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Environmental Effects on Late-Type Galaxies in Nearby Clusters
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.
| A Local Group Polar Ring Galaxy: NGC 6822
Star counts obtained from a 2° × 2° area centered on NGC6822 have revealed an optical image of this galaxy composed of twocomponents: in addition to the well-known H I disk with its youngstellar component, there is a spheroidal stellar structure as extensiveas its H I disk, but with its major axis at roughly right angles to it,that we traced to at least 36'. Radial velocities of over 100intermediate-age carbon stars found within this structure displaykinematics contrasting strongly with those of the H I disk. These Cstars belong to the spheroid. Although devoid of gas, the spheroidrotation is consistent with the I-band Tully-Fisher relation. Theorientation of the rotation axis that minimizes the stellar velocitydispersion coincides with the minor axis of the stellar populationellipsoid, lying very nearly in the plane of the H I disk. We concludethat the H I disk is a polar ring and that the spheroidal component isan erstwhile disk, a fossil remainder of a past close encounter episode.Based on observations obtained with MegaPrime/MegaCam, a joint projectof CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT),which is operated by the National Research Council (NRC) of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique (CNRS) of France, and the University of Hawaii.Based on observations acquired at the du Pont Telescope, from theObservatories of the Carnegie Institution of Washington.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| A Virgo high-resolution Hα kinematical survey. I. NGC 4438
New Hα emission-line observations of the Virgo cluster galaxy NGC4438 are presented. Fabry-Perot interferometry data at an effectiveangular resolution of ~2 arcsec are used to map the kinematics of theionized gas in the galaxy. For the first time we obtain a velocity fieldcovering a large area in NGC 4438, much larger than that deduced fromprevious Hi and CO maps. The kinematics of the extended, low surfacebrightness Hα filaments to the West of the galactic disk isdiscussed. We report on the discovery of a northern Hα structurewhich is clumpier than the other filaments. Evidence for multiplespectral components through the data-cube are presented in a nuclearshell and in the approaching half of the disk. The role of VCC 1040, adwarf elliptical galaxy located to the South of NGC 4438, is presentedto investigate the origin of a small-scale stellar tail of NGC 4438. Itcould be due to a minor tidal interaction between the two galaxies.
| Completing H I observations of galaxies in the Virgo cluster
High sensitivity (rms noise 0.5 mJy) 21-cm H I line observationswere made of 33 galaxies in the Virgo cluster, using the refurbishedArecibo telescope, which resulted in the detection of 12 objects. Thesedata, combined with the measurements available from the literature,provide the first set of H I data that is complete for all 355 late-type(Sa-Im-BCD) galaxies in the Virgo cluster with mp ≤ 18.0mag. The Virgo cluster H I mass function (HIMF) that was derived forthis optically selected galaxy sample is in agreement with the HIMFderived for the Virgo cluster from the blind HIJASS H I survey and isinconsistent with the Field HIMF. This indicates that both in this richcluster and in the general field, neutral hydrogen is primarilyassociated with late-type galaxies, with marginal contributions fromearly-type galaxies and isolated H I clouds. The inconsistency betweenthe cluster and the field HIMF derives primarily from the difference inthe optical luminosity function of late-type galaxies in the twoenvironments, combined with the HI deficiency that is known to occur ingalaxies in rich clusters.Tables \ref{t1, \ref{sample_dat} and Appendix A are only available inelectronic form at http://www.edpsciences.org
| Stellar Populations of Dwarf Elliptical Galaxies: UBVRI Photometry of Dwarf Elliptical Galaxies in the Virgo Cluster
