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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.
| 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.
| Discovery of Ultracompact Dwarf Galaxies in the Virgo Cluster
We have discovered nine ultracompact dwarf galaxies (UCDs) in the VirgoCluster, extending samples of these objects outside the Fornax Cluster.Using the Two Degree Field (2dF) multifiber spectrograph on theAnglo-Australian Telescope, the new Virgo members were found among 1500color-selected, starlike targets with 16.0
| Mass-to-light ratio gradients in early-type galaxy haloes
Owing to the fact that the near future should see a rapidly expandingset of probes of the halo masses of individual early-type galaxies, weintroduce a convenient parameter for characterizing the halo masses fromboth observational and theoretical results:∇lΥ, the logarithmic radial gradient of themass-to-light ratio. Using halo density profiles from Λ-cold darkmatter (CDM) simulations, we derive predictions for this gradient forvarious galaxy luminosities and star formation efficienciesɛSF. As a pilot study, we assemble the available∇lΥ data from kinematics in early-type galaxies- representing the first unbiased study of halo masses in a wide rangeof early-type galaxy luminosities - and find a correlation betweenluminosity and ∇lΥ, such that the brightestgalaxies appear the most dark-matter dominated. We find that thegradients in most of the brightest galaxies may fit in well with theΛCDM predictions, but that there is also a population of faintergalaxies whose gradients are so low as to imply an unreasonably highstar formation efficiency ɛSF > 1. This difficultyis eased if dark haloes are not assumed to have the standard ΛCDMprofiles, but lower central concentrations.
| Evolution of Galaxies and the Tully-Fisher Relation
We study the evolution of the [O/Fe]-[Fe/H] relation and the dependenceof the iron abundance on distance from the galactic plane z in aone-zone model for a disk galaxy, starting from the beginning of starformation. We obtain good agreement with the observational data,including, for the first time, agreement for the [Fe/H]-z relation outto heights of 16 kpc. We also study the influence of the presence ofdark matter in the galaxies on the star-formation rate. Comparison ofthe observed luminosity of the Galaxy with the model prediction placesconstraints on the fractional mass of dark matter, which cannot be muchlarger than the fractional mass of visible matter, at least within theassumed radius of the Galaxy, 20 kpc. We studied the evolution ofdisk galaxies with various masses, which should obey the Tully-Fisherrelation, M ℭ R 2. The Tully-Fisher relation can be explained as acombination of a selection effect related to the observed surfacebrightnesses of galaxies with large radii and the conditions for theformation for elliptical galaxies.
| Bimodal Galaxies and Bimodality in Globular Cluster Systems
Various galaxy properties are not continuous over a large range in massbut rather reveal a remarkable transition or ``bimodality'' at a stellarmass of 3×1010 Msolar. These propertiesinclude colors, stellar populations, X-ray emission, and mass-to-lightratios. This behavior has been interpreted as the transition from hot tocold flows by Dekel & Birnboim. Here we explore whether globularcluster (GC) systems also reveal a bimodal nature with regard to thiscritical mass scale. Globular clusters probe star formation at earlyepochs in the universe and survive subsequent galaxy mergers andaccretions. We use new data from the ACS Virgo Cluster Survey (Peng andcoworkers), which provides a homogeneous sample of the GC systems around100 Virgo early-type galaxies covering a range of 500 in galaxy mass.Their classification of the GC color distributions is taken to examine akey quantity-the number of GCs per unit galaxy luminosity. Below thecritical mass, this quantity (called the GC specific frequency)increases dramatically in its mean value and spread. This increase maybe due to regulated star formation in low-mass galaxies, which in turnis due to mass loss via winds and the transition from hot to cold gasaccretion flows. We also note that above the critical mass, galaxiespossess two GC subpopulations (with blue and red mean colors), but belowthis mass, galaxies reveal an increasing proportion of single (blue) GCsystems.
