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 Completing H I observations of galaxies in the Virgo clusterHigh 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 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. Hα surface photometry of galaxies in the Virgo cluster. IV. The current star formation in nearby clusters of galaxiesHα +[NII] imaging observations of 369 late-type (spiral) galaxiesin the Virgo cluster and in the Coma/A1367 supercluster are analyzed,covering 3 rich nearby clusters (A1367, Coma and Virgo) and nearlyisolated galaxies in the Great-Wall. They constitute an opticallyselected sample (mp<16.0) observed with ~ 60 %completeness. These observations provide us with the current(T<107 yrs) star formation properties of galaxies that westudy as a function of the clustercentric projected distances (Theta ).The expected decrease of the star formation rate (SFR), as traced by theHα EW, with decreasing Theta is found only when galaxies brighterthan Mp ~ -19.5 are considered. Fainter objects show no orreverse trends. We also include in our analysis Near Infrared data,providing information on the old (T>109 yrs) stars. Puttogether, the young and the old stellar indicators give the ratio ofcurrently formed stars over the stars formed in the past, orbirthrate'' parameter b. For the considered galaxies we also determinethe global gas content'' combining HI with CO observations. We definethe gas deficiency'' parameter as the logarithmic difference betweenthe gas content of isolated galaxies of a given Hubble type and themeasured gas content. For the isolated objects we find that b decreaseswith increasing NIR luminosity. In other words less massive galaxies arecurrently forming stars at a higher rate than their giant counterpartswhich experienced most of their star formation activity at earliercosmological epochs. The gas-deficient objects, primarily members of theVirgo cluster, have a birthrate significantly lower than the isolatedobjects with normal gas content and of similar NIR luminosity. Thisindicates that the current star formation is regulated by the gaseouscontent of spirals. Whatever mechanism (most plausibly ram-pressurestripping) is responsible for the pattern of gas deficiency observed inspiral galaxies members of rich clusters, it also produces the observedquenching of the current star formation. A significant fraction of gashealthy'' (i.e. with a gas deficiency parameter less than 0.4) andcurrently star forming galaxies is unexpectedly found projected near thecenter of the Virgo cluster. Their average Tully-Fisher distance isfound approximately one magnitude further away (muo = 31.77)than the distance of their gas-deficient counterparts (muo =30.85), suggesting that the gas healthy objects belong to a cloudprojected onto the cluster center, but in fact lying a few Mpc behindVirgo, thus unaffected by the dense IGM of the cluster. Based onobservations taken at the Observatorio Astronómico Nacional(Mexico), the OHP (France), Calar Alto and NOT (Spain) observatories.Table \ref{tab4} is only available in electronic form athttp://www.edpsciences.org Studies of galaxies in voids. I. H I observations of Blue Compact GalaxiesWe 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. Lopsidedness in dwarf irregular galaxiesWe quantify the amplitude of the lopsidedness, the azimuthal angularasymmetry index and the concentration of star-forming regions, asrepresented by the distribution of the Hα emission, in a sample of78 late-type irregular galaxies. We bin the observed galaxies into twogroups representing blue compact galaxies (BCDs) andlow-surface-brightness dwarf galaxies (LSBs). The light distribution isanalysed with a novel algorithm, which allows detection of details inthe light distribution pattern. We find that while the asymmetry of theunderlying continuum light, representing the older stellar generations,is relatively small, the Hα emission is very asymmetric and iscorrelated in position angle with the continuum light. We show that theconcentration of continuum light is correlated with the Hαconcentration; this implies that the young star formation has the samespatial properties as the older stellar populations, but that theseproperties are more strongly expressed by the young stars. We test amodel of random star formation over the extent of a galaxy by simulatingHii regions in artificial dwarf galaxies. A galaxy is traced by assumingred star clusters distributed on an underlying exponential disc ofradius twice the scalelength. The disc is allowed to change in apparentmagnitude, scaleradius, position angle and ellipticity. We compare theasymmetry-concentration distribution predicted by the simulations withthe real observed distribution; we find