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 Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble TypeWe study the nuclear star clusters (NCs) in spiral galaxies of variousHubble types using spectra obtained with the STIS on board the HubbleSpace Telescope (HST). We observed the nuclear clusters in 40 galaxies,selected from two previous HST WFPC2 imaging surveys. At a spatialresolution of ~0.2" the spectra provide a better separation of clusterlight from underlying galaxy light than is possible with ground-basedspectra. Approximately half of the spectra have a sufficiently highsignal-to-noise ratio for detailed stellar population analysis. For theother half we only measure the continuum slope, as quantified by the B-Vcolor. To infer the star formation history, metallicity, and dustextinction, we fit weighted superpositions of single-age stellarpopulation templates to the high signal-to-noise ratio spectra. We usethe results to determine the luminosity-weighted age, mass-to-lightratio, and masses of the clusters. Approximately half of the sampleclusters contain a population younger than 1 Gyr. Theluminosity-weighted ages range from 10 Myr to 10 Gyr. The stellarpopulations of NCs are generally best fit as a mixture of populations ofdifferent ages. This indicates that NCs did not form in a single event,but that instead they had additional star formation long after theoldest stars formed. On average, the sample clusters in late-typespirals have a younger luminosity-weighted mean age than those inearly-type spirals (L=8.37+/-0.25 vs.9.23+/-0.21). The average mass-weighted ages are older by ~0.7 dex,indicating that there often is an underlying older population that doesnot contribute much light but does contain most of the mass. The averagecluster masses are smaller in late-type spirals than in early-typespirals (logM=6.25+/-0.21 vs. 7.63+/-0.24) and exceed the masses typicalof globular clusters. The cluster mass correlates loosely with totalgalaxy luminosity. It correlates more strongly with both the Hubble typeof the host galaxy and the luminosity of its bulge. The lattercorrelation has the same slope as the well-known correlation betweensupermassive black hole mass and bulge luminosity. The properties ofboth nuclear clusters and black holes in the centers of spiral galaxiesare therefore intimately connected to the properties of the host galaxy,and in particular its bulge component. Plausible formation scenarioshave to account for this. We discuss various possible selection biasesin our results, but conclude that none of them can explain thedifferences seen between clusters in early- and late-type spirals. Theinability to infer spectroscopically the populations of faint clustersdoes introduce a bias toward younger ages, but not necessarily towardhigher masses.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555. These observations areassociated with proposals 9070 and 9783. The structure of galactic disks. Studying late-type spiral galaxies using SDSSUsing imaging data from the SDSS survey, we present the g' and r' radialstellar light distribution of a complete sample of ~90 face-on tointermediate inclined, nearby, late-type (Sb-Sdm) spiral galaxies. Thesurface brightness profiles are reliable (1 σ uncertainty lessthan 0.2 mag) down to μ27 mag/''. Only ~10% of all galaxies havea normal/standard purely exponential disk down to our noise limit. Thesurface brightness distribution of the rest of the galaxies is betterdescribed as a broken exponential. About 60% of the galaxies have abreak in the exponential profile between  1.5-4.5 times thescalelength followed by a downbending, steeper outer region. Another~30% shows also a clear break between  4.0-6.0 times thescalelength but followed by an upbending, shallower outer region. A fewgalaxies have even a more complex surface brightness distribution. Theshape of the profiles correlates with Hubble type. Downbending breaksare more frequent in later Hubble types while the fraction of upbendingbreaks rises towards earlier types. No clear relation is found betweenthe environment, as characterised by the number of neighbours, and theshape of the profiles of the galaxies. The chemical composition of metal-poor emission-line galaxies in the Data Release 3 of the Sloan Digital Sky SurveyWe have re-evaluated empirical expressions for the abundancedetermination of N, O, Ne, S, Cl, Ar and Fe taking into account thelatest atomic data and constructing an appropriate grid ofphotoionization models with state-of-the art model atmospheres. Usingthese expressions we have derived heavy element abundances in the ~310emission-line galaxies from the Data Release 3 of the Sloan Digital SkySurvey (SDSS) with an observed Hβ flux F(Hβ) >10-14 erg s-1 cm-2 and for which the [Oiii] λ4363 emission line was detected at least at a 2σlevel, allowing abundance determination by direct methods. The oxygenabundance 12 + log O/H of the SDSS galaxies lies in the range from ~7.1(Zȯ/30) to ~8.5 (0.7 Zȯ). The SDSSsample is merged with a sample of 109 blue compact dwarf (BCD) galaxieswith high quality spectra, which contains extremely low-metallicityobjects. We use the merged sample to study the abundance patterns oflow-metallicity emission-line galaxies. We find that extremelymetal-poor