תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
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.
| SPITZER : living life to the fullest.
Not Available
| Line-of-sight velocity dispersions and a mass-distribution model of the Sa galaxy NGC 4594
In this paper, we develop an algorithm allowing to calculateline-of-sight velocity dispersions in an axisymmetric galaxy outside thegalactic plane. When constructing a self-consistent model, we take intoaccount the galactic surface brightness distribution, stellar rotationcurve and velocity dispersions. We assume that the velocity dispersionellipsoid is triaxial and lies under a certain angle with respect to thegalactic plane. This algorithm is applied to an Sa galaxy NGC 4594 = M104, for which there exist velocity dispersion measurements outside thegalactic major-axis. The mass-distribution model is constructed in twostages. In the first stage, we construct a luminosity-distributionmodel, where only galactic surface brightness distribution is taken intoaccount. Here, we assume the galaxy to consist of the nucleus, thebulge, the disc and the stellar metal-poor halo and determine structuralparameters of these components. Thereafter, in the second stage, wedevelop, based on the Jeans equations, a detailed mass-distributionmodel and calculate line-of-sight velocity dispersions and the stellarrotation curve. Here, a dark matter (DM) halo is added to visiblecomponents. Calculated dispersions are compared with observations alongdifferent slit positions perpendicular and parallel to the galacticmajor-axis. In the best-fitting model, velocity dispersion ellipsoidsare radially elongated withσθ/σR ~=0.9-0.4,σz/σR ~= 0.7-0.4, and lieunder the angles less than or equal to 30° with respect to thegalactic equatorial plane. Outside the galactic plane, velocitydispersion behaviour is more sensitive to the DM density distributionand allows to estimate dark halo parameters. For visible matter, thetotal M/LB = 4.5 +/- 1.2, M/LR = 3.1 +/- 0.7. Thecentral density of the DM halo is ρDM(0) =0.033Msolarpc-3.
| Lifetime of nuclear velocity dispersion drops in barred galaxies
We have made hydro/N-body simulations with and without star formation toshed some light on the conditions under which a central kinematicallycold stellar component (characterized by a velocity dispersion drop orσ-drop) could be created in a hot medium (e.g. a bulge) andsurvive enough time to be observed. We found that the time-scale for aσ-drop formation could be short (less than 500 Myr), whereas itslifetime could be long (more than 1 Gyr) provided that the centralregion is continuously or regularly fed by fresh gas which leads to acontinuous star formation activity. Star formation in the centralregion, even at a low rate as 1Msolaryr-1, ismandatory to sustain a permanent σ-drop by replacing heatedparticles by new low-σ ones. We moreover show that as soon as starformation is switched off, the σ-drop begins to disappear.
| 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.
| 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.
| 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.
| Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)
We present Spitzer 3.6-160 μm images, Spitzer mid-infrared spectra,and JCMT SCUBA 850 μm images of the Sombrero Galaxy (NGC 4594), an Sagalaxy with a 109 Msolar low-luminosity activegalactic nucleus (AGN). The brightest infrared sources in the galaxy arethe nucleus and the dust ring. The spectral energy distribution of theAGN demonstrates that, while the environment around the AGN is aprominent source of mid-infrared emission, it is a relatively weaksource of far-infrared emission, as had been inferred for AGNs inprevious research. The weak nuclear 160 μm emission and thenegligible polycyclic aromatic hydrocarbon emission from the nucleusalso implies that the nucleus is a site of only weak star formationactivity and the nucleus contains relatively little cool interstellargas needed to fuel such activity. We propose that this galaxy may berepresentative of a subset of low-ionization nuclear emission regiongalaxies that are in a quiescent AGN phase because of the lack of gasneeded to fuel circumnuclear star formation and Seyfert-like AGNactivity. Surprisingly, the AGN is the predominant source of 850 μmemission. We examine the possible emission mechanisms that could giverise to the 850 μm emission and find that neither thermal dustemission, CO line emission, bremsstrahlung emission, nor the synchrotronemission observed at radio wavelengths can adequately explain themeasured 850 μm flux density by themselves. The remainingpossibilities for the source of the 850 μm emission include acombination of known emission mechanisms, synchrotron emission that isself-absorbed at wavelengths longer than 850 μm, or unidentifiedspectral lines in the 850 μm band.
| 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 large scale magnetic field configuration in the Sombrero galaxy - persistence during galaxy evolution?
