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Cold dust and molecular gas towards the centers of Magellanic type galaxies and irregulars. I. The data
We present 1300 μm continuum emission measurements and observationsof the 12CO (1-0) and (2-1) transition towards the centers of64 Magellanic type galaxies (Sdm/Sm) and irregulars (Im/I0/Irr). Thesources are selected to have IRAS flux densities S100 μm≥1000 mJy and optical diameters mainly below 180 arcsec. We wereable to detect 12CO towards 41 and the continuum emissiontowards 28 galaxies. In addition, we obtained the corresponding data fora set of 6 complementary galaxies of different morphological type.Based on observations collected at ESO, La Silla, Chile and IRAM, PicoVeleta, Spain.The full version of Figs. \ref{spec1.fig} and \ref{spec2.fig} is onlyavailable in electronic form at http://www.edpsciences.org
| The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxies
We discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/
| An H I Survey of Actively Star-forming Dwarf Galaxies
We present the results of H I 21 cm observations of 139 activelystar-forming dwarf galaxies obtained with the 305 m radio telescope atArecibo Observatory. Our sample consists of all objects cataloged inobjective-prism surveys for UV-excess or emission-line galaxiespublished prior to the start of the survey that have luminosities belowMB=-17.0 and that are located within the declination limitsof the Arecibo telescope. Galaxies from the Markarian, Michigan, Case,Wasilewski, Haro, and Zwicky lists are included. The sample spans a widerange of both H I gas content and star formation levels. A total of 122objects (88%) were detected; 82 galaxies have been observed for thefirst time in H I. The median velocity width for our sample is 88 kms-1, and the median H I gas mass is 3.0×108Msolar. In general, the sample galaxies are gas-rich, with anaverage MHI/LB=1.3 after correcting for theluminosity enhancement due to the starburst. The progenitors of many ofthe star-forming dwarfs have higher MHI/LB thantypically seen in samples of nearby ``normal'' galaxies, emphasizingtheir distinct nature.
| 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.
| On faint companions in the close environment of star-forming dwarf galaxies. Possible external star formation triggers?
We have searched for companion galaxies in the close environment of 98star-forming dwarf galaxies (SFDGs) from field and low densityenvironments, using the NASA Extragalactic Database. Most of thecompanions are dwarf galaxies which due to observational selectioneffects were previously disregarded in environmental studies of SFDGs. Asubsample at low redshift, cz < 2000 km s-1, was chosen topartially eliminate the observational bias against distant dwarfcompanions. We find companion candidates for approximately 30% of theobjects within a projected linear separation s_p <100 kpc and aredshift difference Delta cz < 500 km s-1. The limitedcompleteness of the available data sets, together with thenon-negligible frequency of H I clouds in the vicinity of SFDGsindicated by recent radio surveys, suggest that a considerably largerfraction of these galaxies may be accompanied by low-mass systems. Thiscasts doubt on the hypothesis that the majority of them can beconsidered truly isolated. The velocity differences between companioncandidates and sample SFDGs amount typically to la 250 kms-1, and show a rising distribution towards lower Delta cz.This is similarly found for dwarf satellites of spiral galaxies,suggesting a physical association between the companion candidates andthe sample SFDGs. SFDGs with a close companion do not show significantdifferences in their Hβ equivalent widths and B-V colours ascompared to isolated ones. However, the available data do not allow usto rule out that interactions with close dwarf companions can influencethe star formation activity in SFDGs.
| Population synthesis of Hii galaxies
We study the stellar population of galaxies with active star formation,determining ages of the stellar components by means of spectralpopulation synthesis of their absorption spectra. The data consist ofoptical spectra of 185 nearby (z<=0.075) emission-line galaxies. Theyare mostly Hii galaxies, but we also include some starbursts and Seyfert2s, for comparison purposes. They were grouped into 19 highsignal-to-noise ratio template spectra, according to their continuumdistribution, absorption- and emission-line characteristics. Thetemplates were then synthesized with a star cluster spectral base. Thesynthesis results indicate that Hii galaxies are typically age-compositestellar systems, presenting important contributions from generations upto as old as 500Myr. We detect a significant contribution of populationswith ages older than 1Gyr in two groups of Hii galaxies. The agedistributions of stellar populations among starbursts can varyconsiderably despite similarities in the emission-line spectra. In thecase of Seyfert 2 groups we obtain important contributions from the oldpopulation, consistent with a bulge. From the diversity of starformation histories, we conclude that typical Hii galaxies in the localUniverse are not systems presently forming their first stellargeneration.