| The ACS Virgo Cluster Survey. X. Half-Light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation
We have measured half-light radii, rh, for thousands ofglobular clusters (GCs) belonging to the 100 early-type galaxiesobserved in the ACS Virgo Cluster Survey and the elliptical galaxy NGC4697. An analysis of the dependencies of the measured half-light radiion both the properties of the GCs themselves and their host galaxiesreveals that, in analogy with GCs in the Galaxy but in a milder fashion,the average half-light radius increases with increasing galactocentricdistance or, alternatively, with decreasing galaxy surface brightness.For the first time, we find that the average half-light radius decreaseswith the host galaxy color. We also show that there is no evidence for avariation of rh with the luminosity of the GCs. Finally, wefind in agreement with previous observations that the averagerh depends on the color of GCs, with red GCs being ~17%smaller than their blue counterparts. We show that this difference isprobably a consequence of an intrinsic mechanism, rather than projectioneffects, and that it is in good agreement with the mechanism proposed byJordán. We discuss these findings in light of two simple picturesfor the origin of the rh of GCs and show that both lead to abehavior in rough agreement with the observations. After accounting forthe dependencies on galaxy color, galactocentric radius, and underlyingsurface brightness, we show that the average GC half-light radii can be successfully used as a standard ruler fordistance estimation. We outline the methodology, provide a calibrationfor its use, and discuss the prospects for this distance estimator withfuture observing facilities. We find =2.7+/-0.35 pcfor GCs with (g-z)=1.2 mag in a galaxy with color(g-z)gal=1.5 mag and at an underlying surface z-bandbrightness of μz=21 mag arcsec-2. Using thistechnique, we place an upper limit of 3.4 Mpc on the 1 σline-of-sight depth of the Virgo Cluster. Finally, we examine the formof the rh distribution for our sample galaxies and provide ananalytic expression that successfully describes this distribution.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.
| UV Properties of Early-Type Galaxies in the Virgo Cluster
We study the UV properties of a volume-limited sample of early-typegalaxies in the Virgo Cluster combining new GALEX far-ultraviolet (1530Å) and near-ultraviolet (2310 Å) data withspectrophotometric data available at other wavelengths. The sampleincludes 264 elliptical, lenticular, and dwarf galaxies spanning a largerange in luminosity (MB<=-15). While the NUV to optical ornear-IR color-magnitude relations (CMRs) are similar to those observedat optical wavelengths, with a monotonic reddening of the color indexwith increasing luminosity, the (FUV-V) and (FUV-H) CMRs show adiscontinuity between massive and dwarf objects. An even more pronounceddichotomy is observed in the (FUV-NUV) CMR. For elliptical galaxies, the(FUV-NUV) color becomes bluer with increasing luminosity and withincreasing reddening of the optical or near-IR color indices. For thedwarfs, the opposite trend is observed. These observational evidencesare consistent with the idea that the UV emission is dominated by hot,evolved stars in giant systems, while in dwarf ellipticals residual starformation activity is more common.
| The Nuclear Disk in the Dwarf Elliptical Galaxy NGC 4486A
Many ellipticals contain nuclear disks of dust and gas. Some ellipticalscontain nuclear disks of stars that are distinct from the rest of thegalaxy. We show that the dwarf E2 galaxy NGC 4486A contains both-it is a``Rosetta stone'' object that tells us how nuclear disks evolve. Itsproperties suggest that, as accreted gas dissipates and settles towardthe center, it forms stars and builds a stellar disk. Secular growth mayexplain not only the most distinct nuclear disks such as the one in NGC4486A but also some of the disky distortions that are commonly seen inelliptical galaxies. That is, density distributions may grow secularlycuspier. This would result in chaotic mixing of stellar orbits in phasespace and would tend to make an elliptical galaxy evolve toward a morenearly axisymmetric shape.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555.
