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The dwarf low surface brightness galaxy population of the Virgo Cluster - II. Colours and HI line observations
In order to investigate the nature of dwarf low surface brightness (LSB)galaxies we have undertaken a deep B- and I-band CCD survey of a14-deg2 strip in the Virgo Cluster and applied a Fourierconvolution technique to explore its dwarf galaxy population down to acentral surface brightness of ~26 B magarcsec-2 and a totalabsolute B mag of ~-10. In this paper we carry out an analysis of theirmorphology, (B-I) colours and atomic hydrogen content. We compare theseproperties with those of dwarf galaxies in other environments to try andassess how the cluster environment has influenced their evolution. Fielddwarfs are generally of a more irregular morphology, are bluer andcontain relatively more gas. We assess the importance that variousphysical processes have on the evolution of cluster dwarf galaxies(ram-pressure stripping, tidal interactions, supernova-driven gas loss).We suggest that enhanced star formation triggered by tidal interactionsis the major reason for the very different general properties of clusterdwarfs: they have undergone accelerated evolution.
| 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.
| Molecular Gas in Tidal Dwarf Galaxies: On-going Galaxy Formation
We investigate the process of galaxy formation as can be observed in theonly currently forming galaxies - the so-called Tidal Dwarf Galaxies,hereafter TDGs - through observations of the molecular gas detected viaits CO (Carbon Monoxide) emission. Molecular gas is a key element in thegalaxy formation process, providing the link between a cloud of gas anda bona fide galaxy. We have now detected CO in 9 TDGs with an overalldetection rate of 80%, showing that molecular gas is abundant in TDGs,up to a few 108 Mȯ. The CO emissioncoincides both spatially and kinematically with the HI emission,indicating that the molecular gas forms from the atomic hydrogen wherethe HI column density is high. A possible trend of more evolved TDGshaving greater molecular gas masses is observed, in accord with thetransformation of HI into H2. Although uncertainties arestill large for individual objects as the geometry is unknown, we findthat the ``dynamical" masses of TDGs, estimated from the CO line widths,do not seem to be greater than the ``visible" masses (HI + H2+ a stellar component), i.e., TDGs require no dark matter. We provideevidence that TDGs are self-gravitating entities, implying that we arewitnessing the ensemble of processes in galaxy formation: concentrationof large amounts of gas in a bound object, condensation of the gas,which is atomic at this point, to form molecular gas and the subsequentstar formation from the dense molecular component.
| Hα Morphologies and Environmental Effects in Virgo Cluster Spiral Galaxies
We describe the various Hα morphologies of Virgo Cluster andisolated spiral galaxies and associate the Hα morphologies withthe types of environmental interactions that have altered the clustergalaxies. The spatial distributions of Hα and R-band emission areused to divide the star formation morphologies of the 52 Virgo Clusterspiral galaxies into several categories: normal (37%), anemic (6%),enhanced (6%), and (spatially) truncated (52%). Truncated galaxies arefurther subdivided on the basis of their inner star formation rates intotruncated/normal (37%), truncated/compact (6%), truncated/anemic (8%),and truncated/enhanced (2%). The fraction of anemic galaxies isrelatively small (6%-13%) in both environments, suggesting thatstarvation is not a major factor in the reduced star formation rates ofVirgo spiral galaxies. The majority of Virgo spiral galaxies have theirHα disks truncated (52%), whereas truncated Hα disks arerarer in isolated galaxies (12%). Most of the Hα-truncatedgalaxies have relatively undisturbed stellar disks and normal toslightly enhanced inner disk star formation rates, suggesting thatintracluster medium-interstellar medium (ICM-ISM) stripping is the mainmechanism causing the reduced star formation rates of Virgo spiralgalaxies. Several of the truncated galaxies are peculiar, with enhancedcentral star formation rates, disturbed stellar disks, and barlikedistributions of luminous H II complexes inside the central 1 kpc but nostar formation beyond, suggesting that recent tidal interactions orminor mergers have also influenced their morphology. Two highly inclinedHα-truncated spiral galaxies have numerous extraplanar H IIregions and are likely in an active phase of ICM-ISM stripping. Severalspiral galaxies have one-sided Hα enhancements at the outer edgeof their truncated Hα disks, suggesting modest local enhancementsin their star formation rates due to ICM-ISM interactions. Low-velocitytidal interactions and perhaps outer cluster H I accretion seem to bethe triggers for enhanced global star formation in four Virgo galaxies.These results indicate that most Virgo spiral galaxies experienceICM-ISM stripping, many experience significant tidal effects, and manyexperience both.
