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Tracing gas motions in the Centaurus cluster
We apply the stochastic model of iron transport developed by Rebusco etal. to the Centaurus cluster. Using this model, we find that aneffective diffusion coefficient D in the range 2 ×1028-4 × 1028cm2s-1can approximately reproduce the observed abundance distribution.Reproducing the flat central profile and sharp drop around 30-70kpc,however, requires a diffusion coefficient that drops rapidly with radiusso that D > 4 × 1028cm2s-1only inside about 25kpc. Assuming that all transport is due to fullydeveloped turbulence, which is also responsible for offsetting coolingin the cluster core, we calculate the length- and velocity-scales ofenergy injection. These length-scales are found to be up to a factor of~10 larger than expected if the turbulence is due to the inflation andrising of a bubble. We also calculate the turbulent thermal conductivityand find it is unlikely to be significant in preventing cooling.
| Near-infrared study of CIZA J1324.7-5736, the second richest cluster of galaxies in the Great Attractor
We present the result of a deep near-infrared survey of the newlyidentified X-ray luminous cluster of galaxies CIZA J1324.7-5736 in theGreat Attractor (GA) region. In a 35 × 35 arcmin2region, 111 galaxy candidates with rKs20 >arcsec are identified. Comparison of the extinction-correctedKs-band luminosity function of CIZA J1324.7-5736 with thoseof nearby clusters indicates that the richness class of CIZAJ1324.7-5736 is almost the same as, or richer than, the Pavo, Centaurusand Hydra clusters but poorer than the Coma, Perseus and Norma clusters.CIZA J1324.7-5736 is possibly the second richest cluster in the GAregion following the Norma cluster. The position of CIZA J1324.7-5736[in the (l, b, v) space] is close to the Centaurus-Crux cluster and theagglomeration of galaxies detected by the Parkes HI survey. CIZAJ1324.7-5736, together with the Centaurus-Crux cluster and the HI galaxyagglomeration, is most likely to be one of the richest localconcentrations in the GA overdensity of galaxies.
| The low-power nucleus of PKS 1246-410 in the Centaurus cluster
We present Chandra, Very Large Array (VLA) and Very Long Baseline Array(VLBA) observations of the nucleus of NGC 4696, a giant elliptical inthe Centaurus cluster of galaxies. Like M87 in the Virgo cluster, PKS1246-410 in the Centaurus cluster is a nearby example of a radio galaxyin a dense cluster environment. In analysing the new X-ray data, we havefound a compact X-ray feature coincident with the optical and radiocore. While nuclear emission from the X-ray source is expected, itsluminosity is low, <1040ergs-1. We estimate theBondi accretion radius to be 30pc and the accretion rate to be0.01Msolaryr-1, which under the canonicalradiative efficiency of 10 per cent would overproduce by 3.5 orders ofmagnitude the radiative luminosity. Much of this energy can be directedinto the kinetic energy of the jet, which over time inflates theobserved cavities seen in the thermal gas. The VLBA observations reveala weak nucleus and a broad, one-sided jet extending over 25pc inposition angle -150°. This jet is deflected on the kiloparsec-scaleto a more east-west orientation (position angle of -80°).
| Direct Measurements of Gas Bulk Flows in the Intracluster Medium of the Centaurus Cluster with the Chandra Satellite
We present the analysis of the velocity structure of the intraclustergas near the core of Abell 3526 obtained with two off-center Chandraobservations, specifically designed to eliminate errors due to spatialvariations of the instrumental gain. We detected a significant velocitygradient along the northeast-southwest direction, roughly perpendicularto the direction of the incoming subgroup Cen 45, in agreement withprevious ASCA SIS measurements. The presence of gas bulk velocities isobserved both with and without the inclusion of the Fe K line complex inthe spectral fittings. The configuration and magnitude of the velocitygradient is consistent with near transonic circulatory motion, eitherbulk or eddylike. The velocity difference obtained using the bestcalibrated central regions of ACIS-S3 is found to be(2.4+/-1.0)×103 km s-1 for rectangularregions 2.4 arcmin×3' roughly diametrically opposedaround the cluster's core. There are also indications of a high-velocityzone toward the southern region with similar magnitudes. The detectionof velocity gradients is significant at >99.4% confidence, andsimulations show that intrachip gain fluctuations >1800 kms-1 are required to explain the velocity gradient by chance.The measurements suggest that >1% of the total merger energy canstill be bulk kinetic 0.4 Gyr after the merging event. This is the firstdirect confirmation of velocity gradients in the intracluster gas withindependent instruments and indicates that strong departure fromhydrostatic equilibrium is possible even for cool clusters that do notshow obvious signs of merging.
| Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters
We present new (B, I) photometry for the globular cluster systems ineight brightest cluster galaxies (BCGs), obtained with the ACS/WFCcamera on the Hubble Space Telescope. In the very rich cluster systemsthat reside within these giant galaxies, we find that all have stronglybimodal color distributions that are clearly resolved by themetallicity-sensitive (B-I) index. Furthermore, the mean colors andinternal color range of the blue subpopulation are remarkably similarfrom one galaxy to the next, to well within the +/-0.02-0.03 maguncertainties in the foreground reddenings and photometric zero points.By contrast, the mean color and internal color range for the redsubpopulation differ from one galaxy to the next by twice as much as theblue population. All the BCGs show population gradients, with muchhigher relative numbers of red clusters within 5 kpc of their centers,consistent with their having formed at later times than the blue,metal-poor population. A striking new feature of the color distributionsemerging from our data is that for the brightest clusters(MI<-10.5) the color distribution becomes broad and lessobviously bimodal. This effect was first noticed by Ostrov et al. andDirsch et al. for the Fornax giant NGC 1399; our data suggest that itmay be a characteristic of many BCGs and perhaps other large galaxies.Our data indicate that the blue (metal-poor) clusters brighter thanMI~=-10 become progressively redder with increasingluminosity, following a mass/metallicity scaling relationZ~M0.55. A basically similar relation has been found for M87by Strader et al. (2005). We argue that these GCS characteristics areconsistent with a hierarchical-merging galaxy formation picture in whichthe metal-poor clusters formed in protogalactic clouds or densestarburst complexes with gas masses in the range107-1010 Msolar, but where the moremassive clusters on average formed in bigger clouds with deeperpotential wells where more preenrichment could occur.
| Magnetic turbulence in cool cores of galaxy clusters
We argue that the recently reported Kolmogorov-like magnetic turbulencespectrum in the cool core of the Hydra A galaxy cluster can beunderstood by kinetic energy injection by active galaxies that drives aturbulent non-helical magnetic dynamo into its saturated state. Althoughdramatic differences exist between small-scale dynamo scenarios, theirsaturated state is expected to be similar, as we show for threescenarios: the flux rope dynamo, the fluctuation dynamo, and theexplosive dynamo. Based on those scenarios, we develop an analyticalmodel of the hydrodynamic and magnetic turbulence in cool cores. Themodel implies magnetic field strengths that fit well with Faradayrotation measurements and minimum energy estimates for the sample ofcool core clusters having such data available. Predictions for magneticfields in clusters for which the appropriate observational informationis still missing, and for yet unobserved quantities like thehydrodynamical turbulence velocity and characteristic length-scale areprovided. The underlying dynamo models suggest magnetic intermittencyand possibly a large-scale hydrodynamic viscosity. We conclude that thesuccess of the model to explain the field strength in cool core clustersindicates that in general cluster magnetic fields directly reflecthydrodynamical turbulence, also in clusters without cool cores.
| 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.
| 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.
| The extended Hα-emitting filaments surrounding NGC 4696, the central galaxy of the Centaurus cluster
We present images of NGC 4696, the central galaxy in the Centauruscluster, showing the large extent of cool filaments that are bright inHα line emission. These filaments share the detailed structure ofboth the central dust lane and the inner regions of the arc-like plumesseen in soft X-ray emission. The X-ray gas is at its coolest and mostabsorbed in this same region. The smoothness of the features impliesthat the local environment is not strongly turbulent. We suggest thatthese filaments are shaped either by confinement due to a strongmagnetic field, or by bulk flows within the intracluster medium. Wepropose that, like similar filamentary systems in the core of clusters,these cooler components have been drawn out of the central galaxy behindbuoyant gas bubbles from previous episodes of radio activity. We find aspur of low-frequency radio emission leading to a region of low X-raypressure within the intracluster medium, supporting this interpretation.
| 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.
