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A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
| A FUSE Survey of High-Latitude Galactic Molecular Hydrogen
Measurements of molecular hydrogen (H2) column densities arepresented for the first six rotational levels (J=0-5) for 73extragalactic targets observed with the Far Ultraviolet SpectroscopicExplorer (FUSE). All of these have a final signal-to-noise ratio largerthan 10 and are located at Galactic latitude |b|>20deg.The individual observations were calibrated with the FUSE calibrationpipeline CalFUSE version 2.1 or higher and then carefully aligned invelocity. The final velocity shifts for all the FUSE segments arelisted. H2 column densities or limits are determined for thesix lowest rotational (J) levels for each H I component in the line ofsight, using a curve-of-growth approach at low column densities(<16.5) and Voigt-profile fitting at higher column densities.Detections include 65 measurements of low-velocity H2 in theGalactic disk and lower halo. Eight sight lines yield nondetections forGalactic H2. The measured column densities range fromlogN(H2)=14 to 20. Strong correlations are found betweenlogN(H2) and T01, the excitation temperature ofthe H2, as well as between logN(H2) and the levelpopulation ratios (log[N(J')/N(J)]). The average fraction ofnuclei in molecular hydrogen [f(H2)] in each sight line iscalculated; however, because there are many H I clouds in each sightline, the physics of the transition from H I to H2 cannot bestudied. Detections also include H2 in 16intermediate-velocity clouds in the Galactic halo (out of 35 IVCs).Molecular hydrogen is seen in one high-velocity cloud (the Leading Armof the Magellanic Stream), although 19 high-velocity clouds areintersected; this strongly suggests that dust is rare or absent in theseobjects. Finally, there are five detections of H2 in externalgalaxies.
| Kinematics of Interstellar Gas in Nearby UV-selected Galaxies Measured with HST STIS Spectroscopy
We measure Doppler shifts of interstellar absorption lines in HST STISspectra of individual star clusters in nearby UV-selected galaxies.Values for systemic velocities, which are needed to quantify outflowspeeds, are taken from the literature and verified with stellar lines.We detect outflowing gas in 8 of 17 galaxies via low-ionization lines(e.g., C II, Si II, Al II), which trace cold and/or warm gas. Thestarbursts in our sample are intermediate in luminosity (and mass) todwarf galaxies and luminous infrared galaxies (LIRGs), and we confirmthat their outflow speeds (ranging from -100 to nearly -520 kms-1, with an accuracy of ~80 km s-1) areintermediate to those previously measured in dwarf starbursts and LIRGs.We do not detect the outflow in high-ionization lines (such as C IV orSi IV); higher quality data will be needed to empirically establish howvelocities vary with the ionization state of the outflow. We do verifythat the low-ionization UV lines and optical Na I doublet give roughlyconsistent outflow velocities, solidifying an important link betweenstudies of galactic winds at low and high redshift. To obtain a highersignal-to-noise ratio (S/N), we create a local average compositespectrum and compare it to the high-z Lyman break composite spectrum. Itis surprising that the low-ionization lines show similar outflowvelocities in the two samples. We attribute this to a combination ofweighting toward higher luminosities in the local composite, as well asboth samples being, on average, brighter than the ``turnover''luminosity in the v-SFR relation.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 NAS 5-26555. These observations areassociated with program GO-9036.
