Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Explaining the Color Distributions of Globular Cluster Systems in Elliptical Galaxies
The colors of globular clusters in most large elliptical galaxies arebimodal. This is generally taken as evidence for the presence of twocluster subpopulations that have different geneses. However, here wefind that, because of the nonlinear nature of the metallicity-to-colortransformation, a coeval group of old clusters with a unimodalmetallicity spread can exhibit color bimodality. The models of clustercolors indicate that horizontal-branch stars are the main drivers behindthe empirical nonlinearity. We show that the scenario gives simple andcohesive explanations for all the key observations and could simplifytheories of elliptical galaxy formation.
| 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.
| Adaptive binning of X-ray data with weighted Voronoi tessellations
We present a technique to adaptively bin sparse data using weightedVoronoi tessellations (WVTs). WVT binning is a generalization of theVoronoi binning algorithm by Cappellari & Copin, developed forintegral field spectroscopy. WVT binning is applicable to many types ofdata and creates unbiased binning structures with compact bins that donot lead the eye. We apply the algorithm to simulated data, as well asseveral X-ray data sets, to create adaptively binned intensity images,hardness ratio maps and temperature maps with constant signal-to-noiseratio per bin. We also illustrate the separation of diffuse gas emissionfrom contributions of unresolved point sources in elliptical galaxies.We compare the performance of WVT binning with other adaptive binningand adaptive smoothing techniques. We find that the csmooth tool in CIAOversions 1.1-3.1 creates serious artefacts and advise against its use tointerpret diffuse X-ray emission.
| 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.
| 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.
| Gas in early-type galaxies: cross-fuelling in late-type-early-type pairs?
We present 12CO (J= 1-0) and 12CO (J= 2-1)observations of eight early-type galaxies, forming part of a sample ofinteracting galaxies, each consisting of one late- and one early-typesystem. All of the early-type galaxies observed are undetected in CO tolow levels, allowing us to place tight constraints on their moleculargas content. Additionally, we present HI absorption data for one system.The implications for possible gas transfer from the late- to theearly-type galaxy during the interaction are discussed.
| The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies
The ACS Virgo Cluster Survey is a Hubble Space Telescope program toobtain high-resolution imaging in widely separated bandpasses (F475W~gand F850LP~z) for 100 early-type members of the Virgo Cluster, spanninga range of ~460 in blue luminosity. We use this large, homogenous dataset to examine the innermost structure of these galaxies and tocharacterize the properties of their compact central nuclei. We presenta sharp upward revision in the frequency of nucleation in early-typegalaxies brighter than MB~-15 (66%<~fn<~82%)and show that ground-based surveys underestimated the number of nucleidue to surface brightness selection effects, limited sensitivity andpoor spatial resolution. We speculate that previously reported claimsthat nucleated dwarfs are more concentrated toward the center of Virgothan their nonnucleated counterparts may be an artifact of theseselection effects. There is no clear evidence from the properties of thenuclei, or from the overall incidence of nucleation, for a change atMB~-17.6, the traditional dividing point between dwarf andgiant galaxies. There does, however, appear to be a fundamentaltransition at MB~-20.5, in the sense that the brighter,``core-Sérsic'' galaxies lack resolved (stellar) nuclei. A searchfor nuclei that may be offset from the photocenters of their hostgalaxies reveals only five candidates with displacements of more than0.5", all of which are in dwarf galaxies. In each case, however, theevidence suggests that these ``nuclei'' are, in fact, globular clustersprojected close to the galaxy photocenter. Working from a sample of 51galaxies with prominent nuclei, we find a median half-light radius of=4.2 pc, with the sizes of individual nucleiranging from 62 pc down to <=2 pc (i.e., unresolved in our images) inabout a half-dozen cases. Excluding these unresolved objects, the nucleisizes are found to depend on nuclear luminosity according to therelation rh L0.50+/-0.03. Because the largemajority of nuclei are resolved, we can rule out low-level AGNs as anexplanation for the central luminosity excess in almost all cases. Onaverage, the nuclei are ~3.5 mag brighter than a typical globularcluster. Based on their broadband colors, the nuclei appear to have oldto intermediate age stellar populations. The colors of the nuclei ingalaxies fainter than MB~-17.6 are tightly correlated