WIKISKY.ORG
 Home Getting Started To Survive in the Universe News@Sky Astro Photo The Collection Forum Blog New! FAQ Press Login

# PGC 41360

Contents

### Images

DSS Images   Other Images

### Related articles

 Mid-Infrared Properties of Low-Metallicity Blue Compact Dwarf Galaxies from the Spitzer Infrared SpectrographWe present a Spitzer-based mid-infrared (MIR) study of a large sample ofblue compact dwarfs (BCDs) using the Infrared Spectrograph (IRS),including the first MIR spectrum of I Zw 18, the archetype for the BCDclass and among the most metal-poor galaxies known. We show the spectraof polycyclic aromatic hydrocarbon (PAH) emission in a low-metallicityenvironment. We find that the equivalent widths (EWs) of PAHs at 6.2,7.7, 8.6, and 11.2 μm are generally weaker in BCDs than in typicalstarburst galaxies and that the fine-structure line ratio, [Ne III]/[NeII], has a weak anticorrelation with the PAH EW. A much strongeranticorrelation is shown between the PAH EW and the product of the [NeIII]/[Ne II] ratio and the UV luminosity density divided by themetallicity. We conclude that the PAH EW in metal-poor high-excitationenvironments is determined by a combination of PAH formation anddestruction effects. Infrared Properties of Star-forming Dwarf Galaxies. II. Blue Compact Dwarf Galaxies in the Virgo ClusterA sample of 16 blue compact dwarf galaxies (BCDs) in the Virgo Clusterhas been imaged in the near-infrared (NIR) in J and Ks on the2.1 m telescope at the Observatorio Astronómico Nacional in theSierra San Pedro Mártir in Mexico. Isophotes as faint asμJ=24 mag arcsec-2 andμKs=23 mag arcsec-2 have beenreached in most of the targets. Surface brightness profiles can befitted across the whole range of radii by the sum of two components: ahyperbolic secant (sech) function, which is known to fit the lightprofiles of dwarf irregular galaxies (dIs), and a Gaussian component,which quantifies the starburst near the center. Isophotal and totalfitted NIR magnitudes have been calculated, along with semimajor axes atμJ=23 mag arcsec-2 andμKs=22 mag arcsec-2. The diffuseunderlying component and the young starburst have been quantified usingthe profile fitting. Most color profiles show a constant color, betweenJ-Ks=0.7 and 0.9 mag. The diffuse component represents theoverwhelming majority of the NIR light for most BCDs, with the starburstenhancing the flux by less than about 0.3 mag. Linear correlations werefound between the sech scale length and the sech magnitude and betweenthe sech semimajor axis and the sech magnitude. Overall, galaxies withmore luminous diffuse components are larger and brighter in the center.The central burst correlates with the diffuse component, with brighterBCDs having stronger starbursts, suggesting that more massive objectsare forming stars more efficiently. BCDs lie on the fundamentalplane'' defined by dIs in Paper I, following the same relation betweensech absolute magnitude, sech central surface brightness, and thehydrogen line width W20, although the scatter is larger thanfor the dIs. On the other hand, correlations between the sech absolutemagnitude and the sech central surface brightness in Ks forBCDs and dIs are equally good, indicating that BCD line widths may beenhanced by turbulence or winds.These data were acquired at the Observatorio Astronómico Nacionalin the Sierra San Pedro Mártir, Mexico. 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. Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic propertiesWe performed a compilation of more than 1000 published spectra of H IIregions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. Theradial distributions of oxygen and nitrogen abundances were derived. Thecorrelations between oxygen abundance and macroscopic properties areexamined. We found that the oxygen abundance in spiral galaxiescorrelates with its luminosity, rotation velocity, and morphologicaltype: the correlation with the rotation velocity may be slightlytighter. There is a significant difference between theluminosity-metallicity relationship obtained here and that based on theoxygen abundances determined through the R23-calibrations.The oxygen abundance of NGC 5457 recently determined using directmeasurements of Te (Kennicutt et al. \cite{Kennicutt2003})agrees with the luminosity-metallicity relationship derived in thispaper, but is in conflict with the luminosity-metallicity relationshipderived with the R23-based oxygen abundances. The obtainedluminosity-metallicity relation for spiral galaxies is compared to thatfor irregular galaxies. Our sample of galaxies shows evidence that theslope of the O/H - MB relationship for spirals (-0.079± 0.018) is slightly more shallow than that for irregulargalaxies (-0.139 ± 0.011). The effective oxygen yields wereestimated for spiral and irregular galaxies. The effective oxygen yieldincreases with increasing luminosity from MB  -11 toMB  -18 (or with increasing rotation velocity fromVrot  10 km s-1 to Vrot  100km s-1) and then remains approximately constant. Irregulargalaxies from our sample have effective oxygen yields lowered by afactor of 3 at maximum, i.e. irregular galaxies usually keep at least1/3 of the oxygen they manufactured during their evolution.Appendix, Tables \ref{table:refero}, \ref{table:referV}, and Figs.