that only LSBs match thedistribution predicted by the model. The reason is that, independentlyof the number of Hii regions, LSBs show no particular location of Hiiregions, whereas BCDs show current star formation activity restrictedvery much to the central parts of the galaxies. A consideration of theproperties of the continuum light leads to the conclusion that most ofLSBs can be approximated by exponential discs of radius twice theirscalelength; BCDs call, however, for much more concentrated underlyingsystems, with smaller scalelengths than assumed in the simulations. Theimplication is that random star formation over the full extent of agalaxy may be generated in LSB dwarf irregular galaxies but not in BCDgalaxies. Late-type dwarf irregular galaxies in the Virgo cluster - I. H alpha and red continuum dataWe 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. Morphology of star formation regions in irregular galaxiesThe location of HII regions, which indicates the locus of present starformation in galaxies, is analysed for a large collection of 110irregular galaxies (Irr) imaged in Hα and nearby continuum. Theanalysis is primarily by visual inspection, although a two-dimensionalquantitative measure is also employed. The two different analyses yieldessentially identical results. HII regions appear preferentially at theedges of the light distribution, predominantly on one side of thegalaxy, contrary to what is expected from stochastic self-propagatingstar formation scenarios. This peculiar distribution of star-formingregions cannot be explained by a scenario of star formation triggered byan interaction with extragalactic gas, or by a strong one-armed spiralpattern. Late-type dwarf galaxies in the Virgo cluster - II. Star formation propertiesWe study star-formation-inducing mechanisms in galaxies throughmultiwavelength measurements of a sample of dwarf galaxies in the Virgocluster described in Paper I. Our main goal is to test howstar-formation-inducing mechanisms depend on several parameters of thegalaxies, such as morphological type and hydrogen content. We derive thestar formation rate and star formation histories of the galaxies, andcheck their dependence on other parameters. Comparison of the samplegalaxies with population synthesis models shows that these objects havesignificantly lower metallicity than the solar value. The colours cangenerally be explained as a combination of two different stellarpopulations: a young (3-20 Myr) metal-poor population which representsthe stars currently forming presumably in a starburst, and an older(0.1-1 Gyr) population of previous stellar generations. There isevidence that the older stellar population was also formed in astarburst. This is consistent with the explanation that star formationin this type of objects takes place in short bursts followed by longquiescent periods. No significant correlation is found between the starformation properties of the sample galaxies and their hydrogen content.Apparently, when star formation occurs in bursts, other parametersinfluence the star formation properties more significantly than theamount of atomic hydrogen. No correlation is found between the projectedVirgocentric distance and the rate of star formation in the galaxies,suggesting that tidal interactions are not significant in triggeringstar formation in cluster dwarf galaxies. Late-type dwarf galaxies in the Virgo cluster - I. The samplesWe selected samples of late-type dwarf galaxies in the Virgo clusterwith HI information. The galaxies were observed at the Wise Observatoryusing several broad-band and Hα bandpasses. UV measurements werecarried out with the IUE Observatory from VILSPA, and with the FAUSTshuttle-borne UV telescope. We describe the observations in detail,paying particular attention to the determination of measurement errors,and present the observational results together with published data andfar-infrared information from IRAS. The sample will be analysed insubsequent papers, in order to study star formation mechanisms ingalaxies. Blue compact dwarf galaxies and new velocities in VirgoWe present new spectral observations of 303 galaxies brighter thanB_J=17.6 in the central 30 deg^2 of the Virgo cluster field. Thegalaxies were selected from two overlapping samples designed for asearch for blue compact dwarf (BCD) galaxies and for a more generalstudy of Virgo dwarf elliptical (dE) galaxies. Many of these galaxieswere designated background' (i.e., not members of the Virgo cluster) onmorphological grounds by Binggeli, Sandage & Tammann in their VirgoCluster Catalog (VCC). Our principal aim was to measure radialvelocities of these galaxies in order to determine which were members ofthe cluster, looking in particular for any