withtheir luminosities, and less so with the luminosities of their hostgalaxies, suggesting that their chemical enrichment histories weregoverned by local or internal factors. Comparing the nuclei to the``nuclear clusters'' found in late-type spiral galaxies reveals a closematch in terms of size, luminosity, and overall frequency. A formationmechanism that is rather insensitive to the detailed properties of thehost galaxy properties is required to explain this ubiquity andhomogeneity. The mean of the frequency function for thenucleus-to-galaxy luminosity ratio in our nucleated galaxies,=-2.49+/-0.09 dex (σ=0.59+/-0.10), isindistinguishable from that of the SBH-to-bulge mass ratio,=-2.61+/-0.07dex (σ=0.45+/-0.09), calculated in 23 early-type galaxies withdetected supermassive black holes (SBHs). We argue that the compactstellar nuclei found in many of our program galaxies are the low-masscounterparts of the SBHs detected in the bright galaxies. If thisinterpretation is correct, then one should think in terms of ``centralmassive objects''-either SBHs or compact stellar nuclei-that accompanythe formation of almost all early-type galaxies and contain a meanfraction ~0.3% of the total bulge mass. In this view, SBHs would be thedominant formation mode above MB~-20.5.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| The ACS Virgo Cluster Survey. VI. Isophotal Analysis and the Structure of Early-Type Galaxies
We present a detailed analysis of the morphology, isophotal parameters,and surface brightness profiles for 100 early-type members of the VirgoCluster, from dwarfs (MB=-15.1 mag) to giants(MB=-21.8 mag), imaged in the g and z passbands using theAdvanced Camera for Surveys on board the Hubble Space Telescope. Dustand complex morphological structures are common. Dust is detected in 42%of galaxies brighter than BT=12.15 mag, whilekiloparsec-scale stellar disk, bars, and nuclear stellar disks are seenin 60% of galaxies with intermediate luminosity. Isophotal parametersare derived typically within 8 kpc from the center for the brightestgalaxies, and 1.5 kpc for the faintest systems, with a resolution of 7pc. For most galaxies, the surface brightness profiles are welldescribed by a Sérsic model with index n that increases steadilywith the galaxy luminosity; only for 8 of the 10 brightest galaxies arethe inner profiles (typically within 100 pc of the center) lower thanexpected based on an extrapolation of the outer Sérsic model, andare better described by a single power-law function. Contrary toprevious claims, we find no evidence in support of a strong bimodalbehavior of the logarithmic slope of the inner surface brightnessprofile, γ in particular the γ distribution for galaxiesthat do not show evidence of multiple morphological components isunimodal across the entire magnitude range spanned by the ACSVCSgalaxies. Although the brightest galaxies have shallow inner profiles,the shallowest profiles are found in faint dwarf systems. The widelyadopted separation of early-type galaxies between ``core'' and``power-law'' types is questioned based on the present study.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theassociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| 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.
| A Chandra View of Dark Matter in Early-Type Galaxies
We present a Chandra study of mass profiles in seven ellipticalgalaxies, of which three have galaxy-scale and four have group-scalehalos, demarcated at 1013 Msolar. These representthe best available data for nearby objects with comparable X-rayluminosities. We measure approximately flat mass-to-light (M/L) profileswithin an optical half-light radius (Reff), rising by anorder of magnitude at ~10 Reff, which confirms the presenceof dark matter (DM). The data indicate hydrostatic equilibrium, which isalso supported by agreement with studies of stellar kinematics inelliptical galaxies. The data are well fitted by a model comprising anNFW DM profile and a baryonic component following the optical light. Thedistribution of DM halo concentration parameters (c) versusMvir agrees with ΛCDM predictions and our observationsof bright groups. Concentrations are slightly higher than expected,which is most likely a selection effect. Omitting the stellar massdrastically increases c, possibly explaining large concentrations foundby some past observers. The stellar M/LK agree withpopulation synthesis models, assuming a Kroupa IMF. Allowing adiabaticcompression (AC) of the DM halo by baryons made M/L more discrepant,casting some doubt on AC. Our best-fitting models imply total baryonfractions ~0.04-0.09, consistent with models of galaxy formationincorporating strong feedback. The groups exhibit positive temperaturegradients, consistent with the ``universal'' profiles found in othergroups and clusters, whereas the galaxies have negative gradients,suggesting a change in the evolutionary history of the systems aroundMvir~=1013 Msolar.
