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Direct Measurements of Gas Bulk Flows in the Intracluster Medium of the Centaurus Cluster with the Chandra Satellite
We present the analysis of the velocity structure of the intraclustergas near the core of Abell 3526 obtained with two off-center Chandraobservations, specifically designed to eliminate errors due to spatialvariations of the instrumental gain. We detected a significant velocitygradient along the northeast-southwest direction, roughly perpendicularto the direction of the incoming subgroup Cen 45, in agreement withprevious ASCA SIS measurements. The presence of gas bulk velocities isobserved both with and without the inclusion of the Fe K line complex inthe spectral fittings. The configuration and magnitude of the velocitygradient is consistent with near transonic circulatory motion, eitherbulk or eddylike. The velocity difference obtained using the bestcalibrated central regions of ACIS-S3 is found to be(2.4+/-1.0)×103 km s-1 for rectangularregions 2.4 arcmin×3' roughly diametrically opposedaround the cluster's core. There are also indications of a high-velocityzone toward the southern region with similar magnitudes. The detectionof velocity gradients is significant at >99.4% confidence, andsimulations show that intrachip gain fluctuations >1800 kms-1 are required to explain the velocity gradient by chance.The measurements suggest that >1% of the total merger energy canstill be bulk kinetic 0.4 Gyr after the merging event. This is the firstdirect confirmation of velocity gradients in the intracluster gas withindependent instruments and indicates that strong departure fromhydrostatic equilibrium is possible even for cool clusters that do notshow obvious signs of merging.
| Nearby early-type galaxies with ionized gas. II. Line-strength indices for 18 additional galaxies
We previously presented a data-set of line-strength indices for 50early-type galaxies in the nearby Universe. The galaxy sample is biasedtoward galaxies showing emission lines, located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. The present addendum enlargesthe above data-set of line-strength indices by analyzing 18 additionalearly-type galaxies (three galaxies, NGC 3607, NGC 5077 and NGC 5898were presented in the previous set). We measured 25 line-strengthindices, defined by the Lick IDS "standard" system (Trager et al. 1998,ApJS, 116, 1; Worthey & Ottaviani 1997, ApJS, 111, 377), for 7luminosity weighted apertures and 4 gradients of each galaxy. Thisaddendum presents the line-strength data-set and compares it with theavailable data in the literature.
| 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.
| A deep Chandra observation of the Centaurus cluster: bubbles, filaments and edges
X-ray images and gas temperatures taken from a deep ~200-ks Chandraobservation of the Centaurus cluster are presented. Multiple innerbubbles and outer semicircular edges are revealed, together with wispyfilaments of soft X-ray emitting gas. The frothy central structure andeastern edge are likely due to the central radio source blowing bubblesin the intracluster gas. The semicircular edges to the surfacebrightness maps 32 kpc to the east and 17.5 kpc to the west are markedby sharp temperature increases and abundance drops. The edges could bedue to sloshing motions of the central potential, or are possiblyenhanced by earlier radio activity. The high abundance of the innermostgas (about 2.5 times solar) limits the amount of diffusion and mixingtaking place.
| 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.
| The distance to Hydra and Centaurus from surface brightness fluctuations: Consequences for the Great Attractor model
We present I-band Surface Brightness Fluctuation (SBF) measurements for16 early-type galaxies (3 giants, 13 dwarfs) in the central region ofthe Hydra cluster, based on deep photometric data in 7 fields obtainedwith VLT FORS1. From the SBF-distances to the galaxies in our sample weestimate the distance of the Hydra cluster to be 41.2 ± 1.4 Mpc((m-M)=33.07 ± 0.07 mag). Based on an improved correction forfluctuations from undetected point sources, we revise the SBF-distanceto the Centaurus cluster from Mieske & Hilker (2003, A&A, 410,455) upwards by 10% to 45.3 ± 2.0 Mpc ((m-M)=33.28 ± 0.09mag). The relative distance modulus of the two clusters then is(m-M)_Cen-(m-M)_Hyd=0.21 ± 0.11 mag. With H_0= 72 ± 4 kms-1 Mpc-1, we estimate a positive peculiarvelocity of 1225 ± 235 km s-1 for Hydra and 210± 295 km s-1 for the Cen30 component of Centaurus.Allowing for a thermal velocity dispersion of 200 km s-1,this rules out a common peculiar flow velocity for both clusters at 98%confidence. We find that the 9× 1015 Mȯ“Great Attractor” at a distance of ≃45 Mpc can explainthe observed peculiar velocities if shifted about 15° towards theHydra cluster position. Our results are inconsistent at 94% confidencewith a scenario where the Centaurus cluster is identical to the GA. Inorder to better restrict partially degenerate Great Attractor parameterslike its mass and distance, a recalculation of the local flow model withupdated distance information over a larger area than covered by us wouldbe needed.
