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Diffuse Light in the Virgo Cluster
We present deep optical imaging of the inner ~1.5d×1.5d of theVirgo Cluster to search for diffuse intracluster light (ICL). Our imagereaches a 1 σ depth of μV=28.5 magarcsec-2, which is 1.5 mag arcsec-2 deeper thanprevious surveys, and reveals an intricate web of diffuse ICL. We seeseveral long (>100 kpc) tidal streamers, as well as a myriad ofsmaller scale tidal tails and bridges between galaxies. The diffuse haloof M87 is traced out to nearly 200 kpc, appearing very irregular onthese scales, while significant diffuse light is also detected aroundthe M84/M86 pair. Several galaxies in the core are embedded in commonenvelopes, suggesting they are true physical subgroups. The complexsubstructure of Virgo's diffuse ICL reflects the hierarchical nature ofcluster assembly, rather than being the product of smooth accretionaround a central 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.
| Stellar Populations of Dwarf Elliptical Galaxies: UBVRI Photometry of Dwarf Elliptical Galaxies in the Virgo Cluster
We present UBVRI surface photometry for 16 dwarf elliptical galaxies inthe Virgo Cluster with previously measured kinematic properties. Theglobal optical colors are red, with median values for the sample of0.24+/-0.03 in U-B, 0.77+/-0.02 in B-V, and 1.02+/-0.03 in V-I. Werecover the well-known color-magnitude relation for cluster galaxies butfind no significant difference in dominant stellar population betweenrotating and nonrotating dwarf elliptical galaxies; the average age ofthe dominant stellar population is 5-7 Gyr in all 16 galaxies in thissample. Analysis of optical spectra confirm these age estimates andindicate Fe and Mg abundances in the range of 1/20 to one-third ofsolar, as expected for low-luminosity galaxies. Based on Lick indicesand simple stellar population models, the derived [α/Fe] ratiosare subsolar to solar, indicating a more gradual chemical enrichmenthistory for dE's as compared with giant elliptical galaxies in the VirgoCluster. These observations confirm the marked difference in stellarpopulation and stellar distribution between dwarf and giant ellipticalgalaxies and further substantiate the need for alternative evolutionaryscenarios for the lowest mass cluster galaxies. We argue that it islikely that several different physical mechanisms played a significantrole in the production of the Virgo Cluster dE galaxies including insitu formation, infall of dE's that were once part of Local Groupanalogs, and transformation of dwarf irregular galaxies by the clusterenvironment. The observations support the hypothesis that a largefraction of the Virgo Cluster dE's are formed by ram pressure strippingof gas from infalling dI's.Based on observations with the VATT: the Alice P. Lennon Telescope andthe Thomas J. Bannan Astrophysics Facility.
| Star Clusters in Virgo and Fornax Dwarf Irregular Galaxies
We present the results of a search for clusters in dwarf irregulargalaxies in the Virgo and Fornax Clusters using Hubble Space Telescope(HST) WFPC2 snapshot data. The galaxy sample includes 28 galaxies, 11 ofwhich are confirmed members of the Virgo and Fornax Clusters. In the 11confirmed members, we detect 237 cluster candidates and determine theirV magnitudes, V-I colors, and core radii. After statistical subtractionof background galaxies and foreground stars, most of the clustercandidates have V-I colors of -0.2 and 1.4, V magnitudes lying between20 and 25 mag, and core radii between 0 and 6 pc. Using Hαobservations, we find that 26% of the blue cluster candidates are mostlikely H II regions. The rest of the cluster candidates are most likelymassive (>104 Msolar) young and old clusters. Acomparison between the red cluster candidates in our sample and theMilky Way globular clusters shows that they have similar luminositydistributions but that the red cluster candidates typically have largercore radii. Assuming that the red cluster candidates are in factglobular clusters, we derive specific frequencies (SN)ranging from ~0 to 9 for the galaxies. Although the values areuncertain, seven of the galaxies appear to have specific frequenciesgreater than 2. These values are more typical of elliptical andnucleated dwarf elliptical galaxies than they are of spiral or LocalGroup dwarf irregular galaxies.