| The Centers of Early-Type Galaxies with Hubble Space Telescope. V. New WFPC2 Photometry
We present observations of 77 early-type galaxies imaged with the PC1CCD of the Hubble Space Telescope (HST) WFPC2. ``Nuker-law'' parametricfits to the surface brightness profiles are used to classify the centralstructure into ``core'' or ``power-law'' forms. Core galaxies aretypically rounder than power-law galaxies. Nearly all power-law galaxieswith central ellipticities ɛ>=0.3 have stellar disks,implying that disks are present in power-law galaxies withɛ<0.3 but are not visible because of unfavorable geometry. Afew low-luminosity flattened core galaxies also have disks; these may betransition forms from power-law galaxies to more luminous core galaxies,which lack disks. Several core galaxies have strong isophote twistsinterior to their break radii, although power-law galaxies have interiortwists of similar physical significance when the photometricperturbations implied by the twists are evaluated. Central colorgradients are typically consistent with the envelope gradients; coregalaxies have somewhat weaker color gradients than power-law galaxies.Nuclei are found in 29% of the core galaxies and 60% of the power-lawgalaxies. Nuclei are typically bluer than the surrounding galaxy. Whilesome nuclei are associated with active galactic nuclei (AGNs), just asmany are not; conversely, not all galaxies known to have a low-level AGNexhibit detectable nuclei in the broadband filters. NGC 4073 and 4382are found to have central minima in their intrinsic starlightdistributions; NGC 4382 resembles the double nucleus of M31. In general,the peak brightness location is coincident with the photocenter of thecore to a typical physical scale of <1 pc. Five galaxies, however,have centers significantly displaced from their surrounding cores; thesemay be unresolved asymmetric double nuclei. Finally, as noted byprevious authors, central dust is visible in about half of the galaxies.The presence and strength of dust correlates with nuclear emission;thus, dust may outline gas that is falling into the central black hole.The prevalence of dust and its morphology suggest that dust clouds form,settle to the center, and disappear repeatedly on ~108 yrtimescales. We discuss the hypothesis that cores are created by thedecay of a massive black hole binary formed in a merger. Apart fromtheir brightness profiles, there are no strong differences between coregalaxies and power-law galaxies that demand this scenario; however, therounder shapes of core, their lack of disks, and their reduced colorgradients may be consistent with it.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withGO and GTO proposals 5236, 5446, 5454, 5512, 5943, 5990, 5999, 6099,6386, 6554, 6587, 6633, 7468, 8683, and 9107.
| Ram Pressure Stripping in the Low-Luminosity Virgo Cluster Elliptical Galaxy NGC 4476
We present a deep VLA search for H I emission from the low-luminosityVirgo Cluster elliptical galaxy NGC 4476, which contains1.1×108 Msolar of molecular gas in anundisturbed disk in regular rotation. No H I was detected. The rms noisein the final image corresponds to a 3 σ column density sensitivityof 1.2×1020 cm-2 at the position of NGC4476, averaged over the 4 kpc beam. The total H I mass is less than1.5×107 Msolar. If we compare our H I upperlimit with the H2 content, we find that NGC 4476 is extremelydeficient in H I compared with other galaxies detected in these twospecies. The H2/HI mass ratio for NGC 4476 is greater than 7,whereas typical H2/HI ratios for elliptical galaxies detectedin both H I and H2 are less than 2. On the basis of thisextreme H I deficiency and the intracluster medium density at theprojected distance from M87, we argue that either NGC 4476 has undergoneram pressure stripping while traveling through the Virgo Cluster core orits average molecular gas density is larger and its interstellar UVfield is smaller than in typical spiral galaxies. NGC 4476 is located12' in projection from M87, which causes extreme continuum confusionproblems. We also discuss in detail the techniques used for continuumsubtraction. The spectral dynamic range of our final image is 50,000 to1.
| Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles
We present a study of the optical brightness profiles of early typegalaxies, using a number of samples of radio galaxies and opticallyselected elliptical galaxies. For the radio galaxy samples - B2 ofFanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined anumber of parameters that describe a "Nuker-law" profile, which werecompared with those already known for the optically selected objects. Wefind that radio active galaxies are always of the "core" type (i.e. aninner Nuker law slope γ < 0.3). However, there are core-typegalaxies which harbor no significant radio source and which areindistinguishable from the radio active galaxies. We do not find anyradio detected galaxy with a power law profile (γ > 0.5). Thisdifference is not due to any effect with absolute magnitude, since in aregion of overlap in magnitude the dichotomy between radio active andradio quiescent galaxies remains. We speculate that core-type objectsrepresent the galaxies that have been, are, or may become, radio activeat some stage in their lives; active and non-active core-type galaxiesare therefore identical in all respects except their eventualradio-activity: on HST scales we do not find any relationship betweenboxiness and radio-activity. There is a fundamental plane, defined bythe parameters of the core (break radius rb and breakbrightness μ_b), which is seen in the strong correlation betweenrb and μ_b. The break radius is also linearly proportionalto the optical Luminosity in the I band. Moreover, for the few galaxieswith an independently measured black hole mass, the break radius turnsout to be tightly correlated with MBH. The black hole masscorrelates even better with the combination of fundamental planeparameters rb and μ_b, which represents the centralvelocity dispersion.