| A Fundamental Relation between Compact Stellar Nuclei, Supermassive Black Holes, and Their Host Galaxies
Imaging surveys with the Hubble Space Telescope (HST) have shown that~50%-80% of low- and intermediate-luminosity galaxies contain a compactstellar nucleus at their center, regardless of host galaxy morphologicaltype. We combine HST imaging for early-type galaxies from the ACS VirgoCluster Survey with ground-based long-slit spectra from KPNO to showthat the masses of compact stellar nuclei in Virgo Cluster galaxies obeya tight correlation with the masses of the host galaxies. The samecorrelation is obeyed by the supermassive black holes (SBHs) found inpredominantly massive galaxies. The compact stellar nuclei in the LocalGroup galaxies M33 and NGC 205 are also found to fall along this samescaling relation. These results indicate that a generic by-product ofgalaxy formation is the creation of a central massive object(CMO)-either an SBH or a compact stellar nucleus-that contains a meanfraction, ~0.2%, of the total galactic mass. In galaxies with massesgreater than Mgal ~ a few × 1010Msolar, SBHs appear to be the dominant mode of CMOformation.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| 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.
| The ACS Virgo Cluster Survey. IX. The Color Distributions of Globular Cluster Systems in Early-Type Galaxies
We present the color distributions of globular cluster (GC) systems for100 early-type galaxies observed in the ACS Virgo Cluster Survey, thedeepest and most homogeneous survey of this kind to date. On average,galaxies at all luminosities in our study (-22
| Type I Ultraluminous Infrared Galaxies: Transition Stage from ULIRGs to QSOs
We examine whether the ultraluminous infrared galaxies that contain atype 1 Seyfert nucleus (a type I ULIRG) are in the transition stage fromULIRGs to quasi-stellar objects (QSOs). To investigate this issue, wecompare the black hole (BH) mass, the bulge luminosity, and thefar-infrared luminosity among type I ULIRGs, QSOs, and ellipticalgalaxies. As a result, we find the following results: (1) The type IULIRGs have systematically smaller BH masses in spite of the comparablebulge luminosity relative to QSOs and elliptical galaxies. (2) Thefar-infrared luminosity of most type I ULIRGs is larger than theEddington luminosity. We show that the above results do not changesignificantly for three type I ULIRGs for which we can estimate thevisual extinction from the column density. Also, for all eight type IULIRGs, we investigate the effect of uncertainties of BH massmeasurements and our sample bias to make sure that our results are notaltered even if we consider the above two effects. In addition, Anabukirecently revealed that their X-ray properties are similar to those ofthe narrow-line Seyfert 1 galaxies. These would indicate that activegalactic nuclei (AGNs) with a high mass accretion rate exist in type IULIRGs. On the basis of all of these findings, we conclude that it wouldbe a natural interpretation that type I ULIRGs are the early phase of BHgrowth, namely, the missing link between ULIRGs and QSOs. Moreover, bycomparing our results with a theoretical model of a coevolution scenarioof a QSO BH and a galactic bulge, we show clearly that this explanationcould be valid.
| On the Correlations of Massive Black Holes with Their Host Galaxies
We address the correlations of black hole (BH) mass with four differenthost-galaxy properties from 11 existing data sets. For the purpose ofguiding theoretical understanding, we first try to quantify thetightness of the intrinsic correlations. We assume that all of therelations are power laws and perform linear regressions that aresymmetric in the two variables on the logarithms of the data points.Given the estimated measurement errors, we evaluate the probabilitydistribution of the residual variance in excess of that expected fromthe measurement errors. Our central result is that the current data setsdo not allow definite conclusions regarding the quality of the truecorrelations because the obtained probability distributions for theresidual variance overlap for most quantities. Velocity dispersion ascollected by Merritt & Ferrarese (σMF) and galaxylight concentration as measured by Graham and coworkers (CRe)are consistent with zero residual variance. Taken at face value, thismeans that these two correlations are better than the others, but thisconclusion is highly sensitive to the assumed measurement errors andwould be undone if the present estimated errors were too large. We thenconsider which of the relations offer the best inferences of BH masswhen there is no direct measurement available. As with the residualvariances, we find that the probability distribution of expecteduncertainty in inferred BH masses overlaps significantly for most of therelations. Photometric methods would then be preferred because the dataare easier to obtain, as long as bulge-disk decomposition or detailedmodeling of the photometric profile (as studied by Graham and coworkers)do not present problems. Determining which correlation offers the bestinferences requires reducing the uncertainty in the expected error inthe inferred BH masses (the ``error on the error''). This uncertainty iscurrently limited by uncertainty in the residual variance for all of therelations. The only quantities for which BH mass inferences are limitedby measurement error are σMF and CRe.Therefore, if these relations are truly better than the others, thennew, improved measurements should allow improved inferences of BHmasses. If they do not, the conclusion must be that the present lowresidual variances for these two relations result from overestimatederror bars.
| 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.
