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The great Milky Way -Andromeda collision.
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| A quasar in every galaxy ?
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| Deciphering the globular-cluster code.
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| GALEX : seeing starbirth, near and far.
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| Astronomie gamma : le ciel revele aux tres hautes energies.
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| Ten Milliparsec-Scale Structure of the Nucleus Region in Centaurus A
We present the results of a VLBI Space Observatory Programme (VSOP)observation of the subparsec structure in Cen A at 4.9GHz. Theobservation produced an image of the subparsec jet components with aresolution of three-times better than images from previous VLBImonitoring campaigns at 8.4GHz, and twice better than the previous 22GHzstudies. Owing to its proximity, our Cen A space-VLBI image is one ofthe highest spatial-resolution images of an AGN ever made -- 0.01pc perbeam -- comparable only to the recent 43GHz VLBI images of M87. Theelongated core region is resolved into several components of over 10milliarcsec long (0.2pc), including a compact component of brightnesstemperature 2.2 × 1010 K. A counterjet was detected: ifwe assume jet-counterjet symmetry, a relatively slow jet speed, and alarge viewing angle, as derived from previous observations, the imageallows us to investigate the distribution of ionized gas around thecore, which is opaque at this frequency due to free-free absorption. Wealso analyzed the jet geometry in terms of collimation. Assuming thestrongest component to be the core, the jet opening angle at 5000rS from the core is estimated to be 12°, with thecollimation of the jet to 3° continuing out to 20000rS. This result is consistent with previous studies of thejet in M87, which favor MHD disk outflow models. Future space VLBIobservations at higher frequencies will probably be able to image thecollimation region, within 1000 rS of the center of Cen A,together with the accretion disk itself.
| Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
| Recent discoveries of bright supernovae at the Bronberg Observatory.
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| The infrared jet in Centaurus A: multiwavelength constraints on emission mechanisms and particle acceleration
We report on Spitzer and Gemini observations of the jet of Centaurus Ain the infrared, which we combine with radio, ultraviolet and X-raydata. Spitzer detects jet emission from about 2 arcmin from the nucleus,the emission becoming particularly bright after the jet flare point at~3.4 arcmin. Where X-ray and infrared emission are seen together thebroad-band data strongly support a synchrotron origin for the X-rays.The jet flare point is marked by a broad, diffuse region of X-rays whichmay be associated with a shock: we discuss possible physical mechanismsfor this. The infrared jet persists after the flare point regionalthough X-ray emission is absent; it is plausible that here we areseeing the effects of particle acceleration followed by downstreamadvection with synchrotron losses. Gemini data probe the inner regionsof the jet, putting limits on the mid-infrared flux of jet knots.
| Can the unresolved X-ray background be explained by the emission from the optically-detected faint galaxies of the GOODS project?
The emission from individual X-ray sources in the Chandra Deep Fieldsand XMM-Newton Lockman Hole shows that almost half of the hard X-raybackground above 6keV is unresolved and implies the existence of amissing population of heavily obscured active galactic nuclei (AGN). Wehave stacked the 0.5-8keV X-ray emission from optical sources in theGreat Observatories Origins Deep Survey (GOODS; which covers the ChandraDeep Fields) to determine whether these galaxies, which are individuallyundetected in X-rays, are hosting the hypothesized missing AGN. In the0.5-6keV energy range, the stacked-source emission corresponds to theremaining 10-20 per cent of the total background - the fraction that hasnot been resolved by Chandra. The spectrum of the stacked emission isconsistent with starburst activity or weak AGN emission. In the 6-8keVband, we find that upper limits to the stacked X-ray intensity from theGOODS galaxies are consistent with the ~40 per cent of the totalbackground that remains unresolved, but further selection refinement isrequired to identify the X-ray sources and confirm their contribution.