Radio polarization observations at 4.86 and 8.35 GHz of the nearbyedge-on galaxy M 104 revealed a large-scale magnetic field in thisearly-type spiral. This is to our knowledge the first detection of aregular magntic field in an Sa galaxy in the radio range. The magneticfield orientation in M 104 is predominantly parallel to the disk but hasalso vertical components at larger z-distances from the disk, i.e. afield configuration typical for normal edge-on spiral galaxies.Bolometer observations at 345 GHz data pertain to the cold dust contentof the galaxy. Despite the optical appearance of the object with thehuge dust lane, its dust content is smaller than that of more late-typespirals.
| Hubble Space Telescope ACS Wide-Field Photometry of the Sombrero Galaxy Globular Cluster System
A detailed imaging analysis of the globular cluster (GC) system of theSombrero galaxy (NGC 4594) has been accomplished using a six-imagemosaic from the Hubble Space Telescope Advanced Camera for Surveys. Thequality of the data is such that contamination by foreground stars andbackground galaxies is negligible for all but the faintest 5% of the GCluminosity function. This enables the study of an effectively puresample of 659 GCs until ~2 mag fainter than the turnover magnitude,which occurs at MTOMV=-7.60+/-0.06 for an assumedm-M=29.77. Two GC metallicity subpopulations are easily distinguishable,with the metal-poor subpopulation exhibiting a smaller intrinsicdispersion in color compared to the metal-rich subpopulation. There arethree new discoveries. (1) A metal-poor GC color-magnitude trend hasbeen observed. (2) The fact that the metal-rich GCs are ~17% smallerthan the metal-poor ones for small projected galactocentric radii (lessthan ~2') has been confirmed. However, the median half-light radii ofthe two subpopulations become identical at ~3' from the center. This ismost easily explained if the size difference is the result of projectioneffects. (3) The brightest (MV<-9.0) members of the GCsystem show a size-magnitude upturn, where the average GC size increaseswith increasing luminosity. Evidence is presented that supports anintrinsic origin for this feature rather than being a result of accreteddwarf elliptical nuclei. In addition, the metal-rich GCs show ashallower positive size-magnitude trend, similar to what is found inprevious studies of young star clusters.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained (from the data archive) at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555. Theseobservations are associated with program 9714.
| Massive Star Cluster Populations in Irregular Galaxies as Probable Younger Counterparts of Old Metal-rich Globular Cluster Populations in Spheroids
Peak metallicities of metal-rich populations of globular clusters(MRGCs) belonging to early-type galaxies and spheroidal subsystems ofspiral galaxies (spheroids) of different mass fall within the somewhatconservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible ageeffects are taken into account, this metallicity range might becomesmaller. Irregular galaxies such as the Large Magellanic Cloud (LMC),with longer timescales of formation and lower star formation (SF)efficiency, do not contain old MRGCs with [Fe/H]>-1.0, but they areobserved to form populations of young/intermediate-age massive starclusters (MSCs) with masses exceeding 104 Msolar.Their formation is widely believed to be an accidental process fullydependent on external factors. From the analysis of available data onthe populations and their hosts, including intermediate-age populousstar clusters in the LMC, we find that their most probable meanmetallicities fall within -0.7<=[Fe/H]<=-0.3, as the peakmetallicities of MRGCs do, irrespective of signs of interaction.Moreover, both the disk giant metallicity distribution function (MDF) inthe LMC and the MDFs for old giants in the halos of massive spheroidsexhibit a significant increase toward [Fe/H]~-0.5. That is in agreementwith a correlation found between SF activity in galaxies and theirmetallicity. The formation of both the old MRGCs in spheroids and MSCpopulations in irregular galaxies probably occurs at approximately thesame stage of the host galaxies' chemical evolution and is related tothe essentially increased SF activity in the hosts around the samemetallicity that is achieved very early in massive spheroids, later inlower mass spheroids, and much later in irregular galaxies. Changes inthe interstellar dust, particularly in elemental abundances in dustgrains and in the mass distribution function of the grains, may be amongthe factors regulating star and MSC formation activity in galaxies.Strong interactions and mergers affecting the MSC formation presumablyplay an additional role, although they can substantially intensify theinternally regulated MSC formation process. Several implications of oursuggestions are briefly discussed.