\ref{figure:sample2}-\ref{figure:sample5} are only available inelectronic form at http://www.edpsciences.org} Spectroscopy of Dwarf Galaxies in the Virgo Cluster. I. Data, Chemical Abundances, and Ionization StructureLong-slit spectroscopy has been obtained for a sample of 22 blue dwarfgalaxies selected in the direction of the Virgo Cluster, as part of alarger sample of Virgo blue dwarf galaxies for which deep Hαimaging has been collected. Most of the galaxies in the present sampleare classified as BCDs or dwarf Irregulars in the Virgo Cluster Catalog.Line fluxes, Hβ equivalent widths, extinction coefficients, spatialemission profiles, ionization structure, and physical conditions arepresented for each galaxy. Chemical abundances have been derived eitherusing a direct determination of the electron temperature or afterdetailed examination of the predictions of different abundancecalibrations. The oxygen abundances derived for the sample of Virgodwarf galaxies span the range 7.6<=12+log(O/H)<=8.9, and thecorresponding nitrogen-to-oxygen abundance ratio ranges from valuestypical of low-metallicity field BCD galaxies to near solar. UV to radio centimetric spectral energy distributions of optically-selected late-type galaxies in the Virgo clusterWe present a multifrequency dataset for an optically-selected,volume-limited, complete sample of 118 late-type galaxies (>=S0a) inthe Virgo cluster. The database includes UV, visible, near-IR, mid-IR,far-IR, radio continuum photometric data as well as spectroscopic dataof Hα , CO and HI lines, homogeneously reduced, obtained from ourown observations or compiled from the literature. Assuming the energybalance between the absorbed stellar light and that radiated in the IRby dust, we calibarte an empirical attenuation law suitable forcorrecting photometric and spectroscopic data of normal galaxies. Thedata, corrected for internal extinction, are used to construct thespectral energy distribution (SED) of each individual galaxy, andcombined to trace the median SED of galaxies in various classes ofmorphological type and luminosity. Low-luminosity, dwarf galaxies haveon average bluer stellar continua and higher far-IR luminosities perunit galaxy mass than giant, early-type spirals. If compared to nearbystarburst galaxies such as M 82 and Arp 220, normal spirals haverelatively similar observed stellar spectra but 10-100 times lower IRluminosities. The temperature of the cold dust component increases withthe far-IR luminosity, from giant spirals to dwarf irregulars. The SEDare used to separate the stellar emission from the dust emission in themid-IR regime. We show that the contribution of the stellar emission at6.75 mu m to the total emission of galaxies is generally important, from~ 80% in Sa to ~ 20% in Sc.Tables 2-5, 7, 8, and Fig. 2 are only available in electronic form athttp://www.edpsciences.orgTables 10-12 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/37 The luminosity function of the Virgo Cluster from MB=-22 to -11We 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 Spectrophotometry of Galaxies in the Virgo Cluster. I. The Star Formation HistoryAs a result of an extensive observational campaign targeting the VirgoCluster, we obtained integrated (drift-scan mode) optical spectra andmultiwavelength (UV, U, B, V, H) photometry for 124 and 330 galaxies,respectively, spanning the whole Hubble sequence, and withmp<=16(Mp<=-15). These data were combined toobtain galaxy Spectral Energy Distributions (SEDs) extending from 2000to 22000 Å. By fitting these SEDs with synthetic ones derivedusing Bruzual & Charlot population synthesis models we try toconstrain observationally the star formation history (SFH) of galaxiesin the rich cluster of galaxies nearest to us. Assuming a Salpeter IMFand an analytical form for the SFH, the fit free parameters are the age(T) of the star formation event, its characteristic timescale (τ),and the initial metallicity (Z). In this work we test the (simplistic)case in which all galaxies have a common age T=13 Gyr, exploring a SFHwith delayed'' exponential form (which we call a la