| A `super' star cluster grown old: the most massive star cluster in the Local Group
We independently redetermine the reddening and age of the globularcluster (GC) 037-B327 in M31 by comparing independently obtainedmulticolour photometry with theoretical stellar population synthesismodels. 037-B327 has long been known to have a very large reddeningvalue, which we confirm to be E(B - V) = 1.360 +/- 0.013, in goodagreement with the previous results. We redetermine its most likely ageat 12.4 +/- 3.2 Gyr.037-B327 is a prime example of an unusually bright early counterpart tothe ubiquitous `super' star clusters presently observed in mosthigh-intensity star-forming regions in the local Universe. In order tohave survived for a Hubble time, we conclude that its stellar initialmass function (IMF) cannot have been top-heavy. Using this constraint,and a variety of simple stellar population (SSP) models, we determine aphotometric mass of , somewhat depending on the SSP models used, themetallicity and age adopted and the IMF representation. This mass, andits relatively small uncertainties, makes this object the most massivestar cluster of any age in the Local Group. Assuming that thephotometric mass estimate thus derived is fairly close to its dynamicalmass, we predict that this GC has a (one-dimensional) velocitydispersion of the order of (72 +/- 13) km s-1. As a surviving`super' star cluster, this object is of prime importance for theoriesaimed at describing massive star cluster evolution.
| Planetary nebulae as tracers of galaxy stellar populations
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
| Gemini/GMOS spectra of globular clusters in the Virgo giant elliptical NGC 4649
NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We haveobtained Gemini/GMOS (Gemini North Multi-Object Spectrograph) spectrafor 38 globular clusters (GCs) associated with this galaxy. Applying themulti-index χ2 minimization technique of Proctor andSansom with the single stellar population models of Thomas, Maraston andKorn, we derive ages, metallicities and α-element abundanceratios. We find several young (2-3 Gyr old) supersolar metallicity GCs,while the majority are old (>10 Gyr), spanning a range ofmetallicities from solar to [Z/H]=-2. At least two of these young GCsare at large projected radii of 17-20 kpc. The galaxy itself shows noobvious signs of a recent starburst, interaction or merger. A trend ofdecreasing α-element ratio with increasing metallicity is found.
| Testing the beamed inverse-Compton model for jet X-ray emission: velocity structure and deceleration
By considering a small sample of core-dominated radio-loud quasars withX-ray jets, I show, as has been argued previously by others, that theobservations require bulk jet deceleration if all of the X-ray emissionis to be explained using the widely adopted beamed inverse-Comptonmodel, and argue that jets even in these powerful objects must havevelocity structure in order to reconcile their radio and X-rayproperties. I then argue that the deceleration model has several seriousweaknesses, and discuss the viability of alternative models for thedecline in X-ray/radio ratio as a function of position. Althoughinverse-Compton scattering from the jets is a required process and mustcome to dominate at high redshifts, adopting an alternative model forthe X-ray emission of some nearby, well-studied objects can greatlyalleviate some of the problems posed by these observations for thebeamed inverse-Compton model.
| J0041+3224: a new double-double radio galaxy
We report the discovery of a double-double radio galaxy (DDRG),J0041+3224, with the Giant Metrewave Radio Telescope (GMRT) andsubsequent high-frequency observations with the Very Large Array (VLA).The inner and outer doubles are aligned within ~4° and arereasonably collinear with the parent optical galaxy. The outer doublehas a steeper radio spectrum compared to the inner one. Using anestimated redshift of 0.45, the projected linear sizes of the outer andinner doubles are 969 and 171 kpc, respectively. The time-scale ofinterruption of jet activity has been estimated to be ~20 Myr, similarto other known DDRGs. We have compiled a sample of known DDRGs, and havere-examined the inverse correlation between the ratio of theluminosities of the outer to the inner double and the size of the innerdouble, lin. Unlike the other DDRGs with lin>~50 kpc, the inner double of J0041+3224 is marginally more luminous thanthe outer one. The two DDRGs with lin<~ few kpc have amore luminous inner double than the outer one, possibly due to a higherefficiency of conversion of beam energy as the jets propagate throughthe dense interstellar medium. We have examined the symmetry parametersand found that the inner doubles appear to be more asymmetric in bothits armlength and its flux density ratios compared to the outer doubles,although they appear marginally more collinear with the core than theouter double. We discuss briefly the possible implications of thesetrends.