| A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
Seyfert galaxies commonly host compact jets spanning 10-100 pc scales,but larger structures are resolved out in long-baseline aperturesynthesis surveys. Previous, targeted studies showed thatkiloparsec-scale radio structures (KSRs) may be a common feature ofSeyfert and LINER galaxies, and the origin of KSRs may be starbursts oractive galactic nuclei (AGNs). We report a new Very Large Array surveyof a complete sample of Seyfert and LINER galaxies. Out of all of thesurveyed radio-quiet sources, we find that 44% (19 out of 43) showextended radio structures at least 1 kpc in total extent that do notmatch the morphology of the disk or its associated star-forming regions.The detection rate is a lower limit owing to the combined effects ofprojection and resolution. The infrared colors of the KSR host galaxiesare unremarkable compared to other Seyfert galaxies, and the large-scaleoutflows orient randomly with respect to the host galaxy axes. The KSRSeyfert galaxies instead stand out by deviating significantly from thefar-infrared-radio correlation for star-forming galaxies, with tendencytoward radio excess, and they are more likely to have a relativelyluminous, compact radio source in the nucleus; these results argue thatKSRs are powered by the AGNs rather than starbursts. The high detectionrate indicates that Seyfert galaxies generate radio outflows over asignificant fraction of their lifetime, which is much longer than thedynamical timescale of an AGN-powered jet but is comparable instead tothe buoyancy timescale. The likely explanation is that the KSRsoriginate from jet plasma that has been decelerated by interaction withthe nuclear interstellar medium (ISM). Based on a simple ram pressureargument, the kinetic power of the jet on kiloparsec scales is about 3orders of magnitude weaker than the power of the jet on 10-100 pcscales. This result is consistent with the interaction model, in whichcase virtually all of the jet power must be lost to the ISM within theinner kiloparsec.
| 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.
| The structure of galactic disks. Studying late-type spiral galaxies using SDSS
Using imaging data from the SDSS survey, we present the g' and r' radialstellar light distribution of a complete sample of ~90 face-on tointermediate inclined, nearby, late-type (Sb-Sdm) spiral galaxies. Thesurface brightness profiles are reliable (1 σ uncertainty lessthan 0.2 mag) down to μ27 mag/''. Only ~10% of all galaxies havea normal/standard purely exponential disk down to our noise limit. Thesurface brightness distribution of the rest of the galaxies is betterdescribed as a broken exponential. About 60% of the galaxies have abreak in the exponential profile between 1.5-4.5 times thescalelength followed by a downbending, steeper outer region. Another~30% shows also a clear break between 4.0-6.0 times thescalelength but followed by an upbending, shallower outer region. A fewgalaxies have even a more complex surface brightness distribution. Theshape of the profiles correlates with Hubble type. Downbending breaksare more frequent in later Hubble types while the fraction of upbendingbreaks rises towards earlier types. No clear relation is found betweenthe environment, as characterised by the number of neighbours, and theshape of the profiles of the galaxies.
