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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.
|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
|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.
|Star Formation Properties of a Large Sample of Irregular Galaxies|
We present the results of Hα imaging of a large sample ofirregular galaxies. Our sample includes 94 galaxies with morphologicalclassifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems.The sample spans a large range in galactic parameters, includingintegrated absolute magnitude (MV of -9 to -19), averagesurface brightness (20-27 mag arcsec-2), current starformation activity (0-1.3 Msolar yr-1kpc-2), and relative gas content(0.02-5Msolar/LB). The Hα images were usedto measure the integrated star formation rates, determine the extents ofstar formation in the disks, and compare azimuthally averaged radialprofiles of current star formation to older starlight. The integratedstar formation rates of Im galaxies normalized to the physical size ofthe galaxy span a range of a factor of 104 with 10% Imgalaxies and one Sm system having no measurable star formation at thepresent time. The BCDs fall, on average, at the high star formation rateend of the range. We find no correlation between star formation activityand proximity to other cataloged galaxies. Two galaxies located in voidsare similar in properties to the Sm group in our sample. The H IIregions in these galaxies are most often found within the Holmbergradius RH, although in a few systems H II regions are tracedas far as 1.7RH. Similarly, most of the star formation isfound within three disk scale lengths RD, but in somegalaxies H II regions are traced as far as 6RD. A comparisonof Hα surface photometry with V-band surface photometry shows thatthe two approximately follow each other with radius in Sm galaxies, butin most BCDs there is an excess of Hα emission in the centers thatdrops with radius. In approximately half of the Im galaxies Hα andV correspond well, and in the rest there are small to large differencesin the relative rate of falloff with radius. The cases with stronggradients in the LHα/LV ratios and with highcentral star formation rate densities, which include most of the BCDs,require a significant fraction of their gas to migrate to the center inthe last gigayear. We discuss possible torques that could have causedthis without leaving an obvious signature, including dark matter barsand past interactions or mergers with small galaxies or H I clouds.There is now a substantial amount of evidence for these processes amongmany surveys of BCDs. We note that such gas migration will also increasethe local pressure and possibly enhance the formation of massive denseclusters but conclude that the star formation process itself does notappear to differ much among BCD, Im, and Sm types. In particular, thereis evidence in the distribution function for Hα surface brightnessthat the turbulent Mach numbers are all about the same in these systems.This follows from the Hα distribution functions corrected forexponential disk gradients, which are log-normal with a nearly constantdispersion. Thus, the influence of shock-triggered star formation isapparently no greater in BCDs than in Im and Sm types.
|Star Formation Histories of Early-Type Galaxies. I. Higher Order Balmer Lines as Age Indicators|
We have obtained blue integrated spectra of 175 nearby early-typegalaxies, covering a wide range in galaxy velocity dispersion andemphasizing those with σ<100 km s-1. Galaxies havebeen observed both in the Virgo Cluster and in lower densityenvironments. The main goals are the evaluation of higher order Balmerlines as age indicators and differences in stellar populations as afunction of mass, environment, and morphology. In this first paper, ouremphasis is on presenting the methods used to characterize the behaviorof the Balmer lines through evolutionary population synthesis models.Lower σ galaxies exhibit a substantially greater intrinsicscatter, in a variety of line-strength indicators, than do higherσ galaxies, with the large intrinsic scatter setting in below aσ of 100 km s-1. Moreover, a greater contrast inscatter is present in the Balmer lines than in the lines of metalfeatures. Evolutionary synthesis modeling of the observed spectralindexes indicates that the strong Balmer lines found primarily among thelow-σ galaxies are caused by young age, rather than by lowmetallicity. Thus we find a trend between the population age and thecentral velocity dispersion, such that low-σ galaxies have youngerluminosity-weighted mean ages. We have repeated this analysis usingseveral different Balmer lines and find consistent results from onespectral indicator to another.