| 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.
| Kinematic Evidence of Minor Mergers in Normal SA Galaxies: NGC 3626, NGC 3900, NGC 4772, and NGC 5854
BVRI and Hα imaging and long-slit optical spectroscopic data arepresented for four morphologically normal and relatively isolated Sagalaxies, NGC 3626, NGC 3900, NGC 4772, and NGC 5854. VLA H I synthesisimaging is presented for the first three objects. In all four galaxies,evidence of kinematic decoupling of ionized gas components is found inthe long-slit spectroscopic data; the degree and circumstances of thedistinct kinematics vary from complete counterrotation of all of the gasfrom all of the stars (NGC 3626) to nuclear gas disks decoupled from thestars (NGC 5854) to anomalous velocity central gas components (NGC 3900and NGC 4772). In the three objects mapped in H I, the neutral gasextends far beyond the optical radius, RHI/R25>=2. In general, the H I surface density isvery low, and the outer H I is patchy and asymmetric (NGC 3900) or foundin a distinct ring, exterior to the optical edge (NGC 3626 and NGC4772). While the overall H I velocity fields are dominated by circularmotions, strong warps are suggested in the outer regions by bending ofthe minor axis isovelocity contours (NGC 3900) and/or systematic shiftsin position angle between inner and outer rings (NGC 3626 and NGC 4772).In the interior, coincidence is found between the Hα and H I inrings, sometimes partial and crisscrossed by dust lanes. Optical imagingis also presented for NGC 4138 previously reported by Jore et al. toshow counterrotating stellar components. The multiwavelength evidence isinterpreted in terms of the kinematic ``memory'' of past minor mergersin objects that otherwise exhibit no morphological signs of interaction.
| H I observations of emission-line galaxies
We present single-dish Lovell telescope H i observations of a sample of67 emission-line and UV-excess galaxies, of which 52 are taken from theUniversity of Michigan (UM) catalogue. In addition, H i observations of24 gas-rich irregular galaxies are presented. We find that emission-linegalaxies are H i-rich with a median H i mass to blue luminosity ratioMHI/LB of ~ 0.45 Msun/Lsun.Within the UM galaxy sample the MHI/LB ratio tendsto increase with decreasing luminosity. Finally, it is found that themost H i-rich UM galaxies are the most metal deficient, implying thatthese objects are less evolved.
| 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.
| 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.
| 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
| On the Spectroscopic Properties of Star-Forming Dwarf Galaxies in Different Environments
A study is presented on the spectroscopic properties of a sample ofstar- forming dwarf galaxies in extreme density environments, as part ofan ongoing project intended to evaluate the influence of the environmenton the evolution of dwarf galaxies. Subsets of dwarf galaxies in nearbyvoids, and in the field of the local supercluster have been selected tocharacterize lower density regions. Conversely, higher densityenvironments have been characterized choosing a subset of galaxies inthe direction of the core of the Virgo Cluster, as well as an isolatedclump of galaxies. Our findings indicate that, overall, the spectra ofthose star-forming dwarf galaxies located in low-density regions tend topresent higher excitations and ionization parameters, higher Hβequivalent widths, and larger total Hβ luminosities than similarobjects located in higher density environments. At the same time, onlymarginal evidence may be found supporting a trend between the gasmetallicity and the density of the environment. The general metallicityluminosity relation for dwarf galaxies appears to be followed by mostgalaxies in the sample. An analysis of the spectroscopic ratios of thegalaxies using recent evolutionary models of giant H II regions suggestsa mixed star-formation history for most of the Virgo star- formingdwarfs, requiring continuous star formation in addition to somecurrently observable bursts.
| The UV properties of normal galaxies. III. Standard luminosity profiles and total magnitudes.
In the previous papers of this series we collected and reduced to thesame system all the available photometric data obtained in theultraviolet (UV) range for normal (i.e. non active) galaxies. Here weuse these data to derive standard UV luminosity profiles for threemorphological bins (E/S0; Sa/Sb; Sc/Sd) and extrapolated totalmagnitudes for almost 400 galaxies. We find that: 1) the UV growthcurves are well matched by the B-band revised standard luminosityprofiles, once a proper shift in the effective radius is applied, and 2)the UV light in early-type galaxies is more centrally concentrated thanthe visible light.
| The UV properties of normal galaxies. II. The ``non-IUE'' data.