| Radio polarization and sub-millimeter observations of the Sombrero galaxy (NGC 4594). Large-scale magnetic field configuration and dust emission
We observed the nearby early-type spiral galaxy NGC 4594 (M 104,Sombrero galaxy) with the Very Large Array at 4.86 GHz, with theEffelsberg 100-m telescope at 8.35 GHz as well as with the HeinrichHertz Telescope at 345 GHz in radio continuum. The 4.86 and 8.35 GHzdata contain polarization information and hence information about themagnetic fields: we detected a large-scale magnetic field which is toour knowledge the first detection of a large-scale magnetic field in anSa galaxy in the radio range. The magnetic field orientation in M 104 ispredominantly parallel to the disk but has also vertical components atlarger z-distances from the disk. This field configuration is typicalfor normal edge-on spiral galaxies. The 345 GHz data pertain to the colddust content of the galaxy. Despite the optical appearance of the objectwith the huge dust lane, its dust content is smaller than that of morelate-type spirals.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| Secular Evolution in Disk Galaxies: The Growth of Pseudobulges and Problems for Cold Dark Matter Galaxy Formation
We review internal secular evolution in galaxy disks - the fundamentalprocess by which isolated disks evolve. We concentrate on the buildup ofdense central features that look like classical, merger-built bulges butthat were made slowly out of disk gas. We call these pseudobulges. As anexistence proof, we review how bars rearrange disk gas into outer rings,inner rings, and gas dumped into the center. In simulations, this gasreaches high densities, and in the observations, many SB and ovalgalaxies show central concentrations of gas. Associated star formationrates imply plausible pseudobulge growth times of a few billion years.If secular processes built dense centers that masquerade as bulges, canwe distinguish them from merger-built bulges? Observations show thatpseudobulges retain a memory of their disky origin. They have one ormore characteristics of disks: (1) flatter shapes than those ofclassical bulges, (2) larger ratios of ordered to random velocities, (3)smaller velocity dispersions, (4) nuclear bars or spiral structure, (5)boxy structure when seen edge-on, (6) nearly exponential brightnessprofiles, and (7) starbursts. These features occur preferentially inbarred and oval galaxies in which secular evolution should be rapid. Sothe cleanest examples of pseudobulges are recognizable. Thusobservations and theory contribute to a new picture of galaxy evolutionthat complements hierarchical clustering and merging.However, an important problem with cold dark matter galaxy formationgets more acute. How can hierarchical clustering produce so many puredisk galaxies with no evidence for merger-built bulges?
| Detection of Neutrinos from Supernovae in Nearby Galaxies
While existing detectors would see a burst of many neutrinos from aMilky Way supernova, the supernova rate is only a few per century. As analternative, we propose the detection of 1 neutrino per supernovafrom galaxies within 10 Mpc, in which there were at least 9core-collapse supernovae since 2002. With a future 1 Mton scaledetector, this could be a faster method for measuring the supernovaneutrino spectrum, which is essential for calibrating numerical modelsand predicting the redshifted diffuse spectrum from distant supernovae.It would also allow a ≳104 times more precise triggertime than optical data alone for high-energy neutrinos and gravitationalwaves.
| Secular Evolution of Barred Galaxies with Massive Central Black Holes
The influence of central black holes on the dynamical evolution of barsin disk galaxies was examined. Once a bar formed by a dynamicalinstability in an infinitesimally thin stellar disk was fully developed,a black hole (BH) was adiabatically added at the center of the disk. Ourresults indicate that a bar can be completely destroyed, in a practicalsense, in a time much smaller than the Hubble time if the central BHmass exceeds about 0.5% of the disk mass. Since this implied minimum BHmass for bar destruction is on the order of 108.5Modot for a typical disk galaxy, this process could occur inthe real Universe. The bar amplitude decreases gradually with time afterthe BH has grown up to its full mass. Surface-of-section plots indicatethat the bar dissolution originates from the chaotic behavior ofbar-supporting orbits. In addition, the scale length and the radialvelocity dispersion of the disk after bar dissolution become much largerthan those of the initial axisymmetric disk. This finding suggests thatit is possible to discriminate between genuine non-barred galaxies andbar-dissolved galaxies induced by massive central BHs from the viewpointof structural properties.