galaxies (12 + log O/H < 7.6, i.e. Z -1.6, implying that theyhave a different nature than the subsample of high-redshift dampedLyα systems with log N/O of ~-2.3 and that their ages are largerthan 100-300 Myr. We confirm the apparent increase in N/O withdecreasing EW(Hβ), already shown in previous studies, and explainit as the signature of gradual nitrogen ejection by massive stars fromthe most recent starburst. Comparison of Star Clusters With and Without Wolf-Rayet Stars in Wolf-Rayet GalaxiesWe compare the properties of young star clusters with and withoutWolf-Rayet (W-R) stars in W-R galaxies using optical, near-infraredimagery and optical spectroscopy. Our work identifies the clusters withW-R stars in these galaxies for the first time. With this information,comparisons of clusters with and without W-R stars are now possible,enabling us to understand the chemical and morphological impact ofmassive stars on their environment and to constrain the parameters formodeling these systems. We find that clusters with W-R stars (W-Rclusters) are systematically younger, bluer clusters. Knowing this agedifference between the two cluster sets, we use an evolutionary scenarioto interpret their other properties. Young clusters, typically W-Rclusters, have a Strömgren sphere-like gas configuration. They alsotend to have H-K colors redder than those of theoretical models. Weinterpret the H-K excess as a combination of thermal emission from hotdust, nebular emission, and molecular emission. Older clusters,typically clusters without W-R stars, have ionized gas in a superbubbleconfiguration caused by the prolonged influence of stellar winds andsupernovae. The H-K excess is generally absent for these clusters. Thenitrogen-to-oxygen abundance ratio (N/O) does not appear to increase asa function of age over the first 10 Myr. Systems without W-R stars doappear to have a significant, elevated N/O over systems with W-R starsin the metallicity range 12+log(O/H)=7.7-7.9. For the entire metallicityrange in our sample, this finding is only marginally significant. Weconcur with previous studies, which find no correlation between thesulfur-to-oxygen abundance ratio and metallicity. Massive star populations in Wolf-Rayet galaxiesWe analyse long-slit spectral observations of 14 Wolf-Rayet (WR)galaxies from the sample of Schaerer, Contini & Pindao. All 14galaxies show broad WR emission in the blue region of the spectrum,consisting of a blend of NIIIλ4640, CIIIλ4650,CIVλ4658 and HeIIλ4686 emission lines, which is a spectralcharacteristic of WN stars. Broad CIVλ5808 emission, termed thered bump, is detected in nine galaxies and CIIIλ5996 is detectedin six galaxies. These emission features are due to WC stars. We derivethe numbers of late WN and early WC stars from the luminosity of theblue and red bumps, respectively. The number of O stars is estimatedfrom the luminosity of the Hβ emission line, after subtracting thecontribution of WR stars. The Schaerer & Vacca models predict thatthe number of WR stars relative to O stars,NWR/NO, increases with metallicity. Forlow-metallicity galaxies, the results agree with predictions ofevolutionary synthesis models for galaxies with a burst of starformation, and indicate an initial mass function (IMF) slope -2<~Γ<~- 2.35 in the low-metallicity regime. Forhigh-metallicity galaxies our observations suggest a Salpeter IMF(Γ=-2.35) and an extended short burst. The main possible sourcesof error are the adopted luminosities for single WCE and WNL stars. Wealso report, for the first time, on NGC 450 as a galaxy with WRcharacteristics. For NGC 450, we estimate the number of WN and WC stars.The number ratio NWR/NO, and the equivalent widthsof the blue bump, EWλ4686, and of the red bump,EWλ5808, in NGC 450 are also in good agreement withthe instantaneous burst model prediction for WR galaxies. Strong Emission Line H II Galaxies in the Sloan Digital Sky Survey. I. Catalog of DR1 Objects with Oxygen Abundances from Te MeasurementsWe present the first edition of the SDSS H II galaxies with Oxygenabundances Catalog (SHOC), which is a listing of strong emission-linegalaxies (ELGs) from the Sloan Digital Sky Survey (SDSS). Oxygenabundances have been obtained with the classic Te method. Wedescribe the method exploiting the SDSS database to construct thissample. The selection procedures are described and discussed in detail,as well as some problems encountered in the process of deriving reliableemission line parameters. The method was applied to the SDSS DataRelease 1 (DR1). We present 612 SDSS emission-line galaxies (624separate SDSS targets in total), for which the oxygen abundances12+log(O/H) have rms uncertainties <=0.20 dex. The subsample of 263ELGs (272 separate SDSS targets) have an uncertainty <=0.10 dex,while 459 ELGs (470 separate SDSS targets) have an uncertainty <=0.15dex. The catalog includes the main parameters of all selected ELGs, theintensities and equivalent widths of hydrogen and oxygen emission lines,as well as oxygen abundances with their uncertainties. The informationon the presence of Wolf-Rayet blue and/or red bumps in 109 galaxies isalso included. With the use of combined g, r, i SDSS images we performedvisual morphological classification of all SHOC galaxies. Four hundredsixty-one