In the last decade several satellite and balloon borne experiments havecollected a large number of ultraviolet fluxes of normal galaxiesmeasured through apertures of various sizes and shapes. We havehomogenized this data set by deriving scale corrections with respect toIUE. In a forthcoming paper these data will be used to derive standardluminosity profiles and total magnitudes.
| Global properties of dwarf galaxies. I. Galaxy sample and IRAS infrared flux-densities
We have selected a sample of 278 dwarf galaxies for which at least Bmagnitudes and preferably also optical colour information are available.For those galaxies that have no previously published IRAS fluxes, wehave used the IRAS database to extract fluxes or upper limits tosensitivity levels significantly better than those of the IRAS PointSource Catalog. New IRAS data include 79 galaxies detected in at leastone band, and 66 galaxies with good upper limits. In total, about 60% ofall dwarf galaxies in the sample now have been detected at 60/100μm.
| Global properties of dwarf galaxies II. Colours and luminosities
We have used a previously determined sample of 278 dwarf galaxies formost of which B magnitudes, optical colours, HI fluxes and IRASflux-densities are known, in order to derive luminosities, colours andsurface brightnesses. Dwarf galaxy properties are compared to those of acontrol sample of 228 larger spiral galaxies. The dwarf galaxies have onaverage higher 60/100μm flux ratios and lower 12/25μm flux ratiosthan the spiral galaxies, indicating that the contribution of `cirrus'to the infrared emission from dwarf galaxies is relativelyinsignificant. In the dwarf galaxies, the 60/100μm flux ratioincreases with increasing optical blueness; spiral galaxies show theopposite. Dwarf galaxies with a low optical surface brightness have low100μm/HI ratios, but the converse is not true. Galaxies with high100μm/HI ratios (indicative of high dust-to-gas ratios) also havehigh FIR/B ratios as well as high 60/100μm flux-density ratios.Although this is true for both spiral and dwarf galaxies, at given100μm/HI ratios the dwarf galaxies have both a lower FIR/B ratio anda higher 60/100μm flux-density ratio. This result is of importance inthe interpretation of FIR/B - 60/100μm diagrams in terms of starformation activity.
| General study of group membership. II - Determination of nearby groups
We present a whole sky catalog of nearby groups of galaxies taken fromthe Lyon-Meudon Extragalactic Database. From the 78,000 objects in thedatabase, we extracted a sample of 6392 galaxies, complete up to thelimiting apparent magnitude B0 = 14.0. Moreover, in order to considersolely the galaxies of the local universe, all the selected galaxieshave a known recession velocity smaller than 5500 km/s. Two methods wereused in group construction: a Huchra-Geller (1982) derived percolationmethod and a Tully (1980) derived hierarchical method. Each method gaveus one catalog. These were then compared and synthesized to obtain asingle catalog containing the most reliable groups. There are 485 groupsof a least three members in the final catalog.
| A spectrophotometric catalogue of HII galaxies
A spectrophotometric catalog is presented of 425 emission-line galaxiesdiscovered in objective prism surveys for which redshifts, emission-lineintensities, equivalent widths, and absolute fluxes are derived. Thevast majority of objects in the catalog are H II region-like galaxies (HII galaxies). In more than 80 H II galaxies the forbidden line O III4363 was measured with accuracy good enough to permit precise electrontemperature determinations. The observational parameters that define theproperties of H II galaxies as a class are characterized and discussed.
| Environmental properties of violently star-forming galaxies
A sample of 71 objects from the Spectrophotometric Catalogue of H IIgalaxies (Terlevich et al., 1990) has been searched for neighboringobjects. According to the metallicity, IRAS colors, morphology, andenvironmental circumstances, the objects have been classified in sixdifferent groups. Specific denominations are proposed for each class,from nuclear starburst galaxies to irregular blue galaxies and bluecompact dwarf galaxies. Blue irregular and blue compact objects havelower metallicities and higher f25 microns/f100 microns color indicesthan nuclear starburst objects, indicating different evolutionaryhistories. All the nuclear starbursts and blue irregulars have massivecompanions, but only 2/3 of the blue compacts do have. The hypothesis ofcloud-cloud collisions or galaxy-galaxy interactions to understand thetriggering mechanism for the violent star formation is brieflydiscussed.