| VLA polarimetry observations of PKS2322-123: estimating magnetic fields in the Abell 2597 cluster
We present 5-, 8-, and 15-GHz total intensity and polarimetricobservations of the radio source PKS2322-123 taken with the Very LargeArray (VLA). This small (11 kpc) source is located at the centre of thecooling-core cluster Abell 2597. The inner X-ray structure, the radiomorphology and the steep spectral index (α=-1.8) in the lobes allsuggest that the radio emission is confined by the ambient X-ray gas. Wedetect a small region of polarized flux in the southern lobe and areable to calculate a Faraday rotation measure (RM) of 3620 radm-2 over this region. Based on these observations and ChandraX-ray data, we suggest that the southern lobe has been deflected fromits original south-western orientation to the south and into our line ofsight. Using the observed rotation measures (RMs) and our calculatedelectron density profiles, and assuming both a uniform and tangledmagnetic field topology, we estimate a lower limit of the line-of-sightcluster magnetic field, B∥= 2.1 μG.
| Detection of Radial Surface Brightness Fluctuations and Color Gradients in Elliptical Galaxies with the Advanced Camera for Surveys
We study surface brightness fluctuations (SBFs) in a sample of eightelliptical galaxies using the Advanced Camera for Surveys (ACS) WideField Channel (WFC) data drawn from the Hubble Space Telescope archive.SBF magnitudes in the F814W bandpass and galaxy colors from F814W,F435W, and F606W images, when available, are presented. Galaxy surfacebrightness profiles are determined as well. We present the firstSBF-broadband color calibration for the ACS WFC F814W bandpass and(relative) distance moduli estimates for seven of our galaxies. Wedetect and study in detail the SBF variations within individual galaxiesas a probe of possible changes in the underlying stellar populations.Inspecting both the SBF and color gradients in comparison to modelpredictions, we argue that SBFs and SBF gradients can in principle beused for unraveling the different evolutionary paths taken by galaxies,although a more comprehensive study of this issue would be required. Weconfirm that the radial variation of galaxy stellar populationproperties is mainly connected to the presence of radial chemicalabundance gradients, with the outer galaxy regions being more metal-poorthan the inner ones.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555. These observations areassociated with programs 9427, 9293, and 9399.
| The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.
| Detection of PAH Emission Features from Nearby Elliptical Galaxies with the Spitzer Infrared Spectrograph
According to the current understanding, the presence of a considerableamount of dust in elliptical galaxies is quite common. Recent studieswith ISO and Spitzer even suggest the presence of polycyclic aromatichydrocarbon (PAH) emission features in the spectral energy distributionsof several elliptical galaxies. Hot ionized gas filling the interstellarspace of elliptical galaxies, however, is expected to easily destroysuch very small grains through sputtering by plasma ions. Here wepresent the results of mid-IR spectroscopic observations of fourelliptical galaxies with the Spitzer Infrared Spectrograph (IRS). Wesucceeded in detecting PAH emission features from elliptical galaxies.The observed spectra seem to be quite unusual; the PAH features at 6.2,7.7, and 8.6 μm are very faint or even absent, in contrast toprominent emission features at 11.3 and 12.7 μm, which may reflectpeculiar physical conditions of the interstellar medium. The detectionof the PAHs provides strong constraints on evolution scenarios for theinterstellar medium of elliptical galaxies.
| Chandra Observation of the Cluster of Galaxies MS 0839.9+2938 at z = 0.194: The Central Excess Iron and Type Ia Supernova Enrichment
We present the Chandra study of the intermediate-redshift (z=0.194)cluster of galaxies MS 0839.9+2938. By performing both projected anddeprojected spectral analyses, we find that the gas temperature isapproximately constant at about 4 keV within 130-444h-170 kpc. In the inner regions, the gastemperature decreases toward the center, reaching <~3 keV in thecentral 37 h-170 kpc. This implies that the lowerand upper limits of the mass deposit rate are 9-34 and 96-126Msolar yr-1, respectively, within 74h-170 kpc, where the gas is significantly colder.Along with the temperature drop, we detect a significant inward ironabundance increase from about 0.4 Zsolar in the outer regionsto ~=1 Zsolar within the central 37h-170 kpc. Thus, MS 0839.9+2938 is the clustershowing the most significant central iron excess at z>~0.2. We arguethat most of the excess iron should have been contributed by SNe Ia.Using the observed SN Ia rate and stellar mass loss rate, we estimatethat the time needed to enrich the central region with excess iron is6.4-7.9 Gyr, which is similar to those found for nearby clusters.Coinciding with the optical extension of the cD galaxy (up to about 30h-170 kpc), the observed X-ray surface brightnessprofile exhibits an excess beyond the distribution expected by eitherthe β model or the Navarro-Frenk-White (NFW) model and can be wellfitted with an empirical two-β model that leads to a relativelyflatter mass profile in the innermost region.