| Galaxy Populations and Evolution in Clusters. IV. Deep H I Observations of Dwarf Elliptical Galaxies in the Virgo Cluster
In this paper we present deep Arecibo H I and WIYN optical observationsof Virgo Cluster dwarf elliptical galaxies. Based on this data we arguethat a significant fraction of low-mass galaxies in the Virgo Clusterrecently underwent evolution. Our new observations consist of H I 21 cmline observations for 22 classified dE galaxies with optical radialvelocities consistent with membership in the Virgo Cluster. Clustermembers VCC 390 and VCC 1713 are detected with H I massesMHI=6×107 and 8×107Msolar, respectively, while MHI values in theremaining 20 dE galaxies have upper limits as low as~5×105 Msolar. We combine our results withthose for 26 other Virgo Cluster dE galaxies with H I observations inthe literature, seven of which have H I detection claims. New opticalimages from the WIYN telescope of five of these H I-detected dEgalaxies, along with archival data, suggest that seven of the claimeddetections are true H I detections, yielding a ~15% detection rate.These H I-detected, classified dE galaxies are preferentially locatednear the periphery of the Virgo Cluster. Three Virgo dE galaxies haveobserved H I velocity widths greater than 200 km s-1,possibly indicating the presence of a large dark matter content ortransient extended H I. We discuss the possible origins of these objectsand argue that they originate from field galaxies accreted onto highangular momentum orbits by Virgo in the last few Gyr. As a result ofthis, we argue, these galaxies are slowly transformed within the clusterby gradual gas-stripping processes, associated truncation of starformation, and passive fading of stellar populations. Low-mass,early-type cluster galaxies are therefore currently being produced asthe product of cluster environmental effects. We utilize our results ina simple model to estimate the recent (past 1-3 Gyr) average massaccretion rate into the Virgo Cluster, deriving a value of M~50Msolar yr-1.
| The luminosity function of the Virgo Cluster from MB=-22 to -11
We measure the galaxy luminosity function (LF) for the Virgo Clusterbetween blue magnitudes MB=-22 and -11 from wide-fieldcharge-coupled device (CCD) imaging data. The LF is only graduallyrising for -22
| Galaxy Populations and Evolution in Clusters. I. Dynamics and the Origin of Low-Mass Galaxies in the Virgo Cluster
Early-type dwarfs are the most common galaxy in the local universe, yettheir origin and evolution remain a mystery. Various cosmologicalscenarios predict that dwarf-like galaxies in dense areas are the firstto form and hence should be the oldest stellar systems in clusters. Byusing radial velocities of early-type dwarfs in the Virgo cluster wedemonstrate that these galaxies are not an old cluster population buthave signatures of production from the infall of field galaxies.Evidence of this includes the combined large dispersions andsubstructure in spatial and kinematic distributions for Virgo early-typedwarfs and a velocity dispersion ratio with giant ellipticals expectedfor virialized and accreted populations. We also argue that thesegalaxies cannot originate from accreted field dwarfs, but must havephysically evolved from a precursor population, of different morphology,that fell into Virgo some time in the past.
| The amorphous galaxy NGC 2777 - H I evidence for tidal interaction with a faint companion
NGC 2777 is an amorphous galaxy possessing the classic signature of anA-type spectrum with superposed emission lines. There is no opticalevidence for an interaction, but observations of the neutral hydrogen inthis system reveal an H I bridge between NGC 2777 and U3, a companionthat is 3 mag fainter. FIR observations indicate that the current rateof massive star formation in NGC 2777 is low, but the optical spectraindicate that it must have been significantly higher in the recent past.We were unable to detect CO in either galaxy. We also consider a sampleof 10 amorphous galaxies, all showing A-type absorption spectra: ninefrom the Revised Shapley-Ames Catalog, plus NGC 2777. All show optical/HI interaction and/or peculiar velocity fields, supporting the view thatamorphous characteristics are the result of interaction. For some of thegalaxies, the interacting companion is much fainter than the amorphoussystem, which indicates that previous concerns raised by the existenceof isolated amorphous galaxies may have been premature.
| Catalogue of HI maps of galaxies. I.