| Properties of the redshift. I - Data and calibrations
Data at 21 cm are presented for 100 galaxies intended to be used forsystem comparisons between the NRAO 140 and 300 foot telescope and the100 m Effelsberg telescope. Data from the 300 foot telescope are alsogiven for galaxies selected for overlap comparisons with older studies.Flux calibrations and measurement uncertainties in flux, redshifts,profile width, and profile shape are discussed.
| IR tracers of star formation and the origin of the 25 micron emission in H II galaxies
A statistical analysis has been performed of a large number of H IIgalaxies, and their optical vs IRAS properties have been compared. It isfound that the most compact, low-metallicity galaxies are not detectedby IRAS. The far-IR luminosity is not a good measure of starburstprocesses, while the ratio I(25)/I(100) is an excellent tracer of recentstar formation events. The emission at 25 microns may be due to hotthermal dust very near the OB associations and/or shocked dusty gas. Theemission at 12 microns fits very well the predictions that follow fromthe model of emission of transiently heated PAH molecules. It isconcluded that the results support the idea that the emission at 25microns is due to, or at least dominated by, a burst of star formationoccurring in the nuclei of these galaxies.
| Observations of a Complete Sample of Emission-Line Galaxies. II. Properties of the UM Survey Galaxies
We utilize our extensive collection of imaging and spectroscopic data toinvestigate the properties of the emission-line galaxies (ELGs) found inLists IV and V of the University of Michigan (UM) objective-prismsurvey. The majority of the ELGs in this deep sample have apparentmagnitudes in the range 16.0 < m_B_ < 19.0. the distribution of B- V colors is nearly identical with that for the Markarian galaxies,indicating that the line-selection technique employed is very efficientat finding the same kinds of activity as found in UV-excess surveys.However, the UM sample differs markedly from complete magnitude-limitedsamples of galaxies in that it contains a very high proportion oflow-luminosity galaxies. The median luminosity is M_B_ = - 18.1, and 25%of the UM galaxies have M_B_ > -16.5. The vast majority are dwarfs,with diameters less than 10 kpc. Each ELG is classified into one of 10natural groups of active galaxies. Seyfert galaxies make up 11% of theentire sample, a result nearly identical with that for the Markariansurvey. The ELG types most commouly found are intermediate- tolow-luminosity objects with irregular or unresolved morphologies and avery intense star-formation region which dominates the optical output ofthe galaxy. Emission-line ratio diagnostic diagrams reveal that the ELGsin the different natural groups tend to have distinctly different lineratios. This suggests that the various ELG types differ from one anotherin terms of the metal abundance of their ionized gas, and we derive aluminosity-metallicity relationship which is singular to those foundpreviously for disk and irregular galaxies.
| Observations of a complete sample of emission-line galaxies. I - CCD imaging and spectroscopy of galaxies in UM lists IV and V. II - Properties of the UM survey galaxies
CCD imaging and spectroscopic observations of emission-line galaxycandidates from lists IV and V of the University of Michiganobjective-prism survey are used to study the properties of the surveyconstituents and the selection characteristics and completeness limitsof the survey. The total magnitudes, B-V colors, apparent diameters, andmorphoplocial and environmental characteristics of 166 objects arepresented. It is found that 95 percent of the objects for which thereare sufficient data exhibit emission lines. Also, contour diagrams ofthe CCD images and plots of the spectra of several galaxies arepresented.
| H I observations in the Virgo cluster area. III - All 'member' spirals
H I observations of 141 spiral galaxies in and around the Virgo Clusterare reported, with major-axis mapping for 65 of them. Heliocentricvelocities, profile widths, and H I fluxes are given for all detectedgalaxies. Spin orientations are given for mapped galaxies and H Idiameters for those sufficiently resolved by the 3.2 arcmin beam. Mappedgalaxy spectra are shown as contour plates of position versus velocity;central beam spectra are shown for the remainder. The distributions ofspin orientations are briefly analyzed and shown to be essentiallyrandom. The distributions of H I luminosity are presented along withindicative dynamical mass for the spirals and a synthesized H Idistribution for the cluster as a whole.