| The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies
The Hubble Space Telescope has revealed a dichotomy in the centralsurface brightness profiles of early-type galaxies, which havesubsequently been grouped into two families: core, boxy, anisotropicsystems; and cuspy (`power-law'), discy, rotating ones. Here weinvestigate whether a dichotomy is also present in the X-ray propertiesof the two families. We consider both their total soft emission(LSX,tot), which is a measure of the galactic hot gascontent, and their nuclear hard emission (LHX,nuc), mostlycoming from Chandra observations, which is a measure of the nuclearactivity. At any optical luminosity, the highest LSX,totvalues are reached by core galaxies; this is explained by their beingthe central dominant galaxies of groups, subclusters or clusters, inmany of the logLSX,tot (ergs-1) >~ 41.5 cases.The highest LHX,nuc values, similar to those of classicalactive galactic nuclei (AGNs), in this sample are hosted only by core orintermediate galaxies; at low luminosity AGN levels, LHX,nucis independent of the central stellar profile shape. The presence ofoptical nuclei (also found by HST) is unrelated to the level ofLHX,nuc, even though the highest LHX,nuc are allassociated with optical nuclei. The implications of these findings forgalaxy evolution and accretion modalities at the present epoch arediscussed.
| On the nature of bulges in general and of box/peanut bulges in particular: input from N-body simulations
Objects designated as bulges in disc galaxies do not form a homogeneousclass. I distinguish three types: the classical bulges, the propertiesof which are similar to those of ellipticals and which form by collapseor merging; boxy and peanut bulges, which are seen in near-edge-ongalaxies and which are in fact just a part of the bar seen edge-on; and,finally, disc-like bulges, which result from the inflow of (mainly) gasto the centre-most parts, and subsequent star formation. I make adetailed comparison of the properties of boxy and peanut bulges withthose of N-body bars seen edge-on, and answer previously voicedobjections about the links between the two. I also present and analysesimulations where a boxy/peanut feature is present at the same time as aclassical spheroidal bulge, and compare them with observations. Finally,I propose a nomenclature that can help to distinguish between the threetypes of bulges and avoid considerable confusion.
| The galaxy luminosity function from MR=-25 to MR=-9
Redshift surveys such as the Sloan Digital Sky Survey (SDSS) have givena very precise measurement of the galaxy luminosity function down toabout MR=-17 (~MB=-16). Fainter absolutemagnitudes cannot be probed because of the flux limit required forspectroscopy. Wide-field surveys of nearby groups using mosaic CCDs onlarge telescopes are able to reach much fainter absolute magnitudes,about MR=-10. These diffuse, spiral-rich groups are thoughtto be typical environments for galaxies, so their luminosity functionsshould be the same as the field luminosity function. The luminosityfunction of the groups at the bright end (MR < -17) islimited by Poisson statistics and is far less precise than that derivedfrom redshift surveys. Here we combine the results of the SDSS and thesurveys of nearby groups, and we supplement the results with studies ofLocal Group galaxies in order to determine the galaxy luminosityfunction over the entire range -25 < MR < -9. Theaverage logarithmic slope of the field luminosity function betweenMR=-19 and MR=-9 is α=-1.26, although asingle power law is a poor fit to the data over the entire magnituderange. We also determine the luminosity function of galaxy clusters anddemonstrate that it is different from the field luminosity function at ahigh level of significance; there are many more dwarf galaxies inclusters than in the field, due to a rise in the cluster luminosityfunction of α~-1.6 between MR=-17 andMR=-14.