| A FUSE Survey of Interstellar Molecular Hydrogen toward High-Latitude AGNs
We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE)survey of interstellar molecular hydrogen (H2) along 45 sightlines to AGNs at high Galactic latitudes (b>20deg). Most(39 out of 45) of the sight lines show detectable Galactic H2absorption from Lyman and Werner bands between 1000 and 1126 Å,with column densities ranging fromNH2=1014.17 to 1019.82cm-2. In the northern Galactic hemisphere, we identify manyregions of low NH2 (<=1015cm-2) between l=60deg and 180° and atb>54deg. These ``H2 holes'' provide valuable,uncontaminated sight lines for extragalactic UV spectroscopy, and a fewmay be related to the ``Northern Chimney'' (low Na I absorption) and the``Lockman Hole'' (low NHI). A comparison of high-latitudeH2 with 139 OB star sight lines surveyed in the Galactic disksuggests that high-latitude and disk H2 clouds may havedifferent rates of heating, cooling, and UV excitation. For rotationalstates J=0 and 1, the mean excitation temperature at high latitude,=124+/-8 K, is somewhat higher thanthat in the Galactic disk, =86+/-20K. For J>=2, the mean =498+/-28 K, and thecolumn-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate acomparable degree of UV excitation in the disk and low halo for sightlines with NH2>=1018cm-2. The distribution of molecular fractions at highlatitude shows a transition at lower total hydrogen column density(logNhlH~20.38+/-0.13) than in the Galactic disk(logNdiskH~20.7). If the UV radiation fields aresimilar in disk and low halo, this suggests an enhanced H2(dust-catalyzed) formation rate in higher density, compressed clouds,which could be detectable as high-latitude, sheetlike infrared cirrus.
| Palomar/Las Campanas Imaging Atlas of Blue Compact Dwarf Galaxies. II. Surface Photometry and the Properties of the Underlying Stellar Population
We present the results from an analysis of surface photometry of B, R,and Hα images of a total of 114 nearby galaxies(vhelio<4000 km s-1) drawn from the Palomar/LasCampanas Imaging Atlas of blue compact dwarf (BCD) galaxies. Surfacebrightness and color profiles for the complete sample have beenobtained. We determine the exponential and Sérsic profiles thatbest fit the surface brightness distribution of the underlying stellarpopulation detected in these galaxies. We also compute the (B-R) colorand total absolute magnitude of the underlying stellar population andcompared them to the integrated properties of the galaxies in thesample. Our analysis shows that the (B-R) color of the underlyingpopulation is systematically redder than the integrated color, except inthose galaxies where the integrated colors are strongly contaminated byline and nebular-continuum emission. We also find that galaxies withrelatively red underlying stellar populations [typically (B-R)>=1mag] show structural properties compatible with those of dwarfelliptical galaxies (i.e., a smooth light distribution, fainterextrapolated central surface brightness, and larger scale lengths thanBCD galaxies with blue underlying stellar populations). At least ~15% ofthe galaxies in the sample are compatible with being dwarf elliptical(dE) galaxies experiencing a burst of star formation. For the remainingBCD galaxies in the sample we do not find any correlation between therecent star formation activity and their structural differences withrespect to other types of dwarf galaxies.
| Metallicity Effects on Mid-Infrared Colors and the 8 μm PAH Emission in Galaxies
We examine colors from 3.6 to 24 μm as a function of metallicity(O/H) for a sample of 34 galaxies. The galaxies range over 2 orders ofmagnitude in metallicity. They display an abrupt shift in the 8μm-to-24 μm color for metallicities between one-third andone-fifth of the solar value. The mean 8-to-24 μm flux density ratiobelow and above 12+log(O/H)=8.2 is 0.08+/-0.04 and 0.70+/-0.53,respectively. We use mid-IR colors and spectroscopy to demonstrate thatthe shift is primarily due to a decrease in the 8 μm flux density, asopposed to an increase in the 24 μm flux density. This result is mostsimply interpreted as being due to a weakening at low metallicity of themid-IR emission bands usually attributed to PAHs (polycyclic aromatichydrocarbons) relative to the small-grain dust emission. However,existing empirical spectral energy distribution models cannot accountfor the observed short-wavelength (below 8 μm) colors of thelow-metallicity galaxies merely by reducing the strength of the PAHfeatures; some other emission source (e.g., hot dust) is required.