| A large H I cloud near the centre of the Virgo cluster
We report the discovery of a large H I cloud in the central regions ofthe Virgo cluster. It is 110× 25 kpc in size and contains3.4× 108 Mȯ of H I. The morphology andkinematics of this cloud strongly suggest that it consists of H Iremoved from the galaxy NGC 4388 by ram-pressure stripping. It is morelikely the result of an interaction of the ISM of NGC 4388 with the hothalo of the M 86 group and not with the ICM centred on M 87. The largeextent of the plume suggests that gas stripped from cluster galaxies canremain neutral for at least 108 yr. Locally, the columndensity is well above 1020 cm-2, suggesting thatthe intra-cluster H II regions known to exist in Virgo may have formedfrom gas stripped from cluster galaxies. The existence of the H I plumesuggests that stripping of infalling spirals contributes to theenrichment of the ICM. The H I object in the Virgo cluster recentlyreported by Minchin et al. (2005, ApJ, 622, L21) may have a similarorigin and may therefore not be a "dark galaxy".
| The Ghosts of Galaxies: Tidal Debris in Clusters
Gravitational interactions in rich clusters can strip material from theouter parts of galaxies or even completely disrupt entire systems,giving rise to large scale, low surface brightness ghostly featuresstretching across intergalactic space. The nearby Coma and Centaurusclusters both have striking examples of galaxy ghosts, in the form of100 kpc-long plumes of intergalactic debris. By searching HST archivalimages, we have found numerous other examples of galaxy ghosts in richclusters at low redshift, evidence that galaxy destruction and recyclingare ubiquitous, important in cluster formation and evolution, andcontinue to mold clusters at the present epoch. Many ghosts appear inX-ray bright clusters, perhaps signaling a connection with energeticsubcluster mergers. The fate of such material has importantramifications for cluster evolution. Our new HST WFPC2 V & I imagesof a portion of the Centaurus plume reveal that it contains an excess ofdiscrete objects with -12 < MV < -6, consistent withbeing globular clusters or smaller dwarf galaxies. This tidallyliberated material is being recycled directly into the intraclusterpopulation of stars, dwarf galaxies, globular clusters, and gas, whichmay have been built largely from a multitude of similar events over thelife of the cluster.
| Dynamical Evolution of Galaxies in Clusters
Tidal forces acting on galaxies in clusters lead to a strong dynamicalevolution. In order to quantify the amount of evolution, I runself-consistent N-body simulations of disk galaxies for a variety ofmodels in the hierarchically forming clusters. The tidal field along thegalactic orbits is extracted from the simulations of cluster formationin the Ω0=1, 0.4, and 0.4,ΩΛ=0.6 cosmological scenarios. For large spiralgalaxies with a rotation speed of 250 km s-1, tidalinteractions truncate massive dark matter halos at 30+/-6 kpc andthicken stellar disks by a factor of 2-3, increasing Toomre's parameterto Q>~2 and halting star formation. Low-density galaxies, such as thedwarf spheroidals with a circular velocity of 20 km s-1 andthe extended low surface brightness galaxies with a scale length of10-15 kpc, are completely disrupted by tidal shocks. Their debriscontribute to the diffuse intracluster light. The tidal effects aresignificant not only in the core but throughout the cluster and can beparametrized by the critical tidal density. The tidally inducedevolution results in the transformation of the infalling spirals into S0galaxies and in the depletion of the low surface brightness galaxypopulation. In the low-Ω0 cosmological models, clustersform earlier and produce stronger evolution of galaxies.
| Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations
To empirically calibrate the IR surface brightness fluctuation (SBF)distance scale and probe the properties of unresolved stellarpopulations, we measured fluctuations in 65 galaxies using NICMOS on theHubble Space Telescope. The early-type galaxies in this sample includeelliptical and S0 galaxies and spiral bulges in a variety ofenvironments. Absolute fluctuation magnitudes in the F160W (1.6 μm)filter (MF160W) were derived for each galaxy using previouslymeasured I-band SBF and Cepheid variable star distances. F160W SBFs canbe used to measure distances to early-type galaxies with a relativeaccuracy of ~10%, provided that the galaxy color is known to ~0.035 magor better. Near-IR fluctuations can also reveal the properties of themost luminous stellar populations in galaxies. Comparison of F160Wfluctuation magnitudes and optical colors to stellar population modelpredictions suggests that bluer elliptical and S0 galaxies havesignificantly younger populations than redder ones and may also be moremetal-rich. There are no galaxies in this sample with fluctuationmagnitudes consistent with old, metal-poor (t>5 Gyr, [Fe/H]<-0.7)stellar population models. Composite stellar population models implythat bright fluctuations in the bluer galaxies may be the result of anepisode of recent star formation in a fraction of the total mass of agalaxy. Age estimates from the F160W fluctuation magnitudes areconsistent with those measured using the Hβ Balmer-line index. Thetwo types of measurements make use of completely different techniquesand are sensitive to stars in different evolutionary phases. Bothtechniques reveal the presence of intermediate-age stars in theearly-type galaxies of this sample.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 NAS 5-26555.
| The Ringed Spiral Galaxy NGC 4622. I. Photometry, Kinematics, and the Case for Two Strong Leading Outer Spiral Arms
The intriguing nearly face-on southern ringed spiral galaxy NGC 4622,the first galaxy definitively shown to have leading spiral structure, isrevisited in this paper with new images from the Hubble SpaceTelescope's (HST) WFPC2, together with ground-based optical and near-IRimaging, and a Fabry-Perot Hα velocity field. The data provide newinformation on the disk/bulge/halo mix, rotation curve, star formationin the galaxy, and the sense of winding of its prominent spiral arms.Previously, we suggested that the weaker, inner single arm most likelyhas the leading sense, based on a numerical simulation. Now, takingadvantage of HST resolution and using de Vaucouleurs' standardextinction and reddening technique to determine the near side of thegalaxy's slightly tilted disk, we come to the more surprising conclusionthat the two strong outer arms have the leading sense. We suggest thatthis highly unusual configuration may be the result of a past minormerger or mild tidal encounter. Possible evidence for a minor merger isfound in a short, central dust lane, although this is purelycircumstantial and an unrelated interaction with a different companioncould also be relevant. The leading arms may be allowed to persistbecause NGC 4622 is dark halo dominated (i.e., not ``maximum disk'' inthe inner regions) and displays a significantly rising rotation curve.The new HST observations also reveal a rich globular cluster system inthe galaxy. The mean color of these clusters is (V-I)0=1.04,and the specific frequency is 3.4+/-0.6. The luminosity function ofthese clusters confirms the membership of NGC 4622 in the CentaurusCluster.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., undercontract NAS 5-26555.
| Distance to the Centaurus cluster and its subcomponents from surface brightness fluctuations
We present I-band Surface Brightness Fluctuations (SBF) measurements for15 early type galaxies (3 giants, 12 dwarfs) in the central region ofthe Centaurus cluster, based on deep photometric data in 7 fieldsobtained with VLT FORS1 and with very good seeing. From theSBF-distances to our sample galaxies we determine the distance of theCentaurus cluster to be 41.3 +/- 2.1 Mpc (33.08 +/- 0.11 mag). Thisplaces the Centaurus cluster at about the same distance as the ``GreatAttractor''. We find a distance difference of 0.27 +/- 0.34 mag betweenthe two subcomponents Cen 30 and Cen 45, ruling out that both componentsare separated by their Hubble flow distance. A distance difference of0.48 +/- 0.21 mag is found between the central galaxies NGC 4696 (Cen30) and NGC 4709 (Cen 45) of both components, supported by the differentturn-over magnitudes of their respective globular cluster systems. Thissuggests that Cen 45 is falling into but has not yet reached Cen 30,supporting the idea of a large scale filament along the line of sighttowards Centaurus (Churazov et al. \cite{Churaz99}). H_0= 83.0 +/- 8.3km s-1 Mpc-1 is obtained for our Cen 30 sampletaking into account the peculiar motion of the Local Group into thedirection of the Centaurus cluster. This value of H_0 corresponds to amuch smaller Hubble flow distortion in the direction of Centaurus thandetermined by Tonry et al. (\cite{Tonry00}), implying that the GA massestimate by Tonry et al. may be too high and/or that the Centauruscluster falls into the GA almost perpendicularly to the line of sight.As our mean single measurement error is very close to the measureddistance scatter of the investigated galaxies, we can only derive anupper limit of +/-10 Mpc radial extension for the Centaurus cluster,corresponding to a five times larger radial than tangential extension.No evidence for an infall pattern into the Great Attractor is foundwithin the uncertainties for the 11 galaxies with measured redshifts.Based on observations obtained at the European Southern Observatory,Chile (Observing Programme 67.A-0358).