cluster members previouslymisclassified as background galaxies. We measured reliable velocitiesfor 291 galaxies, of which nine were found to be cluster members. Fiveof these were listed in the VCC as members' and four as possiblemembers'. We also confirmed that 10 VCC background' galaxies werecorrectly identified, and determined that an additional three VCCpossible members' were background galaxies. These results show that theVCC membership estimates were generally correct. Our sample of candidateBCD galaxies was defined by objective criteria from digitized platematerial and therefore allows us to place new limits on the dwarf-galaxypopulation. We confirm the drop in the BCD luminosity function at anabsolute magnitude of M_B=-14 reported by Binggeli, Sandage &Tammann (using their Virgo distance modulus). We also show that if BCDsfade at the end of the current burst of star formation, such `dead'BCDs, if they exist, would all have to be fainter than our sample limitof B_J=17.6. We use a (U-B_J)(B_J-R_F) colour-colour diagram plottedfrom our photographic data to separate the different dwarf-galaxy typeseffectively and use this colour information to argue that there are noblue star-forming progenitors of dE galaxies in the cluster. Most of thegalaxies of compact appearance we selected were background objects whichwe show to be significantly younger than general field galaxies; thissample also shows considerable clustering in redshift space. Optical structure and star formation in blue compact dwarf galaxies. II. Relations between photometric components and evolutionary implications.In a preceding paper, we have derived the structural parameters of theyoung high-surface-brightness stellar component formed in starbursts andthat of the old underlying low-surface-brightness component, by applyinga 3-component decomposition scheme to the surface brightness profiles of12 blue compact dwarf galaxies (BCDs) and 2 starburst galaxies. Here wecompare the various properties of these two components. We find that thesize of the starburst component depends on the size and luminosity ofthe underlying component, as well as on the HI mass of the BCD.Furthermore, the fractional surface occupied by the star-forming regionsdecreases with increasing luminosity of the underlying host galaxy. Wecompare the structural properties of BCDs with those of other types ofdwarf galaxies. We find that, at equal B luminosity, the underlyingcomponent of a BCD has a central surface brightness brighter by ~1.5 magand an exponential scale length smaller by a factor of ~2 than that ofdwarf irregulars (dIs) and dwarf ellipticals (dEs). Thus there can beevolutionary connections between BCDs and dEs and dIs only if the BCDunderlying component can modify its structural properties. This mayoccur in response to changes in the global gravitational potential ofthe dwarf galaxy, caused by such events as mass infall from the outergas halo and/or mass loss in starburst-driven galactic winds. Near Infrared Imaging of Dwarf Ellipticals Irregulars and Blue Compact Galaxies in the Virgo ClusterNew near-IR images are presented for 13 dwarf galaxies in the Virgocluster. Together with previous data these provide a data base of JHKimaging for 26 dwarf ellipticals (dEs), dwarf irregulars (dIs) and bluecompact dwarfs (BCDs). These images show the dIs to be highly asymmetricand unrelaxed, implying that they are dynamically young and unevolved.This is consistent with their blue near-IR and optical-IR colours whichare most easily explained by young stellar populations. The dEs aresymmetrical and apparently relaxed, with very uniform colours indicatingthat they are dominated by old stars. They generally have exponentiallight profiles, but the brighter galaxies tend to exhibit more cuspedlight distributions, similar to the de Vaucouleurs profiles of brightellipticals. The BCDs have moderately asymmetric light profiles, andparadoxically red colours, possibly indicating an intermediate-agestellar population. They are probably dEs which have undergone bursts ofstar formation in the last few X 10^9^ years, whilst the dIs are afundamentally distinct population. Colour gradients are present in manyof the galaxies, invariably in the sense that the nuclei are redder thanthe surrounding galaxy light. The kinematics of the Virgo cluster revisitedThe 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. Extreme blue compact dwarf galaxies in the Virgo ClusterCCD images and surface photometry are presented for six extreme(especially compact) blue compact dwarf (BCD) galaxies in the VirgoCluster. The photometric parameters of the galaxies are used to studytheir possible fate once star formation has ended. It is found that theymay fade to form dwarf elliptical galaxies, as has previously beensuggested, but the dE galaxies formed from these BCDs would lie amongthe most compact in existing samples. Two of the BCDs studied have nosurrounding low-surface-brightness envelopes detected to a limit ofmu(v) = 27 mag/sq arcsec. The Virgo cluster as a test for quantization of extragalactic redshiftsTifft'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. IRAS observations of galaxies in the Virgo cluster areaIRAS 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. 