| The Stellar Content of Nearby Star-forming Galaxies. III. Unravelling the Nature of the Diffuse Ultraviolet Light
We investigate the nature of the diffuse intracluster ultraviolet lightseen in 12 local starburst galaxies, using long-slit ultravioletspectroscopy obtained with the Space Telescope Imaging Spectrograph(STIS) aboard the Hubble Space Telescope (HST). We take this faintintracluster light to be the field in each galaxy and compare itsspectroscopic signature with Starburst99 evolutionary synthesis modelsand with neighboring star clusters. Our main result is that the diffuseultraviolet light in 11 of the 12 starbursts lacks the strong O starwind features that are clearly visible in spectra of luminous clustersin the same galaxies. The difference in stellar features dominatingcluster and field spectra indicates that the field light comes primarilyfrom a different stellar population and not from scattering of UVphotons originating in the massive clusters. A cut along the spatialdirection of the UV spectra establishes that the field light is notsmooth but rather shows numerous ``bumps and wiggles.'' Roughly 30%-60%of these faint peaks seen in field regions of the closest (<4 Mpc)starbursts appear to be resolved, suggesting a contribution fromsuperpositions of stars and/or faint star clusters. Complementary WFPC2UVI imaging for the three nearest target galaxies, NGC 4214, NGC 4449,and NGC 5253, is used to obtain a broader picture and establish that allthree galaxies have a dispersed population of unresolved, luminous bluesources. Because the field spectra are dominated by B stars, we suggestthat the individual sources observed in the WFPC2 images are individualB stars (rather than O stars) or small star clusters. We considerseveral scenarios to understand the lack of observed massive stars inthe field and their implications for the origin of the field stellarpopulation. If the field stellar populations formed in situ, the fieldmust have either an IMF that is steeper than Salpeter (α~-3.0 to-3.5) or a Salpeter slope with an upper mass cutoff of 30-50Msolar. If star formation occurs primarily in star clusters,the field could be composed of older, faded clusters and/or a populationthat is coeval with the luminous clusters but lower in mass. We usethese benchmark populations to place constraints on the field stellarpopulation origin. Although the field probably includes stars ofdifferent ages, the UV light is dominated by the youngest stellarpopulations in the field. If the field is composed of older, dissolvingclusters, we estimate that star clusters (regardless of mass) need todissolve on timescales 7-10 Myr to create the field. If the field iscomposed of young clusters that fall below the detection limit ofindividual sources in our spectroscopy, they would have to be severalhundred solar masses or less, in order to be deficient in O stars,despite their extreme youth.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Optimization of Starburst99 for Intermediate-Age and Old Stellar Populations
We have incorporated the latest release of the Padova models into theevolutionary synthesis code Starburst99. The Padova tracks were extendedto include the full asymptotic giant branch (AGB) evolution until thefinal thermal pulse over the mass range 0.9-5 Msolar. Withthis addition, Starburst99 accounts for all stellar phases thatcontribute to the integrated light of a stellar population witharbitrary age from the extreme-ultraviolet to the near-infrared. AGBstars are important for ages between 0.1 and 2 Gyr, with theircontribution increasing at longer wavelengths. We investigatesimilarities and differences between the model predictions by the Genevaand the Padova tracks. The differences are particularly pronounced atages >1 Gyr, when incompleteness sets in for the Geneva models. Wealso perform detailed comparisons with the predictions of other majorsynthesis codes and find excellent agreement. Our synthesized opticalcolors are compared to observations of old, intermediate-age, and youngpopulations. Excellent agreement is found for the old globular clustersystem of NGC 5128 and for old and intermediate-age clusters in NGC4038/4039. In contrast, the models fail for red supergiant-dominatedpopulations with subsolar abundances. This failure can be traced back toincorrect red supergiant parameters in the stellar evolutionary tracks.Our models and the synthesis code are publicly available as version 5.0of Starburst99 at http://www.stsci.edu/science/starburst99.
| Gas and Stars in an H I-Selected Galaxy Sample
We present the results of a J-band study of the H I-selected AreciboDual-Beam Survey and Arecibo Slice Survey galaxy samples using TwoMicron All Sky Survey data. We find that these galaxies span a widerange of stellar and gas properties. However, despite the diversitywithin the samples, we find a very tight correlation between luminosityand size in the J band, similar to that found in a previous paper byRosenberg & Schneider between the H I mass and size. We also findthat the correlation between the baryonic mass and the J-band diameteris even tighter than that between the baryonic mass and the rotationalvelocity.