Sandage''), thusallowing for an increasing SFR with time. This SFH is consistent withthe full range of observed SEDs, provided that the characteristictimescale τ is let free to vary between 0.1 (quasi-instantaneousburst) and 25 Gyr (increasing SFR) and Z between 1/50 and 2.5 Zsolar.Elliptical galaxies (including dEs) are best fitted with shorttimescales (τ~3 Gyr) and metallicity varying between 1/5 and Zsolar.The model metallicity is found to increase as a function of H-bandluminosity. Spiral galaxies require that both τ and metallicitycorrelate with H-band luminosity: low-mass Im+BCD have subsolar Z andτ>=10 Gyr, whereas giant spirals have solar metallicities andτ~3 Gyr, consistent with elliptical galaxies. Moreover, we find thatthe SFH of spiral galaxies in the Virgo Cluster depends upon thepresence at their interior of fresh gas capable of sustaining the starformation. In fact, the residuals of the τ vs. LHrelation depend significantly on the H I content. H I deficient galaxieshave shorter (up to a factor of 4) τ (truncated SFH) than spiralswith normal H I content. Based on observations collected at theObservatoire de Haute Provence (OHP) (France), operated by the CNRS,France, and at the European Southern Observatory (Chile) (programme66.B-0026). 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 The metallicities of UM 151, UM 408 and A 1228+12 revisitedWe present the results of new spectrophotometry and heavy elementabundance determinations for 3 dwarf galaxies UM 151, UM 408 and A1228+12 (RMB 132). These galaxies have been claimed in the literature tohave very low metallicities, corresponding to log(O/H)+12 le 7.65, thatare in the metallicity range of some candidate local young galaxies. Wepresent higher S/N data for these three galaxies. UM 151 and UM 408 havesignificantly larger metallicities: log(O/H)+12 = 8.5 and 7.93,respectively. For A 1228+12 our new log(O/H)+12= 7.73 is close to thatrecalculated from earlier data (7.68). Thus, the rederived metallicitiesallow us to remove these objects from the list of galaxies with Z <~1/20 Zsun. Hα surface photometry of galaxies in the Virgo cluster III. Observations with INT and NOT 2.5 m telescopesWe present Hα line imaging observations of 30 galaxies obtained atthe 2.5 m INT and NOT telescopes. The observed galaxies are mostly BCDVirgo cluster galaxies. Hα +[NII] fluxes and equivalent widths, aswell as images of all the detected targets are presented. With theseobservations, Hα data are available for =~ 50% of the BCD galaxieslisted in the VCC. Based on observations taken at the INT and NOTtelescopes, operated on the island of La Palma by the ING and NOT teamsin the Spanish Observatorio del Roque de Los Muchachos of the Institutode AstrofÃ­sica de Canarias. Figure 1 is only available inelectronic form at http://www.edpsciences.org Oxygen abundances in dwarf irregular galaxies and the metallicity-luminosity relationshipThe low-luminosity dwarf irregular galaxies are considered. The oxygenabundances in H ii regions of dwarf irregular galaxies were recalculatedfrom published spectra through the recently suggested P-method. It hasbeen found that the metallicity of low-luminosity dwarf irregulargalaxies, with a few exceptions, correlates well with galaxy luminosity.The dispersion of oxygen abundances around the metallicity-luminosityrelationship increases with decreasing galaxy luminosity, as was foundby Richer & McCall (1995). The absence of a relationship between theoxygen abundance and the absolute magnitude in the blue band forirregular galaxies obtained by Hidalgo-Gámez & Olofsson(1998) can be explained by the large uncertainties in the oxygenabundances derived through the Te-method, that in turn can beexplained by the large uncertainties in the measurements of thestrengths of the weak oxygen line [OIII] lambda 4363 used in theTe-method. The most metal-poor galaxiesMetallicity is a key parameter that controls many aspects in theformation and evolution of stars and galaxies. In this review we focuson the metal deficient galaxies, in particular the most metal-poor ones,because they play a crucial rôle in the cosmic scenery. We firstset the stage by discussing the difficult problem of defining a globalmetallicity and how this quantity can be measured for a given galaxy.The mechanisms that control the metallicity in a galaxy are reviewed indetail and involve many aspects of modern astrophysics: galaxy formationand evolution, massive star formation, stellar winds, chemical yields,outflows and inflows etc. Because metallicity roughly scales as thegalactic mass, it is among the dwarfs that the most metal-poor galaxiesare found. The core of our paper reviews the considerable progress madein