galaxies (~75%) are classified as confident or probable bluecompact galaxies (BCG/BCG?), 78 as irregular ones, 20 as low surfacebrightness galaxies (LSBG), 10 as obviously interacting, and 43 asspiral galaxies. In creating the catalog, 30 narrow-line active galacticnuclei and 69 LINERs were also identified; these are also presentedapart of the main catalog. We outline briefly the content of thecatalog, and the prospects of its use for statistical studies of thestar formation and chemical evolution issues. Some of these studies willbe presented in the forthcoming paper. Finally, we show that the methodpresented by Kniazev et al. for calculating O+/H+using intensities of the [O II] λλ7320, 7330 lines forSDSS emission-line spectra in the absence of [O II] λ3727 lineappears to yield reliable results over a wide range of studied oxygenabundances: 7.10<12+log(O/H)<8.5. Systematic Effects and a New Determination of the Primordial Abundance of 4He and dY/dZ from Observations of Blue Compact GalaxiesWe use spectroscopic observations of a sample of 82 H II regions in 76blue compact galaxies to determine the primordial helium abundanceYp and the slope dY/dZ from the Y-O/H linear regression. Toimprove the accuracy of the dY/dZ measurement, we have included newspectrophotometric observations of 33 H II regions that span a largemetallicity range, with oxygen abundance 12+log(O/H) varying between7.43 and 8.30 (Zsolar/30<=Z<=Zsolar/4). Mostof the new galaxies were selected from the First Byurakan, theHamburg/SAO, and the University of Michigan objective prism surveys. Fora subsample of seven H II regions, we derive the He mass fraction takinginto account known systematic effects, including collisional andfluorescent enhancements of He I emission lines, collisional excitationof hydrogen emission, underlying stellar He I absorption, and thedifference between the temperatures Te(He II) in theHe+ zone and Te(O III) derived from thecollisionally excited [O III] lines. We find that the net result of allthe systematic effects combined is small, changing the He mass fractionby less than 0.6%. By extrapolating the Y versus O/H linear regressionto O/H=0 for seven H II regions of this subsample, we obtainYp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds todY/dZ=3.7+/-1.2, assuming the oxygen mass fraction to be O=0.66Z. In theframework of the standard big bang nucleosynthesis theory, thisYp corresponds toΩbh2=0.012+0.003-0.002,where h is the Hubble constant in units of 100 km s-1Mpc-1. This is smaller at the 2 σ level than the valueobtained from recent deuterium abundance and microwave backgroundradiation measurements. The linear regression slope dY/dO=4.3+/-0.7(corresponding to dY/dZ=2.8+/-0.5) for the whole sample of 82 H IIregions is similar to that derived for the subsample of seven H IIregions, although it has a considerably smaller uncertainty. A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. II. Cluster Sizes and Structural Parameter CorrelationsWe investigate the structural properties of nuclear star clusters inlate-type spiral galaxies. More specifically, we fit analytical modelsto Hubble Space Telescope images of 39 nuclear clusters in order todetermine their effective radii after correction for the instrumentalpoint-spread function. We use the results of this analysis to comparethe luminosities and sizes of nuclear star clusters to those of otherellipsoidal stellar systems, in particular the Milky Way globularclusters. Our nuclear clusters have a median effective radius ofre=3.5 pc, with 50% of the sample falling in the range2.4pc<=re<=5.0pc. This narrow size distribution isstatistically indistinguishable from that of Galactic globular clusters,even though the nuclear clusters are, on average, 4 mag brighter thanthe old globular clusters. We discuss some possible interpretations ofthis result. From a comparison of nuclear cluster luminosities withvarious properties of their host galaxies, we confirm that more luminousgalaxies harbor more luminous nuclear clusters. It remains unclearwhether this correlation mainly reflects the influence of galaxy size,mass, and/or star formation rate. Since the brighter galaxies in oursample typically have stellar disks with a higher central surfacebrightness, nuclear cluster luminosity also correlates with thisproperty of their hosts. On the other hand, we find no evidence for acorrelation between the presence of a nuclear star cluster and thepresence of a large-scale stellar bar. The ISOPHOT 170 μm Serendipity Survey II. The catalog of optically identified galaxies%The ISOPHOT Serendipity Sky Survey strip-scanning measurements covering≈15% of the far-infrared (FIR) sky at 170 μm were searched forcompact sources associated with optically identified galaxies. CompactSerendipity Survey sources with a high signal-to-noise ratio in at leasttwo ISOPHOT C200 detector pixels were selected that have a positionalassociation with a galaxy identification in the NED and/or Simbaddatabases and a galaxy counterpart visible on the Digitized Sky Surveyplates. A catalog with 170 μm fluxes for more than 1900 galaxies hasbeen established, 200 of which were measured several times. The faintest170 μm fluxes reach values just below 0.5 Jy, while the brightest,already somewhat