A catalogue is presented of galaxies having large-scale observations inthe HI line. This catalogue collects from the literature the informationthat characterizes the observations in the 21-cm line and the way thatthese data were presented by means of maps, graphics and tables, forshowing the distribution and kinematics of the gas. It containsfurthermore a measure of the HI extension that is detected at the levelof the maximum sensitivity reached in the observations. This catalogueis intended as a guide for references on the HI maps published in theliterature from 1953 to 1995 and is the basis for the analysis of thedata presented in Paper II. The catalogue is only available inelectronic form at the CDS via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Arecibo H i Mapping of a Large Sample of Dwarf Irregular Galaxies
Neutral hydrogen mapping of 70 dwarf irregular (Sm, Im, and BCD)galaxies is reported, with position-velocity contour maps (and a fewcontour maps in R.A. and decl.) presented for those resolved by theArecibo beam. The galaxies were selected either from the Virgo ClusterCatalog, from similarly identified field galaxies, or from a distance-limited sample within the Arecibo declination range. We do not find anyisolated dwarfs with a larger H I to optical radius ratio than DDO 154;the "protogalaxy" H I 1225+01 of Giovanelli & Haynes continues to bea unique object among dwarfs that have been mapped in H I. For alldwarfs with significant rotation, we are able to determine the sense ofthe spin. For a number of better resolved dwarfs, we are able todetermine rotation curves, in most cases extending well beyond the lastmeasured point in available synthesis array maps. Correlations among theseveral measures of galaxy size and mass are studied; in a companionpaper we combine these data with those for the set of all availablemapped dwarf irregular galaxies and for mapped spirals spanning asimilar range of redshifts to investigate variations in Tully-Fisherrelations and in surface densities as functions of galaxy size andluminosity or mass.
| Correlation Statistics of Irregular and Spiral Galaxies Mapped in H i
Several measures of galaxy size and mass obtained from the neutralhydrogen mapping of 70 dwarf irregular galaxies (Sm, Im, and BCD,hereafter generically called "dwarfs" irrespective of size orluminosity) presented in an associated paper are compared statisticallyto those for the set of all available H I-mapped dwarfs and H I-mappedspirals distributed within the same spatial volume to investigatevariations in Tully-Fisher relations and in surface densities asfunctions of galaxy size and luminosity or mass. Some ambiguities due tothe "non-commutativity" of the correlations among the variables areaddressed. From linear regressions of logarithms we find that l isproportional to r^2.68^ is proportional to v^3.73^ is proportional tom_H_^1.35^ is proportional to m_dyn_^1.16^ where l is blue luminosity, ris the geometric mean of the radius to the outermost detectable H I andthe optical radius, v is the velocity profile half-width incorporatingrotation and random motions, m_H_ is the mass of H I, and m_dyn_ =v^2^r/G. All are normalized by the values appropriate to a galaxy withblue luminosity of 10^9^ L_sun_, typical of the region of overlapbetween dwarf and spiral galaxies. The surface density {SIGMA}_H_ ofneutral hydrogen (averaged within the "isophotal" radius r) is almostconstant along the sequence of size/mass/luminosity, while surfacedensity {SIGMA}_L_ of blue luminosity increases with galaxy size. Forquantities not involving H I we find no evidence for a "break" betweendwarfs and spirals, but we do find some curvature in log v versus log rand in the Tully-Fisher relation, log v versus log l. Two consequencesare the following: (1) l is proportional to v^3.7^ is more appropriatefor the whole sequence than is l is proportional to v^2.5^ as found forlarge spirals alone; (2) the surface density {SIGMA})dyn_ of totaldynamic mass (is proportional to v^2^/r) is almost constant along thelower portion of the luminosity sequence, but increases appreciably withl along the upper portion. There is an indication for a difference inthe correlations involving H I mass or radius between dwarfs alone andspirals alone, in the sense that irregular have somewhat more H I massor slightly larger H I radii than spirals at a given blue luminosity,optical radius, or velocity profile width. It is not clear whether thisis a true morphological effect or is merely due to m~ varying lessstrongly than l.