| Spatially resolved X-ray spectroscopy of the core of the Centaurus cluster
We present Chandra data from a 31.7-ks observation of the Centauruscluster, using the ACIS-S detector. Images of the X-ray emission show aplume-like feature at the centre of the cluster, of extent 60arcsec(20kpc in projection). The feature has the same metallicity as gas at asimilar radius, but is cooler. Using adaptive binning, we generatetemperature, abundance and absorption maps of the cluster core. Theradial abundance profile shows that the previously known, steepabundance gradient peaks with a metallicity of 1.3-1.8Zsolarat a radius of about 45arcsec (15kpc), before falling back to0.4Zsolar at the centre of the cluster. A radial temperatureprofile shows that the temperature decreases inwards. We determine thespatial distributions of each of two temperature components, whereapplicable. The radiative cooling time of the cooler component withinthe inner 10arcsec (3kpc) is less than 2×107yr. X-rayholes in the image coincident with the radio lobes are seen, as well astwo outer sharp temperature drops, or cold fronts. The origin of theplume is unclear. The existence of the strong abundance gradient is astrong constraint on extensive convection or gas motion driven by acentral radio source.
| Prospects for Detection of Intracluster Gas Bulk Velocities through the Sunyaev-Zeldovich Effect
Intracluster gas velocity gradients have been recently detected by theauthors in the Centaurus Cluster (A3526) using the Doppler shift ofX-ray spectral lines with ASCA Solid-State Imaging Spectrometers. Thevelocity gradient was found to be maximum along a line roughlyperpendicular to the direction of the incoming subgroup Cen 45 and has acorrespondent velocity difference of ~(3.4+/-1.1)×103km s-1 within a ~10' diameter region centered on the cDgalaxy NGC 4696. Such bulk velocities should Comptonize the cosmicmicrowave background radiation (CMBR) producing variations of intensityand temperature that can be detectable in the near future withbolometers such as BOLOCAM. In this paper we realistically estimate theexpected CMBR Comptonization for the central region of A3526, using ASCAand ROSAT data to constrain the Sunyaev-Zeldovich parameterexpectations.
| Redshift-Distance Survey of Early-Type Galaxies. I. The ENEARc Cluster Sample
This paper presents data on the ENEARc subsample of the larger ENEARsurvey of nearby early-type galaxies. The ENEARc galaxies belong toclusters and were specifically chosen to be used for the construction ofa Dn-σ template. The ENEARc sample includes newmeasurements of spectroscopic and photometric parameters (redshift,velocity dispersion, line index Mg2, and the angular diameterdn), as well as data from the literature. New spectroscopicdata are given for 229 cluster early-type galaxies, and new photometryis presented for 348 objects. Repeat and overlap observations withexternal data sets are used to construct a final merged catalogconsisting of 640 early-type galaxies in 28 clusters. Objectivecriteria, based on catalogs of groups of galaxies derived from completeredshift surveys of the nearby universe, are used to assign galaxies toclusters. In a companion paper, these data are used to construct thetemplate Dn-σ distance relation for early-typegalaxies, which has been used to estimate galaxy distances and derivepeculiar velocities for the ENEAR all-sky sample. Based on observationsat Complejo Astronomico El Leoncito, operated under agreement betweenthe Consejo Nacional de Investigaciones Científicas de laRepública Argentina and the National Universities of La Plata,Córdoba, and San Juan; Cerro Tololo Inter-American Observatory,National Optical Astronomical Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation; the EuropeanSouthern Observatory (ESO), partially under the ESO-ON agreement; theFred Lawrence Whipple Observatory; the Observatório do Pico dosDias, operated by the Laboratório Nacional de Astrofísicaand the MDM Observatory at Kitt Peak.
