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Recent discoveries of bright supernovae at the Bronberg Observatory.
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| 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.
| 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.
| Supernovae 2006db, 2006dc, 2006dd
IAUC 8723 available at Central Bureau for Astronomical Telegrams.
| Swift Observations of SN 2006dd in NGC 1316
We report on Swift Ultraviolet/Optical Telescope (UVOT) and X-RayTelescope (XRT) observations of SN 2006dd (cf. CBET #533) obtained onJune 20.71 UT. We confirm the detection of a new object at R.A. =03h22m41s.64, Decl. = -37o12'13".2 (equinox 2000.0) as compared toDigitized Sky Survey images, consistent with the position reported byMonard in CBET #553, with UVOT magnitudes of V = 14.0 (167 s exposuretime), B = 14.5 (259 s), U = 13.7 (258 s), UVW1 [181-321 nm] = 15.5 (520s), UVM2 [166-268 nm] = 17.9 (881 s), and UVW2 [112-264 nm] = 18.1 (881s).
| 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 XMM-Newton Examination of Energetics in the East Lobe of the Nearby Radio Galaxy Fornax A (NGC 1316)
An XMM-Newton observation of the east radio lobe of the nearby radiogalaxy Fornax A is reported. The diffuse hard X-ray emission associatedwith the east lobe, which was initially discovered by ASCA and ROSAT, isconfirmed with significant signal statistics, after strictly removing 59sources detected within the MOS field of view. Its X-ray spectrum isdescribed by a single power-law model, which is absorbed by a mediumwith a column density consistent with that toward the object. Thebest-fit X-ray photon index,ΓX=1.62+0.24-0.15, agrees withthe synchrotron radio index, ΓR=1.68+/-0.1, determinedfrom the radio spectrum between 29.9 MHz and 5 GHz. Hence, the inverseCompton interpretation for the diffuse X-rays is justified. The X-rayflux density in the east lobe is measured to be 90+/-21 nJy at 1 keV(including both statistical and systematic errors) with the index fixedat the radio value. This gives electron and magnetic energy densities of3.0+1.5-1.0×10-13 and6.1+5.7-3.5×10-14 ergscm-3, respectively. The latter corresponds to a magneticfield strength of 1.24+0.50-0.40 μG, which issmaller than the field estimated under the minimum energy condition,1.55 μG, although with a slightly large error. Reevaluation is alsomade of the ASCA result on the west lobe, to show that both lobes sharea similar physical condition in terms of energetics.
| 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.
| Two Populations of Young Massive Star Clusters in Arp 220
We present new optical observations of young massive star clusters inArp 220, the nearest ultraluminous infrared galaxy, taken in UBVI withthe Hubble Space Telescope ACS HRC camera. We find a total of 206probable clusters whose spatial distribution is centrally concentratedtoward the nucleus of Arp 220. We use model star cluster tracks todetermine ages, luminosities, and masses for 14 clusters with completeUBVI indices or previously published near-infrared data. We estimaterough masses for 24 additional clusters with I<24 mag from BVIindices alone. The clusters with useful ages fall into two distinctgroups: a ``young'' population (<10 Myr) and an intermediate-agepopulation (~=300 Myr). There are many clusters with masses clearlyabove 106 Msolar and possibly even above107 Msolar in the most extreme instances. Thesemasses are high enough that the clusters being formed in the Arp 220starburst can be considered to be genuine young globular clusters. Inaddition, this study allows us to extend the observed correlationbetween global star formation rate and maximum cluster luminosity bymore than 1 order of magnitude in star formation rate.
| Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. II. Optical Study and Interpretation
Our X-ray study of the nuclear activity in a new sample of six quiescentearly-type galaxies, as well as in a larger sample from the literature,confirmed (Paper I) that the Bondi accretion rate of diffuse hot gas isnot a good indicator of the SMBH X-ray luminosity. Here we suggest thata more reliable estimate of the accretion rate must include the gasreleased by the stellar population inside the sphere of influence of theSMBH, in addition to the Bondi inflow of hot gas across that surface. Weuse optical surface brightness profiles to estimate the mass-loss ratefrom stars in the nuclear region: we show that for our sample ofgalaxies it is an order of magnitude higher (~10-4 to10-3 Msolar yr-1) than the Bondi inflowrate of hot gas, as estimated from Chandra (Paper I). Only by takinginto account both sources of fuel can we constrain the true accretionrate, the accretion efficiency, and the power budget. Radiativelyefficient accretion is ruled out, for quiescent SMBHs. For typicalradiatively inefficient flows, the observed X-ray luminosities of theSMBHs imply accretion fractions ~1%-10% (i.e., ~90%-99% of the availablegas does not reach the SMBH) for at least five of our six targetgalaxies and most of the other galaxies with known SMBH masses. Wediscuss the conditions for mass conservation inside the sphere ofinfluence, so that the total gas injection is balanced by accretion plusoutflows. We show that a fraction of the total accretion power(mechanical plus radiative) would be sufficient to sustain aself-regulating, slow outflow that removes from the nuclear region allthe gas that does not sink into the BH (``BH feedback''). The rest ofthe accretion power may be carried out in a jet or advected. We alsodiscuss scenarios that would lead to an intermittent nuclear activity.
| Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. I. X-Ray Study
We have studied the nuclear activity in a sample of six quiescentearly-type galaxies, with new Chandra data and archival HST opticalimages. Their nuclear sources have X-ray luminosities~1038-1039 ergs s-1(LX/LEdd~10-8 to 10-7) andcolors or spectra consistent with accreting supermassive black holes(SMBHs), except for the nucleus of NGC 4486B, which is softer thantypical AGN spectra. In a few cases, the X-ray morphology of the nuclearsources shows hints of marginally extended structures, in addition tothe surrounding diffuse thermal emission from hot gas, which isdetectable on scales >~1 kpc. In one case (NGC 5845), a dusty diskmay partially obstruct our direct view of the SMBH. We have estimatedthe temperature and density of the hot interstellar medium, which is onemajor source of fuel for the accreting SMBH; typical central densitiesare ne~(0.02+/-0.01) cm-3. Assuming that the hotgas is captured by the SMBH at the Bondi rate, we show that the observedX-ray luminosities are too faint to be consistent with standard diskaccretion, but brighter than predicted by radiatively inefficientsolutions (e.g., advection-dominated accretion flows [ADAFs]). In total,there are ~20 galaxies for which SMBH mass, hot gas density, and nuclearX-ray luminosity are simultaneously known. In some cases, the nuclearsources are brighter than predicted by the ADAF model; in other cases,they are consistent or fainter. We discuss the apparent lack ofcorrelations between Bondi rate and X-ray luminosity and suggest that,in order to understand the observed distribution, we need to know twoadditional parameters: the amount of gas supplied by the stellarpopulation inside the accretion radius, and the fraction (possibly<<1) of the total gas available that is accreted by the SMBH. Weleave a detailed study of these issues to a subsequent paper.
| A Chandra Survey of Early-Type Galaxies. I. Metal Enrichment in the Interstellar Medium
We present a Chandra study of the emission-weighted metal abundances in28 early-type galaxies, spanning ~3 orders of magnitude in X-rayluminosity (LX). We report constraints for Fe, O, Ne, Mg, Si,S, and Ni. We find no evidence of the very subsolar Fe abundance(ZFe) historically reported, confirming a trend in recentobservations of bright galaxies and groups, nor do we find anycorrelation between ZFe and luminosity. Excepting one case,the ISM is single-phase, indicating that multitemperature fits foundwith ASCA reflected temperature gradients that we resolve with Chandra.We find no evidence that ZFe (ISM) is substantially lowerthan the stellar metallicity estimated from simple stellar populationmodels. In general, these quantities are similar, which is inconsistentwith galactic wind models and recent hierarchical chemical enrichmentsimulations. Our abundance ratio constraints imply that 66%+/-11% of theISM Fe was produced in SNe Ia, similar to the solar neighborhood,indicating similar enrichment histories for elliptical galaxies and theMilky Way. Although these values are sensitive to the considerablesystematic uncertainty in the supernova yields, they are in agreementwith observations of more massive systems. This indicates considerablehomology in the enrichment process operating from cluster scales tolow-to-intermediate-LX galaxies. The data uniformly exhibitlow ZO/ZMg ratios, which have been reported insome clusters, groups, and galaxies. This is inconsistent with standardSN II metal yield calculations and may indicate an additional source ofenrichment, such as Population III hypernovae.