We present UBVRI surface photometry for 16 dwarf elliptical galaxies inthe Virgo Cluster with previously measured kinematic properties. Theglobal optical colors are red, with median values for the sample of0.24+/-0.03 in U-B, 0.77+/-0.02 in B-V, and 1.02+/-0.03 in V-I. Werecover the well-known color-magnitude relation for cluster galaxies butfind no significant difference in dominant stellar population betweenrotating and nonrotating dwarf elliptical galaxies; the average age ofthe dominant stellar population is 5-7 Gyr in all 16 galaxies in thissample. Analysis of optical spectra confirm these age estimates andindicate Fe and Mg abundances in the range of 1/20 to one-third ofsolar, as expected for low-luminosity galaxies. Based on Lick indicesand simple stellar population models, the derived [α/Fe] ratiosare subsolar to solar, indicating a more gradual chemical enrichmenthistory for dE's as compared with giant elliptical galaxies in the VirgoCluster. These observations confirm the marked difference in stellarpopulation and stellar distribution between dwarf and giant ellipticalgalaxies and further substantiate the need for alternative evolutionaryscenarios for the lowest mass cluster galaxies. We argue that it islikely that several different physical mechanisms played a significantrole in the production of the Virgo Cluster dE galaxies including insitu formation, infall of dE's that were once part of Local Groupanalogs, and transformation of dwarf irregular galaxies by the clusterenvironment. The observations support the hypothesis that a largefraction of the Virgo Cluster dE's are formed by ram pressure strippingof gas from infalling dI's.Based on observations with the VATT: the Alice P. Lennon Telescope andthe Thomas J. Bannan Astrophysics Facility.
| Rotationally Supported Virgo Cluster Dwarf Elliptical Galaxies: Stripped Dwarf Irregular Galaxies?
New observations of 16 dwarf elliptical galaxies (dEs) in the VirgoCluster indicate that at least seven dEs have significant velocitygradients along their optical major axis, with typical rotationamplitudes of 20-30 km s-1. Of the remaining nine galaxies inthis sample, six have velocity gradients of less than 20 kms-1 kpc-1, while the other three observations hadtoo low a signal-to-noise ratio to determine an accurate velocitygradient. Typical velocity dispersions for these galaxies are ~44+/-5 kms-1, indicating that rotation can be a significant componentof the stellar dynamics of Virgo dEs. When corrected for the limitedspatial extent of the spectral data, the rotation amplitudes of therotating dEs are comparable to those of similar-brightness dwarfirregular galaxies (dIs). Evidence of a relationship between therotation amplitude and galaxy luminosity is found and, in fact, agreeswell with the Tully-Fisher relation. The similarity in the scalingrelations of dIs and dEs implies that it is unlikely that dEs evolvefrom significantly more luminous galaxies. These observations reaffirmthe possibility that some cluster dEs may be formed when the neutralgaseous medium is stripped from dIs in the cluster environment. Wehypothesize that several different mechanisms are involved in thecreation of the overall population of dEs and that stripping ofinfalling dIs may be the dominant process in the creation of dEs inclusters like Virgo.
| The Current Health of Dwarf Irregular Galaxies in the Virgo Cluster
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.
| Hot Gas in and around Elliptical Galaxies
We review the origin, evolution, and physical nature of hot gas inelliptical galaxies and associated galaxy groups. Unanticipated recentX-ray observations with Chandra and XMM indicate much less cooling thanpreviously expected. Consequently, many long-held assumptions must bereexamined or discarded and new approaches must be explored. Chief amongthese are the role of heating by active galactic nuclei, the influenceof radio lobes on the hot gas, details of the cooling process, possiblerelation between the hot and colder gas in elliptical galaxies, and thecomplexities of stellar enrichment of the hot gas.
| Spectroscopy of Dwarf Galaxies in the Virgo Cluster. I. Data, Chemical Abundances, and Ionization Structure
Long-slit spectroscopy has been obtained for a sample of 22 blue dwarfgalaxies selected in the direction of the Virgo Cluster, as part of alarger sample of Virgo blue dwarf galaxies for which deep Hαimaging has been collected. Most of the galaxies in the present sampleare classified as BCDs or dwarf Irregulars in the Virgo Cluster Catalog.Line fluxes, Hβ equivalent widths, extinction coefficients, spatialemission profiles, ionization structure, and physical conditions arepresented for each galaxy. Chemical abundances have been derived eitherusing a direct determination of the electron temperature or afterdetailed examination of the predictions of different abundancecalibrations. The oxygen abundances derived for the sample of Virgodwarf galaxies span the range 7.6<=12+log(O/H)<=8.9, and thecorresponding nitrogen-to-oxygen abundance ratio ranges from valuestypical of low-metallicity field BCD galaxies to near solar.