|The UZC-SSRS2 Group Catalog|
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
|Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies|
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
|Ellipticals with Kinematically Distinct Cores: V-I Color Images with WFPC2|
We have analyzed Hubble Space Telescope WFPC2 F555W and F814W (i.e., Vand I) images for 15 elliptical galaxies with kinematically distinctcores. For each of them, we have derived surface brightness andisophotal parameter profiles in the two bands, color maps, and radialprofiles in V - I. Most galaxies show patchy dust absorption close totheir nuclei. However, there are generally no indications ofhomogeneous, diffuse dust components close to the nuclei. The nuclearcolors in the unobscured regions are most likely representative of thecentral stellar populations. We have detected photometric evidence forfaint stellar disks, on scales of a few tens to a few arcseconds, inseven galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494, and 5322.In NGC 1700, the isophotes are slightly boxy at the scale of thecounterrotating component and disky at larger radii. We find nodifference in V - I color greater than 0.02 mag between these disks andthe surrounding galactic regions. Hence, the stellar populations in thekinematically distinct cores are not strongly deviant from thepopulation of the main body. Specifically, there is no evidence for adominating population of blue, very metal weak stars as predicted bysome of the formation scenarios. This argues against models in whichsmall galaxies fall in and survive in the nuclei, unless supermassiveblack holes are present. These would likely disrupt the accreted smallsystems. For one galaxy, NGC 4365, the innermost region is bluer thanthe surrounding regions. This area extends to ~15 pc and contains aluminosity of ~2.5 x 106 Lȯ. If interpreted as a stellar populationeffect, an age difference of ~3--4 Gyr, or an [Fe/H] variation of about0.2 dex, is derived. The nuclear intensity profiles show a largevariety: some galaxies have steep cusp profiles, while others haveshallow cusps and a "break radius." The nuclear cusps of galaxies withkinematically distinct cores follow the same trends as the nuclei ofnormal galaxies. We have not been able to identify a unique, qualifyingfeature in the WFPC2 images that distinguishes the galaxies withkinematically distinct cores from the kinematically normal cores. It ispossible that statistical differences exist: possibly, the kinematicallydistinct cores have a higher fraction of nuclear disks. The similarityof both types of cores puts strong constraints on the formationscenarios. Simulations of galaxy mergers, with the inclusion of starformation and nuclear black holes, are needed to resolve the question ofhow these structures may have formed. Spectra with high spatialresolution are needed to study the nuclear structure of the distinctcomponent in detail.
|The Centers of Early-Type Galaxies with HST. IV. Central Parameter Relations.|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1771F&db_key=AST
|The Centers of Early-Type Galaxies With HST. III. Non-Parametric Recovery of Stellar Luminosity Distribution|
We have non-parametrically determined the luminosity density profilesand their logarithmic slopes for 42 early-type galaxies observed withHST. Assuming that the isodensity contours are spheroidal, then theluminosity density is uniquely determined from the surface brightnessdata through the Abel equation. For nearly all the galaxies in oursample, the logarithmic slope of the luminosity density (S = d log v/dlog r) measured at 0.1" (the innermost reliable measurement with theuncorrected HST) is significantly different from zero; i.e., mostelliptical galaxies have cusps. There are only two galaxies for which ananalytic core (S approaches 0) cannot be excluded. The distribution oflogarithmic slopes at 0.1" appears to be bimodal, confirming theconclusion of Lauer et al. [AJ, 110,2622(1995)] that early-type galaxiescan be divided into two types based on their surface brightnessprofiles; i.e., those with cuspy cores and those whose steep power-lawprofiles continue essentially unchanged in to the resolution limit. Thepeaks in the slope distribution occur at S = -0.8 and - 1.9. More thanhalf of the galaxies have slopes steeper than - 1.0. Taken together withthe recent theoretical work of Merritt and Fridman, these resultssuggest that many (and maybe most) elliptical galaxies are either nearlyaxisymmetric or spherical near the center, or slowly evolve due to theinfluence of stochastic orbits.
|The Centers of Early-Type Galaxies With HST. II. Empirical Models and Structural Parameters|
We present a set of structural parameters for the central parts of 57early-type galaxies observed with the Planetary Camera of the HubbleSpace Telescope. These parameters are based on a new empirical law thatsuccessfully characterizes the centers of early-type galaxies. Thisempirical law assumes that the surface brightness profile is acombination of two power laws with different slopes γ and βfor the inner and outer regions. Conventional structural parameters suchas core radius and central surface brightness are replaced by breakradius r_b_, where the transition between power-law slopes takes place,and surface brightness μ_b_ at that radius. An additional parameterα describes the sharpness of the break. The structural parametersare derived using a X^2^ minimization process applied to the meansurface brightness profiles. The resulting model profiles generally givevery good agreement to the observed profiles out to the radius of ~10"imaged by the Planetary Camera. Exceptions include galaxies which departfrom pure power laws at large radius, those with strong nuclearcomponents, and galaxies partly obscured by dust. The uncertainties inthe derived parameters are estimated using Monte Carlo simulations whichtest the stability of solutions in the face of photon noise and theeffects of the deconvolution process. The covariance of the structuralparameters is examined by computing contours of constant X^2^ inmulti-dimensional parameter space.
