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|Chandra and ROSAT Observations of NGC 5044: Profile of Dark Halos in Galaxy Groups|
We combined spatially resolved Chandra data with the ROSAT data toconstrain the dark-matter distribution in the galaxy group NGC 5044.Within 250 h-150kpc, the total mass is found to be 1.6 × 1013 Mȯ, 12% of whichconstitutes baryonic mass. Within the inner central regions, the totalmass profile exhibits a double structure, typical for groups containinga cD galaxy. Following previous studies of mostly rich galaxy clusters,we studied in detail the nature of such a double structure, whereby weinferred likely interface between the cD galaxy and the surroundinggalaxies. For this interesting group, we determined for the first timethe galaxy-group interface, which is around 7.5kpc from the peak of theX-ray emission. The total mass internal to this interface radius isfound to be 7.1 × 1010 Mȯ. Beyondthis radius, the total mass profile becomes DM-dominated and thecorresponding DM profile is reasonably fitted with the NFW model,yielding results consistent with the observed scatter expected for CDMhalos. A power-law fit to the DM mass profile gives α = 1.88± 0.32, a slope that is within the observed range, but issignificantly larger than that of low surface brightness galaxies andself-interacting DM halos.
|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.
|Globular cluster systems in low-luminosity early-type galaxies near the Fornax cluster centre|
We present a photometric study of the globular cluster systems (GCSs) ofthe Fornax cluster galaxies NGC1374, NGC1379 and NGC1387. The dataconsist of images from the wide-field MOSAIC imager of the Cerro TololoInter-American Observatory (CTIO) 4-m telescope, obtained withWashington C and Kron-Cousins R filters. The images cover a field of 36× 36arcmin2, corresponding to 200 ×200kpc2 at the Fornax distance. Two of the galaxies, NGC1374and NGC1379, are low-luminosity ellipticals while NGC1387 is alow-luminosity lenticular. Their cluster systems are still embedded inthe cluster system of NGC1399. Therefore, the use of a large field iscrucial and some differences to previous work can be explained by this.The colour distributions of all GCSs are bimodal. NGC1387 presents aparticularly distinct separation between red and blue clusters and anoverproportionally large population of red clusters. The radialdistribution is different for blue and red clusters, red clusters beingmore concentrated towards the respective galaxies. The different colourand radial distributions point to the existence of two globular clustersubpopulations in these galaxies. Specific frequencies are in the rangeSN= 1.4-2.4, smaller than the typical values for ellipticalgalaxies. These galaxies might have suffered tidal stripping of blueglobular clusters by NGC1399.
|The central kinematics of NGC 1399 measured with 14 pc resolution|
We present near-infrared (NIR) adaptive optics-assisted spectroscopicobservations of the CO(Δμ= 2) absorption bands towards thecentre of the giant elliptical galaxy NGC 1399. The observations weremade with NAOS-CONICA (on the European Southern Observatory's Very LargeTelescope) and have a full width at half-maximum resolution of0.15arcsec (14pc). Kinematic analysis of the observations reveals adecoupled core and strongly non-Gaussian line-of-sight velocity profilesin the central 0.2arcsec (19pc). NIR imaging also indicates anasymmetric elongation of the central isophotes in the same region.We use spherical orbit-superposition models to interpret the kinematics,using a set of orthogonal `eigen-velocity profiles' that allow us to fitmodels directly to spectra. The models require a central black hole ofmass 1.2+0.5-0.6× 109Msolar, with a strongly tangentially biased orbitdistribution in the inner 40pc.
|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.