| Uncovering Additional Clues to Galaxy Evolution. II. The Environmental Impact of the Virgo Cluster on the Evolution of Dwarf Irregular Galaxies
The impact of the cluster environment on the evolution of dwarf galaxiesis investigated by comparing the properties of a sample of dwarfirregular galaxies (dI's) in the Virgo Cluster with a control sample ofnearby (``field'') dI's having oxygen abundances derived from [O III]λ4363 measurements and measured distances from resolved stellarconstituents. Spectroscopic data are obtained for H II regions in 11Virgo dI's distributed in the central and outer regions of the cluster.To ensure that oxygen abundances are derived in a homogeneous manner,oxygen abundances for field and Virgo dI's are computed using thebright-line method and compared with abundances directly obtained from[O III] λ4363, where available. They are found to agree to withinabout 0.2 dex, with no systematic offset. At a given optical luminosity,there is no systematic difference in oxygen abundance between the sampleof Virgo dI's and the sample of nearby dI's. However, five of the 11Virgo dI's exhibit much lower baryonic gas fractions than field dI's atcomparable oxygen abundances. Using field dI's as a reference, agas-deficiency index for dI's is constructed, making it possiblequantitatively to identify which galaxies have lost gas. For the Virgosample, some of the dwarfs are gas-deficient by a factor of 30. The gasdeficiency correlates roughly with the X-ray surface brightness of theintracluster gas. Ram pressure stripping can best explain the observedgas-poor dI's in the cluster sample. Together with the lack ofsignificant fading and reddening of the gas-poor dI's compared withgas-normal dI's, these observations suggest that the gas-poor dI's inVirgo have recently encountered the intracluster medium for the firsttime. Faded remnants of gas-poor dI's in Virgo will resemble brightdwarf elliptical galaxies currently seen in the cluster core.
| Is HCG 31 undergoing a merger or a fly-by interaction?
We present Fabry-Perot and multi-object spectroscopy of the galaxies inHickson compact group 31 (HCG 31). Based upon our Hα data cubes,galaxies A and C are a single entity, showing no discontinuity in theirkinematics. Kinematically, galaxy E is probably a component of the A+Ccomplex; otherwise it is a recently detached fragment. Galaxy F appears,both kinematically and chemically, to have formed from material tidallyremoved from the A+C complex. Galaxies B and G are kinematicallydistinct from this complex. Galaxy Q also has a radial velocitycompatible with group membership. Galaxies A, B, C, and F have nearlyidentical oxygen abundances, despite spanning a luminosity range of 5mag. Galaxy B's oxygen abundance is normal for its luminosity, whilegalaxy F's abundance is that expected given its origin as a tidalfragment of the A+C complex. The oxygen abundances in galaxies A and Care also understandable if the A+C complex is a late-type spiralsuffering strong gas inflow and star formation as a result of a tidalinteraction. Given the kinematics of both the galaxies and the H I gas,the oxygen abundances, and the position of galaxy G, we propose that aninteraction of galaxy G with the A+C complex, rather than a merger ofgalaxies A and C, is a more complete explanation for the tidal featuresand other properties of HCG 31. In this case, the A+C complex need notbe a merger in progress, though this is not ruled out.
| 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.
| The current health of dwarf irregular galaxies in the Virgo cluster.