| 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.
| A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion
We present results from a high-resolution X-ray imaging study of nearbyLINERs observed by ACIS on board Chandra. This study complements andextends previous X-ray studies of LINERs, focusing on the underexploredpopulation of nearby dust-enshrouded infrared-bright LINERs. The sampleconsists of 15 IR-bright LINERs (LFIR/LB>3),with distances that range from 11 to 26 Mpc. Combining our sample withprevious Chandra studies, we find that ~51% (28/55) of the LINERsdisplay compact hard X-ray cores. The nuclear 2-10 keV luminosities ofthe galaxies in this expanded sample range from ~2×1038to ~2×1044 ergs s-1. We find that the mostextreme IR-faint LINERs are exclusively active galactic nuclei (AGNs).The fraction of LINERs containing AGNs appears to decrease with IRbrightness and increase again at the highest values ofLFIR/LB. We find that of the 24 LINERs showingcompact nuclear hard X-ray cores in the expanded sample that wereobserved at Hα wavelengths, only eight actually show evidence of abroad line. Similarly, of the 14 LINERs showing compact nuclear hardX-ray cores with corresponding radio observations, only eight display acompact flat spectrum radio core. These findings emphasize the need forhigh-resolution X-ray imaging observations in the study of IR-brightLINERs. Finally, we find an intriguing trend in the Eddington ratioversus LFIR and LFIR/LB for theAGN-LINERs in the expanded sample that extends over 7 orders ofmagnitude in L/LEdd. This correlation may imply a linkbetween black hole growth, as measured by the Eddington ratio, and thestar formation rate, as measured by the far-IR luminosity andIR-brightness ratio. If the far-IR luminosity is an indicator of themolecular gas content in our sample of LINERs, our results may furtherindicate that the mass accretion rate scales with the host galaxy's fuelsupply. We discuss the potential implications of our results in theframework of black hole growth and AGN fueling in low-luminosity AGNs.
| Near infra-red and optical colour gradients in E-type galaxies. Inferences on dust content
Colour gradients are considered for a sample of circa 50 E-type galaxiesin the Local Supercluster. The new data includes isophotal colourprofiles in J-H, J-K, V-J and V-K, measured using 2MASS frames mostlyfrom the Large Galaxies Atlas, V frames from previous work and Vprofiles from the literature. This is supplemented by U-B, B-V, B-R, V-Icolour gradients obtained anew from published photometric data. Colourgradients in E galaxies show remarkably large variations from object toobject and do not correlate with other properties. Metallicity gradientsare the primary cause as shown before. Age gradients with oppositeeffects are possibly needed to explain objects with small colourgradients. Some empirical evidence of such age effects has been foundfor a subset of objects with morphological peculiarities and youngerstars mixed. Dust has only modest effects on colour gradients, as shownby the fact that objects with zero IRAS 100 μ flux have the sameaverage values of the gradients, except in V-J and V-K, as those withnon zero flux (cf. Table 7). This last subsample however exhibits poorbut definite correlations between IRAS flux and gradients, which mightbe caused by the presence of a few relatively dusty galaxies in thesample. Given the absence of a correlation between any gradients andgalaxy velocity dispersion (and hence mass), the observations do notagree with the predictions of the monolithic scenario for the formationof E galaxies. Simulated datasets of “dummy” objectsmimicking the hierarchical scenario have been obtained, and used to testa technique for estimating the dust content of E-galaxies from thecomparison of the V-K (or V-J) colour gradients with the U-B (or B-V)ones: the contents of diffuse dust, gauged in terms of published models,are obtained for a dozen objects.
| 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.
