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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
| Principal component analysis of International Ultraviolet Explorer galaxy spectra
We analyse the UV spectral energy distribution of a sample of normalgalaxies listed in the International Ultraviolet Explorer (IUE) NewlyExtracted Spectra (INES) Guide No. 2 - Normal Galaxies using a principalcomponent analysis. The sample consists of the IUE short-wavelength (SW)spectra of the central regions of 118 galaxies, where the IUE apertureincluded more than 1 per cent of the galaxy size. The principalcomponents are associated with the main components observed in theultraviolet (UV) spectra of galaxies. The first component, accountingfor the largest source of diversity, may be associated with the UVcontinuum emission. The second component represents the UV contributionof an underlying evolved stellar population. The third component issensitive to the amount of activity in the central regions of galaxiesand measures the strength of star-formation events.In all the samples analysed here, the principal component representativeof star-forming activity accounts for a significant percentage of thevariance. The fractional contribution to the spectral energydistribution (SED) by the evolved stars and by the young population aresimilar.Projecting the SEDs on to their eigenspectra, we find that none of thecoefficients of the principal components can outline an internalcorrelation or can correlate with the optical morphological types. In asubsample of 43 galaxies, consisting of almost only compact and BCDgalaxies, the third principal component defines a sequence related tothe degree of starburst activity of the galaxy.
| Are interactions the primary triggers of star formation in dwarf galaxies?
We investigate the assumption that the trigger of star formation indwarf galaxies is interactions with other galaxies, in the context of asearch for a `primary' trigger of a first generation of stars. This iscosmologically relevant because the galaxy formation process consistsnot only of the accumulation of gas in a gravitational potential wellbut also of the triggering of star formation in this gas mass, and alsobecause some high-z potentially primeval galaxy blocks look like nearbystar-forming dwarf galaxies. We review theoretical ideas proposed toaccount for the tidal interaction triggering mechanism and present aseries of observational tests of this assumption using published data.We also show results of a search in the vicinity of a composite sampleof 96 dwarf late-type galaxies for interaction candidates showing starformation. The small number of possible perturbing galaxies identifiedin the neighbourhood of our sample galaxies, along with similar findingsfrom other studies, supports the view that tidal interactions may not berelevant as primary triggers of star formation. We conclude thatinteractions between galaxies may explain some forms of star formationtriggering, perhaps in central regions of large galaxies, but they donot seem to be significant for dwarf galaxies and, by inference, forfirst-time galaxies forming at high redshifts. Intuitive reasoning,based on an analogy with stellar dynamics, shows that conditions forprimary star formation triggering may occur in gas masses oscillating ina dark-matter gravitational potential. We propose this mechanism as aplausible primary trigger scenario, which would be worth investigatingtheoretically.
| Neutral Hydrogen Mapping of Virgo Cluster Blue Compact Dwarf Galaxies
A new installment of neutral hydrogen mappings of blue compact dwarf(BCD) galaxies, as defined by optical morphology, in and near the VirgoCluster is presented. The primary motivation was to search for outlyingclouds of H I as potential interactive triggers of enhanced starformation, and therefore the mapped galaxies were selected for large H Imass, large optical diameter, and large velocity profile width.Approximately half the sample proved to have one or more small, lowcolumn density, star-free companion clouds, either detached or appearingas an appendage in our maps, at a resolution on the order of 4 kpc.Comparison is made with a sample of similarly mapped field BCD galaxiesdrawn from the literature; however, the Virgo Cluster sample of mappedBCDs is still too small for conclusive comparisons to be made. We found,on the one hand, little or no evidence for ram-pressure stripping nor,on the other, for extremely extended low column density H I envelopes.The H I rotation curves in most cases rise approximately linearly andslowly, as far out as we can trace the gas.
| Studies of galaxies in voids. I. H I observations of Blue Compact Galaxies
We present here results of studies of the properties of galaxies locatedin very low density environments. We observed 26 blue compact galaxies(BCGs) from the Second Byurakan (SBS) and Case surveys located in voidswith the radial velocities Vhel <~ 11 000 kms-1, two BCGs in the void behind the Virgo cluster and 11BCGs in denser environments. H I fluxes and profile widths, as well asestimates of total H I masses, are presented for the 27 detectedgalaxies (of which 6 are in three galaxy pairs and are not resolved bythe radiotelescope beam). Preliminary comparisons of void BCGs withsimilar objects from intermediate density regions - in the general fieldand the Local Supercluster (sub-samples of BCGs in the SBS zone) and inthe dense environment of the Virgo Cluster (a BCD sample) - areperformed using the hydrogen-to-blue-luminosity ratio M(ion{H}i)/LB. We find that for the same blue luminosity, forMB > -18.0m, BCGs in lower density environment have onaverage more H I. The slope beta of the M(ion {H}i)/LBvarpropto Lbeta for BCGs shows a trend of steepening withdecreasing bright galaxy density, being very close to zero for thedensest environment considered here and reaching beta = -0.4 for voids.