| A catalogue and analysis of X-ray luminosities of early-type galaxies
We present a catalogue of X-ray luminosities for 401 early-typegalaxies, of which 136 are based on newly analysed ROSAT PSPC pointedobservations. The remaining luminosities are taken from the literatureand converted to a common energy band, spectral model and distancescale. Using this sample we fit the LX:LB relationfor early-type galaxies and find a best-fit slope for the catalogue of~2.2. We demonstrate the influence of group-dominant galaxies on the fitand present evidence that the relation is not well modelled by a singlepower-law fit. We also derive estimates of the contribution to galaxyX-ray luminosities from discrete-sources and conclude that they provideLdscr/LB~=29.5ergs-1LBsolar-1. Wecompare this result with luminosities from our catalogue. Lastly, weexamine the influence of environment on galaxy X-ray luminosity and onthe form of the LX:LB relation. We conclude thatalthough environment undoubtedly affects the X-ray properties ofindividual galaxies, particularly those in the centres of groups andclusters, it does not change the nature of whole populations.
| A synthesis of data from fundamental plane and surface brightness fluctuation surveys
We perform a series of comparisons between distance-independentphotometric and spectroscopic properties used in the surface brightnessfluctuation (SBF) and fundamental plane (FP) methods of early-typegalaxy distance estimation. The data are taken from two recent surveys:the SBF Survey of Galaxy Distances and the Streaming Motions of AbellClusters (SMAC) FP survey. We derive a relation between(V-I)0 colour and Mg2 index using nearly 200galaxies and discuss implications for Galactic extinction estimates andearly-type galaxy stellar populations. We find that the reddenings fromSchlegel et al. for galaxies with E(B-V)>~0.2mag appear to beoverestimated by 5-10 per cent, but we do not find significant evidencefor large-scale dipole errors in the extinction map. In comparison withstellar population models having solar elemental abundance ratios, thegalaxies in our sample are generally too blue at a given Mg2;we ascribe this to the well-known enhancement of the α-elements inluminous early-type galaxies. We confirm a tight relation betweenstellar velocity dispersion σ and the SBF `fluctuation count'parameter N, which is a luminosity-weighted measure of the total numberof stars in a galaxy. The correlation between N and σ is eventighter than that between Mg2 and σ. Finally, we deriveFP photometric parameters for 280 galaxies from the SBF survey data set.Comparisons with external sources allow us to estimate the errors onthese parameters and derive the correction necessary to bring them on tothe SMAC system. The data are used in a forthcoming paper, whichcompares the distances derived from the FP and SBF methods.
| Detection of Bulk Motions in the Intracluster Medium of the Centaurus Cluster
Several recent numerical simulations of off-center cluster mergerspredict that significant angular momentum with associated velocities ofa few ×103 km s-1 can be imparted to theresulting cluster. Such gas bulk velocities can be detected by theDoppler shift of X-ray spectral lines with ASCA spectrometers. Using twoASCA observations of the Centaurus cluster (Abell 3526), we produced avelocity map for the gas in the cluster's central regions. We alsodetected radial and azimuthal gradients in temperature and metalabundance distributions, which seem to be associated with the infallingsubgroup centered at NGC 4709 (Cen 45). More importantly, we found asignificant (>99.8% confidence level) velocity gradient along a linenear-perpendicular to the direction of the incoming subgroup and with amaximum velocity difference of ~3.4+/-1.1×103 kms-1. It is unlikely (P<0.002) that the observed velocitygradient is generated by gain fluctuations across the detectors. Whilethe observed azimuthal temperature and abundance variations can beattributed to the interaction with Cen 45, we argue that theintracluster gas velocity gradient is more likely due to a previousoff-center merging event in the main body of the Centaurus cluster.
| The Infrared Surface Brightness Fluctuation Hubble Constant
We measured infrared surface brightness fluctuation (SBF) distances toan isotropically distributed sample of 16 distant galaxies withredshifts reaching 10,000 km s-1 using the near-IR camera andmultiobject spectrometer (NICMOS) on the Hubble Space Telescope (HST).The excellent spatial resolution, very low background, and brightness ofthe IR fluctuations yielded the most distant SBF measurements to date.Twelve nearby galaxies were also observed and used to calibrate theF160W (1.6 μm) SBF distance scale. Of these, three have Cepheidvariable star distances measured with HST and eleven have optical I-bandSBF distance measurements. A distance modulus of 18.5 mag to the LargeMagellanic Cloud was adopted for this calibration. We present the F160WSBF Hubble diagram and find a Hubble constant H0=76+/-1.3 (1σ statistical) +/-6 (systematic) km s-1Mpc-1. This result is insensitive to the velocity model usedto correct for local bulk motions. Restricting the fit to the six mostdistant galaxies yields the smallest value of H0=72+/-2.3 kms-1 Mpc-1 that is consistent with the data. This6% decrease in the Hubble constant is consistent with the hypothesisthat the Local Group inhabits an underdense region of the universe, butis also consistent with the best-fit value of H0=76 kms-1 Mpc-1 at the 1.5 σ level. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by AURA, Inc.,under NASA contract NAS 5-26555.