extended galaxies have fluxes up to ≈600 Jy. For thevast majority of listed galaxies, the 170 μm fluxes were measured forthe first time. While most of the galaxies are spirals, about 70 of thesources are classified as ellipticals or lenticulars. This is the onlycurrently available large-scale galaxy catalog containing a sufficientnumber of sources with 170 μm fluxes to allow further statisticalstudies of various FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) areMPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena,Imperial College London.Full Table 4 and Table 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/cgi-bin/qcat?J/A+A/422/39 The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxiesWe discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/ Star Formation Rates of Local Blue Compact Dwarf Galaxies. I. 1.4 GHz and 60 Micron LuminositiesWe determine and examine the star formation rates of 50 well-known,local blue compact dwarf (BCD) galaxies based on their 1.4 GHz and 60μm luminosities. We find that in cases for which both radio andfar-infrared luminosities are available, the resulting star formationrates agree extremely well with one another. We determine that the starformation rates of the BCD galaxies in our sample span nearly 5 ordersof magnitude, from approximately a few times 10-3 to severaltimes 101 Msolar yr-1, with a medianSFR of about 0.3 Msolar yr-1. We discuss trends ofmetallicity (primarily oxygen abundance) with star formation rate andexplore the connections between SFR and galaxy mass estimates. Bar Galaxies and Their EnvironmentsThe prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment. A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. I. Observations and Image AnalysisWe present new Hubble Space Telescope I-band images of a sample of 77nearby late-type spiral galaxies with low inclination. The main purposeof this catalog is to study the frequency and properties of nuclear starclusters. In 59 galaxies of our sample, we have identified a distinct,compact (but resolved), and dominant source at or very close to thephotocenter. In many cases, these clusters are the only prominent sourcewithin a few kiloparsecs from the galaxy nucleus. We present surfacebrightness profiles, derived from elliptical isophote fits, of allgalaxies for which the fit was successful. We use the fitted isophotesat radii larger than 2" to check whether the location of the clustercoincides with the photocenter of the galaxy and confirm that in nearlyall cases, we are truly dealing with nuclear'' star clusters. Fromanalytical fits to the surface brightness profiles, we derive thecluster luminosities after subtraction of the light contribution fromthe underlying galaxy disk and/or bulge. Based on observations made withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS 5-26555. Theseobservations are associated with proposal 8599. On the oxygen abundance determination in HII regions. High-metallicity regionsThis is our second paper devoted to the problem of line intensity -oxygen abundance calibration starting from the idea of McGaugh(\cite{mcg91}) that the strong oxygen lines ([OII] lambda lambda 3727,3729 and [OIII] lambda lambda 4959, 5007) contain the necessaryinformation to determine accurate abundances in HII regions. In theprevious study (Pilyugin 2000) the corresponding relations were obtainedfor the low-metallicity HII regions (12+log O/H <= 7.95, the lowerbranch of the O/H - R23 diagram). The high-metallicity HIIregions (12+log O/H >= 8.2, the upper branch of the O/H -R23 diagram) are considered in the present study. A relationof the type O/H=f(P, R23) between oxygen abundance and thevalue of abundance index R23, introduced by Pagel et al.(\cite{pag79}), and the excitation parameter P (which is defined here asthe contribution of the radiation in [OIII] lambda lambda 4959, 5007lines to the total" oxygen radiation) has been derived empiricallyusing the available oxygen abundances determined via measurement of atemperature-sensitive line ratio [OIII]4959,5007/[OIII]4363(Te-method). By comparing oxygen abundances inhigh-metallicity HII regions derived with the Te-method andthose derived with the suggested relations (P-method), it was found thatthe precision of oxygen abundance determination with the P-method isaround 0.1 dex (the mean difference for the 38 HII regions considered is~ 0.08 dex) and is comparable to that of the Te-method. Arelation of the type Te=f(P, R23) between electrontemperature and the values of abundance index R23 and theexcitation parameter P was derived empirically using the availableelectron temperatures determined via measurement oftemperature-sensitive line ratios. The maximum value of differencesbetween electron temperatures determined via measurement oftemperature-sensitive line ratios and those derived with the suggestedrelation is around 1000 K for HII regions considered here, the meanvalue of differences for 38 HII regions is ~ 500 K, which is the sameorder of magnitude as the uncertainties of electron temperaturedeterminations in high-metallicity HII regions via measuredtemperature-sensitive line ratios. Population synthesis of Hii galaxiesWe study the stellar population of galaxies with active star formation,determining ages of the