| Late-type dwarf irregular galaxies in the Virgo cluster - I. H alpha and red continuum data
We present H alpha and red continuum observations for a sample oflate-type low surface brightness (LSB) dwarf irregular galaxies,consisting of all the ImIV and V galaxies with m_B<= 17.2 in theVirgo cluster, and compare them with similar data for a representativesample of high surface brightness (HSB) dwarf irregular galaxies, alsoin the Virgo cluster. Line fluxes and equivalent widths are listed forindividual HII regions, and total H alpha emission is measured for theentire galaxy. Although significant line emission originates in the HIIregions that we have identified, it does not make up the entire H alphaoutput of all galaxies. For those objects in the LSB sample withHα emission, we find typical star formation rates (SFRs) from6.9x10^-3 to as high as 4.3x10^-2 M_solar yr^-1. This is, on average,one order of magnitude weaker than for HSB objects, although the SFRsoverlap. On average, ~2 HII regions are detected per LSB galaxy, for atotal of 38 HII regions among 17 galaxies with Hα emission. TheHII regions are smaller and fainter than in HSB galaxies in the sameVirgo cluster environment, have Hα line equivalent widths about 50per cent of those in HSBs, and cover similar fractions of the galaxies.When more than one HII region is present in a galaxy, we observe astrong intensity difference between the brightest and the secondbrightest HII regions. The line-emitting regions of LSB galaxies arepreferentially located at the periphery of the galaxy, while in HSBsthey tend to be central. The Hα line strength of an HII region iscorrelated with the red continuum light underneath the region; thisholds for both LSBs and HSBs. We do not identify fundamental differencesin the star formation properties of the LSB and HSB dwarf galaxies thatwe have studied, and we infer that these galaxies must be similar, withthe difference being the intensity of the present star formation burst.
| 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
| H I envelopes around low luminosity galaxies
H I mapping observations of a sample of isolated low luminosity galaxieswere undertaken with the Arecibo 305 m telescope in order to determinethe extent of their H I distribution. The sample included both compactemission-line galaxies (BCDs) and galaxies with hydrogen mass to-blueluminosity ratios, MH/LB, greater than five. Of 78targeted galaxies, 49 were found to have extended H I envelopes. Whereasmost (25 of 41) of the BCDs are compact in both optical light and in HI, 29 of the 33 high MH/LB galaxies has someevidence of extended hydrogen. As a class, the highMH/LB galaxies are low luminosity, low opticalsurface brightness, gas-rich dwarfs. Their extreme values ofMH/LB and large H I extent make them similar to HI 1225+01 and I Zw 18, but the possibility that the highMH/LB galaxies are BCDs in a quiescent phase isstill unresolved.
| 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.
| The Virgo cluster as a test for quantization of extragalactic redshifts
Tifft's (1972, 1977) hypothesis that redshifts are partially quantizedwith a periodicity in the range 70-75 km/s is tested for samples ofbright spiral and dwarf irregular galaxies with accurate H I redshiftsin the region of the Virgo cluster. The heliocentric redshifts arecorrected for solar motion, first by adopting an estimate of the sun'smotion with respect to the centroid of the Local Group, and then byallowing the solar velocity vector to vary in direction over the wholesky. Power spectrum analyses of the corrected redshifts are used tosearch for a significant periodicity in the prescribed range 70-75 km/s.No such periodicity is found for the dwarf irregulars, but there is apossible periodicity of about 71.1 km/s for the bright spirals. In afurther exploratory study, the sample of 112 spirals is divided upaccording to environment. The spirals in high-density regions of thecluster show no quantization, whereas those in low-density regionsappear to be partially quantized in intervals of about 71.0 km/s.
| Spectrophotometrical study of the blue compact dwarf galaxies in the Virgo cluster.
Not Available
| IRAS observations of galaxies in the Virgo cluster area
IRAS data on 196 galaxies in the area of the Virgo Cluster arepresented. The data derive from combining all available surveyobservations for each object and therefore achieve greater sensitivitythan the IRAS Point Source Catalog (PSC). The enhanced sensitivityallows 78 galaxies to be detected at 12 microns, 82 at 25 microns, 139at 60 microns, and 135 at 100 microns, compared to 16, 23, 88, and 95detections listed in the PSC. From the blue compact dwarf galaxy sample,23 and 19 objects are detected at 60 and 100 microns, compared to threeand two detections listed in the PSC. The emission in three close pairsof galaxies which are reported as single sources in the PSC areseparated here. These statistics demonstrate the importance andpotential of a detailed examination of IRAS data, especially forpossibly resolved sources and, in particular, for galaxies out toredshifts of 0.008 or galaxies with D(25) of 3 arcmin or greater.
| H I detection survey of a complete magnitude-limited sample of dwarf irregular galaxies in the Virgo Cluster area
New single-beam Arecibo H I observations of 298 late-type galaxies inthe Virgo Cluster drawn mostly from the new catalog of Binggeli,Sandage, and Tammann (1985) are presented. Two hundred seventeen ofthese constitute a magnitude-limited 'complete sample' of such galaxies,types Sdm through Im and BCD. Sixty-one percent of this 'completesample' was detected, greatly enhancing the store of redshifts and H Imasses for such galaxies in the Virgo Cluster. For detected galaxies,heliocentric velocities, 50 percent profile widths, and single-beamfluxes are presented. For those that escaped detection, upper limits arecomputed to the flux appropriate to the redshift range (-600 to +3000km/s).
| 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.
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