| Spectrophotometry of galaxies in the Virgo cluster. II. The data
Drift-scan mode (3600-6800 Å) spectra with 500
| Spectroscopy of Dwarf Galaxies in the Virgo Cluster. I. Data, Chemical Abundances, and Ionization Structure
Long-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.
| Uncovering Additional Clues to Galaxy Evolution. II. The Environmental Impact of the Virgo Cluster on the Evolution of Dwarf Irregular Galaxies
The impact of the cluster environment on the evolution of dwarf galaxiesis investigated by comparing the properties of a sample of dwarfirregular galaxies (dI's) in the Virgo Cluster with a control sample ofnearby (``field'') dI's having oxygen abundances derived from [O III]λ4363 measurements and measured distances from resolved stellarconstituents. Spectroscopic data are obtained for H II regions in 11Virgo dI's distributed in the central and outer regions of the cluster.To ensure that oxygen abundances are derived in a homogeneous manner,oxygen abundances for field and Virgo dI's are computed using thebright-line method and compared with abundances directly obtained from[O III] λ4363, where available. They are found to agree to withinabout 0.2 dex, with no systematic offset. At a given optical luminosity,there is no systematic difference in oxygen abundance between the sampleof Virgo dI's and the sample of nearby dI's. However, five of the 11Virgo dI's exhibit much lower baryonic gas fractions than field dI's atcomparable oxygen abundances. Using field dI's as a reference, agas-deficiency index for dI's is constructed, making it possiblequantitatively to identify which galaxies have lost gas. For the Virgosample, some of the dwarfs are gas-deficient by a factor of 30. The gasdeficiency correlates roughly with the X-ray surface brightness of theintracluster gas. Ram pressure stripping can best explain the observedgas-poor dI's in the cluster sample. Together with the lack ofsignificant fading and reddening of the gas-poor dI's compared withgas-normal dI's, these observations suggest that the gas-poor dI's inVirgo have recently encountered the intracluster medium for the firsttime. Faded remnants of gas-poor dI's in Virgo will resemble brightdwarf elliptical galaxies currently seen in the cluster core.
| UV to radio centimetric spectral energy distributions of optically-selected late-type galaxies in the Virgo cluster
We 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 -11
We measure the galaxy luminosity function (LF) for the Virgo Clusterbetween blue magnitudes MB=-22 and -11 from wide-fieldcharge-coupled device (CCD) imaging data. The LF is only graduallyrising for -22
| Ram-pressure stripping of the interstellar medium in NGC 4485
This paper describes submm, 12CO (J=2-1) observations of theinteracting pair of galaxies NGC 4490 and 4485, and together with highresolution Hi and multifrequency radio continuum data we investigate theevolution of the ISM in this system. We find the following. (i) Thesmaller member of the pair, NGC 4485, has had the atomic, molecular anddust components of its ISM stripped via ram pressure during its recentpassage through the extended Hi distribution of NGC 4490. A bow-shock isidentified in the Hi ahead of the stripped gas. (ii) Within the disc ofNGC 4490 we find a very low H2-to-Hi ratio as well as astrong correlation between thermal emission and mass of H2suggesting that the star formation rate is limited in this case by theconversion of Hi to H2. (iii) 12CO emission froman Hi and radio-continuum bridge between the two galaxies is detected.
| 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.
| The Dwarf Irregular Galaxy UGC 7636 Exposed: Stripping at Work in the Virgo Cluster
We present the results of optical spectroscopy of a newly discovered HII region residing in the H I gas cloud located between the dwarfirregular galaxy UGC 7636 and the giant elliptical galaxy NGC 4472 inthe Virgo Cluster. By comparing UGC 7636 with dwarf irregular galaxiesin the field, we show that the H I cloud must have originated from UGC7636 because (1) the oxygen abundance of the cloud agrees with thatexpected for a galaxy with the blue luminosity of UGC 7636 and (2)MHI/LB for UGC 7636 becomes consistent with themeasured oxygen abundance of the cloud if the H I mass of the cloud isadded back into UGC 7636. It is likely that tides from NGC 4472 firstloosened the H I gas, after which ram pressure stripping removed the gasfrom UGC 7636.