Not Available
| Hα surface photometry of galaxies in the Virgo cluster I. Observations with the San Pedro Martir 2.1 m telescope
Hα imaging observations of 125 galaxies obtained with the 2.1 mtelescope of the San Pedro Martir Observatory (SPM) (Baja California,Mexico) are presented. The observed galaxies are mostly Virgo clustermembers (77), with 36 objects in the Coma/A1367 supercluster and 12 inthe clusters A2197 and A2199 taken as fillers. Hα +[NII] fluxesand equivalent widths, as well as images of the detected targets arepresented. The observatory of San Pedro Martir (Mexico) belongs to theObservatorio Astronómico Nacional, UNAM. Figure 4 is onlyavailable in electronic form at http://www.edpsciences.org
| Extragalactic Planetary Nebula Kinematics with the WHT
In spiral galaxies, the observability to large radii of cold gas diskshas facilitated a general understanding of the distribution of mass intheir outer parts. Elliptical galaxies, being gas poor, are not amenableto this approach, and other tracers are necessary. The standard approachis to observe the kinematics of the stars using integrated lightspectroscopy. However, an elliptical's surface brightness drops offrapidly at large radii, making such observations quite difficult in theouter parts where any dark matter would become dominant. One approachespecially suitable for elliptical galaxies is to measure the kinematicsof bright objects within their outer parts: globular clusters (GCs) andplanetary nebulae (PNe). With a suitably large number of measuredvelocities, and with careful dynamical models, these objects can be usedto effectively constrain the mass distribution of ellipticals. A newtechnique called counter-dispersed imaging allows us to detect andmeasure velocity of the Pne, combined into one observational step. Weare members of a team which is building a specialised instrument to usethis technique: the Planetary Nebula Spectrograph, soon to becommissioned at the WHT.
| The Dwarf Irregular Galaxy UGC 7636 Exposed: Stripping at Work in the Virgo Cluster
We present the results of optical spectroscopy of a newly discovered HII region residing in the H I gas cloud located between the dwarfirregular galaxy UGC 7636 and the giant elliptical galaxy NGC 4472 inthe Virgo Cluster. By comparing UGC 7636 with dwarf irregular galaxiesin the field, we show that the H I cloud must have originated from UGC7636 because (1) the oxygen abundance of the cloud agrees with thatexpected for a galaxy with the blue luminosity of UGC 7636 and (2)MHI/LB for UGC 7636 becomes consistent with themeasured oxygen abundance of the cloud if the H I mass of the cloud isadded back into UGC 7636. It is likely that tides from NGC 4472 firstloosened the H I gas, after which ram pressure stripping removed the gasfrom UGC 7636.
| Dust Streamers in the Virgo Galaxy M86 from Ram Pressure Stripping of Its Companion VCC 882
The giant elliptical galaxy M86 in Virgo has a ~28 kpc long dust trailinside its optical halo that points toward the nucleated dwarfelliptical galaxy VCC 882. The trail seems to be stripped material fromthe dwarf. Extinction measurements suggest that the ratio of the totalgas mass in the trail to the blue luminosity of the dwarf is aboutunity, which is comparable to such ratios in dwarf irregular galaxies.The ram pressure experienced by the dwarf galaxy in the hot gaseous haloof M86 was comparable to the internal gravitational binding energydensity of the presumed former gas disk in VCC 882. Published numericalmodels of this case are consistent with the overall trail-likemorphology observed here. Three concentrations in the trail may beevidence for the predicted periodicity of the mass loss. The evaporationtime of the trail is comparable to its age obtained from the relativespeed of the galaxies and the trail length. Thus the trail could becontinuously formed from stripped replenished gas if the VCC 882 orbitis bound. However, the high gas mass and the low expected replenishmentrate suggest that this is only the first stripping event. Implicationsfor the origin of nucleated dwarf ellipticals are briefly discussed.