stellar components by means of spectralpopulation synthesis of their absorption spectra. The data consist ofoptical spectra of 185 nearby (z<=0.075) emission-line galaxies. Theyare mostly Hii galaxies, but we also include some starbursts and Seyfert2s, for comparison purposes. They were grouped into 19 highsignal-to-noise ratio template spectra, according to their continuumdistribution, absorption- and emission-line characteristics. Thetemplates were then synthesized with a star cluster spectral base. Thesynthesis results indicate that Hii galaxies are typically age-compositestellar systems, presenting important contributions from generations upto as old as 500Myr. We detect a significant contribution of populationswith ages older than 1Gyr in two groups of Hii galaxies. The agedistributions of stellar populations among starbursts can varyconsiderably despite similarities in the emission-line spectra. In thecase of Seyfert 2 groups we obtain important contributions from the oldpopulation, consistent with a bulge. From the diversity of starformation histories, we conclude that typical Hii galaxies in the localUniverse are not systems presently forming their first stellargeneration. Nearby Optical Galaxies: Selection of the Sample and Identification of GroupsIn 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. Seeing Galaxies through Thick and Thin. I. Optical Opacity Measures in Overlapping GalaxiesWe describe the use of partially overlapping galaxies to provide directmeasurements of the effective absorption in galaxy disks, independent ofassumptions about internal disk structure. The nonoverlapping parts ofthe galaxies and symmetry considerations are used to reconstruct, viadifferential photometry, how much background galaxy light is lost inpassing through the foreground disks. Extensive catalog searches andfollow-up imaging yield ~15-25 nearby galaxy pairs suitable for varyingdegrees of our analysis; 11 of the best such examples are presentedhere. From these pairs, we find that interarm extinction is modest,declining from AB~1 mag at 0.3RB25 toessentially zero by RB25; the interarm dust has ascale length consistent with that of the disk starlight. In contrast,dust in spiral arms and resonance rings may be optically thick(AB>2) at virtually any radius. Some disks have flatterextinction curves than the Galaxy, with AB/AI~1.6this is probably the signature of clumpy dust distributions. Even thoughtypical spirals are not optically thick throughout their disks, wherethey are optically thick is correlated with where they are mostluminous: in spiral arms and inner disks. This correlation betweenabsorption and emission regions may account for their apparent surfacebrightness being only mildly dependent on inclination, erroneouslyindicating that spirals are generally optically thick. Taken as anensemble, the opacities of spiral galaxies may be just great enough tosignificantly affect QSO counts, though not enough to cause theirhigh-redshift cutoff. Based in part on archival observations with theNASA/ESA Hubble Space Telescope (HST) obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS5-26555. Ionization Corrections for Low-Metallicity H II Regions and the Primordial Helium AbundanceHelium and hydrogen recombination lines observed in low-metallicity,extragalactic H II regions provide the data used to infer the primordialhelium mass fraction, YP. The ionizationcorrections for unseen neutral helium (or hydrogen) are usually assumedto be absent; i.e., the ionization correction factor (ICF) is taken tobe unity (ICF=1). In this paper we revisit the question of the ICF for HII regions ionized by clusters of young, hot, metal-poor stars. Our keyresult is that for the H II regions used in the determination ofYP, there is a reverse'' ionizationcorrection: ICF<1. We explore the effect on the ICF of more realisticinhomogeneous H II region models and find that for those regions ionizedby young stars, with hard'' radiation spectra, the ICF is reducedfurther below unity. In Monte Carlos using H II region data from theliterature (Izotov & Thuan) we estimate a reduction in the publishedvalue of YP of order 0.003, which is roughlytwice as large as the quoted statistical error in theYP determination. A Spectroscopic Study of a Large Sample Of Wolf-Rayet GalaxiesWe analyze long-slit spectral observations of 39 Wolf-Rayet (WR)galaxies with heavy element mass fraction ranging over 2 orders ofmagnitude, from Zsolar/50 to 2Zsolar. Nearly allgalaxies in our sample show broad WR emission in the blue region of thespectrum (the blue bump) consisting of an unresolved blend of N IIIλ4640, C III λ4650, C IV λ4658, and He IIλ4686 emission lines. Broad C IV λ5808 emission (the redbump) is detected in 30 galaxies. Additionally, weaker WR emission linesare identified, most often the N III λ4512 and Si IIIλ4565 lines, which have very rarely or never been seen anddiscussed before in WR galaxies. These emission features arecharacteristic of WN7-WN8 and WN9-WN11 stars, respectively. We derivethe numbers of early WC (WCE) and late WN (WNL) stars from theluminosities of the red and blue bumps, and the number of O stars fromthe luminosity of the Hβ emission line. Additionally, we propose anew technique for deriving the numbers of WNL