| Discovery of two M 32 twins in Abell 1689
The M 31 satellite galaxy M 32 has long been considered an object ofunique properties, being the most extreme example of the very rarecompact elliptical galaxy class. Here we present the spectroscopicdiscovery of two M 32 twins in the massive galaxy cluster Abell 1689. Asthese objects are so rare, this is an important step towards a betterunderstanding of the nature of compact galaxies. The two M 32 twins hadfirst been detected within our photometric search for ultra compactdwarf galaxy (UCDs) candidates in A1689 (Mieske et al. \cite{Mieske04b},AJ, 128, 1529) with the Advanced Camera for Surveys (ACS). Theirluminosities (MV≃ -17 mag) are very similar to M 32 andtheir surface brightness profiles are consistent with that of M 32projected to A1689's distance. From our ACS imaging we detect severalfainter compact galaxy candidates with luminosities intermediate betweenM 32 and the Fornax UCDs. If spectroscopically confirmed as clustermembers, this would almost close the gap in the mag-μ plane betweenthe region of UCDs and the compact ellipticals, implying a sequence offaint compact galaxies well separated from that of dwarf ellipticals.Based on observations obtained at the European Southern Observatory,Chile (Observing Programme 273.B-5008).
| The structure of elliptical galaxies in the Virgo cluster. Results from the INT Wide Field Survey
We report on a complete CCD imaging survey of 226 elliptical galaxies inthe North-East quadrant of the Virgo cluster, representative of theproperties of giant and dwarf elliptical galaxies in this cluster. Wefit their radial light profiles with the Sersic r1/n model oflight distribution. We confirm the result of Graham & Guzman(\cite{Graham03}, AJ, 125, 2936) that the apparent dichotomy between Eand dE galaxies in the luminosity-< μ>e plane nolonger appears when other structural parameters are considered and canbe entirely attributed to the onset of ``core'' galaxies atBT -20.5 mag. When ``core'' galaxies are notconsidered, E and dE form a unique family with n linearly increasingwith the luminosity. For 90 galaxies we analyze the B-I color indices,both in the nuclear and in the outer regions. Both indices are bluertoward fainter luminosities. We find also that the outer color gradientsdo not show any significant correlation with the luminosity. The scatterin all color indicators increases significantly toward lowerluminosities, e.g. galaxies fainter than BT -15 have aB-I spread > 0.5 mag.Table 2, Figs. 13 and 14 are only available in electronic form athttp://www.edpsciences.org
| Observational and research progress of the M31 galaxy
Observational properties of the largest Local Group spiral M31(Andromeda galaxy) are presented. Similar to the Milky Way Galaxy, themain components of M31 are nucleus, bulge, disc and halo. In this paper,we have summarized the most recent observational and research progressfor each component. It is concentrated on the M31 disk stellarpopulations and star formation history, the globular clustersdistribution and the halo properties. Finally, a detailed comparison forthe observed values of each component and formation mechanism betweenM31 and our Galaxy is given.
| The ACS Virgo Cluster Survey. II. Data Reduction Procedures
The ACS Virgo Cluster Survey is a large program to carry out multicolorimaging of 100 early-type members of the Virgo Cluster using theAdvanced Camera for Surveys (ACS) on the Hubble Space Telescope. DeepF475W and F850LP images (~SDSS g and z) are being used to study thecentral regions of the program galaxies, their globular cluster systems,and the three-dimensional structure of Virgo itself. In this paper, wedescribe in detail the data reduction procedures used for the survey,including image registration, drizzling strategies, the computation ofweight images, object detection, the identification of globular clustercandidates, and the measurement of their photometric and structuralparameters.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.