| Magellanic Cloud Periphery Carbon Stars. IV. The SMC
The kinematics of 150 carbon stars observed at moderate dispersion onthe periphery of the Small Magellanic Cloud are compared with themotions of neutral hydrogen and early-type stars in the intercloudregion. The distribution of radial velocities implies a configuration ofthese stars as a sheet inclined at 73°+/-4° to the plane of thesky. The near side, to the south, is dominated by a stellar component;to the north, the far side contains fewer carbon stars and is dominatedby the neutral gas. The upper velocity envelope of the stars is closelythe same as that of the gas. This configuration is shown to beconsistent with the known extension of the SMC along the line of sightand is attributed to a tidally induced disruption of the SMC thatoriginated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago.The dearth of gas on the near side of the sheet is attributed toablation processes akin to those inferred in 1996 by Weiner &Williams to collisional excitation of the leading edges of MagellanicStream clouds. Comparison with the 1989 kinematic data of Hardy,Suntzeff, & Azzopardi and Maurice, Martin, & Bouchet and the1986 and 1988 data of Mathewson et al. leaves little doubt that forcesother than gravity play a role in the dynamics of the H I.
| HI observations of nearby galaxies . I. The first list of the Karachentsev catalog
We present HI observations of the galaxies in the first list of theKarachentsev catalog of previously unknown nearby dwarf galaxies(Karachentseva & Karachentsev 1998). This survey covers all knownnearby galaxy groups within the Local Volume (i.e. within 10 Mpc) andtheir environment, that is about 25% of the total sky. A total of 257galaxies have been observed with a detection rate of 60%. We searched afrequency band corresponding to heliocentric radial velocities from -470km s-1 to ~ +4000 km s-1. Non-detections areeither due to limited coverage in radial velocity, confusion with LocalHI (mainly in the velocity range -140 km s-1 to +20 kms-1), or lack of sensitivity for very weak emission. 25% ofthe detected galaxies are located within the Local Volume. Thosegalaxies are dwarf galaxies judged by their optical linear diameter (1.4+/- 0.2 kpc on the average), their mean total HI mass (4.6107 Msun), and their observed linewidths (39 kms-1).
| A neutral hydrogen survey of polar ring galaxies. III. Nançay observations and comparison with published data
A total of 50 optically selected polar ring galaxies, polar ring galaxycandidates and related objects were observed in the 21-cm H i line withthe Nançay decimetric radio telescope and 31 were detected. Theobjects, selected by their optical morphology, are all north ofdeclination -39o, and generally relatively nearby (V< 8000km s-1) and/or bright (mB< 15.5). The H i linedata are presented for all 74 galaxies observed for the survey with theEffelsberg, Green Bank or Nanç radio telescopes, as well as allother published H i line parameters of these objects. Three objects wereobserved and detected by us at Parkes. A total of 59 objects weredetected. For each object a brief description is given based on aliterature search.
| Galaxy collisions.
Theories of how galaxies, the fundamental constituents of large-scalestructure, form and evolve have undergone a dramatic paradigm shift inthe last few decades. Earlier views were of rapid, early collapse andformation of basic structures, followed by slow evolution of the stellarpopulations and steady buildup of the chemical elements. Currenttheories emphasize hierarchical buildup via recurrent collisions andmergers, separated by long periods of relaxation and secularrestructuring. Thus, collisions between galaxies are now seen as aprimary process in their evolution. This article begins with a briefhistory; we then tour parts of the vast array of collisional forms thathave been discovered to date. Many examples are provided to illustratehow detailed numerical models and multiwaveband observations haveallowed the general chronological sequence of collisional morphologiesto be deciphered, and how these forms are produced by the processes oftidal kinematics, hypersonic gas dynamics, collective dynamical frictionand violent relaxation. Galaxy collisions may trigger the formation of alarge fraction of all the stars ever formed, and play a key role infueling active galactic nuclei. Current understanding of the processesinvolved is reviewed. The last decade has seen exciting new discoveriesabout how collisions are orchestrated by their environment, howcollisional processes depend on environment, and how these environmentsdepend on redshift or cosmological time.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| Formation of Low-Mass Stars in Elliptical Galaxy Cooling Flows