| Gemini/GMOS spectra of globular clusters in the Leo group elliptical NGC 3379
The Leo group elliptical NGC 3379 is one of the few normal ellipticalgalaxies close enough to make possible observations of resolved stellarpopulations, deep globular cluster (GC) photometry and highsignal-to-noise ratio GC spectra. We have obtained Gemini/GMOS spectrafor 22 GCs associated with NGC 3379. We derive ages, metallicities andα-element abundance ratios from simple stellar population modelsusing the recent multi-index χ2 minimization method ofProctor & Sansom. All of these GCs are found to be consistent withold ages, i.e. >~10Gyr, with a wide range of metallicities. This iscomparable to the ages and metallicities that Gregg et al. found acouple of years ago for resolved stellar populations in the outerregions of this elliptical. A trend of decreasing α-elementabundance ratio with increasing metallicity is indicated.The projected velocity dispersion of the GC system is consistent withbeing constant with radius. Non-parametric, isotropic models require asignificant increase in the mass-to-light ratio at large radii. Thisresult is in contrast to that of Romanowsky et al., who recently found adecrease in the velocity dispersion profile as determined from planetarynebulae (PN). Our constant dispersion requires a normal-sized dark halo,although without anisotropic models we cannot rigorously determine thedark halo mass.A two-sided χ2 test over all radii gives a 2σdifference between the mass profile derived from our GCs compared to thePN-derived mass model of Romanowsky et al. However, if we restrict ouranalysis to radii beyond one effective radius and test if the GCvelocity dispersion is consistently higher, we determine a > 3σdifference between the mass models, and hence we favour the conclusionthat NGC 3379 does indeed have dark matter at large radii in its halo.
| X-rays from the radio-quiet quasar PG 1407+265: relativistic jet or accretion disc emission?
We present two XMM-Newton observations of the luminous (Lx> 1046ergs-1), radio-quiet quasar PG 1407+265,separated by 11 months. The data indicate two distinct states: a highlyvariable, bright state (first epoch); and a quiescent, low-flux one(second epoch). During the low-flux state the spectrum is consistentwith a single, unabsorbed power law. However, during the brighter statea highly variable, steep component is statistically required.Contemporaneous ultraviolet data from the Optical Monitor allow anestimate of the optical-to-X-ray spectral index (αox),which appears typical of radio-quiet quasars during the low-flux state,but extremely flat during the high-flux state. The XMM-Newton data canbe described as originating from a combination of jet and accretion discprocesses, in which the (relativistic) X-ray jet only worksintermittently. The scenario could help to describe some of thecomplexities seen in the broad-band spectral energy distribution of PG1407+265, such as weak high-ionization emission lines, strong FeII,unbeamed continuum and the weak radio emission relative to the optical.
| A Multiwavelength Study of the Jets in FR-I Radio Galaxies: I. Data and Analysis
We compile a sample of 11 Fanaroff-Riley type I Radio Galaxies (FR-IRGs) with multi-wavelength observations to address the dynamic behaviorof jets in these objects. Optical images acquired by the Hubble SpaceTelescope (HST) are carefully analyzed. The method and reductionprocedure are described in detail. Unresolved optical cores emerge afterhaving properly removed starlight from the host galaxies in eight of theFR-I RGs, of which five are new identifications. Broad band spectralproperties of these newly identified compact cores are compared withthat previously found in FR-I RGs, as well as the low-energy-peaked BLLac objects. The similarity between them argues for the same non-thermalsynchrotron origin. Well-resolved optical jets with knotty morphologiesare found in three FR-I RGs in our sample, namely 3C 15, 3C 66B and B20755+37. The optical counterparts to the inner radio/X-ray jets areidentified and a clear one-to-one correspondence between the optical,radio and X-ray knots is found. The structure and information on theoptical jets are discussed. Physical parameters such as the knotsposition, flux and size are also presented. Detailed comparison betweenthe multi-wavelength data and radiative and dynamic models of jet willbe made in a forthcoming paper.