| XMM-Newton Observation of Diffuse Gas and Low-Mass X-Ray Binaries in the Elliptical Galaxy NGC 4649 (M60)
We present an XMM-Newton X-ray observation of the X-ray-bright E2elliptical galaxy NGC 4649. In addition to bright diffuse emission, weresolve 158 discrete sources, ~50 of which are likely to be LMXBsassociated with NGC 4649. We find evidence for variability in threesources between this observation and a previous Chandra observation.Additionally, we detect five sources that were not detected with Chandradespite its better detection limit, suggesting that these sources havesince brightened. The total X-ray spectrum of the resolved sources iswell fit by a hard power law, while the diffuse spectrum requires a hardand a soft component, presumably due to the relatively soft diffuse gasand the harder unresolved sources. A deprojection of the diffuseemission revealed a radial temperature gradient that is hot in thecenter, drops to a minimum at about 20"-50" (1.6-4.1 kpc), and risesagain in the outer regions. The diffuse emission appears to require atwo-temperature model with heavy-element abundance ratios that differfrom the solar values. We have verified the existence of faint radialfeatures extending out from the core of NGC 4649 that had previouslybeen seen with Chandra. The fingers are morphologically similar toradial features seen in hydrodynamic simulations of cooling flows inelliptical galaxies, and although their other properties do not matchthe predictions of the particular simulations used, we conclude that theradial fingers might be due to convective motions of hot outflowing gasand cooler inflowing gas. We also find evidence for a longer, previouslyundetected filament that extends to the northeastern edge of NGC 4649.The diffuse gas in the region of the filament appears to have a lowertemperature and may also have a higher abundance as compared to nearbyregions. There also appears to be an excess of X-ray sources along thefilament, although the excess is not statistically significant. Weconclude that the filament may be the result of a tidal interaction,possibly with NGC 4647, although more work is necessary to verify thisconclusion.
| 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.
| The Effects of Interactions on the Structure and Morphology of Elliptical/Lenticular Galaxies in Pairs
We present a structural and photometric analysis of 42elliptical/lenticular galaxies in E/S0 + S pairs observed in the BVRIcolor bands. The aim of the analysis is to empirically determine theeffects of interactions on the galaxies' morphology, structure, andstellar populations as seen from the CAS parameters (light concentrationC, asymmetry A, and clumpiness S). We further compare these values to acontrol sample of 67 mostly isolated noninteracting E/S0 galaxies. Wefind that the paired E/S0 galaxies occupy more scattered loci in CASspace than noninteracting E/S0s and that the structural effects ofinteractions on E/S0s are minor, in contrast to disk galaxies involvedin interactions. This suggests that observational methods forrecognizing interactions, such the CAS methodology of Conselice, wouldnot detect E/S0s involved in interactions (related to early phases ofthe so-called dry mergers), and that the majority of interactinggalaxies identified at high redshift must be gas-dominated systems.However, we find statistical differences in the asymmetry index whencomparing isolated and interacting E/S0s. On average, paired E/S0galaxies have A-values 2.96+/-0.72 times larger than those ofnoninteracting E/S0s. For the subset of presumably strongly interactingE/S0s, A and S can be several times larger than the typical values ofthe isolated E/S0s. We show that the asymmetries are consistent withseveral internal and external morphological distortions. We concludethat the subsample of interacting E/S0s should be dense, gas-poorgalaxies in systems spanning a wide range of interaction stages, withtypical merging timescales >~0.1-0.5 Gyr. We use the observedphenomenology of this subsample to predict the approximate loci of drypremergers in the CAS parameter space.
| Low-Luminosity Active Galaxies and Their Central Black Holes
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
| 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.
| 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.
| Large-scale study of the NGC 1399 globular cluster system in Fornax
We present a Washington C and Kron-Cousins R photometric study of theglobular cluster system of NGC 1399, the central galaxy of the Fornaxcluster. A large areal coverage of 1 square degree around NGC 1399 isachieved with three adjoining fields of the MOSAIC II Imager at the CTIO4-m telescope. Working on such a large field, we can perform the firstindicative determination of the total size of the NGC 1399 globularcluster system. The estimated angular extent, measured from the NGC 1399centre and up to a limiting radius where the areal density of blueglobular clusters falls to 30 per cent of the background level, is 45± 5 arcmin, which corresponds to 220-275 kpc at the Fornaxdistance. The bimodal colour distribution of this globular clustersystem, as well as the different radial distribution of blue and redclusters, up to these large distances from the parent galaxy, areconfirmed. The azimuthal globular cluster distribution exhibitsasymmetries that might be understood in terms of tidal stripping ofglobulars from NGC 1387, a nearby galaxy. The good agreement between theareal density profile of blue clusters and a projected dark-matter NFWdensity profile is emphasized.
| 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.
| 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.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
INHABITED SKY