Thermal X-ray emission from cooling flows in elliptical galaxiesindicates that ~1 Msolar of hot (T~107 K)interstellar gas cools each year, accumulating ~1010Msolar over a Hubble time. Paradoxically, optical and radiofrequency emission from the cooled gas is lacking, indicating that lessthan ~10-3 of the cooled gas remains. Many have speculatedthat the cooled gas has formed into relatively invisible low-mass stars,particularly in the context of massive cooling flows in galaxy clusters.We focus here on cooling flows in elliptical galaxies like NGC 4472where the cooled gas is made visible in emission lines from H II regionsionized and heated (THii~104 K) by stellarultraviolet radiation. The low filling factor of H II gas requires thatthe hot gas cools at ~106 cooling sites within several kpc ofthe galactic center. H II mass slowly increases at each site at~10-6 Msolar yr-1 until a neutral coredevelops. Neutral cores are heated (THii~15 K) and ionized(x~10-6) by thermal X-rays from the entire interstellarcooling flow. We show that the maximum mass of spherical H I cores thatbecome gravitationally unstable is only ~2 Msolar. No starcan exceed this mass, and fragmentation of collapsing cores producesstars of even lower mass. By this means we establish with someconfidence that the hypothesis of low-mass star formation is indeedcorrect-the initial mass function is bottom heavy, but its stars may beoptically luminous. Slightly more massive stars <~4.5Msolar can form near the effective radius (r=8.57 kpc in NGC4472) if sufficient masses of interstellar gas cool there, producing aluminous population of intermediate-mass stars perhaps with radialorbits that may contribute to the stellar Hβ index. The degree ofionization in gravitationally collapsing cores is sufficiently low toallow magnetic fields to disconnect by ambipolar diffusion. Low-massstar formation is very efficient, involving ~106Msolar of galactic cold gas at any time, in agreement withobserved upper limits on cold gas mass. We discuss the cooling regionsurrounding a typical cooling site and show that the total X-rayabsorption in cold and cooling gas is much less than that indicated byrecent X-ray observations. Using a mass dropout scheme consistent withX-ray observations and dynamical mass-to-light ratios, we plot theglobal Hβ surface brightness profile in NGC 4472 and compare itwith the smaller contribution from H II gas recently ejected from redgiant stars. The lifetime of cooled gas at each cooling site,~105 yr, is too short to permit dust formation and perhapsalso gas phase formation of molecules.
| Evolution of Hot Gas and Dark Halos in Group-dominant Elliptical Galaxies: Influence of Cosmic Inflow
Hot interstellar gas in elliptical galaxies has two sources: mass lostfrom evolving stars and a much older component that accompanied galaxyformation or arrived subsequently by secondary cosmic infall toward thegalaxy group containing the elliptical. We present here an approximatebut comprehensive study of the dynamical evolution of the hot gas inmassive elliptical galaxies born into a simple flat universe. Baryonicand dark matter are both conserved. We use NGC 4472 as a prototypicalmassive elliptical having a well-observed hot interstellar medium. Weallow for star formation in a simple single burst using a Salpeterinitial mass function but treat the gasdynamics in detail. The galaxyhas a de Vaucouleurs stellar core and a Navarro-Frenk-White dark halosurrounded by inflowing cosmic matter. Using rather standard assumptionsand parameters, we are able to successfully reproduce the gas-densityand temperature distributions-n(r) and T(r)-in the hot interstellar gasdetermined from recent X-ray observations. Our model is sensitive to thebaryon fraction of the universe, the Type II supernova energy releasedper unit stellar mass, and the time of galaxy formation. However, thereis some degeneracy; as each of these parameters is varied, the effect onmodel fits to n(r) and T(r) is similar. Nevertheless, secondary inflowof cosmic gas is essential for successful fits to n(r) and T(r). Somegas is expelled from the stellar galactic core at early times when theType II supernova energy is released. As a result, the present daybaryonic fraction has a deep minimum in the outer galactic halo.Interstellar gas that cooled since the time of maximum star formationcannot all have collected at the galactic center but must be spatiallydispersed; otherwise both gas temperatures and stellar dispersions inthe galactic center would be larger than those observed. Finally, whenrelatively gas-rich, X-ray luminous models are spatially truncated atearly times, simulating tidal events that may have occurred duringgalaxy group dynamics, the current locus of truncated models lies justalong the L_X-X-ray size correlation among well-observed ellipticals.This is another striking confirmation of our model of ellipticalevolution.