| Dust Streamers in the Virgo Galaxy M86 from Ram Pressure Stripping of Its Companion VCC 882
The giant elliptical galaxy M86 in Virgo has a ~28 kpc long dust trailinside its optical halo that points toward the nucleated dwarfelliptical galaxy VCC 882. The trail seems to be stripped material fromthe dwarf. Extinction measurements suggest that the ratio of the totalgas mass in the trail to the blue luminosity of the dwarf is aboutunity, which is comparable to such ratios in dwarf irregular galaxies.The ram pressure experienced by the dwarf galaxy in the hot gaseous haloof M86 was comparable to the internal gravitational binding energydensity of the presumed former gas disk in VCC 882. Published numericalmodels of this case are consistent with the overall trail-likemorphology observed here. Three concentrations in the trail may beevidence for the predicted periodicity of the mass loss. The evaporationtime of the trail is comparable to its age obtained from the relativespeed of the galaxies and the trail length. Thus the trail could becontinuously formed from stripped replenished gas if the VCC 882 orbitis bound. However, the high gas mass and the low expected replenishmentrate suggest that this is only the first stripping event. Implicationsfor the origin of nucleated dwarf ellipticals are briefly discussed.
| 1.65 μm (H-band) surface photometry of galaxies. V. Profile decomposition of 1157 galaxies
We present near-infrared H-band (1.65 μm) surface brightness profiledecomposition for 1157 galaxies in five nearby clusters of galaxies:Coma, A1367, Virgo, A262 and Cancer, and in the bridge between Coma andA1367 in the ``Great Wall". The optically selected (mpg≤16.0) sample is representative of all Hubble types, from E to Irr+BCD,except dE and of significantly different environments, spanning fromisolated regions to rich clusters of galaxies. We model the surfacebrightness profiles with a de Vaucouleurs r1/4 law (dV), withan exponential disk law (E), or with a combination of the two (B+D).From the fitted quantities we derive the H band effective surfacebrightness (μe) and radius (re) of each component, theasymptotic magnitude HT and the light concentration indexC31. We find that: i) Less than 50% of the Ellipticalgalaxies have pure dV profiles. The majority of E to Sb galaxies is bestrepresented by a B+D profile. All Scd to BCD galaxies have pureexponential profiles. ii) The type of decomposition is a strong functionof the total H band luminosity (mass), independent of the Hubbleclassification: the fraction of pure exponential decompositionsdecreases with increasing luminosity, that of B+D increases withluminosity. Pure dV profiles are absent in the low luminosity rangeLH<1010 L\odot and become dominantabove 1011 L\odot . Based on observations taken atTIRGO, Gornergrat, Switzerland (operated by CAISMI-CNR, Arcetri,Firenze, Italy) and at the Calar Alto Observatory (operated by theMax-Planck-Institut für Astronomie (Heidelberg) jointly with theSpanish National Commission for Astronomy). Table 2 and Figs. 2, 3, 4are available in their entirety only in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Evolutionary Status of Dwarf ``Transition'' Galaxies
We present deep B-band, R-band, and Hα imaging of three dwarfgalaxies: NGC 3377A, NGC 4286, and IC 3475. Based on previous broadbandimaging and H I studies, these mixed morphology galaxies have beenproposed to be, respectively, a gas-rich low surface brightness Imdwarf, a nucleated dwarf that has lost most of its gas and is intransition from Im to dS0, N, and the prototypical example of a gas-poor``huge low surface brightness'' early-type galaxy. From the combinationof our broadband and Hα imaging with the published information onthe neutral gas content of these three galaxies, we find that (1) NGC3377A is a dwarf spiral, similar to those found by Schombert andcoworkers and Matthews & Gallagher; (2) both NGC 3377A and NGC 4286have comparable amounts of ongoing star formation, as indicated by theirHα emission, while IC 3475 has no detected H II regions to a verylow limit; (3) the global star formation rates are at least a factor of20 below those of 30 Doradus for NGC 3377A and NGC 4286; (4) while theamount of star formation is comparable, the distribution of star-formingregions is very different between NGC 3377A and NGC 4286, with Hαemission scattered over most of the optical face of NGC 3377A and allcontained within the inner half of the optical disk of NGC 4286; (5)given their current star formation rates and gas contents, both NGC3377A and NGC 4286 can continue to form stars for more than a Hubbletime; (6) both NGC 3377A and NGC 4286 have integrated total B-R colorsthat are redder than the integrated total B-R color for IC 3475 and thusit is unlikely that either galaxy will ever evolve into an IC 3475counterpart; and (7) IC 3475 is too blue to be a dE. We thus concludethat we have not identified potential precursors to galaxies such as IC3475, and unless significant changes occur in the star formation rates,neither NGC 3377A nor NGC 4286 will evolve into a dwarf elliptical ordwarf spheroidal within a Hubble time. Furthermore, optical morphology,even when coupled with the knowledge of the neutral gas content of adwarf galaxy, is not sufficient to determine its evolutionary phase. Theevolutionary states of NGC 3377A and NGC 4286 are thus unclear and morecomplicated than might be inferred from either previous broadbandimaging or H I content alone.