| The ACS Virgo Cluster Survey. I. Introduction to the Survey
The Virgo Cluster is the dominant mass concentration in the LocalSupercluster and the largest collection of elliptical and lenticulargalaxies in the nearby universe. In this paper, we present anintroduction to the ACS Virgo Cluster Survey: a program to image, in theF475W and F850LP bandpasses (~Sloan g and z), 100 early-type galaxies inthe Virgo Cluster using the Advanced Camera for Surveys on the HubbleSpace Telescope. We describe the selection of the program galaxies andtheir ensemble properties, the choice of filters, the field placementand orientation, the limiting magnitudes of the survey, coordinatedparallel observations of 100 ``intergalactic'' fields with WFPC2, andsupporting ground-based spectroscopic observations of the programgalaxies. In terms of depth, spatial resolution, sample size, andhomogeneity, this represents the most comprehensive imaging survey todate of early-type galaxies in a cluster environment. We brieflydescribe the main scientific goals of the survey, which include themeasurement of luminosities, metallicities, ages, and structuralparameters for the many thousands of globular clusters associated withthese galaxies, a high-resolution isophotal analysis of galaxiesspanning a factor of ~450 in luminosity and sharing a commonenvironment, the measurement of accurate distances for the full sampleof galaxies using the method of surface brightness fluctuations, and adetermination of the three-dimensional structure of Virgo itself.ID="FN1"> 1Based on observations with the NASA/ESA Hubble SpaceTelescope obtained at the Space Telescope Science Institute, which isoperated by the association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| Self-Gravitating Eccentric Disk Models for the Double Nucleus of M31
We present new dynamical models of weakly self-gravitating, finitedispersion eccentric stellar disks around central black holes for thedouble nucleus of M31. The disk is fixed in a frame rotating at constantprecession speed and is populated by stars on quasi-periodic orbitswhose parents are numerically integrated periodic orbits in the totalpotential. A distribution of quasi-periodic orbits about a given parentis approximated by a distribution of Kepler orbits dispersed ineccentricity and orientation, using an approximate phase-spacedistribution function written in terms of the integrals of motion in theKepler problem. We use these models, along with an optimization routine,to fit available published kinematics and photometry in the inner 2" ofthe nucleus. A grid of 24 best-fit models is computed to accuratelyconstrain the mass of the central black hole and nuclear diskparameters. We find that the supermassive black hole in M31 has massMBH=5.62+/-0.66×107 Msolar, whichis consistent with the observed correlation between the central blackhole mass and the velocity dispersion of its host spheroid. Our modelsprecess rapidly, at Ω=36.5+/-4.2 km s-1pc-1, and possess a characteristic radial eccentricitydistribution, which gives rise to multimodal line-of-sight velocitydistributions along lines of sight near the black hole. These featurescan be used as sensitive discriminants of disk structure.
| Revised Rates of Stellar Disruption in Galactic Nuclei
We compute rates of tidal disruption of stars by supermassive blackholes in galactic nuclei, using downwardly revised black hole massesfrom the MBH-σ relation. In galaxies with steep nucleardensity profiles, which dominate the overall event rate, the disruptionfrequency varies inversely with assumed black hole mass. We compute atotal rate for nondwarf galaxies of ~10-5 yr-1Mpc-3, about a factor of 10 higher than in earlier studies.Disruption rates are predicted to be highest in nucleated dwarfgalaxies, assuming that such galaxies contain black holes. Monitoring ofa rich galaxy cluster for a few years could rule out the existence ofintermediate-mass black holes in dwarf galaxies.