stars from the N IIIλ4512 and Si III λ4565 emission lines. This technique ispotentially more precise than the blue-bump method because it does notsuffer from contamination of WCE and early WN (WNE) stars and nebulargaseous emission. It is found that the relative number of WR starsN(WR)/N(O+WR) decreases with decreasing metallicity, in agreement withpredictions of evolutionary synthesis models. The relative number ratiosN(WC)/N(WN) and the equivalent widths of the blue bump EW(λ4650)and of the red bump EW(λ5808) derived from observations are alsoin satisfactory agreement with theoretical predictions, except for themost metal-deficient WR galaxies. A possible source of disagreement istoo low a line emission luminosity adopted for a single WCE star inlow-metallicity models. We assemble a sample of 30 H II regions withdetected He II λ4686 nebular emission to analyze the possibleconnection of this emission with the hard UV radiation of the WR stars.The theoretical predictions satisfactorily reproduce the observedintensities and equivalent widths of the He II λ4686 nebularemission line. However, galaxies with nebular He II λ4686emission do not always show WR emission. Therefore, in addition to theionization of He+ in the H II region by WR stars, othermechanisms for the origin of He II λ4686 such as radiative shocksprobably need to be invoked. H I observations of emission-line galaxiesWe present single-dish Lovell telescope H i observations of a sample of67 emission-line and UV-excess galaxies, of which 52 are taken from theUniversity of Michigan (UM) catalogue. In addition, H i observations of24 gas-rich irregular galaxies are presented. We find that emission-linegalaxies are H i-rich with a median H i mass to blue luminosity ratioMHI/LB of ~ 0.45 Msun/Lsun.Within the UM galaxy sample the MHI/LB ratio tendsto increase with decreasing luminosity. Finally, it is found that themost H i-rich UM galaxies are the most metal deficient, implying thatthese objects are less evolved. Spectral classification of emission-line galaxiesThe main goal of this work is to further investigate the classificationof emission-line galaxies from the Spectrophotometric Catalogue of HII galaxies'' by Terlevich et al. (1991) in a homogeneous and objectiveway, using the three line-ratio diagrams, called diagnostic diagrams, ofVeilleux & Osterbrock (1987). On the basis of the resultingcatalogue, we critically discuss the classification methods in theoptical range. In particular we compare our classification scheme to theone done by Rola et al. (1997) which is efficient for the classificationof redshifted galaxies. We also propose a new diagnostic diagraminvolving the known intensity ratio R23=([O II],l 3727+[OIII] l 4959+{[O III] l 5007)/Hb which appears to be a very goodcriterion allowing to discriminate the Seyfert 2 from H ii galaxies. Therevised catalogue including 314 narrow-emission-line galaxies contains HII galaxies, Seyfert 2 galaxies, Low Ionization Nuclear Emission-LineRegions (hereafter LINERs) galaxies and some particular types ofgalaxies with the most intriguing ones, called ambiguous'', due to theambiguity of their location in the diagnostic diagrams. These galaxiesappear as H II galaxies and as active galactic nuclei (hereafter AGNs)in different diagrams of Veilleux & Osterbrock and constitutecertainly a sample of particularly interesting candidates for a thoroughstudy of connections between starbursts and AGNs. Available inelectronic form only via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html Arcsecond Positions of UGC GalaxiesWe 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. The Mass-to-Light Ratio of Binary GalaxiesWe report on the mass-to-light ratio determination based on a newlyselected binary galaxy sample, which includes a large number of pairswhose separations exceed a few hundred kpc. The probabilitydistributions of the projected separation and the velocity differencehave been calculated considering the contamination of optical pairs, andthe mass-to-light (M/L) ratio has been determined based on the maximumlikelihood method. The best estimate of the M/L in the B band for 57pairs is found to be 28-36 depending on the orbital parameters and thedistribution of optical pairs (solar unit: H_0=50 km s^-1 Mpc^-1). Thebest estimate of the M/L for 30 pure spiral pairs is found to be 12-16.These results are relatively smaller than those obtained in previousstudies but are consistent with each other within the errors. Althoughthe number of pairs with large separation is significantly increasedcompared with previous samples, the M/L does not show any tendency ofincrease but is found to be almost independent of the separation ofpairs beyond 100 kpc. The constancy of the M/L beyond 100 kpc mayindicate that the typical halo size of spiral galaxies is less than ~100kpc. Heavy-Element Abundances in Blue Compact GalaxiesWe present high-quality ground-based spectroscopic observations of 54supergiant H II regions in 50 low-metallicity blue compact galaxies withoxygen abundances 12+logO/H between 7.1 and 8.3. We use the data todetermine abundances for the elements N, O, Ne, S, Ar, and Fe. We alsoanalyze Hubble Space Telescope (HST) Faint Object Spectrograph archivalspectra of 10 supergiant H II regions to