|An imaging study of the globular cluster systems of NGC 1407 and 1400|
We present wide-field Keck telescope imaging of the globular cluster(GC) systems around NGC 1407 and 1400 in the Eridanus galaxy cloud. Thisis complemented by Hubble Space Telescope (HST) images from the AdvancedCamera for Surveys of NGC 1407 and Wide Field and Planetary Camera 2images of NGC 1400. We clearly detect bimodality in the GC colourdistribution of NGC 1407. The blue GC subpopulation has a mean colour ofB-I= 1.61 and a relative contribution of around 40 per cent, whereas thered subpopulation with B-I= 2.06 contributes 60 per cent to the overallGC system. Assuming old ages, this corresponds to [Fe/H]=-1.45 and-0.19. Both subpopulations are intrinsically broad in colour (indicatinga range in ages and/or metallicities), with the red subpopulation beingbroader than the blue. The GC colour distribution for NGC 1400 is lessclear cut than for NGC 1407, however, we also find evidence for abimodal distribution. We find the NGC 1407 red GCs to be 20 per centsmaller in size than the blue ones. This is consistent with theexpectations of mass segregation in an old coeval GC system. Half adozen large objects (20-40 pc), with GC-like colours are identified,which are probably background galaxies.The HST data sets allow us to probe to small galactocentric radii. Herewe find both GC systems to possess a GC surface density distributionwhich is largely constant in these inner galaxy regions. We fitisothermal-like profiles and derive GC system core radii of 9.4 kpc forNGC 1407 and 5.8 kpc for NGC 1400. For NGC 1407 we are able to separatethe surface density distribution into blue and red subpopulations,giving 17.8 and 7.6 kpc, respectively. Outside this central region, theradial profile of the GC surface density is similar to that of thegalaxy light for NGC 1407 but it is flatter for NGC 1400. The mean GCmagnitude appears to be constant with galactocentric radius. We findthat for both galaxies, the GC systems have a similar ellipticity andazimuthal distribution as the underlying galaxy starlight. A fit to theGC luminosity function gives a distance modulus of 31.6, which is ingood agreement with distances based on the Faber-Jackson relation andthe Virgo infall corrected velocity.
|NGC 4435: a bulge-dominated galaxy with an unforeseen low-mass central black hole|
We present the ionized gas kinematics of the SB0 galaxy NGC 4435 fromspectra obtained with the Space Telescope Imaging Spectrograph. Thisgalaxy has been selected on the basis of its ground-based spectroscopy,for displaying a position-velocity diagram consistent with the presenceof a circumnuclear Keplerian disc rotating around a supermassive blackhole (SMBH). We obtained the Hα and [NII]λ6583 kinematicsin the galaxy nucleus along the major axis and two parallel offsetpositions. We built a dynamical model of the gaseous disc taking intoaccount the whole bi-dimensional velocity field and the instrumentalsetup. For the mass of the central SMBH, we found an upper limit of 7.5× 106Msolar at the 3σ level. Thisindicates that the mass of the SMBH of NGC 4435 is lower than the oneexpected from the M•-σc (5 ×107Msolar) and near-infraredM•-Lbulge (4 ×107Msolar) relationships.
|New Candidate Ehb Stars in the Open Cluster NGC 6791: Looking Locally Into the Uv-Upturn Phenomenon|
Relying on U and B imagery at the Italian Telescopio Nazionale Galileo(TNG), we report here the discovery of a sample of 13 new UV-brightpost-HB candidate stars in the field of the Galactic open cluster NGC6791. Owing to its super-solar metal content ([Fe/H] ≳ 0.2 dex) andestimated age (t ≳ 8 Gyr), this cluster represents the nearest andideal stellar aggregate to match the distinctive properties of theevolved stellar populations possibly ruling the UV-upturn phenomenon inelliptical galaxies and bulges of spirals. Our ongoing spectroscopicfollow-up of this unique UV-bright sample will allow us to assess --once cluster membership of the candidates is properly checked -- thereal nature (e.g., sdB, sdO, AGB-manqué or EHB stars) of thesehot sources and their link with the ultraviolet excess emerging fromlow-mass, metal-rich evolutionary environments of external galaxies.
|Binary Population Synthesis, Sdb Stars and the UV Upturn|
We aim to use the methods of binary population synthesis (BPS) to studythe ultraviolet upturn (UVX) in the spectra of giant elliptical galaxieswith emphasis being placed on those binary channels which could lead tothe formation of stars that can account for it. This project willcombine Sukyoung Yi's single star population synthesis methodology withthat of Zhanwen Han's binary population synthesis methods in order toprovide a more coherent study of the origin of the UV upturn, the focusbeing on sdB stars as the possible source of the UV upturn. The primaryapproach of the project will be to explore the UV upturn, with sdBsincluded, for several different metallicities in order to examine theUVX in composite populations.