| A Complete Redshift Survey to the Zwicky Catalog Limit in a 2^h X 15 deg Region around 3C 273
We compile 1113 redshifts (648 new measurements, 465 from theliterature) for Zwicky catalog galaxies in the region (-3.5d <= delta<= 8.5d, 11h5 <= alpha <= 13h5). We include redshifts for 114component objects in 78 Zwicky catalog multiplets. The redshift surveyin this region is 99.5% complete to the Zwicky catalog limit, m_Zw =15.7. It is 99.9% complete to m_Zw = 15.5, the CfA Redshift Survey(CfA2) magnitude limit. The survey region is adjacent to the northernportion of CfA2, overlaps the northernmost slice of the Las CampanasRedshift Survey, includes the southern extent of the Virgo Cluster, andis roughly centered on the QSO 3C 273. As in other portions of theZwicky catalog, bright and faint galaxies trace the same large-scalestructure.
| The Intrinsic Shapes of Low Surface Brightness Dwarf Irregular Galaxies and Comparison to Other Types of Dwarf Galaxies
In this paper, we measure the ellipticities of 30 low surface brightness(LSB) dwarf irregular (dI) galaxies and compare the ellipticitydistribution with that of 80 dwarf elliptical (dEs) and 62 blue-compactdwarfs (BCDs). We find that the ellipticity distribution of LSB dIs isvery similar to that of BCDs, and marginally different from that of dEs.We then determine the distribution of intrinsic shapes of dI galaxiesand compare this to the distributions of other types of dwarf galaxiesunder various assumptions. First, we assume that LSB dIs are either alloblate or all prolate, and use a nonparametric analysis to find thebest-fitting distribution of intrinsic shapes. With this assumption, wefind that the scarcity of nearly circular LSB dIs implies, at the 99%confidence level, that they cannot be a population of randomly orientedoblate or prolate objects, implying that LSB dIs are highly unlikely tobe disk-shaped systems. Next, we assume that dIs are triaxial, and use aparametric analysis to find permissible distributions of intrinsicshapes. We find that if the intrinsic axis ratios beta and gamma aredistributed according to a Gaussian with means beta_0 and gamma_0 and acommon standard deviation of sigma, the best-fitting set of parametersfor LSB dIs is (beta_0, gamma_0, sigma) = (0.66, 0.50, 0.15), and thebest fit for BCDs is (beta_0, gamma_0, sigma) = (0.66, 0.55, 0.16),while the best fit for dEs is (beta_0, gamma_0, sigma) = (0.78, 0.69,0.24). The dIs and BCDs thus have very similar shape distributions,given this triaxial hypothesis, while the dEs peak at a somewhat morespherical shape. Therefore, our results provide strong observationalevidence to support the evolutionary scenario in which the three typesof dwarf galaxy have a close relation with each other.
| A list of new nearby dwarf galaxy candidates
To increase completeness of the distance limited sample of nearbygalaxies from the \cite[Kraan-Korteweg & Tammann (1979)]{Kra79}catalogue we undertook a search for small companions of larger knowngalaxies which have corrected radial velocities within 500 km/s. Basedprimarily on the POSS-II and ESO/SERC films we found 260 nearby dwarfgalaxy candidates with angular diameters aga0 .5 arcmin. More than 50%of the objects were revealed for the first time. As we suppose, asignificant part of them (about 30%) may really belong to the LocalVolume sample. Tables 1 and 2 also available in electronic form at CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via\breakftp://cdsweb.u-strasbg.fr/Abstract.html
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Datos observacionales y astrométricos
Catálogos y designaciones:
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INHABITED SKY