derive C and Si abundances in asubsample of seven BCGs. The main result of the present study is thatnone of the heavy element-to-oxygen abundance ratios studied here (C/O,N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGswith 12+logO/H<=7.6 (Z<=Z_solar/20). This constancy implies thatall of these heavy elements have a primary origin and are produced bythe same massive (M>=10 M_solar) stars responsible for O production.The dispersion of the ratios C/O and N/O in these galaxies is found tobe remarkably small, being only +/-0.03 and +/-0.02 dex, respectively.This very small dispersion is strong evidence against any time-delayedproduction of C and primary N in the lowest metallicity BCGs (secondaryN production is negligible at these low metallicities). The absence of atime-delayed production of C and N is consistent with the scenario thatgalaxies with 12+logO/H<=7.6 are now undergoing their first burst ofstar formation, and that they are therefore young, with ages notexceeding 40 Myr. If very low metallicity BCGs are indeed young, thiswould argue against the commonly held belief that C and N are producedby intermediate-mass (3 M_solar<=M<=9 M_solar) stars at very lowmetallicities, as these stars would not have yet completed theirevolution in these lowest metallicity galaxies. In higher metallicityBCGs (7.6<12+logO/H<8.2), the abundance ratios Ne/O, Si/O, S/O,Ar/O, and Fe/O retain the same constant value they had at lowermetallicities. By contrast, there is an increase of C/O and N/O alongwith their dispersions at a given O. We interpret this increase as dueto the additional contribution of C and primary N production inintermediate-mass stars, on top of that by high-mass stars. The aboveresults lead to the following timeline for galaxy evolution: (1) allobjects with 12+logO/H<=7.6 began to form stars less than 40 Myr ago;(2) after 40 Myr, all galaxies have evolved so that 12+logO/H>7.6 (3)by the time intermediate-mass stars have evolved and released theirnucleosynthetic products (100-500 Myr), all galaxies have becomeenriched to 7.6<12+logO/H<8.2. The delayed release of primary N atthese metallicities greatly increases the scatter in N/O; (4) later,when galaxies get enriched to 12+logO/H>8.2, secondary N productionbecomes important. BCGs show the same O/Fe overabundance with respect tothe Sun (~0.4 dex) as Galactic halo stars, suggesting the same chemicalenrichment history. We compare heavy elements yields derived from theobserved abundance ratios with theoretical yields for massive stars andfind general good agreement. However, the theoretical models are unableto reproduce the observed N/O and Fe/O. Further theoretical developmentsare necessary, in particular to solve the problem of primary nitrogenproduction in low-metallicity massive stars. We discuss the apparentdiscrepancy between abundance ratios N/O measured in BCGs and those inhigh-redshift damped Lyalpha galaxies, which are up to 1 order ofmagnitude smaller. We argue that this large discrepancy may arise fromthe unknown physical conditions of the gas responsible for the metallicabsorption lines in high-redshift damped Lyalpha systems. While it iswidely assumed that the absorbing gas is neutral, we propose that itcould be ionized. In this case, ionization correction factors can boostN/O in damped Lyalpha galaxies into the range of those measured in BCGs. The I-Band Tully-Fisher Relation for SC Galaxies: 21 Centimeter H I Line DataA compilation of 21 cm line spectral parameters specifically designedfor application of the Tully-Fisher (TF) distance method is presentedfor 1201 spiral galaxies, primarily field Sc galaxies, for which opticalI-band photometric imaging is also available. New H I line spectra havebeen obtained for 881 galaxies. For an additional 320 galaxies, spectraavailable in a digital archive have been reexamined to allow applicationof a single algorithm for the derivation of the TF velocity widthparameter. A velocity width algorithm is used that provides a robustmeasurement of rotational velocity and permits an estimate of the erroron that width taking into account the effects of instrumental broadeningand signal-to-noise. The digital data are used to establish regressionrelations between measurements of velocity widths using other commonprescriptions so that comparable widths can be derived throughconversion of values published in the literature. The uniform H I linewidths presented here provide the rotational velocity measurement to beused in deriving peculiar velocities via the TF method. The I-Band Tully-Fisher Relation for SC Galaxies: Optical Imaging DataProperties derived from the analysis of photometric I-band imagingobservations are presented for 1727 inclined spiral galaxies, mostly oftypes Sbc and Sc. The reduction, parameter extraction, and errorestimation procedures are discussed in detail. The asymptotic behaviorof the magnitude curve of growth and the radial variation in ellipticityand position angle are used in combination with the linearity of thesurface brightness falloff to fit the disk portion of the profile. TotalI-band magnitudes are calculated by extrapolating the detected surfacebrightness profile to a radius of eight disk scale lengths. Errors inthe magnitudes, typically ~0.04 mag, are dominated by