| The distance to Hydra and Centaurus from surface brightness fluctuations: Consequences for the Great Attractor model
We present I-band Surface Brightness Fluctuation (SBF) measurements for16 early-type galaxies (3 giants, 13 dwarfs) in the central region ofthe Hydra cluster, based on deep photometric data in 7 fields obtainedwith VLT FORS1. From the SBF-distances to the galaxies in our sample weestimate the distance of the Hydra cluster to be 41.2 ± 1.4 Mpc((m-M)=33.07 ± 0.07 mag). Based on an improved correction forfluctuations from undetected point sources, we revise the SBF-distanceto the Centaurus cluster from Mieske & Hilker (2003, A&A, 410,455) upwards by 10% to 45.3 ± 2.0 Mpc ((m-M)=33.28 ± 0.09mag). The relative distance modulus of the two clusters then is(m-M)_Cen-(m-M)_Hyd=0.21 ± 0.11 mag. With H_0= 72 ± 4 kms-1 Mpc-1, we estimate a positive peculiarvelocity of 1225 ± 235 km s-1 for Hydra and 210± 295 km s-1 for the Cen30 component of Centaurus.Allowing for a thermal velocity dispersion of 200 km s-1,this rules out a common peculiar flow velocity for both clusters at 98%confidence. We find that the 9× 1015 Mȯ“Great Attractor” at a distance of ≃45 Mpc can explainthe observed peculiar velocities if shifted about 15° towards theHydra cluster position. Our results are inconsistent at 94% confidencewith a scenario where the Centaurus cluster is identical to the GA. Inorder to better restrict partially degenerate Great Attractor parameterslike its mass and distance, a recalculation of the local flow model withupdated distance information over a larger area than covered by us wouldbe needed.
| Cosmic magnetic fields - as observed in the Universe, in galactic dynamos, and in the Milky Way
Cosmic magnetism has that exotic ``Je ne sais quoi''! Magnetism has beenobserved in various objects, located near the edge of the Universe andall the way down to the Milky Way's center. The observed magnetic fieldcan take the cell-type shape in randomly-oriented large blobs found inintracluster gas or outside of clusters of galaxies, the helix shape insynchrotron jets, the longitudinal shape in ram-pressured shocks inradio lobes near elliptical galaxies, the spiral shape of logarithmicarms in spiral galaxies, or the egg shape of an enlarged interstellarbubble. In strength, the magnetic field varies from 0.1 nG(cosmological), to 20 μG (galaxies, jets, superbubbles), and to 1 mGin the Milky Way filaments.Magnetism plays a small physical role in the formation of largestructures. It acts as a tracer of the dynamical histories ofcosmological and intracluster events, it guides the motion of theinterstellar ionised gas, and it aligns the charged dust particles.Batteries and dynamos are often employed in models to create and amplifyseed magnetic fields. Starting soon after the Big Bang (redshiftz>2000), this review covers the cosmological background surface(z~1100, distance ~4.3 Gpc), the epoch of first stars (z~20 distance~4.1 Gpc), the currently observable Universe (z~10, distance ~3.9 Gpc),superclusters of galaxies (size ~50 Mpc), intracluster gas (size ~10Mpc), galaxies (~30 kpc), spiral arms (~10 kpc), interstellarsuperbubbles (~100 pc), synchrotron filaments (~10 pc), and the MilkyWay's center.
| A deep near-infrared survey around the giant radio galaxy PKS 1343-601
We present the results of a deep near-infrared survey of a 36 × 36arcmin2 region centred on the giant elliptical radio galaxyPKS 1343-601, suggested to be the core of an unknown rich clusterlocated at the low Galactic latitude of b= 1.°73 in the GreatAttractor (GA) region. 19 obvious galaxies and 38 galaxy candidates havebeen detected; only three of them were previously identified as agalaxy. The total Galactic extinction AK towards our surveyarea is estimated to be 0.6-0.8 mag from the J-K colour of foregroundgiants. This is systematically lower by about 0.4 mag than AKtaken from the IRAS/DIRBE extinction map. The number density of galaxiesbrighter than an extinction-corrected Ks band magnitude of 13 is 42galaxies deg-2, five times as high as the overall average inthe GA region. However, the number of galaxies within the central270-kpc radius is less than that of the Norma, Pavo and Centaurusclusters in the GA region. We found no evidence that a rich cluster isassociated with PKS 1343-601.
| 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.