| An Engineer Becomes an Astronomer
The early days of radio astronomy in Australia are revisited. Theevolution of ideas and the way they led to various instrumentaldevelopments and some of the results of these developments arepresented. Besides these personal reminiscences, an indication of thepolitical background that sometimes influenced developments is givenand, as a coda, an account of a different approach to relativity throughthe so-called twin paradox.
| Optical Counterparts of Ultraluminous X-Ray Sources Identified from Archival HST WFPC2 Images
We present a systematic analysis of archival HST WFPC2 ``Association''data sets that correlate with the Chandra positions of a set of 44ultraluminous X-ray sources (ULXs) of nearby galaxies. The mainmotivation is to address the nature of ULXs by searching for opticalcounterparts. Sixteen of the ULXs are found in early-type galaxies (RC3Hubble type <3). We have improved the Chandra/HST relative astrometrywhenever possible, resulting in errors circles of 0.3"-1.7" in size.Disparate numbers of potential ULX counterparts are found, and in somecases none are found. The lack of or low number of counterparts in somecases may be due to insufficient depth in the WFPC2 images. Particularlyin late-type galaxies, the HST image in the ULX region was often complexor crowded, requiring source detection to be performed manually. Wetherefore address various scenarios for the nature of the ULX since itis not known which, if any, of the sources found are true counterparts.The optical luminosities of the sources are typically in the range104-106 Lsolar, with (effective) Vmagnitudes typically in the range 22-24. In several cases colorinformation is available, with the colors roughly tending to be more redin early-type galaxies. This suggests that, in general, the (potential)counterparts found in early-type galaxies are likely to be older stellarpopulations and are probably globular clusters. Several early-typegalaxy counterparts have blue colors, which may be due to youngerstellar populations in the host galaxies, however, these could also bebackground sources. In spiral galaxies the sources may also be due tolocalized structure in the disks rather than bound stellar systems.Alternatively, some of the counterparts in late-type galaxies may beisolated supergiant stars. The observed X-ray/optical flux ratio isdiluted by the optical emission of the cluster in cases where the systemis an X-ray binary in a cluster, particularly in the case of a low-massX-ray binaries in an old cluster. If any of the counterparts are boundsystems with ~104-106 stars and are the truecounterparts to the ULX sources, then the X-ray luminosities of the ULXare generally well below the Eddington limit for a black hole with mass~0.1% of the cluster mass. Finally, we find that the optical flux of thecounterparts is consistent with being dominated by emission from anaccretion disk around an intermediate-mass black hole if the black holehappens to have a mass >~102 Msolar and isaccreting at close to the Eddington rate, unless the accretion disk isirradiated (which would result in high optical disk luminosities atlower black hole masses).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. This project isassociated with Archival proposal 9545.
| VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
We discuss the abundances of interstellar CH, CH+, and CN inthe Magellanic Clouds, derived from spectra of seven SMC and 13 LMCstars obtained (mostly) with the VLT UVES. CH and/or CH+ havenow been detected toward three SMC and nine LMC stars; CN is detectedtoward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly allthe optical detections of these molecular species in interstellar mediabeyond the Milky Way. In the LMC, the CH/H2 ratio iscomparable to that found for diffuse Galactic molecular clouds in foursight lines but is lower by factors of 2.5-4.0 in two others. In theSMC, the CH/H2 ratio is comparable to the local Galacticvalue in one sight line but is lower by factors of 10-15 in two others.The abundance of CH in the Magellanic Clouds thus appears to depend onlocal physical conditions and not just on metallicity. In both the SMCand the LMC, the observed relationships between the column density of CHand those of CN, CH+, Na I, and K I are generally consistentwith the trends observed in our Galaxy.Using existing data for the rotational populations of H2 inthese sight lines, we estimate temperatures, radiation field strengths,and local hydrogen densities for the diffuse molecular gas. The inferredtemperatures range from about 45 to 90 K, the radiation fields rangefrom about 1 to 900 times the typical local Galactic field, and thedensities (in most cases) lie between 100 and 600 cm-3.Densities estimated from the observed N(CH), under the assumption thatCH is produced via steady state gas-phase reactions, are considerablyhigher than those derived from H2. Much better agreement isfound by assuming that the CH is made via the (still undetermined)process(es) responsible for the observed CH+. A significantfraction of the CH and CH+ in diffuse molecular material inthe SMC and LMC may be produced in photon-dominated regions. Theexcitation temperature obtained from the populations of the two lowestCN rotational levels toward Sk -67 2 is quite consistent with thetemperature of the cosmic microwave background radiation measured withCOBE.Toward most of our targets, the UVES spectra also reveal absorption atvelocities corresponding to the Magellanic Clouds ISM from several ofthe strongest of the diffuse interstellar bands (DIBs; at 5780, 5797,and 6284 Å). On average, the three DIBs are weaker by factors of7-9 (LMC) and about 20 (SMC), compared to those typically observed inGalactic sight lines with similar N(H I), presumably due to the lowermetallicities and stronger radiation fields in the LMC and SMC. Thethree DIBs are also weaker (on average, but with some exceptions), byfactors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in theMagellanic Clouds. The detection of several of the so-calledC2 DIBs toward Sk 143 and Sk -67 2 with strengths similar tothose in comparable Galactic sight lines, however, indicates that nosingle, uniform scaling factor (e.g., one related to metallicity)applies to all DIBs (or for all sight lines) in the Magellanic Clouds.Based on observations collected at the European Southern Observatory,Chile, under programs 67.C-0281, 70.D-0164, 72.C-0064, 72.C-0682, and74.D-0109.
| 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.
| Spitzer Reveals Hidden Quasar Nuclei in Some Powerful FR II Radio Galaxies
We present a Spitzer mid-infrared survey of 42 Fanaroff-Riley class IIradio galaxies and quasars from the 3CRR catalog at redshift z<1. Allof the quasars and 45%+/-12% of the narrow-line radio galaxies have amid-IR luminosity of νLν(15μm)>8×1043 ergs s-1, indicating strongthermal emission from hot dust in the active galactic nucleus. Ourresults demonstrate the power of Spitzer to unveil dust-obscuredquasars. The ratio of ``mid-IR luminous'' narrow-line radio galaxies toquasars indicates a mean dust covering fraction of 0.56+/-0.15, assumingrelatively isotropic emission. We analyze Spitzer spectra of the 14mid-IR luminous narrow-line radio galaxies thought to host hidden quasarnuclei. Dust temperatures of 210-660 K are estimated fromsingle-temperature blackbody fits to the low- and high-frequency ends ofthe mid-IR bump. Most of the mid-IR luminous radio galaxies have a 9.7μm silicate absorption trough with optical depth <0.2, attributedto dust in a molecular torus. Forbidden emission lines fromhigh-ionization oxygen, neon, and sulfur indicate a source of far-UVphotons in the hidden nucleus. However, we find that the other 55%+/-13%of narrow-line FR II radio galaxies are weak at 15 μm, contrary tosingle-population unification schemes. Most of these galaxies are alsoweak at 30 μm. Mid-IR weak radio galaxies may constitute a separatepopulation of nonthermal, jet-dominated sources with low accretionpower.