| The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances
We report data for I-band surface brightness fluctuation (SBF)magnitudes, (V-I) colors, and distance moduli for 300 galaxies. Thesurvey contains E, S0, and early-type spiral galaxies in the proportionsof 49:42:9 and is essentially complete for E galaxies to Hubblevelocities of 2000 km s-1, with a substantial sampling of Egalaxies out to 4000 km s-1. The median error in distancemodulus is 0.22 mag. We also present two new results from the survey.(1) We compare the mean peculiar flow velocity (bulk flow) implied byour distances with predictions of typical cold dark matter transferfunctions as a function of scale, and we find very good agreement withcold, dark matter cosmologies if the transfer function scale parameterΓ and the power spectrum normalization σ8 arerelated by σ8Γ-0.5~2+/-0.5. Deriveddirectly from velocities, this result is independent of the distributionof galaxies or models for biasing. This modest bulk flow contradictsreports of large-scale, large-amplitude flows in the ~200 Mpc diametervolume surrounding our survey volume. (2) We present adistance-independent measure of absolute galaxy luminosity, N and showhow it correlates with galaxy properties such as color and velocitydispersion, demonstrating its utility for measuring galaxy distancesthrough large and unknown extinction. Observations in part from theMichigan-Dartmouth-MIT (MDM) Observatory.
| Galaxy destruction and diffuse light in clusters
Deep images of the Centaurus and Coma clusters reveal two spectaculararcs of diffuse light that stretch for over 100kpc, yet are just a fewkiloparsecs wide. At a surface brightness ofmb~27-28magarcsec-2, the Centaurus arc is the moststriking example known of structure in the diffuse light component of arich galaxy cluster. We use numerical simulations to show that theCentaurus feature can be reproduced by the tidal debris of a spiralgalaxy that has been tidally disrupted by the gravitational potential ofNGC 4709. The surface brightness and narrow dimensions of the diffuselight suggest that the disc was corotating with its orbital path pastpericentre. Features this prominent in clusters will be relatively rare,although at fainter surface brightness levels the diffuse light willreveal a wealth of structure. Deeper imaging surveys may be able totrace this feature for several times its presently observed extent, andsomewhere along the tidal debris, a fraction of the original stellarcomponent of the disc will remain bound, but transformed into a faintspheroidal galaxy. It should be possible to confirm the galactic originof the Centaurus arc by observing planetary nebulae along its lengthwith redshifts close to that of NGC 4709.
| Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| The Surface Brightness Fluctuation Survey of Galaxy Distances. II. Local and Large-Scale Flows
We present results from the Surface Brightness Fluctuation (SBF) Surveyfor the distances to 300 early-type galaxies, of which approximatelyhalf are ellipticals. A modest change in the zero point of the SBFrelation, derived by using Cepheid distances to spirals with SBFmeasurements, yields a Hubble constant H0=77+/-4+/-7 kms-1 Mpc-1, somewhat larger than the HST KeyProject result. We discuss how this difference arises from a differentchoice of zero point, a larger sample of galaxies, and a different modelfor large-scale flows. Our result is 4% larger than found in a recentcomparison of the SBF Survey peculiar velocities with predictionsderived from the galaxy density field measured by redshift surveys(Blakeslee et al. 1999b). The zero point of the SBF relation is thelargest source of uncertainty, and our value for H0 issubject to all the systematic uncertainties of the Key Project zeropoint, including a 5% decrease if a metallicity correction for theCepheids is adopted. To analyze local and large-scale flows-departuresfrom smooth Hubble flow-we use a parametric model for the distributionfunction of mean velocity and velocity dispersion at each point inspace. These models include a uniform thermal velocity dispersion andspherical attractors whose position, amplitude, and radial shape arefree to vary. Our modeling procedure performs a maximum likelihood fitof the model to the observations. Our models rule out a uniform Hubbleflow as an acceptable fit to the data. Inclusion of two attractors, oneof which having a best-fit location coincident with the Virgo clusterand the other having a fit location slightly beyond the Centaurusclusters (which we refer to by convention as the Great Attractor),reduces χ2/N from 2.1 to 1.1. The fits to theseattractors both have radial profiles such that v~r-1 (i.e.,isothermal) over a range of overdensity between about 10 and 1, but falloff more steeply at larger radius. The best-fit value for thesmall-scale, cosmic thermal velocity is 180+/-14 km s-1. Thequality of the fit can be further improved by the addition of aquadrupole correction to the Hubble flow. The dipole velocity offsetfrom the CMB frame for the volume we survey (amplitude ~150 kms-1) and the quadrupole may be genuine (though weak)manifestations of more distant density fluctuations, but we findevidence that they are more likely due to the inadequacy of sphericalmodels to describe the density profile of the attractors. The residualdipole we find is comparable to the systematic error in these simple,parametrized models; in other words, our survey volume of R<3000 kms-1 is, in a mass averaged sense, essentially at rest withrespect to the CMB. This contradicts claims of large amplitude flows inmuch larger volumes that include our sample. Our best-fitting model,which uses attenuated power-law mass distributions for the twoattractors, has enclosed mass overdensities at the Local Group of7x1014 Msolar for the Virgo Attractor and9x1015 Msolar for the Great Attractor. Withoutrecourse to information about the overdensities of these attractors withrespect to the cosmic mean we cannot provide a good constraint onΩM, but our data do give us accurate measurements interms of δ, the overdensities of the enclosed masses with respectto the background: δ Ω2/3M=0.33 forthe Virgo Attractor and δ Ω2/3M=0.27for the Great Attractor. Observations in part from theMichigan-Dartmouth-MIT (MDM) Observatory.
| A Test for Large-Scale Systematic Errors in Maps of Galactic Reddening
Accurate maps of Galactic reddening are important for a number ofapplications, such as mapping the peculiar velocity field in the nearbyuniverse. Of particular concern are systematic errors which vary slowlyas a function of position on the sky, as these would induce spuriousbulk flow. We have compared the reddenings of Burstein & Heiles (BH)and those of Schlegel, Finkbeiner, & Davis (SFD) to independentestimates of the reddening, for Galactic latitudes |b|>10^deg. Ourprimary source of Galactic reddening estimates comes from comparing thedifference between the observed B-V colors of early-type galaxies, andthe predicted B-V color determined from the B-V-Mg_2 relation. We havefitted a dipole to the residuals in order to look for large-scalesystematic deviations. There is marginal evidence for a dipolar residualin the comparison between the SFD maps and the observed early-typegalaxy reddenings. If this is due to an error in the SFD maps, then itcan be corrected with a small (13%) multiplicative dipole term. Weargue, however, that this difference is more likely to be due to a small(0.01 mag) systematic error in the measured B-V colors of the early-typegalaxies. This interpretation is supported by a smaller, independentdata set (globular cluster and RR Lyrae stars), which yields a resultinconsistent with the early-type galaxy residual dipole. BH reddeningsare found to have no significant systematic residuals, apart from theknown problem in the region 230^deg
| Evidence for Merging in the Centaurus Cluster
We present a two-dimensional map of the gas temperature distribution inthe Centaurus cluster, based on Advanced Satellite for Cosmology andAstrophysics (ASCA) observations derived using a novel approach toaccount for the energy-dependent point-spread function. Along with acool region centered on NGC 4696, asymmetric temperature variations ofmoderate amplitude are detected. The hottest region roughly coincideswith the position of the second-brightest galaxy, NGC 4709, known to bethe dominant galaxy of one of the subgroups in the Centaurus cluster.ROSAT images show faint surface brightness emission also centered onthis galaxy. The imaging and spectral results suggest that a subclustercentered on NGC 4709 is merging with the main cluster centered on NGC4696, in agreement with earlier suggestion that these two systems arelocated at the same distance, despite their different line-of-sightvelocities.