| Distances, Metallicities, and Ages of Dwarf Elliptical Galaxies in the Virgo Cluster from Surface Brightness Fluctuations
We have employed FORS1 and 2 at the Very Large Telescope at the EuropeanSouthern Observatory (ESO) to acquire deep B and R-band CCD images of 16dwarf elliptical (dE) galaxies in the direction of the Virgo cluster.For each dwarf, we measure the apparent R-band surface brightnessfluctuation (SBF) magnitude mR and the (B-R)0color in a number of fields at different galactocentric distances. Fromthe field-to-field variation of the two quantities, we determine the SBFdistance by means of the (B-R)0-MR relation. Thederived distances of the dwarfs range from 14.9 to 21.3 Mpc, with a mean1 σ uncertainty of 1.4 Mpc or 8% of the distance, which confirmsthat there is considerable depth in the distance distribution ofearly-type cluster members. For VCC 1104 (IC 3388), our SBF distancemodulus of (m-M)SBF=31.15+/-0.19 (17.0+/-1.5 Mpc) is in goodagreement with the Harris et al result of(m-M)TRGB=30.98+/-0.19 mag (15.7+/-1.5 Mpc) based on HubbleSpace Telescope (HST) observations and the tip magnitude of the redgiant branch. Combining our results with existing distances for giantVirgo ellipticals, we identify two major galaxy concentrations in thedistance distribution: a broad primary clump around (M-m)=31.0 mag (15.8Mpc) and a narrow secondary clump around 31.33 mag (18.5 Mpc). Anadaptive kernel analysis finds the two concentrations to be significantat the 99% (2.5 σ) and 89% (1.6 σ) levels, respectively.While the near-side clump of Virgo early-type galaxies can be associatedwith the subcluster centered on M87, the second clump is believed to bemainly due to the far side infalling group of galaxies around M86. Theages and metallicities of the dE stellar populations are estimated bycombining the observed (B-R)0 colors with Worthey's stellarpopulation synthesis models. It appears that the Virgo dE galaxies covera wider range in metallicity, from [Fe/H]~-1.4 (VCC 0815) to -0.5 (NGC4415), than Fornax cluster dEs. The derived metallicities place theVirgo dEs on the extension of the metallicity-luminosity relationdefined by the low-luminosity Local Group dEs. The data further suggestan age range from genuinely old (~17 Gyr) stellar systems such as IC3019 and IC 0783 to intermediate-age (8-12 Gyr) dwarfs such as NGC 4431and IC 3468.
| The origin of H I-deficiency in galaxies on the outskirts of the Virgo cluster. II. Companions and uncertainties in distances and deficiencies
The origin of the deficiency in neutral hydrogen of 13 spiral galaxieslying in the outskirts of the Virgo cluster is reassessed. If thesegalaxies have passed through the core of the cluster, their interstellargas should have been lost through ram pressure stripping by the hotX-ray emitting gas of the cluster. We analyze the positions of these HI-deficient and other spiral galaxies in velocity-distance plots, inwhich we include our compilation of velocity-distance data on 61elliptical galaxies, and compare with simulated velocity-distancediagrams obtained from cosmological N-body simulations. We find that20% relative Tully-Fisher distance errors are consistent with thegreat majority of the spirals, except for a small number of objectswhose positions in the velocity-distance diagram suggest grosslyincorrect distances, implying that the Tully-Fisher error distributionfunction has non-Gaussian wings. Moreover, we find that the distanceerrors may lead to an incorrect fitting of the Tolman-Bondi solutionthat can generate significant errors in the distance and especially themass estimates of the cluster. We suggest 4 possibilities for theoutlying H I-deficient spirals (in decreasing frequency): 1) they havelarge relative distance errors and are in fact close enough (atdistances between 12.7 and 20.9 Mpc from us) to the cluster to havepassed through its core and seen their gas removed by ram pressurestripping; 2) their gas is converted to stars by tidal interactions withother galaxies; 3) their gas is heated during recent mergers withsmaller galaxies; and 4) they are not truly H I-deficient (e.g. S0/amisclassified as Sa).Appendix A is only available in electronic form athttp://www.edpsciences.org
| Spectrophotometry of galaxies in the Virgo cluster. II. The data
Drift-scan mode (3600-6800 Å) spectra with 500
| The Binary Black Hole Scenario for the BL Lacertae Object AO~0235+16
Recent analysis of the long term radio light curve of the extremelyvariable BL Lacertae object AO 0235+16 by Raiteri et al. have revealedthe presence of recurrent outbursts with a period of 5.7±0.5 yr. Periodicity analysis of the optical light curve also showsevidence for a shorter period. Here we discuss whether such a behaviorcan be explained by a binary black hole model where the accretion diskof one of the supermassive black holes is precessing due to the tidaleffects of the companion. We estimate the mass of the accreting hole andanalyze constraints on the secondary mass and the orbital parameters ofthe system. It is possible to provide a viable interpretation of theavailable multiwavelength data.