uncertainties inthe sky subtraction and disk-fitting procedures. Comparison is made withthe similar imaging database of Mathewson, Ford, & Buchhorn, both aspresented originally by those authors and after reanalyzing theirdigital reduction files using identical disk-fitting procedures. Directcomparison is made of profile details for 292 galaxies observed incommon. Although some differences occur, good agreement is found,proving that the two data sets can be used in combination with onlyminor accommodation of those differences. The compilation of opticalproperties presented here is optimized for use in applications of theTully-Fisher relation as a secondary distance indicator in studies ofthe local peculiar velocity field. Galaxy coordinates. II. Accurate equatorial coordinates for 17298 galaxiesUsing images of the Digitized Sky Survey we measured coodinates for17298 galaxies having poorly defined coordinates. As a control, wemeasured with the same method 1522 galaxies having accurate coordinates.The comparison with our own measurements shows that the accuracy of themethod is about 6 arcsec on each axis (RA and DEC). New catalogue of Wolf-Rayet galaxies and high-excitation extra-galactic HII regionsWe present a new compilation of Wolf-Rayet (WR) galaxies andextra-galactic Hii regions showing broad He ii lambda 4686 emissiondrawn from the literature. Relevant information on the presence of otherbroad emission lines ([N i] lambda 5199ii, C iv lambda 5808 and others)from WR stars of WN and WC subtypes, and other existing broad nebularlines is provided. In total we include 139 known WR galaxies. Amongthese, 57 objects show both broad He ii lambda 4686 and C iv lambda 5808features. In addition to the broad (stellar) He ii lambda 4686 emission,a nebular He ii component is well established (suspected) in 44 (54)objects. We find 19 extra-galatic Hii regions without WR detectionsshowing nebular He ii lambda 4686 emission. The present sample can beused for a variety of studies on massive stars, interactions of massivestars with the ISM, stellar populations, starburst galaxies etc. Thedata is accessible electronically and will be updated periodically. Thecatalogue is available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Effects of interaction on the properties of spiral galaxies. II. Isolated galaxies: The zero pointWe analyse the properties of a sample of 22 bright isolated spiralgalaxies on the basis of Johnson B,V,I images and optical rotationcurves. The fraction of early morphological types in our sample ofisolated galaxies (or in other samples of non-interacting spiralgalaxies) appears to be smaller than in samples including interactingsystems. The overall morphological aspect is regular and symmetric, butall the galaxies present non-axisymmetric components in the form of barsor rings. We find that the color indices become bluer towards the outerparts and that their central values are well correlated with the totalcolors. The properties of the bulges span a larger range than those ofthe disks, that thus are more alike between them. None of the galaxiesshows a truncated, type II disk profile. It is found that the relationbetween surface brightness and size for the bulges, the Kormendyrelation, is tighter when only isolated galaxies are considered. We finda similar relation for the disk parameters with an unprecedented lowscatter. A Principal Component Analysis of the measured parameters showsthat 2 eigenvectors suffice to explain more than 95 % of the totalvariance. These are, as found for other samples including spiralgalaxies in different environmental situations, a scale parameter givenby the mass or, equivalently, the luminosity or the size; and a formparameter given by the bulge to disk luminosity ratio, B/D, or,equivalently, by the gradient of the solid-body rotation region of therotation curve, the G-parameter. We report here a tight correlationbetween G and B/D for our sample of isolated spirals that could be usedas a new distance indicator. Based on data obtained at the 1.5mtelescope of the Estacion de Observacion de Calar Alto, InstitutoGeografico Nacional, which is jointly operated by the InstitutoGeografico Nacional and the Consejo Superior de InvestigacionesCientificas through the Instituto de Astrofisica de Andalucia B and R CCD surface photometry of late-type galaxiesWe present B and R CCD observations of 16 late-type galaxies, mainlyMagellanic irregulars and blue compact galaxies, at a limiting surfacebrightness of mu_ {B} ~ 26-27 mag arcsec(-2) . The sample is derivedfrom a H I-rich subset of an H I survey of late-type dwarfs and consistsof objects with -19.5<=MB<=-14.4, nine galaxies beingfainter than MB=-17 (H0=75 km s(-1) Mpc(-1) ).Radial luminosity distributions for all the sample objects arepresented. We find that the blue compacts become redder in (B-R) withincreasing distances from their centres confirming other studiesindicating that recent star formation in these galaxies may be centrallyconcentrated. The Im and Sm galaxies show flatter (B-R) colour profilesindicating less centrally concentrated recent star formation. Themajority of the galaxies have an identifiable exponential disk whilehalf also have a central bulge.
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