| A linear near-IR Tully-Fisher relation for giant and dwarf late-type galaxies
We present the near-IR (K(') -band, i.e.: lambda = 2.1 ^mu m)Tully-Fisher (TF) relation for a sample of 50 giant and dwarf late-typegalaxies, selected from the Virgo Cluster Catalogue. We find that L ~VMax(4) along a range of 8 K(') -mag for galaxies withdifferent Hubble types (from Sa to Im-BCD), phenomenologies, structures,star-formation histories, masses, dark-to-luminous mass ratios,metallicities and, perhaps, ages. The linearity of the near-IR TFrelation is in contrast with recent determinations of the optical TFrelations for samples of extreme late-type and dwarf galaxies. Thenear-IR TF law is in agreement both with the expectation from theFundamental Plane for disk systems and with the scenario ofself-regulating star-formation in disks. The previous results suggestthat the TF relation reflects the connection between thestructural/dynamical properties and the star-formation process of bothgiant and dwarf late-type galaxies, through a gas supply forstar-formation regulated by the gravitational potential of the galaxy.
| On the local radio luminosity function of galaxies. I. The Virgo cluster
We cross-correlate the galaxies brighter than m_B=18 in the Virgocluster with the radio sources in the NVSS survey (1.4 GHz), resultingin 180 radio-optical identifications. We determine the radio luminosityfunction of the Virgo galaxies, separately for the early- andlate-types. Late-type galaxies develop radio sources with a probabilityproportional to their optical luminosity. In fact their radio/optical(R_B) distribution is gaussian, centered at log R_B ~ -0.5, i.e. theradio luminosity is ~ 0.3 of the optical one. The probability oflate-type galaxies to develop radio sources is almost independent oftheir detailed Hubble type, except for Sa (and S0+S0a) which are afactor of ~ 5 less frequent than later types at any R_B. Giantelliptical galaxies feed ``monster" radio sources with a probabilitystrongly increasing with mass. However the frequency of fainter radiosources is progressively less sensitive on the system mass. The faintestgiant E galaxies (M_B=-17) have a probability of feeding low power radiosources similar to that of dwarf E galaxies as faint as M_B=-13. Table~1is only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| One Arc Degree Core Substructure of the Virgo Cluster
Not Available
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We 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.