our understanding of the properties and the physical processes thatare at work in these objects. The question on how they are related andmay evolve from one class of objects to another is discussed. Whilediscussing metal-poor galaxies in general, we present a more detaileddiscussion of a few very metal-poor blue compact dwarf galaxies likeIZw18. Although most of what is known relates to our local universe, weshow that it pertains to our quest for primeval galaxies and isconnected to the question of the origin of structure in the universe. Wediscuss what do QSO absorption lines and known distant galaxies tell usalready? We illustrate the importance of star-forming metal-poorgalaxies for the determination of the primordial helium abundance, theiruse as distance indicator and discuss the possibility to detect nearlymetal-free galaxies at high redshift from Lyα emission. The filament of ionized gas in the outskirt of M87We report on the filament of ionized gas found at the NE periphery ofM87, the second brightest elliptical galaxy in the Virgo cluster. Theobject lies at 3.16 arcmin (15.7 kpc) projected distance of the nucleusof M87, and it coincides in position with the Eastern radio lobe of M87.Long slit spectroscopy confirmed the gaseous nature of this object,showing Hbeta, [OIII], [OI], [NII], Halpha and[SII] lines, without any underlying continuum. The redshift of theobject, V = 1170 +/- 25 km s-1, agrees with the velocity ofM87 itself (V = 1280 km s-1), and with the bulk of theionized material found near the center of M87. The Halphaflux of this filament corresponds to 1% of the ionized material in M87.The radio-optical coincidence together with the spectral characteristicssuggest that the filament is ionized by a shock. Based on observationstaken at INT and OHP. The INT is operated by the I.N.G. at the SpanishObservatorio del Roque de Los Muchachos of the I.A.C.. The OHP isoperated by the French CNRS. Kinematic Disturbances in Optical Rotation Curves among 89 Virgo Disk GalaxiesFor 89 galaxies, mostly spirals, in the Virgo Cluster region, we haveobtained optical long-slit major-axis spectra of the ionized gas. Wefind the following: (1) One-half of the Virgo galaxies we observed haveregular rotation patterns, while the other half exhibit kinematicdisturbances ranging from mild to major. Velocity complexities aregenerally consistent with those resulting from tidal encounters oraccretion events. Since kinematic disturbances are expected to fadewithin ~10^9 yr, many Virgo galaxies have experienced severalsignificant kinematic disturbances during their lifetimes. (2) There isno strong correlation of rotation curve complexity with Hubble type,with galaxy luminosity, with local galaxy density, or with H Ideficiency. (3) A few Virgo galaxies have ionized gas of limited extent,with velocities exceptionally low for their luminosities. In thesegalaxies the gas must be not rotationally supported. (4) There is aremarkable difference in the distribution of galaxy systemic velocityfor galaxies with regular rotation curves and galaxies with disturbedrotation curves. Galaxies with regular rotation patterns show a flatdistribution with velocities ranging from V_0=-300 km s^-1 to V_0=+2500km s^-1 galaxies with disturbed kinematics have a Gaussian distributionthat peaks at V_0=+1172+/-100 km s^-1, close to the cluster meanvelocity. This latter distribution is virtually identical to thedistribution of systemic velocity for elliptical galaxies in Virgo.However, disturbed galaxies are less concentrated to the cluster corethan are the ellipticals; those near the periphery have velocitiescloser to the mean cluster velocity. Thus, spirals with disturbedkinematics are preferentially on radial orbits, which bring them to thedenser core, where tidal interactions are strong and/or more common.Because they spend much time near apocenter, we observe them near thecluster periphery. Some may be falling into the core for the first time.These observations suggest that for a nonvirialized cluster like Virgo,galaxies may encounter either local (nearby galaxies) or global(cluster-related) interactions. These interactions may alter themorphology of the galaxy and may also play a role in driving the VirgoCluster toward dynamical equilibrium. One Arc Degree Core Substructure of the Virgo ClusterNot Available Total magnitude, radius, colour indices, colour gradients and photometric type of galaxiesWe present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory. 