| The Ghosts of Galaxies: Tidal Debris in Clusters
Gravitational interactions in rich clusters can strip material from theouter parts of galaxies or even completely disrupt entire systems,giving rise to large scale, low surface brightness ghostly featuresstretching across intergalactic space. The nearby Coma and Centaurusclusters both have striking examples of galaxy ghosts, in the form of100 kpc-long plumes of intergalactic debris. By searching HST archivalimages, we have found numerous other examples of galaxy ghosts in richclusters at low redshift, evidence that galaxy destruction and recyclingare ubiquitous, important in cluster formation and evolution, andcontinue to mold clusters at the present epoch. Many ghosts appear inX-ray bright clusters, perhaps signaling a connection with energeticsubcluster mergers. The fate of such material has importantramifications for cluster evolution. Our new HST WFPC2 V & I imagesof a portion of the Centaurus plume reveal that it contains an excess ofdiscrete objects with -12 < MV < -6, consistent withbeing globular clusters or smaller dwarf galaxies. This tidallyliberated material is being recycled directly into the intraclusterpopulation of stars, dwarf galaxies, globular clusters, and gas, whichmay have been built largely from a multitude of similar events over thelife of the cluster.
| 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.
| Hubble Space Telescope STIS Far-Ultraviolet Observations of the Central Nebulae in the Cooling-Core Clusters A1795 and A2597
We present Hubble Space Telescope STIS FUV images of the Lyα andFUV continuum emission of the luminous emission-line nebulae in twocooling-core clusters, A1795 and A2597. The Lyα and FUV continuumemission consist of a diffuse component (~60%) and more compact features(knots and filaments), which lie preferentially along the radio sourceedges. There are correlations between the FUV continuum flux and theemission-line fluxes of Lyα and Hα that imply that thebrighter parts of the nebulae are ionized locally. We suggest that theFUV knots are star clusters with ongoing star formation of several solarmasses per year. The bolometric luminosity of such a starburst (ifabsorbed by dust) would be detectable with the Spitzer Space TelescopeMIPS. It appears that star formation occurs throughout the nebula,although it is strongly enhanced along the edges of the radio source. Wefind that young hot stars (e.g., O5) probably provide the bulk of thephotons that ionize the nebula, although other sources of ionization maycontribute in selected regions of the nebula. Constraints on diagnosticUV emission lines are consistent with photoionization from a starburstpopulation and perhaps marginally with intermediate-velocity shocks,~400 km s-1. We suggest that the mass accretion rates arecomparable to the star formation rates (of order 10 Msolaryr-1). This is consistent with the lack ofintermediate-temperature gas (<1 keV) being due to energy input tothe cooling gas rather than to ``hiding'' the cooling gas.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, Inc., underNASA contract NAS5-26555. These observations are associated withprograms 8107 and 7760.
| X-Ray and Optical Filaments in M87
We compare a very deep X-ray image of M87, at the center of the VirgoCluster, to high-quality optical images of the low-excitationemission-line gas in the same region. There are striking coincidences ofdetail between the two. We explore the possibility that this representsa thermal interaction between hot gas at 107 K and warm gasat 104 K. We find that two temperatures are present in theX-ray gas, with the lower more prevalent in the vicinity of the opticalfilaments. Electron conduction from the hot phase to the cooler oneprovides a quantitatively acceptable energy source for the opticalfilaments, and we show additionally that it can do so for the brightestX-ray cluster, Perseus. If operative, conduction in the presence ofgas-rich galaxy mergers may explain the presence of ``cool cores'' inclusters of galaxies.
| Very High Iron Abundance of the Hot Gas around the cD Galaxy of the MKW 4 Cluster
We observed a nearby X-ray-bright poor cluster MKW 4 with Chandra. Theiron abundance is found to increase dramatically from 0.3 to 1.3 solarwithin 150" of the cD galaxy NGC 4073. Then MKW 4 is the second clusterto exhibit a significantly high metallicity of greater than 1 solar,after the Centaurus Cluster. Weakness of the central cool componenthelps us to obtain the metallicity free from the multitemperatureeffects. The scale of temperature decrease toward the center region isclearly different from that of abundance increment, indicating thatthese are not the same phenomena. Since the stellar-to-intracluster massratio around NGC 4073 is very high compared to other cD type clusters,it is no doubt that the abundance increment is due to metal ejectionfrom the cD galaxy. The iron-to-stellar mass ratio around NGC 4073 is6.2×10-3, and thus most of the ejected iron stillremains in the vicinity of NGC 4073. By comparing the stellardistribution with that of iron, the steady state outflow speed of ironis roughly estimated to be 2.4 km s-1, which is faster thanthe Coulomb diffusion.
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