is consistent withthe Bondi accretion rate of the system. Star formation may thereforehelp to suppress accretion on to the central engines of massiveellipticals. We also discuss some implications for the fuelling of theGalactic Centre and quasars.
| Dark matter in elliptical galaxies - I. Is the total mass density profile of the NFW form or even steeper?
Elliptical galaxies are modelled as Sérsic luminositydistributions with density profiles (DPs) for the total mass adoptedfrom the DPs of haloes within dissipationless ΛCDM (cold darkmatter) N-body simulations. Ellipticals turn out to be inconsistent withcuspy low-concentration NFW models representing the total massdistribution, neither are they consistent with a steeper -1.5 innerslope, nor with the shallower models proposed by Navarro et al., norwith NFW models 10 times more concentrated than predicted, as deducedfrom several X-ray observations - the mass models, extrapolated inwards,lead to local mass-to-light ratios that are smaller than the stellarvalue inside an effective radius (Re), and to centralaperture velocity dispersions that are much smaller than observed. Thisconclusion remains true as long as there is no sharp steepening (slope< -2) of the dark matter DPs just inside 0.01 virial radii.The very low total mass and velocity dispersion produced withinRe by an NFW-like total mass profile suggests that thestellar component should dominate the dark matter component out to atleast Re. It should then be difficult to kinematicallyconstrain the inner slope of the DP of ellipticals. Thehigh-concentration parameters deduced from X-ray observations appear tobe a consequence of fitting an NFW model to the total mass DP made up ofa stellar component that dominates inside and a dark matter componentthat dominates outwards.An appendix gives the virial mass dependence of the concentrationparameter, central density and total mass of the Navarro et al. model.In a second appendix are given single integral expressions for thevelocity dispersions averaged along the line of sight, in circularapertures and in thin slits, for general luminosity density and massdistributions, with isotropic orbits.
| Radio spectra of the low-luminosity active galactic nucleus NGC 266 at centimetre-to-submillimetre wavelengths
We report multi-frequency and multi-epoch radio continuum observationswith multi-spatial resolution for the low-luminosity active galacticnucleus (LLAGN) NGC 266. In the centimetre regime, we find diffusecomponents with Very Large Array (VLA) observations, and a variablecompact core with a rising spectrum with Very Long Baseline Array (VLBA)observations. Although the spectral index of the rising spectrum isconsistent with the prediction of the simple advection-dominatedaccretion flow (ADAF) model, the observed radio power is slightly highcompared with that of the model prediction. A spectral break atcentimetre-to-millimetre wavelengths is inferred from the upper limitsof flux densities from Nobeyama Millimetre Array (NMA) and James ClerkMaxwell Telescope (JCMT) data at millimetre and submillimetrewavelengths, respectively. More complicated considerations are requiredfor the theoretical model to interpret such observed radio properties.
| The interaction of 3C 401 with the surrounding intracluster medium
We present an observation of the radio galaxy 3C 401 and the surroundingintracluster medium (ICM) of its host galaxy cluster by the ChandraX-ray Observatory. This luminous radio galaxy is notable in that it hascharacteristics intermediate between the Fanaroff-Riley type I and IImorphologies. We clearly detect point-like emission coincident with theradio core of 3C 401, although the spatial resolution of even Chandra isonly 2 kpc at the distance of 3C 401 (z= 0.201) and so the possibilityremains that this is a dense (and rapidly cooling) thermal gaseous corein the centre of the ICM atmosphere. Strong departures from sphericalsymmetry in the central 10-20 kpc of the ICM clearly suggest interactionbetween the ICM and the radio lobes of 3C 401. A central X-ray barprobably results from the evacuation of two ICM cavities by theexpanding radio lobes. Beyond these central regions, the clusterpossesses a flatter profile than many clusters of comparable mass,suggesting the importance of ICM heating and entropy injection by 3C401. We detect an interesting cross-like structure in the ICM on 100 kpcscales. We speculate that this could be a radio-galaxy-induceddisturbance corresponding to a time when 3C 401 was substantially morepowerful. A particularly exciting possibility is that this cross-likestructure corresponds to a large-scale global g-mode oscillation excitedby a past outburst of 3C 401.
| Evidence for radio-source heating of groups
We report evidence that the gas properties of X-ray groups containingradio galaxies differ from those of radio-quiet groups. For awell-studied sample of ROSAT-observed groups, we found that more thanhalf of the elliptical-dominated groups can be considered `radio-loud',and that radio-loud groups are likely to be hotter at a given X-rayluminosity than radio-quiet groups. We tested three different models forthe origin of the effect and conclude that radio-source heating is themost likely explanation. We found several examples of groups where thereis strong evidence from Chandra or XMM-Newton images for interactionsbetween the radio source and the group gas. A variety of radio-sourceheating processes are important, including shock-heating by youngsources and gentler heating by larger sources. The heating effects canbe longer-lasting than the radio emission. We show that the sample ofX-ray groups used in our study is not significantly biased in thefraction of radio-loud groups that it contains. This allows us toconclude that the energy per particle that low-power radio galaxies caninject over the group lifetime is comparable to the requirements ofstructure formation models.
| Spherical models for early-type galaxies with cuspy mass densities
Spherical mass density models are used to fit the central surfacebrightness profiles of early-type galaxies which are generated fromNuker law parameters obtained from the literature. The mass density andthe corresponding potential are in an analytical form. It is shown thatonly a few mass density components are necessary to obtain a good fitand that for all power-law galaxies and for the core galaxies that weconsider, most or all of the mass density components must have cusps toprovide good fits. The applied quadratic programming fitting allows fora method of deprojection, which is reliable and convenient. The resultscan be used directly for further dynamical modelling.
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