21 centimeter line width distances of cluster galaxies and the value of H0Locally calibrated blue and infrared Tully-Fisher (TF) relations areapplied to an 82 percent complete sample of 81 Sab-Sm galaxies which arebona fide members of the Virgo Cluster. A nearly unbiased Virgo modulusof 31.60 + or - 0.15 can be derived in perfect agreement withindependent recent determinations. It is shown that the blue TF andinfrared TF relations give almost identical distance moduli from anyselected Virgo subsample. The intrinsic scatter about the two TFrelations is 0.7 mag, considerably larger than the observed scatter inthe UMa Cluster and in 10 more distant clusters. Distance determinationof these clusters therefore can be achieved only by fitting the upperenvelopes of their TF relations onto the blue and infrared upperenvelopes of the Virgo Cluster. The resulting distances define a linearexpansion law with a small scatter. The present cluster data require H0= 56.6 + or - 0.9 km/s/Mpc. H I detection survey of a complete magnitude-limited sample of dwarf irregular galaxies in the Virgo Cluster areaNew 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). Comparative photometric parameters of dwarf irregular and elliptical galaxies in the Virgo Cluster - Two different classes of dwarf galaxies?The possible evolutionary relationships of dwarf irregular (dI) anddwarf elliptical (dE) galaxies is explored with a data base thatconsists of 21 cm data, infrared photometry, and CCD multicolor surfacephotometry of both dIs and dEs in the Virgo cluster. In general, dIs areat or slightly below the same surface-brightness level as dEs, yet theyare significantly bluer. To have the same optical and IR colordistribution as the dEs requires that dIs fade by 1.5 mag in the blue.This will in turn produce a population of dwarf galaxies that will haveaverage surface brightness below the detection threshold of conventionalphotographic plates. It is concluded that the progenitors of thebrighter dE galaxies in the Virgo cluster are not to be found among thecurrent generation of dIs with the exception that some of the largerblue compact dwarf galaxies are probably gas-rich analogs to dEs.Furthermore, it is probable that dIs form a parallel sequence of dwarfgalaxies with surface mass densities significantly lower than the dEs. HI-observations of galaxies in the Virgo cluster of galaxies. I - The dataNew H I-data for a large number of bright galaxies inside the 10 degradius area of the Virgo cluster of galaxies have been obtained with the100 m radiotelescope at Effelsberg. A total of 234 galaxies was observedfor the first time. Among them, 53 have been detected providing newaccurate radial velocities. Data from the literature have been compiled.Together with the new data, they form a (nearly homogeneous) set of H Iobservations for more than 450 galaxies. HI-observations of galaxies in the Kraan-Korteweg - Tammann catalogue of nearby galaxies. I - The dataAbstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1986A&AS...63..323H&db_key=AST The structure of the Virgo cluster of galaxiesA 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. 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. On the Largescale Distribution of Faint Blue GalaxiesNot Available Radial Velocities of Galaxies in the Virgo ClusterNot Available Middle-ultraviolet photometry of Virgo cluster galaxiesPhotographic photometry of Virgo cluster galaxies has been performed ina wavelength band extending from 1620-3200 A using sounding rockettechniques. The observational results are middle ultraviolet magnitudes,U2421, or faint limits for U2421, for 201 galaxies within 5.5 arcmin ofthe cluster center. A strong negative correlation is found between U2421- V and V for all observed S0 galaxies and a similar but weakercorrelation for the observed ellipticals. No such correlation is foundfor spiral galaxies regardless of their projection angles on the sky.The measured colors are generally compatible with colors computed frompreviously generated composite spectra. Faint blue objects in the Virgo cluster region.Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1967AJ.....72...59R&db_key=AST
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