| The galaxy cluster Abell 3581 as seen by Chandra
We present results from an analysis of a Chandra observation of thecluster of galaxies Abell 3581. We discover the presence of apoint-source in the central dominant galaxy that is coincident with thecore of the radio source PKS 1404-267. The emission from theintracluster medium is analysed, both as seen in projection on the sky,and after correcting for projection effects, to determine the spatialdistribution of gas temperature, density and metallicity. We find thatthe cluster, despite hosting a moderately powerful radio source, shows atemperature decline to around 0.4 Tmax within the central 5kpc. The cluster is notable for the low entropy within its core. We testand validate the XSPEC PROJCT model for determining the intrinsiccluster gas properties.
| Infrared Luminosities of Local-Volume Galaxies
Based on data from the Two-Micrometer All-Sky Survey (2MASS), weanalyzed the infrared properties of 451 Local-Volume galaxies atdistances D ≤ 10 Mpc. We determined the K-band luminosity functionof the galaxies in the range of absolute magnitudes from -25m to -11m.The local luminosity density within 8 Mpc is 6.8 × 108 L ȯMpc-3, a factor of 1.5 ± 0.1 higher than the global mean K-bandluminosity density. We determined the ratios of the virial mass to theK-band luminosity for nearby groups and clusters of galaxies. In theluminosity range from 5 × 1010 to 2 × 1013 L ȯ, thedependence log(M/L K) ∝ (0.27 ± 0.03) log L K with adispersion of 0.1 comparable to the measurement errors of themasses and luminosities of the systems of galaxies holds for the groupsand clusters of galaxies. The ensemble-averaged ratio, ≃ (20 25) M ȯ/L ȯ, was found to be much smaller than theexpected global ratio, (80 90)M ȯ/L ȯ, in the standard modelwith Ωm = 0.27. This discrepancy can be eliminated if the bulk ofthe dark matter in the Universe is not associated with galaxies andtheir systems.
| Supermassive Black Holes: Relation to Dark Halos
Estimates of the masses of supermassive black holes (M bh ) in thenuclei of disk galaxies with known rotation curves are compared withestimates of the rotational velocities V m and the“indicative” masses of the galaxies M i . Although there isa correlation between M bh and V m or M i , it is appreciably weakerthan the correlation with the central velocity dispersion. The values ofM bh for early-type galaxies (S0-Sab), which have more massive bulges,are, on average, higher than the values for late-type galaxies with thesame rotational velocities. We conclude that the black-hole masses aredetermined primarily by the properties of the bulge and not therotational velocity or the mass of the galaxy.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| M82, Starbursts, Star Clusters, and the Formation of Globular Clusters
We observed the nearby starburst galaxy M82 in CO in the higherfrequency (2-1) transition to achieve an angular resolution below 1" or17 pc at the target. We resolved the molecular gas into a large numberof compact clouds, with masses ranging from ~2×103 to2×106 Msolar. The mass spectrum scales asN(M)~M-1.5+/-0.1, similar to the mass spectra of youngmassive star clusters, suggesting that individual molecular clouds aretransformed in the starburst into individual star clusters. The largerclouds are surrounded by supernovae and H II regions, suggesting thatstar formation proceeds from the outside of the clouds and progressesinward, consistent with triggering by a sudden increase in externalpressure. The clouds with internal star formation have velocitygradients and inverse P Cygni spectral line profiles indicating inwardmotions of 35 km s-1, consistent with shock-drivencompression. Diffuse free-free radio emission and X-ray emission aroundthe clouds provide evidence for superheated ionized gas sufficient todrive the compression. Clouds with spectral lines indicating expansionshow little internal star formation, suggesting that the dynamicsprecedes and is responsible for the star formation rather than theinverse. M82 is known to be in interaction with the neighboring galaxyM81. The overall picture is consistent with the formation of massivestar clusters from individual giant molecular clouds crushed by a suddengalactic-scale increase in external pressure generated by the changingdynamics that result from a near-collision with a neighboring galaxy.Present-day globular clusters may have formed in a similar fashion inprimordial galaxies.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
קטלוגים וכינוים:
|