| Multicolor Surface Photometry of Lenticular Galaxies. I. The Data
We present multicolor surface and aperture photometry in the B, V, R,and K' bands for a sample of 34 lenticular galaxies from the UppsalaGeneral Catalogue. From surface photometric analysis, we obtain radialprofiles of surface brightness, colors, ellipticity, position angle, andthe Fourier coefficients that describe the departure of isophotal shapesfrom a purely elliptical form; we find the presence of dust lanes,patches, and ringlike structure in several galaxies in the sample. Weobtain total integrated magnitudes and colors and find that these are ingood agreement with the values from the Third Reference Catalogue.Isophotal colors are correlated with each other, following the sequenceexpected for early-type galaxies. The color gradients in lenticulargalaxies are more negative than the corresponding gradients inelliptical galaxies. There is a good correlation between B-V and B-Rcolor gradients, and the mean gradients in the B-V, B-R, and V-K' colorsare -0.13+/-0.06, -0.18+/-0.06, and -0.25+/-0.11 mag dex-1 inradius, respectively.
| Dust properties of UV bright galaxies at z ~ 2
We investigate the properties of the extinction curve in the rest-frameUV for a sample of 34 UV-luminous galaxies at 2 < z < 2.5,selected from the FORS Deep Field (FDF) spectroscopic survey. A newparametric description of the rest-frame UV spectral energy distributionis adopted; its sensitivity to properties of the stellar populations orof dust attenuation is established with the use of models. The latterare computed by combining composite stellar population models andcalculations of radiative transfer of the stellar and scatteredradiation through the dusty interstellar medium (ISM) for a dust/starsconfiguration describing dust attenuation in local starbursts. In thefavoured configuration the stars are enveloped by a shell with atwo-phase, clumpy, dusty ISM. The distribution of the z 2UV-luminous FDF galaxies in several diagnostic diagrams shows that theirextinction curves range between those typical of the Small and LargeMagellanic Clouds (SMC and LMC, respectively). For the majority ofstrongly reddened objects having a UV continuum slope β > -0.4 asignificant 2175 Å absorption feature (or "UV bump") is inferred,indicating an LMC-like extinction curve. On the other hand, the UVcontinua of the least reddened objects are mostly consistent withSMC-like extinction curves, lacking a significant UV bump, as for thesample of local starbursts investigated by Calzetti and collaborators.Furthermore, the most opaque (β 0) and, thus (for ourmodels), dustiest UV-luminous FDF galaxies tend to be among the mostmetal-rich, most massive, and largest systems at z 2, indicating< Z > 0.5 {-} 1 Zȯ, < Mstars> 6 × 1010 Mȯ, and 4 kpc, respectively. The presence of the UVbump does not seem to depend on the total metallicity, as given by theequivalent width (EW) of the C IV doublet. Conversely, it seems to beassociated with a large average EW of the six most prominentinterstellar low-ionisation absorption lines falling in the FORSspectra. The average EW of these saturated lines offers a proxy for theISM topology. We interpret these results as the evidence for adifference in the properties of the dusty ISM among the most evolvedUV-luminous, massive galaxies at z 2.