| On the Fraction of X-Ray-obscured Quasars in the Local Universe
Recent wide-area hard X-ray and soft gamma-ray surveys have shown thatthe fraction of X-ray-obscured active galactic nuclei (AGNs) in thelocal universe significantly decreases with intrinsic luminosity. Inthis Letter we point out that two corrections have to be made to thesamples: (1) radio-loud AGNs have to be excluded, since their X-rayemission might be dominated by the jet component, and (2) Compton-thicksources have to be excluded too, since their hard X-ray and softgamma-ray emission are also strongly attenuated by Compton scattering.The soft gamma-ray-selected AGN samples obtained by Swift and INTEGRALprovide the best opportunity to study the fraction of obscured AGNs inthe local universe in the least biased way. We choose these samples tocheck if the corrections could alter the above result on the fraction ofobscured AGNs. We find that before the corrections both samples showsignificant anticorrelation between LX and NH,indicating an obvious decrease in the fraction of obscured AGNs withluminosity. However, after the corrections, we find only marginalevidence of anticorrelation (at the 98% confidence level) in the Swiftsample and no evidence at all in the INTEGRAL sample, which consists ofa comparable number of objects. We conclude that current samples onlyshow a marginal decrease in the fraction of obscured AGNs in the localuniverse and that much larger samples are required in order to reach amore robust conclusion.
| Spitzer Observations of Centaurus A: Infrared Synchrotron Emission from the Northern Lobe
We present measurements obtained with the Spitzer Space Telescope infive bands from 3.6 to 24 μm of the northern inner radio lobe ofCentaurus A, the nearest powerful radio galaxy. We show that thisemission is synchrotron in origin. Comparison with ultravioletobservations from GALEX shows that diffuse ultraviolet emission existsin a smaller region than the infrared but also coincides with the radiojet. We discuss the possibility that synchrotron emission is responsiblefor the ultraviolet emission, and we conclude that further data arerequired to confirm this.
| A Comprehensive Search for Gamma-Ray Lines in the First Year of Data from the INTEGRAL Spectrometer
We have carried out an extensive search for gamma-ray lines in the firstyear of public data from the spectrometer (SPI) on the INTEGRAL mission.INTEGRAL has spent a large fraction of its observing time in theGalactic plane with particular concentration in the Galactic center (GC)region (~3 Ms in the first year). Hence the most sensitive searchregions are in the Galactic plane and center. The phase space of thesearch spans the energy range 20-8000 keV and line widths from 0 to 1000keV (FWHM). It includes both diffuse and pointlike emission. We havesearched for variable emission on timescales down to ~1000 s. Diffuseemission has been searched for on a range of different spatial scalesfrom ~20° (the approximate field of view of the spectrometer) up tothe entire Galactic plane. Our search procedures were verified by therecovery of the known gamma-ray lines at 511 and 1809 keV at theappropriate intensities and significances. We find no evidence for anypreviously unknown gamma-ray lines. The upper limits range from afew×10-5 to a few×10-2 cm-2s-1 depending on line width, energy, and exposure; regions ofstrong instrumental background lines were excluded from the search.Comparison is made between our results and various prior predictions ofastrophysical lines.
| A Search for Radio Emission from Type Ia Supernovae
We present and discuss the radio observations of 27 Type Ia supernovae(SNe Ia), observed over two decades with the Very Large Array. No SN Iahas been detected so far in the radio, implying a very low density forany possible circumstellar material established by the progenitor, orprogenitor system, before explosion. We derive 2 σ upper limits toa steady mass-loss rate for individual SN systems as low as~3×10-8 Msolar yr-1, which arguesstrongly against white dwarf accretion via a stellar wind from a massivebinary companion in the symbiotic star, an example of the``single-degenerate'' scenario. However, a white dwarf accreting from arelatively low mass companion via a sufficiently high efficiency(>60%-80%) Roche lobe overflow is still consistent with our limits.The ``double-degenerate'' merger scenario also cannot be excluded.