| A catalogue of Mg_2 indices of galaxies and globular clusters
We present a catalogue of published absorption-line Mg_2 indices ofgalaxies and globular clusters. The catalogue is maintained up-to-datein the HYPERCAT database. The measurements are listed together with thereferences to the articles where the data were published. A codeddescription of the observations is provided. The catalogue gathers 3541measurements for 1491 objects (galaxies or globular clusters) from 55datasets. Compiled raw data for 1060 galaxies are zero-point correctedand transformed to a homogeneous system. Tables 1, 3, and 4 areavailable in electronic form only at the CDS, Strasbourg, via anonymousftp 130.79.128.5. Table 2 is available both in text and electronic form.
| 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.
| An HI survey for protogalaxies in the Centaurus and Fornax galaxy clusters
The results of 21-cm neutral hydrogen survey observations, made usingthe 64-m Parkes telescope, are presented for two 8 deg by 8 deg fields,centred on the Centaurus and Fornax galaxy clusters, and a smaller 1deg.5 field in Eridanus. The purpose of the observations was to searchfor extended Hi clouds with no clear optical counterparts. 31 previouslycatalogued galaxies were detected, with Hi parameters for 16 beinglisted for the first time. One previously uncatalogued dwarf galaxy(`Wombat I', J0341-3851) was found near the Fornax cluster. AustraliaTelescope Compact Array observations give an Hi mass of 8x10^7 Msolarand a diameter of 4kpc for this object, which is also visible on UKSTsurvey plates. However, no clouds with optically invisible counterpartswere detected. We deduce a 99 per cent confidence limit on the total Hidensity of such objects in the cluster and near-cluster environment ofOmega_HI<10^-2h^-1(deltaV/100kms^-1.
| Studies of the Centaurus cluster. I. A catalogue of galaxies in the central region of the Centaurus cluster
The central region of the Centaurus cluster was surveyed on a film copyof a high-resolution photographic plate taken with the 100-inch du PontLas Campanas telescope. From visual inspection of the film, a largenumber of cluster galaxy candidates were identified down to a limitingmagnitude of 21.5 in B. Between cluster members and background objectswere distinguished by applying morphological criteria established inprevious studies of the Virgo and Fornax clusters. Following themorphological classification of each member, the images of all 296cluster galaxies on a SRC deep-blue sky survey plate were digitized,processed and analysed with programs written in the image processingpackage MIDAS. Aperture photometry yielded growth curves and totalinstrumental magnitudes of the galaxies. The magnitudes were calibratedwith independent photometric data and subsequently transformed intostandard B-magnitudes. We further established a set of structureparameters for each cluster galaxy: effective surface brightness,effective radius, central exponential surface brightness, and scalelength. For the ellipticals we additionally determined the shapeparameters of the light profiles. The catalogue (CCC) is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Velocity structure of the dwarf galaxy population in the Centaurus cluster
Based on the photometric survey of the inner region of the Centauruscluster (Jerjen & Dressler 1997a) we measured redshifts for a deep,surface brightness limited sample of galaxies using the MEFOS multifibrespectrograph at the ESO 3.6m telescope. With the new data set radialvelocities for 120 centrally located cluster members become availablewhich is equivalent to 78% of all known cluster galaxies in the regionbrighter than B_T=18.5. The relevant aspect of this investigation isthat new redshifts for 32 dwarf galaxies have been measured, rising thetotal number to 48. We investigate the prominent bimodal velocitydistribution of Centaurus in more detail, discussing the very differentcharacteristics of the velocity distributions for the main Hubble typesE&S0, spirals, Im&BCD, and dE&dS0. The nucleated, brightdwarf ellipticals are the only galaxies with a Gaussian-likedistribution centred at 3148 +/- 98 kms(-1) . The remarkable coincidenceof this velocity with the mean velocity of Cen30 and the redshift ofNGC4696 in particular strongly suggests a connection of the dE&dS0sto the gravitational centre of the Centaurus cluster and/or to thecluster dominant E galaxy. The application of statistical tests revealsthe existence of a population dwarf galaxies bound to NGC4696. Thedynamical parameters for the two velocity components suggest that Cen30is the real Centaurus cluster whereas Cen45 can only be a loosely boundgroup of galaxies. This conclusion is followed up with a type-mixtureanalysis. All results are fully consistent with the cluster-groupscenario. Whether Cen45 is merging with the cluster or is located in theclose background remains unclear. We show that the poorness of Cen45represents an intrinsic problem which makes it difficult to approachthis question. Based on observations made at the European SouthernObservatory, La Silla, Chile.
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