|An image database. II. Catalogue between δ=-30deg and δ=70deg.|
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
|The Centers of Early-Type Galaxies with HST.I.An Observational Survey|
We have obtained V-band images of 45 nearby elliptical galaxies andbulges using the original Planetary Camera of the Hubble SpaceTelescope. The majority of the sample is at distances of 10-20 Mpc. Thisrepresents a substantial increase in the number of galaxies that havebeen studied at a resolution of a few parsecs. At this resolution, manygalaxies reveal previously unknown central disks, dust clouds, andnuclear components. We find that galaxies have two types of brightnessprofiles. The first type consists of galaxies that have cores. Thesegalaxies have brightness profiles that "break" from steep outer powerlaws to shallow inner cusps. The core class includes many galaxies thathad cores apparently resolved from the ground. The second type consistsof galaxies that have profiles that continue into the resolution limitas steep power laws, showing no evidence of cores of any sort. We thusfind that all galaxies studied so far have singular brightness profilesin the sense that I(r) ~ r^-γ^ as r - 0.1", with 0 < γ< 0.3 at the few parsec scale for galaxies with cores, and γ ~1 for power-law galaxies. No galaxies in our sample have a centralregion that is constant in surface brightness. This implies that thestellar density in these systems is still increasing steeply at the HSTresolution limit. Many galaxies reach stellar mass densities of ~5 x10^4^ M_sun_ pc^-3^ at the resolution limit, appearing similar in formto M32 at radii of a few parsecs. The core and power-law profile classescorrespond to the Jaffe et al. (AJ, 108, 1567 (1994)] Type I and IIprofiles; however, we disagree with their suggestion that the presenceof a central stellar disk is closely related to, or even determines,profile type. Power-law galaxies are seen at all ellipticities, and themajority of them show no evidence for central disks.
|Dwarf galaxies in the Virgo cluster. II - Photometric techniques and basic data|
Results are presented of photographic surface photometry carried out for305 (mostly dwarf) galaxies in the Virgo cluster, in which the galaxyimages were digitized on 14 of the 67 du Pont plates used for the Virgocluster survey. Azimuthally averaged surface brightness profiles areshown for all galaxies. The following model-free photometric parametersare derived and listed for each galaxy: total apparent blue magnitude,mean effective radius and surface brightness, and various isophotalradii, ellipticity, and position angle. Most galaxies were fitted by anexponential form and/or a King model profile. The best-fittingparameters, including the 'nuclear' (central residual) magnitudes fordE+dS0 galaxies, are listed.
|Dwarf galaxies in the Virgo cluster. I - The systematic photometric properties of early-type dwarfs|
The azimuthally averaged surface brightness profiles of 200 faintearly-type Virgo cluster galaxies have been analyzed. Faint dwarfs arevery well described by an exponential or a King model. The magnitudes ofthe nuclei vary greatly at a given galaxian magnitude, but the maximumnuclear luminosity is a strong function of M(T). In the 0.1-1 kpc radiusrange, the logarithmically plotted profiles of all early-type galaxiescome in two well-defined classes identified with classical types versusdwarf types. The former are all classified E or S0, while the lattercomprise all galaxies classified dE or dS0, all morphologically'intermediate' types, and even two classified 'E'. The mean SB profilesof dS0 galaxies are indistinguishable from bright dE profiles. In 2D,the dS0s appear highly flattened and/or show asymmetric and irregularfeatures which may indicate their disk nature.
|The influence of absorption on galaxy diameters and magnitudes|
A sample of spiral and elliptical galaxies with known surface photometrywas artificially absorbed. The newly calculated intensity profiles,resulting from an ever increasing artificial absorption, were thencompared with the original ones and relationships between the absorptionand a diameter reduction factor as well as a magnitude correction factorwere established. These relationships differ according to galaxian type.For absorptions larger than one magnitude this additional correction dueto the diameter reduction is significant. At an absorption of 3.5 mag,an elliptical galaxy will be additionally dimmed by 0.8 mag, a spiralgalaxy by as much as 1.6 mag.
|Studies of the Virgo Cluster. II - A catalog of 2096 galaxies in the Virgo Cluster area.|
The present catalog of 2096 galaxies within an area of about 140 sq degapproximately centered on the Virgo cluster should be an essentiallycomplete listing of all certain and possible cluster members,independent of morphological type. Cluster membership is essentiallydecided by galaxy morphology; for giants and the rare class of highsurface brightness dwarfs, membership rests on velocity data. While 1277of the catalog entries are considered members of the Virgo cluster, 574are possible members and 245 appear to be background Zwicky galaxies.Major-to-minor axis ratios are given for all galaxies brighter than B(T)= 18, as well as for many fainter ones.
|Neutral hydrogen in isolated galaxies. IV - Results for the Arecibo sample|
A standard sample for the comparison of the H I content of galaxies invarious intergalactic environments is presently defined by means ofobservations of 324 isolated galaxies lying in the declination rangeaccessible to the Arecibo 305-m telescope. Both mapping and single pointspectra are used to compute the integral properties of these galaxies.Neutral hydrogen was detected in 288 of the 324 galaxies surveyed, andit is noted that the optical diameter of a spiral disk is bettercorrelated with the hydrogen mass than the morphological type. When usedto define a measure of H I content, the isolated galaxy sample canpredict 'normalcy' with an accuracy that carries a standard error ofabout 0.20 in the log of the H I mass, if a dependence on disk size, aswell as type, is taken into account.
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