|The Look-Back Time Evolution of the UV Upturn Phenomenon|
In order to investigate the origin of the far-UV (FUV) flux fromearly-type galaxies, Galaxy Evolution Explorer (GALEX) is collecting theUV data for the elliptical-rich clusters at moderate redshifts (z <0.25) where the dominant FUV source is predicted to be hothorizontal-branch (HB) stars and their post-HB progeny. The earlyresults show that the FUV flux of quiescent early-type galaxies doesevolve substantially during the last 1--2 Gyr of look-back time, and theobserved UV fading is consistent with the variation predicted by thepopulation synthesis models where the mean temperature of HB starsdeclines rapidly with increasing look-back time.
|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.
|A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds|
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
|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 Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions|
The mean ages of early-type galaxies obtained from the analysis ofoptical spectra give a mean age of 8 Gyr at z=0, with 40% being youngerthan 6 Gyr. Independent age determinations are possible by usinginfrared spectra (5-21 μm), which we have obtained with the InfraredSpectrograph on Spitzer. This age indicator is based on the collectivemass-loss rate of stars, in which mass loss from AGB stars produces asilicate emission feature at 9-12 μm. This feature decreases morerapidly than the shorter wavelength continuum as a stellar populationages, providing an age indicator. From observations of 30 nearbyearly-type galaxies, 29 show a spectral energy distribution dominated bystars, and one has significant emission from the ISM and is excluded.The infrared age indicators for the 29 galaxies show them all to be old,with a mean age of about 10 Gyr and a standard deviation of only a fewGyr. This is consistent with the ages inferred from the values ofM/LB, but is inconsistent with the ages derived from theoptical line indices, which can be much younger. All of these ageindicators are luminosity weighted and should be correlated, even ifmultiple-age components are considered. The inconsistency indicates thatthere is a significant problem with either the infrared and theM/LB ages, which agree, or with the ages inferred from theoptical absorption lines.
|Chandra Multiwavelength Project: Normal Galaxies at Intermediate Redshift|
We have investigated 136 Chandra extragalactic sources, including 93galaxies with narrow emission lines (NELGs) and 43 with only absorptionlines (ALGs). Based on fX/fO, LX, X-rayspectral hardness, and optical emission-line diagnostics, we haveconservatively classified 36 normal galaxies and 71 AGNs. Their redshiftranges from 0.01 to 1.2, with normal galaxies in the range z=0.01-0.3.Our normal galaxies appear to share characteristics with local galaxies,as expected from the X-ray binary populations and the hot interstellarmatter (ISM). In conjunction with normal galaxies found in othersurveys, we found no statistically significant evolution inLX/LB, within the limited z range (<~0.1). Thebest-fit slope of our log(N)-log(S) relationship is -1.5 for both S(0.5-2 keV) and B (0.5-8 keV) energy bands, which is considerablysteeper than that of the AGN-dominated cosmic background sources, butslightly flatter than the previous estimate, indicating that normalgalaxies will not exceed the AGN population until fX(0.5-2.0keV)~2×10-18 ergs s-1 cm-2 (afactor of ~5 lower than the previous estimate). A group of NELGs appearto be heavily obscured in X-rays. After correcting for intrinsicabsorption, their X-ray luminosities could beLX>1044 ergs s-1, making them type 2quasar candidates. While most X-ray-luminous ALGs do not appear to besignificantly absorbed, we found two heavily obscured objects that couldbe as luminous as an unobscured broad-line quasar. Among 43 ALGs, wefound two E+A galaxy candidates. The X-ray spectra of both galaxies aresoft, and one of them has a nearby close companion galaxy, supportingthe merger/interaction scenario rather than the dusty starbursthypothesis.