| Principal component analysis of International Ultraviolet Explorer galaxy spectra
We analyse the UV spectral energy distribution of a sample of normalgalaxies listed in the International Ultraviolet Explorer (IUE) NewlyExtracted Spectra (INES) Guide No. 2 - Normal Galaxies using a principalcomponent analysis. The sample consists of the IUE short-wavelength (SW)spectra of the central regions of 118 galaxies, where the IUE apertureincluded more than 1 per cent of the galaxy size. The principalcomponents are associated with the main components observed in theultraviolet (UV) spectra of galaxies. The first component, accountingfor the largest source of diversity, may be associated with the UVcontinuum emission. The second component represents the UV contributionof an underlying evolved stellar population. The third component issensitive to the amount of activity in the central regions of galaxiesand measures the strength of star-formation events.In all the samples analysed here, the principal component representativeof star-forming activity accounts for a significant percentage of thevariance. The fractional contribution to the spectral energydistribution (SED) by the evolved stars and by the young population aresimilar.Projecting the SEDs on to their eigenspectra, we find that none of thecoefficients of the principal components can outline an internalcorrelation or can correlate with the optical morphological types. In asubsample of 43 galaxies, consisting of almost only compact and BCDgalaxies, the third principal component defines a sequence related tothe degree of starburst activity of the galaxy.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| C II Radiative Cooling of the Diffuse Gas in the Milky Way
The heating and cooling of the interstellar medium (ISM) allow the gasin the ISM to coexist at very different temperatures in thermal pressureequilibrium. The rate at which the gas cools or heats is therefore afundamental ingredient for any theory of the ISM. The heating cannot bedirectly determined, but the cooling can be inferred from observationsof .CII*, which is an important coolant in differentenvironments. The amount of cooling can be measured through either theintensity of the 157.7 μm [C II] emission line or the CII*absorption lines at 1037.018 and 1335.708 Å, observable with theFar Ultraviolet Spectroscopic Explorer and the Space Telescope ImagingSpectrograph on board the Hubble Space Telescope, respectively. Wepresent the results of a survey of these far-UV absorption lines in 43objects situated at |b|>~30deg. Measured column densitiesof CII*, S II, P II, and Fe II are combined with H I 21 cmemission measurements to derive the cooling rates (per H atom using H Iand per nucleon using S II) and to analyze the ionization structure,depletion, and metallicity content of the low-, intermediate-, andhigh-velocity clouds (LVCs, IVCs, and HVCs) along the different sightlines. Based on the depletion and the ionization structure, the LVCs,IVCs, and HVCs consist mostly of warm neutral and ionized clouds. Forthe LVCs, the mean cooling rate in ergs s-1 per H atom is-25.70+0.19-0.36 dex (1 σ dispersion). Witha smaller sample and a bias toward high H I column density, the coolingrate per nucleon is similar. The corresponding total Galactic C IIluminosity in the 157.7 μm emission line isL~2.6×107 Lsolar. CombiningN(CII*) with the intensity of Hα emission, we derivethat ~50% of the CII* radiative cooling comes from the warmionized medium (WIM). The large dispersion in the cooling rates iscertainly due to a combination of differences in the ionizationfraction, in the dust-to-gas fraction, and physical conditions betweensight lines. For the IVC Intermediate-Velocity (IV) Arch at z~1 kpc wefind that on average the cooling is a factor of 2 lower than in the LVCsthat probe gas at lower z. For an HVC (complex C, at z>6 kpc) we findthe much lower rate of -26.99+0.21-0.53 dex,similar to the rates observed in a sample of damped Lyα absorbersystems (DLAs). The fact that in the Milky Way a substantial fraction ofthe C II cooling comes from the WIM implies that this is probably alsotrue in the DLAs. We also derive the electron density, assuming atypical temperature of the warm gas of 6000 K: for the LVCs,=0.08+/-0.04 cm-3, and for the IV Arch,=0.03+/-0.01 cm-3 (1 σdispersion). Finally, we measured the column densities N(S II) and N(PII) in many sight lines and confirm that sulphur appears undepleted inthe ISM. Phosphorus becomes progressively more deficient whenlogN(HI)>19.7 dex, which can mean that either P becomes more depletedinto dust as more neutral gas is present or P is always depleted byabout -0.3 dex, but the higher value of P II at lower H I column densityindicates the need for an ionization correction.