| Spitzer Observations of the Dusty Warped Disk of Centaurus A
Spitzer mid-infrared images of the dusty warped disk in the galaxyCentaurus A show a parallelogram-shaped structure. We successfully modelthe observed mid-infrared morphology by integrating the light from anemitting, thin, and warped disk, similar to that inferred from previouskinematic studies. The models with the best match to the morphology lackdust emission within the inner 0.1-0.8 kpc, suggesting that energeticprocesses near the nucleus have disturbed the inner molecular disk,creating a gap in the molecular gas distribution.
| VLT Diffraction-limited Imaging and Spectroscopy in the NIR: Weighing the Black Hole in Centaurus A with NACO
We present high spatial resolution near-infrared spectra and images ofthe nucleus of Centaurus A (NGC 5128) obtained with NAOS-CONICA at theVLT. The adaptive optics-corrected data have a spatial resolution of0.06" (FWHM) in K and 0.11" in H band, 4 times higher than previousstudies. The kinematics of the ionized gas ([Fe II]) are mapped alongfour slit positions. The observed gas motions suggest a kinematicallyhot disk that is orbiting a central object and is oriented nearlyperpendicular to the nuclear jet. We model the central rotation andvelocity dispersion curves of the [Fe II] gas in the combined potentialof the stellar mass and the (dominant) black hole. Our physically mostplausible model, a dynamically hot and geometrically thin gas disk,yields a black hole mass ofMbh=6.1+0.6-0.8×107Msolar. As the physical state of the gas is not wellunderstood, we also consider two limiting cases: first, a cold diskmodel, which completely neglects the velocity dispersion but is in linewith many earlier gas disk models; it yields an Mbh estimatethat is almost 2 times lower. Second, a spherical gas distribution inhydrostatic equilibrium via the Jeans equation; the best-fit black holemass increases by a factor of 1.5. This wide mass range spanned by thelimiting cases shows how important the gas physics is even forhigh-resolution data. Our overall best-fitting black hole mass is afactor of 2-4 lower than previous measurements. A substantially lowerMbh estimate when using higher resolution kinematics was alsofound for many other black hole mass measurements as HST data becameavailable. With our revised Mbh estimate, Cen A's offset fromthe Mbh-σ relation is significantly reduced.Based on observations collected at the European Southern Observatory,Paranal, Chile, ESO Program 72.B.0294A.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT HRI Observations. II. Statistical Properties
The statistical properties of the nonnuclear X-ray point sources fromthe ROSAT HRI survey of nearby galaxies in Paper I are studied, withparticular attention to the contamination from background and/orforeground objects. This study reveals a statistical preference for theultraluminous X-ray sources (ULXs) to occur in late-type galaxies overearly-type galaxies, and in starburst/H II galaxies over nonstarburstgalaxies. There is a trend of greater occurrence frequencies and ULXrates for galaxies with increasing star formation rates, confirming theconnection between the ULX phenomenon and the star formation. Anonlinear correlation is found between the number of ULXs and the starformation rate, with significantly more ULXs at low star formation ratesthan the ULX population expected from the high-mass X-ray binaries(HMXBs) as an indicator of the star formation and the accompanying youngstellar population, suggestive of another population of ULXs associatedwith the low-mass X-ray binaries (LMXBs) and the old stellar population.There are no breaks around 1039 ergs s-1 in theluminosity functions of ULXs in all galaxies or in late-type galaxies,suggesting the regular ULXs below 1040 ergs s-1are a high-luminosity extension of the ordinary HMXB/LMXB populationsbelow 1039 ergs s-1. There is evidence that theextreme ULXs above 1040 ergs s-1 might be adifferent ULX class from the regular ULXs below 1040 ergss-1, although a larger sample with more ULXs is needed toestablish the statistical properties of the extreme ULXs as a class.
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