| The Realm of the Supermassive Black Holes
Not Available
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
| Kinematics of 10 Early-Type Galaxies from Hubble Space Telescope and Ground-based Spectroscopy
We present stellar kinematics for a sample of 10 early-type galaxiesobserved using the Space Telescope Imaging Spectrograph (STIS) aboardthe Hubble Space Telescope and the Modular Spectrograph on the MDMObservatory 2.4 m telescope. These observations are a part of an ongoingprogram to understand the coevolution of supermassive black holes andtheir host galaxies. Our spectral ranges include either the calciumtriplet absorption lines at 8498, 8542, and 8662 Å or the Mg babsorption at 5175 Å. The lines are used to derive line-of-sightvelocity distributions (LOSVDs) of the stars using a maximum penalizedlikelihood method. We use Gauss-Hermite polynomials to parameterize theLOSVDs and find predominantly negative h4 values (boxy distributions) inthe central regions of our galaxies. One galaxy, NGC 4697, hassignificantly positive central h4 (high tail weight). The majority ofgalaxies have a central velocity dispersion excess in the STISkinematics over ground-based velocity dispersions. The galaxies with thestrongest rotational support, as quantified withvmax/σSTIS, have the smallest dispersionexcess at STIS resolution. The best-fitting, general, axisymmetricdynamical models (described in a companion paper) require black holes inall cases, with masses ranging from 106.5 to 109.3Msolar. We replot these updated masses on theMBH-σ relation and show that the fit to only these 10galaxies has a slope consistent with the fits to larger samples. Thegreatest outlier is NGC 2778, a dwarf elliptical with relatively poorlyconstrained black hole mass. The two best candidates for pseudobulges,NGC 3384 and NGC 7457, do not deviate significantly from the establishedrelation between MBH and σ. Neither do the threegalaxies that show the most evidence of a recent merger, NGC 3608, NGC4473, and NGC 4697.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal GO-7388.
| Simulations of Galaxy Formation in a ΛCDM Universe. III. The Dissipative Formation of an Elliptical Galaxy
We examine the dynamical structure of an elliptical galaxy simulated inthe ΛCDM scenario. The morphology of the galaxy evolvesdramatically over time in response to the mode and timing of massaccretion; smooth deposition of cooled gas leads to the formation ofcentrifugally supported disks, while major mergers disperse stellardisks into spheroids. In the case we consider here, these two modes ofaccretion alternate successively until z~0.6, when the galaxy undergoesone last major (1:3) merger. The event triggers a starburst thatconsumes much of the remaining gas into stars. Little gas cools andaccretes subsequently, and as a result, most stars at z=0 are rather old(75% are older than 9 Gyr) and distributed in a spheroidal componentthat resembles present-day elliptical galaxies. Dynamically, the galaxyis well approximated by an E4 oblate rotator, with rotational supportincreasing steadily from the center outward. The apparent rotationsupport, as measured by Vrot/σ, correlates stronglywith isophotal deviations from perfect ellipses. Boxy isophotes areobtained when the galaxy is seen face-on andVrot/σ<<1. On the other hand, disky isophotes arefound for inclinations that maximize Vrot/σ. Theline-of-sight velocity distribution is nearly Gaussian at all radii,although small but significant deviations are robustly measured. Thesign of the Gauss-Hermite skewness parameter h3 isanticorrelated with the apparent sense of rotation, in agreement withobserved trends. Despite its relatively recent assembly, the simulatedgalaxy has much higher effective surface brightness than normalelliptical galaxies of similar luminosity, in a way reminiscent of theless common type of M32-like ``compact elliptical galaxies.'' This islikely a direct consequence of our star formation and feedbackalgorithm, which allows for efficient transformation of gas into starsin dense, early collapsing progenitors rather than a definitiveprediction for the structure of galaxies assembled in this ΛCDMhalo. Despite this limitation, our simulation suggests that dark matterplays a minor role in the luminous regions of compact ellipticalgalaxies, whose dynamical mass-to-light ratios are thus not dissimilarto those of normal elliptical galaxies. This explains the proximity ofcompact elliptical galaxies to edge-on projections of the fundamentalplane, despite their far greater velocity dispersion at givenluminosity. Overall, our simulation shows that repeated episodes ofdissipational collapse, followed by merger events, lead to stellarspheroids that are only mildly triaxial and of relatively simplekinematic structure. This is in better agreement with observation thanearlier models based on dissipationless mergers of stellar disks and apositive step toward reconciling the observed structure of ellipticalgalaxies with a hierarchical assembly process where mergers play asubstantial role.
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