|O VI Observations of Galaxy Clusters: Evidence for Modest Cooling Flows|
A prediction of the galaxy-cluster cooling flow model is that as gascools from the ambient cluster temperature, emission lines are producedin gas at subsequently decreasing temperatures. Gas passing through105.5 K emits in the lines of O VI λλ1032,1035, and here we report a FUSE study of these lines in three coolingflow clusters, Abell 426, Abell 1795, and AWM 7. No emission wasdetected from AWM 7, but O VI is detected from the centers of Abell 426and Abell 1795, and possibly to the south of the center in Abell 1795,where X-ray and optical emission line filaments lie. In Abell 426 theseline luminosities imply a cooling rate of 32+/-6 Msolaryr-1 within the central r=6.2 kpc region, while for Abell1795 the central cooling rate is 26+/-7 Msolaryr-1 (within r=22 kpc), and about 42+/-9 Msolaryr-1 including the southern pointing. Including otherstudies, three of six clusters have O VI emission, and they also havestar formation as well as emission lines from 104 K gas.These observations are generally consistent with the cooling flow model,but at a rate closer to 30 Msolar yr-1 than to theoriginally suggested values of 102-10 3Msolar yr-1.
|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.
|Three-Body Dynamics with Gravitational Wave Emission|
We present numerical three-body experiments that include the effects ofgravitational radiation reaction by using equations of motion thatinclude the 2.5-order post-Newtonian force terms, which are theleading-order terms of energy loss from gravitational waves. We simulatebinary-single interactions and show that close-approach cross sectionsfor three 1 Msolar objects are unchanged from the purelyNewtonian dynamics except for close approaches smaller than10-5 times the initial semimajor axis of the binary. We alsopresent cross sections for mergers resulting from gravitationalradiation during three-body encounters for a range of binary semimajoraxes and mass ratios including those of interest for intermediate-massblack holes (IMBHs). Building on previous work, we simulate sequences ofhigh-mass-ratio three-body encounters that include the effects ofgravitational radiation. The simulations show that the binaries mergewith extremely high eccentricity such that when the gravitational wavesare detectable by LISA, most of the binaries will have eccentricitiese>0.9, although all will have circularized by the time they aredetectable by LIGO. We also investigate the implications for theformation and growth of IMBHs and find that the inclusion ofgravitational waves during the encounter results in roughly half as manyblack holes ejected from the host cluster for each black hole accretedonto the growing IMBH.
|Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. II. Optical Study and Interpretation|
Our X-ray study of the nuclear activity in a new sample of six quiescentearly-type galaxies, as well as in a larger sample from the literature,confirmed (Paper I) that the Bondi accretion rate of diffuse hot gas isnot a good indicator of the SMBH X-ray luminosity. Here we suggest thata more reliable estimate of the accretion rate must include the gasreleased by the stellar population inside the sphere of influence of theSMBH, in addition to the Bondi inflow of hot gas across that surface. Weuse optical surface brightness profiles to estimate the mass-loss ratefrom stars in the nuclear region: we show that for our sample ofgalaxies it is an order of magnitude higher (~10-4 to10-3 Msolar yr-1) than the Bondi inflowrate of hot gas, as estimated from Chandra (Paper I). Only by takinginto account both sources of fuel can we constrain the true accretionrate, the accretion efficiency, and the power budget. Radiativelyefficient accretion is ruled out, for quiescent SMBHs. For typicalradiatively inefficient flows, the observed X-ray luminosities of theSMBHs imply accretion fractions ~1%-10% (i.e., ~90%-99% of the availablegas does not reach the SMBH) for at least five of our six targetgalaxies and most of the other galaxies with known SMBH masses. Wediscuss the conditions for mass conservation inside the sphere ofinfluence, so that the total gas injection is balanced by accretion plusoutflows. We show that a fraction of the total accretion power(mechanical plus radiative) would be sufficient to sustain aself-regulating, slow outflow that removes from the nuclear region allthe gas that does not sink into the BH (``BH feedback''). The rest ofthe accretion power may be carried out in a jet or advected. We alsodiscuss scenarios that would lead to an intermittent nuclear activity.
|Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. I. X-Ray Study|
We have studied the nuclear activity in a sample of six quiescentearly-type galaxies, with new Chandra data and archival HST opticalimages. Their nuclear sources have X-ray luminosities~1038-1039 ergs s-1(LX/LEdd~10-8 to 10-7) andcolors or spectra consistent with accreting supermassive black holes(SMBHs), except for the nucleus of NGC 4486B, which is softer thantypical AGN spectra. In a few cases, the X-ray morphology of the nuclearsources shows hints of marginally extended structures, in addition tothe surrounding diffuse thermal emission from hot gas, which isdetectable on scales >~1 kpc. In one case (NGC 5845), a dusty diskmay partially obstruct our direct view of the SMBH. We have estimatedthe temperature and density of the hot interstellar medium, which is onemajor source of fuel for the accreting SMBH; typical central densitiesare ne~(0.02+/-0.01) cm-3. Assuming that the hotgas is captured by the SMBH at the Bondi rate, we show that the observedX-ray luminosities are too faint to be consistent with standard diskaccretion, but brighter than predicted by radiatively inefficientsolutions (e.g., advection-dominated accretion flows [ADAFs]). In total,there are ~20 galaxies for which SMBH mass, hot gas density, and nuclearX-ray luminosity are simultaneously known. In some cases, the nuclearsources are brighter than predicted by the ADAF model; in other cases,they are consistent or fainter. We discuss the apparent lack ofcorrelations between Bondi rate and X-ray luminosity and suggest that,in order to understand the observed distribution, we need to know twoadditional parameters: the amount of gas supplied by the stellarpopulation inside the accretion radius, and the fraction (possibly<<1) of the total gas available that is accreted by the SMBH. Weleave a detailed study of these issues to a subsequent paper.
|Spitzer IRS spectra of Virgo Early-Type Galaxies: Detection of Stellar Silicate Emission|
We present high signal-to-noise ratio Spitzer Infrared Spectrographobservations of 17 Virgo early-type galaxies. The galaxies were selectedfrom those that define the color-magnitude relation of the cluster, withthe aim of detecting the silicate emission of their dusty, mass-losingevolved stars. To flux calibrate these extended sources, we have deviseda new procedure that allows us to obtain the intrinsic spectral energydistribution and to disentangle resolved and unresolved emission withinthe same object. We have found that 13 objects of the sample (76%) arepassively evolving galaxies with a pronounced broad silicate featurethat is spatially extended and likely of stellar origin, in agreementwith model predictions. The other four objects (24%) are characterizedby different levels of activity. In NGC 4486 (M87), the line emissionand the broad silicate emission are evidently unresolved, and, givenalso the typical shape of the continuum, they likely originate in thenuclear torus. NGC 4636 shows emission lines superposed on extended(i.e., stellar) silicate emission, thus pushing the percentage ofgalaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435are characterized by polycyclic aromatic hydrocarbon (PAH) and lineemission, arising from a central unresolved region. A more detailedanalysis of our sample, with updated models, will be presented in aforthcoming paper.
|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.
|The Rest-Frame Far-Ultraviolet Morphologies of Star-forming Galaxies at z ~ 1.5 and 4|
We apply a new approach to quantifying galaxy morphology and identifyinggalaxy mergers to the rest-frame far-ultraviolet images of 82 z~4 Lymanbreak galaxies (LBGs) and 55 1.22.5 and Petrosian radii >0.3". Ten of the 82 LBGs haveM20>=-1.1 and possess bright double or multiple nuclei,implying a major-merger fraction of star-forming galaxies ~10%-25% atMFUV<-20, depending on our incompleteness corrections.Galaxies with bulge-like morphologies (G>=0.55,M20<-1.6) make up ~30% of the z~4 LBG sample, while theremaining ~50% have G- and M20-values higher than expectedfor smooth bulges and disks and may be star-forming disks, minormergers, or postmergers. The star-forming z~1.5 galaxy sample has amorphological distribution that is similar to the UDF z~4 LBGs, with anidentical fraction of major-merger candidates but fewer spheroids. Theobserved morphological distributions are roughly consistent with currenthierarchical model predictions for the major-merger rates andminor-merger-induced starbursts at z~1.5 and ~4. We also examine therest-frame FUV-NUV and FUV-B colors as a function of morphology and findno strong correlations at either epoch.
|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.
|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.