| 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.
| Arecibo mapping of three extended H I galaxy disks and conjectures on what lies beyond
We present Arecibo H I mapping of three dwarf irregular galaxies with HII envelopes more than five times the diameter of their opticallydetectable stellar regions. These include one isolated system, DDO 154,and two systems that are apparently interacting tidally with a largercompanion. DDO 137 and its companion NGC 4532 exhibit an enormoushorseshoe-shaped cloud with quite complex kinematics. The H I around VCC2062 appears on the surface to be a plume swept from NGC 4694 by tidalor ram-pressure mechanisms; however, the coincidence of a faint stellarpatch with the peak H I column density bears striking resemblance to the'protogalaxy' H I 1225+0146. Drawing on theoretical considerations andon the observation that DDO 154 and other extended disks appear totruncate abruptly at an H I column density around 2 x 10 exp 19/sq cm,we conjecture that the total hydrogen column density declines moreslowly than exponentially beyond that point. Extended partially ionizeddisks may then account for recently reported 1ow-redshift Ly-alphaabsorption features.
| The kinematics of the Virgo cluster revisited
The paper updates the velocity data of Virgo cluster galaxies andreconsiders the kinematic structure of the Virgo cluster. New velocitiesare given for 144 galaxies listed in the Virgo Cluster Catalog (VCC).Improved velocities are given for another 131 VCC galaxies. The Virgocluster is disentangled from its surrounding clouds of galaxies, and thelikely members of each of these clouds are listed. The velocitydistribution of dwarf elliptical cluster members is found to be highlyasymmetric. This phenomenon is interpreted as evidence for the imminentmerging of two subclusters in the core region, which points to thedynamical youth of the Virgo cluster. The mean heliocentric velocity ofthe Virgo cluster is estimated at 1050 +/- 35 km/s.
| A huge H I cloud near a galaxy pair
NGC 4532 and DDO 137 are a pair of Sm galaxies near the edge of theVirgo Cluster. Arecibo observations of a nearby H I cloud of enormousextent and unusual velocity structure are reported. The overall diameterof the cloud is almost 30 arcmin (about 200 kpc). Emission near the twogalaxies in both position and velocity accounts for about 80 percent ofthe total H I mass of 2.1 x 10 exp 10 solar. The remaining extendedemission is mainly at lower velocity; none of the extended emissionappears at velocities higher than the systemic velocity of the pair. Thekinematics of the extended cloud are too complex for a unique dynamicinterpretation of the system; however, the 'indicative dynamic mass' isof order 10 exp 12 solar.
| 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.
| A catalog of dwarf galaxies in Virgo
A catalog listing the location, apparent angular diameter, type,estimated central light concentration, and estimated brightness of 846dwarf galaxies in a 200-deg-sq region in Virgo is presented. Thegalaxies comprise 634 ellipticals, 137 IC-3475-type galaxies, 73 dwarfspirals and irregulars, and two objects which are jets of normalgalaxies, and were found on nine long-exposure IIIa-J-emulsion platesmade with the 1.2-m-Schmidt telescope at Palomar Observatory from 1971to 1976. Concordances to other catalogs, tables of additionalparameters, maps, graphs, and photographs are provided. The projecteddistributions of normal and dwarf galaxies and the dependence ofapparent luminosity on central light concentration are discussed. It isfound that dwarf ellipticals and IC-3475-type galaxies are probablemembers of the Virgo cluster, while dwarf spirals and possibly dwarfirregulars are not.
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