| 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.
| SGRS J0515-8100: A Fat-Double Giant Radio Galaxy
We present here the first detailed study of a giant radio galaxy of thefat-double type. The lobes of the double radio galaxy SGRS J0515-8100have transverse widths that are 1.3 times their extent from the center,their surface brightness is the lowest among known giant radio sources,and the lobes have relatively steep radio spectra. We infer that thesewide lobes were created as a result of a highly variable andintermittent jet whose axis direction also varied significantly: thefat-double lobes in this giant radio source are a result of the ejectionand deposition of synchrotron plasma over a wide range of angles overtime rather than the expansion of relic lobes. In addition, the opticalhost shows evidence for an ongoing galaxy-galaxy interaction. SGRSJ0515-8100 supports the hypothesis that interactions with companionsmight perturb the inner accretion disk that produces and sustains thejets at the centers of active galactic nuclei. As a result, it appearsunnecessary to invoke black hole coalescence to explain suchmorphologies, implying that the corresponding event rates predicted forgravitational wave detectors may be overestimates.
| First Results from SAPAC: Toward a Three-dimensional Picture of the Fornax Cluster Core
A sophisticated surface brightness fluctuation (SBF) analysis packagehas been developed, designed to measure distances of early-type galaxiesby means of SBFs of unresolved stars. This suite of programs, calledSAPAC, is made readily available to the astronomical community forextensive testing, with the long-term goal of providing the necessarytools for systematic distance surveys of early-type galaxies usingmodern optical/near-IR telescopes equipped with wide-field cameras. Wediscuss the technical and scientific concepts of SAPAC and demonstrateits capabilities by analyzing deep B- and R-band CCD images of 10 dwarfelliptical galaxy candidates in the Fornax Cluster obtained with FORS1at the Very Large Telescope. All candidates are confirmed as clustermembers. We then turn our attention to the innermost region of theFornax Cluster. A total of 29 early-type galaxies closer than threecluster core radii (2deg) to the central galaxy NGC 1399 haveradial velocities and SBF distances. Their Hubble diagram exhibits apronounced S-shaped infall pattern, suggesting that Fornax is still inthe process of formation during the present epoch through a generalcollapse and possible accretion of distinct groups of galaxies. Fromfitting a model we estimate the cluster mass within 720 kpc projecteddistance of NGC 1399 to be 2.3+/-0.3×1014Msolar. The associated collapse time istcoll=2.9+1.6-0.9 Gyr. After cleansing our galaxy sample of afew kinematical outliers, the true distance of the Fornax Cluster coreis determined at 20.13+/-0.40 Mpc [(m-M)0=31.51+/-0.04 mag].Applying a bootstrap resampling technique on the distance distributionwith individual distance errors taken into account further reveals asmall intrinsic cluster depth of σint=0.74+0.52-0.74Mpc, in best agreement with the cluster's linear extension in the sky:σR.A.=σdecl.~0.5 Mpc. We conclude thatthe early-type galaxy population in the Fornax Cluster must be spatiallywell constrained, with no evidence of elongation along the line ofsight, in contrast to the Virgo Cluster. Moreover, we find marginalevidence for substructure, a result that is consistent with the youngevolutionary state of the cluster and the overall galaxy infall.Combining the kinematically defined cluster distance with the meancosmological velocity for the central cluster galaxy sample yields aHubble constant of H0=63+/-5 km s-1Mpc-1.Based on observations collected at the ESO Very Large Telescope, underprogram ESO 68.A-0176.