| IRAS observations of an optically selected sample of interacting galaxies
IRAS observations of a large, morphologically selected sample ofstrongly interacting disk-type galaxies have demonstrated thatgalaxy-galaxy collisions can lead to enhanced infrared emission, but notin all cases. Infrared luminosities of the interacting galaxies span alarge range, but are about a factor of 2 higher, on average, than thoseof isolated disk galaxies. The data suggest the existence of a cutoff inblue luminosity, below which no galaxies show markedly enhanced infraredemission. Only the most strongly interacting systems in the sample showextreme values of infrared excess, suggesting that deep,interpenetrating collisions are necessary to drive infrared emission toextreme levels. Comparisons with optical indicators of star formationshow that infrared excess and color temperatures correlate with thelevel of star-formation activity in the interacting galaxies. Allinteracting galaxies in our sample that exhibit an infrared excess andhave higher than normal color temperatures also have optical indicatorsof high levels of star formation. It is not necessary to invokeprocesses other than star formation to account for the enhanced infraredluminosity in this sample of interacting galaxies.
| The far-infrared properties of optically selected emission-line galaxies
A high-sensitivity sample of IRAS-detected emission-line galaxies (ELGs)is used to investigate the FIR properties of these galaxies and therelative selection biases of the UM Curtis Schmidt and IRAS surveys.Color-color diagrams indicate that the FIR emission from the UM ELGs isdue primarily to warm dust heated directly by the active star-formingregions that are responsible for the strong optical emission lines. TheFIR luminosity of the UM ELGs is found to correlate with both H-beta andblue luminosities, suggesting that the current star-formation rate inthese objects is more closely coupled to the past star-formation historythan is generally acknowledged.
| Ultraviolet observations and star-formation rate in galaxies
The present 149 galaxies, essentially of spiral and irregular types wereobserved at about 2000 A in a survey using a balloon-borne imagingtelescope. Total ultraviolet fluxes in an about 125-A wide bandpass havebeen obtained by comparison with field stars. On average, the m(2000)-Bcolor gets redder from late to early morphological types. For a giventype, this color exhibits a large scatter which increases from late toearly types. As expected, the galaxies show a relation between them(2000)-B and B-V colors. After a correction for dust extinction basedon the neutral hydrogen content of each galaxy, the observed fluxes areused to obtain quantitative estimates of the current star formation rate(SFR). A good correlation is found between the SFR and the total gascontent. A weak correlation observed between the SFR per unit area andthe average gas surface density might be compatible with the existenceof a power-law relation of exponent 1, between birthrate and gasdensity. The SFR per unit gas mass shows both a significant dispersionand a decrease toward later types. Implications in terms of starformation history are discussed.
| The structure of the Virgo cluster of galaxies
A new detailed analysis of galaxies in the Virgo area shows that theconventional Virgo cluster comprises two different clouds of galaxies:the Virgo cluster I and the Southern cloud II by the definition proposedhere. Excluding the galaxies in the Southern cloud II, the Virgo clusterI centered at 12 h 27.6 m, +13 deg 07 arcmin (1950), has a mean radialvelocity of 980 + or - 60 km/s and there is no significant velocitydifference between elliptical-lenticular and spiral-irregular galaxiesin the Virgo cluster I. The Southern cloud II is centered at about 12 h25 m, +7 deg 30 arcmin (1950) and all types of galaxies in the cloud arediffusely distributed, while elliptical galaxies in the Virgo cluster Iare concentrated to the center. The mean radial velocity within a radiusof 3.5 deg of the Southern cloud II is +1240 + or - 80 km/s, which issignificantly higher than the mean velocity of the Virgo cluster I.
| Infall of galaxies into the Virgo cluster and some cosmological constraints
The observed infall of galaxies in the Virgo Southern Extension towardthe Virgo Cluster is characterized by a family of mass models satisfyingthe requirement of an explanation of the Milky Way's motion with respectto the Virgo Cluster, in order to furnish constraints of cosmologicalinterest. If the age of the universe is 10-15 Gyr, there is a roughcoincidence of the mass required to explain the infall pattern ofgalaxies near the cluster, the mass needed to account for the motion ofthe Milky Way, and the mass implied for the central cluster by thevirial theorem, in which case most of the supercluster's mass mustreside in the Virgo Cluster. With the mass-age models and a census ofthe distribution of galaxies near the Virgo Cluster, it is possible toestimate the near-future accretion rate of galaxies into the centralcluster.
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