|Spectroscopic Metallicities for Fornax Ultracompact Dwarf Galaxies, Globular Clusters, and Nucleated Dwarf Elliptical Galaxies|
Various formation channels for the puzzling ultracompact dwarf galaxies(UCDs) have been proposed in the last few years. To better judge some ofthe competing scenarios, we present spectroscopic [Fe/H] estimates for asample of 26 compact objects in the central region of the FornaxCluster, covering the magnitude range of UCDs and bright globularclusters (18 mag-11 mag. This metallicity break is accompanied by achange in the size-luminosity relation for compact objects, as deducedfrom Hubble Space Telescope imaging: for MV<-11 mag,rh scales with luminosity, while for MV>-11mag, rh is almost luminosity-independent. In our study wetherefore assume a limiting absolute magnitude of MV=-11 magbetween UCDs and globular clusters. The mean metallicity of five Fornaxnucleated dwarf elliptical galaxy (dE,N) nuclei included in our study isabout 0.8 dex lower than that of the UCDs, a difference significant atthe 4.5 σ level. This difference is marginally higher thanexpected from a comparison of their (V-I) colors, indicating that UCDsare younger than or at most coeval to dE,N nuclei. Because of the largemetallicity discrepancy between UCDs and nuclei, we disfavor thehypothesis that most of the Fornax UCDs are the remnant nuclei oftidally stripped dE,Ns. Our metallicity estimates for UCDs are closer tobut slightly below those derived for young massive clusters (YMCs) ofcomparable masses. We therefore favor a scenario in which most UCDs inFornax are successors of merged YMCs produced in the course of violentgalaxy-galaxy mergers. It is noted that, in contrast, the properties ofVirgo UCDs are more consistent with the stripping scenario, suggestingthat different UCD formation channels may dominate in either cluster.Based partly on observations 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 NAS 5-26555.
|Hot Populations in M87 Globular Clusters|
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.
|The Globular Cluster System of the Virgo Dwarf Elliptical Galaxy VCC 1087|
We present an analysis of the globular cluster (GC) system of thenucleated dwarf elliptical galaxy VCC 1087 in the Virgo Cluster based onKeck LRIS spectroscopy and archival Hubble Space Telescope AdvancedCamera for Surveys imaging. We estimate that VCC 1087 hosts a totalpopulation of 77+/-19 GCs, which corresponds to a relatively high V-bandspecific frequency of 5.8+/-1.4. The g475-z850color distribution of the GCs shows a blue (metal-poor) peak with a tailof redder (metal-rich) clusters similar in color to those seen inluminous elliptical galaxies. The luminosity function of the GCs islognormal and peaks atMTOg475=-7.2+/-0.3,MTOz850=-8.1+/-0.2. These peakpositions are consistent with those found for luminous Virgo ellipticalgalaxies, suggesting either the lack of or, surprisingly similarly, thedynamical destruction processes of GCs among dwarf and giant galaxies.Spectroscopy of a subsample of 12 GCs suggests that the GC system is oldand coeval (>~10 Gyr), with a fairly broad metallicity distribution(-1.8<~[M/H]<~-0.8). In contrast, an integrated spectrum of theunderlying galaxy starlight reveals that its optical luminosity isdominated by metal-rich, intermediate-age stars. The radial velocitiesof the GCs suggest rotation close to the major axis of the galaxy, andthis rotation is dynamically significant with(vrot/σlos)*>1. A compilationof the kinematics of the GC systems of nine early-type galaxies showssurprising diversity in the (vrot/σlos)parameter for GC systems. In this context, the GC system of VCC 1087exhibits the most significant rotation-to-velocity dispersion signature.Dynamical mass modeling of the velocity dispersion profile of the GCsand galaxy stars suggests fairly constant mass-to-light ratios of ~3 outto 6.5 kpc. The present observations can entertain both baryonic andnonbaryonic solutions, and GC velocities at larger radii would be mostvaluable with regard to this issue. Finally, we discuss the evolution ofVCC 1087 in terms of the galaxy ``harassment'' scenario and concludethat this galaxy may well be the remains of a faded, tidally perturbedSc spiral.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
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