| The Molonglo Southern 4 Jy Sample (MS4). II. ATCA Imaging and Optical Identification
Of the 228 sources in the Molonglo Southern 4 Jy sample (MS4), the 133with angular sizes <35" have been imaged at 5 GHz at 2"-4" resolutionwith the Australia Telescope Compact Array. More than 90% of the samplehas been reliably optically identified, either on the plates of the UKSchmidt Southern Sky Survey or on R-band CCD images made with theAnglo-Australian Telescope. A subsample of 137 sources, the SMS4,defined to be a close southern equivalent of the northern 3CRR sample,was found to have global properties mostly consistent with the northernsample. Linear sizes of MS4 galaxies and quasars were found to beconsistent with galaxy-quasar unification models of orientation andevolution.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.
| Dynamical mass estimates for two luminous star clusters in galactic merger remnants
We present high-dispersion spectra of two extremely massive starclusters in galactic merger remnants, obtained using the UVESspectrograph mounted on the ESO Very Large Telescope. One cluster, W30,is located in the ~500 Myr old merger remnant NGC 7252 and has avelocity dispersion and effective radius of σ=27.5±2.5 kms-1 and Reff=9.3±1.7 pc, respectively. Theother cluster, G114, located in the ~3 Gyr old merger remnant NGC 1316,is much more compact, Reff=4.08±0.55 pc, and has avelocity dispersion of σ=42.1±2.8 km s-1. Thesemeasurements allow an estimate of the virial mass of the two clusters,yielding Mdyn(W30)=1.59(±0.26)× 10^7Mȯ and Mdyn(G114)=1.64(±0.13)×10^7 Mȯ. Both clusters are extremely massive, being morethan three times heavier than the most massive globular clusters in theGalaxy. For both clusters we measure light-to-mass ratios, which whencompared to simple stellar population (SSP) models of the appropriateage, are consistent with a Kroupa-type stellar mass function. Usingmeasurements from the literature we find a strong age dependence on howwell SSP models (with underlying Kroupa or Salpeter-type stellar massfunctions) fit the light-to-mass ratio of clusters. Based on this resultwe suggest that the large scatter in the light-to-mass ratio of theyoungest clusters is not due to variations in the underlying stellarmass function, but instead to the rapidly changing internal dynamics ofyoung clusters. Based on sampling statistics we argue that while W30 andG114 are extremely massive, they are consistent with being the mostmassive clusters formed in a continuous power-law cluster massdistribution. Finally, based on the positions of old globular clusters,young massive clusters (YMCs), ultra-compact dwarf galaxies (UCDs) anddwarf-globular transition objects (DGTOs) in κ-space we concludethat 1) UCDs and DGTOs are consistent with the high mass end of starclusters and 2) YMCs occupy a much larger parameter space than oldglobular clusters, consistent with the idea of preferential disruptionof star clusters.
| 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.
| Young star cluster complexes in NGC 4038/39. Integral field spectroscopy using VIMOS-VLT
We present the first results of a survey to obtain Integral FieldSpectroscopy of merging galaxies along the Toomre Sequence. In thepresent work, we concentrate on the star cluster complexes in theAntennae galaxies (NGC 4038/39) in the overlap region as well as thenuclear region of NGC 4038. Using optical spectroscopy we derive theextinction, age, metallicity, velocity, velocity dispersion of the gasand star formation rate for each of the eight complexes detected. Wesupplement this study with archival HST-WFPC2 U, B, V, Hα, and Iband imaging. Correcting the observed colours of the star clusterswithin the complexes for extinction, measured through our opticalspectra, we compare the clusters with simple stellar population models,with which we find an excellent agreement, and hence proceed to derivethe ages and masses of the clusters from comparison with the models. Infive of the complexes we detect strong Wolf-Rayet emission features,indicating young ages (3-5 Myr). The ionized gas surrounding thecomplexes is expanding at speeds of 20{-}40 km s-1. This slowexpansion can be understood as a bubble, caused by the stellar winds andsupernovae within the complexes, expanding into the remnant of theprogenitor giant molecular cloud. We also find that the complexesthemselves are grouped, at about the largest scale of which young starclusters are correlated, representing the largest coherent star formingregion. We show that the area normalized star formation rates of thesecomplexes clearly place them in the regime of star forming regions instarburst galaxies, thereby justifying the label of localizedstarbursts. Finally, we estimate the stability of the complexes, andfind that they will probably loose a large fraction of their mass to thesurrounding environment, although the central regions may merge into asingle large star cluster.