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. Dwarf elliptical galaxiesDwarf elliptical (dE) galaxies, with blue absolute magnitudes typicallyfainter than MB = -16, are the most numerous type of galaxyin the nearby universe. Tremendous advances have been made over the pastseveral years in delineating the properties of both Local Groupsatellite dE's and the large dE populations of nearby clusters. Wereview some of these advances, with particular attention to how wellcurrently availiable data can constrain (a) models for the formation ofdE's, (b) the physical and evolutionary connections between differenttypes of galaxies that overlap in the same portion of the mass-spectrumof galaxies, (c) the contribution of dE's to the galaxy luminosityfunctions in clusters and the field, (d) the star-forming histories ofdE's and their possible contribution to faint galaxy counts, and (e) theclustering properties of dE's. In addressing these issues, we highlightthe extent to which selection effects temper these constraints, andoutline areas where new data would be particularly valuable. General study of group membership. II - Determination of nearby groupsWe 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. Dwarf galaxies in the Virgo cluster. II - Photometric techniques and basic dataResults are presented of photographic surface photometry carried out for305 (mostly dwarf) galaxies in the Virgo cluster, in which the galaxyimages were digitized on 14 of the 67 du Pont plates used for the Virgocluster survey. Azimuthally averaged surface brightness profiles areshown for all galaxies. The following model-free photometric parametersare derived and listed for each galaxy: total apparent blue magnitude,mean effective radius and surface brightness, and various isophotalradii, ellipticity, and position angle. Most galaxies were fitted by anexponential form and/or a King model profile. The best-fittingparameters, including the 'nuclear' (central residual) magnitudes fordE+dS0 galaxies, are listed. 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. Evolutionary models for disk galaxiesThe evolution of disk galaxies is studied within the framework of a gasturbulence energy balance between the supernovae input energy and theturbulence collisional dissipation. The initial mass, the angularmomentum, and the gas accretion rate determine the evolution of themodels. We predict the present gas content and the star formationhistory of a galaxy. The role of gas accretion is discussed. Thecomparison between models and observations shows that gas accretion maybe very important to explain the high gas content of the dwarf irregulargalaxies. A Schmidt's law with a star formation rate per surface unitproportional to the square of the gas surface density provides a goodagreement with the models at the Hubble time as well as during theirevolution. 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. A density-gradient model for the H II galaxy I ZW 18Most of the lowest-metallicity galactic systems known have emission-lineratios that are not well-fitted by a simple H II region model. Theseobjects are similar to the low-abundance H II galaxy I Zw 18 in that theobserved F(3727 A)/F(H-beta) does not agree with the predictions ofmodels that fit F(4363 A)/F(H-beta), and F(4363)/F(4959, 5007), castingdoubt on the oxygen abundance estimated from the line ratios. Here, theobserved emission-line ratios of I Zw 18 are compared with those ofmodel nebulae incorporating a density gradient. An attempt is made todevelop a self-consistent model capable of explaining all existingobservations of I Zw 18 and to investigate the possibility that the trueoxygen abundance of this object is higher than the very low valueobtained from the estimated electron temperature of O(2+). Empirical oxygen abundances and physical conditions for relatively low abundance H II regionsThe utility of the emission-line ratio (3727 + 4959 + 5007 A)/H-beta asan estimate of the total oxygen abundance in H II regions of lowabundance (less than 10 percent of the solar value) is discussed. Usingboth observational data where the 4363A line is measured and model H IIregions it is concluded that, for low abundance systems, total oxygenabundances can be determined with an accuracy of + or - 0.2 dex in theabsence of a 4363A measurement. An attempt is made to study the averagebehavior of the stellar effective temperature (Teff) and ionizationparameter (U) with changing abundance in low abundance systems. It isshown that some diagnostic methods which are viable for high abundancesystems are not capable of uniquely determining Teff and U in lowabundance systems. The most promising method of determining Teff and Urequires measuring emission lines of forbidden O II, O III, S II, and SIII. 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. 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).
Submit a new article