| The ``Virgo photometry catalogue''; a catalogue of 1180 galaxies in the direction of the Virgo Cluster's core
We present a new catalogue of galaxies in the direction of the VirgoCluster's core: the Virgo Photometry Catalogue (VPC)*. This cataloguecontains 1180 galaxies (including background objects) within a 23square-degree area of the sky centred on R.A._{1950.0} = 12h 26m anddec._{1950.0} = 13(deg) 08'. The VPC galaxy sample comprises ofnon-stellar objects brighter than B_J25 = 19.0; thecompleteness limits being B_J25 ~18.5 for the northern halfof the survey area and B_J25 ~18.0 for the southern half.Independently-calibrated photographic surface photometry is presentedfor over 1000 galaxies in the U, B_J and R_C bands. Parameters listedfor catalogued galaxies include: equatorial coordinates, morphologicaltypes, surface-brightness profile parameters (which preserve themajority of the original surface photometry information), U, B_J &R_C isophotal magnitudes, B_J and [transformed] B total magnitudes,(U-B_J) and (B_J-R_C) equal-area and total colours, apparent angularradii, ellipticities, position angles, heliocentric radial velocitiesand alternative designations. All total magnitudes and total colours areextrapolated according to a new system denoted t in order to distinguishit from the T system already in use. The VPC is based primarily on four(one U, two B_J and one R_C) UK-Schmidt plates, all of which weredigitised using the Royal Observatory Edinburgh's (ROE) COSMOS measuringmachine. All magnitudes, colours and surface-brightness parameters arederived from numerical integrations of segmented plate-scan data, exceptfor (in 109 cases) saturated or (in 51 cases) inextricably-mergedimages; our segmentation software being able to cope with the vastmajority of image mergers. * Appendices B, C and E, which contain thesurface photometry, the main catalogue and the summary cataloguerespectively, are only available in electronic form. They can beobtained from La Centre des Donees astronomiques de Strasbourg (CDS) viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html.
| Near infrared surface photometry of late-type Virgo cluster galaxies
Near Infrared (K' band) surface photometry has been obtained for 102 (88late-type) Virgo cluster galaxies. A subset of 20 galaxies was alsoimaged in the H band. Magnitudes and diameters within the 21.5 and 22.0mag arcsec$^{-2}$ isophote, concentration indices and total H and K'magnitudes are derived. Basic statistical properties of a completesample of spiral galaxies spanning the range 6.3 < K'_T < 13.5 aregiven. Tables 3, 5 and 6 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html} Based on observations taken atthe Calar Alto Observatory, operated by the Max-Planck-Institut furAstronomie (Heidelberg) jointly with the Spanish National Commission forAstronomy.
| On the Size and Formation Mechanism of Star Complexes in Sm, Im, and BCD Galaxies
The diameters D_c_ of the largest star-forming complexes in 67Magellanic spiral and irregular galaxies and 16 blue compact dwarf (BCD)galaxies are found to scale approximately with the square root of thegalaxy luminosity for each type, i.e., smaller galaxies haveproportionately smaller star-forming regions. This is the same relationas for the largest complexes in bright spiral galaxies found previously,although Sm/Im galaxies have complexes that, on average, are a factor of2 larger than the extrapolation for spiral galaxies at the same absolutemagnitude, and the BCD galaxies have complexes that are ~2 times largerthan those typical of the Sm/Im galaxies at the same absolute magnitude.These results are consistent with the interpretation that the largestcomplexes form at the gravitational length scale in a marginally stableinterstellar medium with a nearly constant velocity dispersion c ~ 5-10km s^-1^. The luminosity scaling is then the result of higher averagetotal densities in smaller galaxies compared with the outer regions ofgiant spirals. This total density correlation is shown using published HI line widths and optical galaxy sizes. The implication of these resultsis that star formation begins when the ratio of the gas density ρ tothe total density (gas + stars + dark matter) exceeds several tenths. Ifstar formation lasts for a time scaling with (Grho_)^-1/2^ ~D_c_/c, then the main morphological differences between star formationin galaxies of various sizes can be explained: large galaxies have largestar complexes that form groups of OB associations slowly for up to 50Myr; small galaxies have small complexes (in terms of absolute size)that form dense associations quickly, in bursts spanning less than 5Myr.
| An image database. II. Catalogue between δ=-30deg and δ=70deg.
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
| 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.
| Distance calibration for E-galaxies. I. Elliptical calibrators.
We present a sample of elliptical galaxies selected in nearby groups ofgalaxies having unbiased distances. These galaxies can be considered assecondary calibrators for the extragalactic distance scale. Their meanerror in absolute magnitude is less than 0.4mag.
| Chemical Evolution of the Galactic Disk: Evidence for a Gradient Perpendicular to the Galactic Plane
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2813P&db_key=AST
| 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.
| Distribution of the spin vectors of the disk galaxies of the Virgo cluster. I. The catalogue of 310 disk galaxies in the Virgo area.
Not Available
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