| Gamma-ray emissions of AGN and cosmological standard candles
In this work, we compile a sample which contains 71 GeV Gamma-ray-loudActive Galactic Nuclei (AGNs) (14 BL Lacs and 57 FSRQs), 53 FR I radiogalaxies and 63 FR II radio galaxies. We make a nonlinear least-squarefit to this sample, and find that the best fit value of the Hubbleconstant is H0=71.5±3.8 kms-1Mpc-1 with a reduced χ ~= 2.46 by assumingMv = -23.0 and accepting q0 = 1.0, and thecorresponding regression line has a correlation index R ~= 0.78. Thebest fit value of H0 = 71.5±3.8 kms-1Mpc-1 is in well agreement with H0 =72±8 km s-1 obtained by the Hubble Space TelescopeKey Project. Our results show that the GeV Gamma-ray emissions of AGNscan be used as cosmological standard candles indeed.
| Gordon James Stanley and the Early Development of Radio Astronomy in Australia and the United States
Following the end of the Second World War, the CSIRO RadiophysicsLaboratory applied the expertise and surplus radar equipment acquiredduring the war to problems of astronomy. Gordon Stanley was among thefirst group of scientists and engineers to work in the exciting newfield of radio astronomy. Like many of his contemporaries, he had astrong background in radio and electronics but none in astronomy. At theRadiophysics Laboratory, and later at Caltech, Stanley developedinnovative new radio telescopes and sophisticated instrumentation whichresulted in important new discoveries that changed, in a fundamentalway, our understanding of the Universe. He was one of those who played akey role in the early development of radio astronomy both in Australiaand the United States.
| The initial mass distribution of the M82 star cluster system
We explore whether we can constrain the shape of the initial massdistribution of the star cluster population in M82's ~1-Gyr-oldpost-starburst region `B', in which the present-day cluster massfunction (CMF) is closely approximated by a lognormal distribution. Weconclude that the M82 B initial CMF must have had a mean mass very closeto that of the `equilibrium' CMF of Vesperini. Consequently, if thepresently observed M82 B CMF has remained approximately constant sinceits formation, as predicted, then the initial CMF must have beencharacterized by a mean mass that was only slightly larger than thepresent mean mass. From our detailed analysis of the expected evolutionof CMFs, we conclude that our observations of the M82 B CMF areinconsistent with a scenario in which the 1-Gyr-old cluster populationoriginated from an initial power-law mass distribution. Our conclusionis supported by arguments related to the initial density in M82 B, whichwould have been unphysically high if the present cluster population werethe remains of an initial power-law distribution.
| 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.
| The Chandra view of extended X-ray emission from Pictor A
We discuss the extended X-ray emission seen in three archival Chandraobservations, and one archival XMM-Newton observation, of theFanaroff-Riley type II radio galaxy, Pictor A. The overall properties ofthe X-ray lobes are consistent with the conclusions of earlier worksthat the extended X-ray emission is largely due to the inverse-Comptonprocess, and the implied departure from equipartition is in the rangeseen by us in other sources. In detail, we show that the X-ray/radioflux ratio varies quite strongly as a function of position throughoutthe source, and we discuss possible implications of this observation forthe spatial variation of electron energy spectra and magnetic fieldstrength through the lobe. We show that the radio and X-ray propertiesof the lobe are not consistent with a simple model in which variationsin the magnetic field strength alone are responsible for the observeddifferences between emission at different frequencies. We also discussthe origins of the extended emission seen around the eastern hotspot,arguing that it may be diffuse synchrotron radiation tracing a region ofdistributed particle acceleration, and the implications of a possibleweak X-ray counterjet detection, which, taken together with the otherproperties of the bright X-ray jet, leads us to suggest that the X-rayjet and possible counterjet are also produced by synchrotron emission.
| Non-thermal X-rays, a high-abundance ridge and fossil bubbles in the core of the Perseus cluster of galaxies
Using a deep Chandra observation of the Perseus cluster of galaxies, wefind a high-abundance shell 250 arcsec (93 kpc) from the centralnucleus. This ridge lies at the edge of the Perseus radio mini-halo. Inaddition we identify two Hα filaments pointing towards this shell.We hypothesize that this ridge is the edge of a fossil radio bubble,formed by entrained enriched material lifted from the core of thecluster. There is a temperature jump outside the shell, but the pressureis continuous indicating a cold front. A non-thermal component is mappedover the core of the cluster with a morphology similar to the mini-halo.Its total luminosity is 4.8 × 1043 erg s-1,extending in radius to ~75 kpc. Assuming the non-thermal emission to bethe result of inverse Compton scattering of the cosmic microwavebackground and infrared emission from NGC 1275, we map the magneticfield over the core of the cluster.
| Evolutionary history of the elliptical galaxy NGC 1052
We have obtained Keck spectra for 16 globular clusters (GCs) associatedwith the merger remnant elliptical NGC 1052, as well as a long-slitspectrum of the galaxy. We derive ages, metallicities and abundanceratios from simple stellar population models using the recentlypublished methods of Proctor & Sansom, applied to extragalactic GCsfor the first time. A number of GCs indicate the presence of strong bluehorizontal branches that are not fully accounted for in the currentstellar population models. We find all of the GCs to be ~13 Gyr oldaccording to simple stellar populations, with a large range ofmetallicities. From the galaxy spectrum we find NGC 1052 to have aluminosity-weighted central age of ~2 Gyr and metallicity of[Fe/H]~+0.6. No strong gradients in either age or metallicity were foundto the maximum radius measured (0.3re~= 1 kpc). However, wedo find a strong radial gradient in α-element abundance, whichreaches a very high central value. The young central starburst age isconsistent with the age inferred from the HI tidal tails and infallinggas of ~1 Gyr. Thus, although NGC 1052 shows substantial evidence for arecent merger and an associated starburst, it appears that the mergerdid not induce the formation of new GCs, perhaps suggesting that littlerecent star formation occurred. This interpretation is consistent with`frosting' models for early-type galaxy formation.
| Mass-to-light ratio gradients in early-type galaxy haloes
Owing to the fact that the near future should see a rapidly expandingset of probes of the halo masses of individual early-type galaxies, weintroduce a convenient parameter for characterizing the halo masses fromboth observational and theoretical results:∇lΥ, the logarithmic radial gradient of themass-to-light ratio. Using halo density profiles from Λ-cold darkmatter (CDM) simulations, we derive predictions for this gradient forvarious galaxy luminosities and star formation efficienciesɛSF. As a pilot study, we assemble the available∇lΥ data from kinematics in early-type galaxies- representing the first unbiased study of halo masses in a wide rangeof early-type galaxy luminosities - and find a correlation betweenluminosity and ∇lΥ, such that the brightestgalaxies appear the most dark-matter dominated. We find that thegradients in most of the brightest galaxies may fit in well with theΛCDM predictions, but that there is also a population of faintergalaxies whose gradients are so low as to imply an unreasonably highstar formation efficiency ɛSF > 1. This difficultyis eased if dark haloes are not assumed to have the standard ΛCDMprofiles, but lower central concentrations.
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