| NGC 3125-1: The Most Extreme Wolf-Rayet Star Cluster Known in the Local Universe
We use Space Telescope Imaging Spectrograph long-slit ultravioletspectroscopy of local starburst galaxies to study the massive starcontent of a representative sample of super-star clusters, with aprimary focus on their Wolf-Rayet (W-R) content as measured from the HeII λ1640 emission feature. The goals of this work are threefold.First, we quantify the W-R and O-star content for selected massive youngstar clusters. These results are compared with similar estimates madefrom optical spectroscopy available in the literature. We conclude thatthe He II λ4686 equivalent width is a poor diagnostic measure ofthe true W-R content. Second, we present the strongest known He IIλ1640 emission feature in a local starburst galaxy. This featureis clearly of stellar origin in the massive cluster NGC 3125-1, as it isbroadened (~1000 km s-1). Strong N IV λ1488 and N Vλ1720 emission lines commonly found in the spectra of individualW-R stars of WN subtype are also observed in the spectrum of NGC 3125-1.Finally, we create empirical spectral templates to gain a basicunderstanding of the recently observed strong He II λ1640 featureseen in Lyman break galaxies (LBGs) at redshifts z~3. The UV fieldobserved in local starbursts provides a good overall match to thecontinuum and weak photospheric features in LBGs in the spectral rangeλλ1300-1700 but cannot reproduce the He II λ1640emission seen in the composite LBG sample of Shapley et al. Anadditional (ad hoc) 10%-15% contribution from ``extreme'' W-R clusterssimilar to NGC 3125-1 on top of the field provides a good match to thestrength of this feature.Based on observations with the NASA ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| 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
| Minor-axis velocity gradients in disk galaxies
We present the ionized-gas kinematics and photometry of a sample of 4spiral galaxies which are characterized by a zero-velocity plateau alongthe major axis and a velocity gradient along the minor axis,respectively. By combining these new kinematical data with thoseavailable in the literature for the ionized-gas component of the S0s andspirals listed in the Revised Shapley-Ames Catalog of Bright Galaxies werealized that about 50% of unbarred galaxies show a remarkable gasvelocity gradient along the optical minor axis. This fraction rises toabout 60% if we include unbarred galaxies with an irregular velocityprofile along the minor axis. This phenomenon is observed all along theHubble sequence of disk galaxies, and it is particularly frequent inearly-type spirals. Since minor-axis velocity gradients are unexpectedif the gas is moving onto circular orbits in a disk coplanar to thestellar one, we conclude that non-circular and off-plane gas motions arenot rare in the inner regions of disk galaxies.Based on observations carried out at the European Southern Observatoryin La Silla (Chile) (ESO 69.B-0706 and 70.B-0338), with the MultipleMirror Telescope which is a joint facility of the SmithsonianInstitution and the University of Arizona, and with the ItalianTelescopio Nazionale Galileo (AOT-5, 3-18) at the Observatorio del Roquede los Muchachos in La Palma (Spain).Table 1 is only available in electronic form athttp://www.edpsciences.org. Table 5 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/507
| Palomar/Las Campanas Imaging Atlas of Blue Compact Dwarf Galaxies. I. Images and Integrated Photometry
We present B, R, and Hα images for a total of 114 nearby galaxies(vhelio<4000 km s-1) that, with exception ofnine objects, are classified as blue compact dwarfs (BCDs). BRintegrated magnitudes, Hα fluxes and Hα equivalent widthsfor all the objects in the sample are presented. A new set ofquantitative, observational criteria for a galaxy to be classified as aBCD is proposed. These criteria include a limit on the K-band luminosity(i.e., stellar mass; MK>-21 mag), peak surface brightness(μB,peak<22 mag arcsec-2), and color at thepeak surface brightness(μB,peak-μR,peak<~1). Hα emissionis detected in all but three sample galaxies. Typical color, absolutemagnitude, and Hα luminosity are (B-R)=0.7+/-0.3 mag,MB=-16.1+/-1.4 mag, and log (LHα)=40.0+/-0.6(ergs s-1). Galaxies morphologically classified as nE and iEBCDs within our sample show lower Hα equivalent widths and reddercolors, on average, than the iI- and i0-type BCDs. For most of thegalaxies the presence of an evolved stellar population is required toexplain their observed properties; only the most metal-poor BCDs (e.g.,I Zw 18, Tol 65) are still compatible with a pure, young burst. Theflux-calibrated and WCS-compliant images in this Atlas are individuallyavailable through the NASA/IPAC Extragalactic Database (NED) imageserver and collectively through a dedicated Web page.
| Highly Ionized High-Velocity Gas in the Vicinity of the Galaxy
We report the results of a FUSE study of high-velocity O VI absorptionalong complete sight lines through the Galactic halo in directionstoward 100 extragalactic objects and two halo stars. The high-velocity OVI traces a variety of phenomena, including tidal interactions with theMagellanic Clouds, accretion of gas, outflowing material from theGalactic disk, warm/hot gas interactions in a highly extended Galacticcorona, and intergalactic gas in the Local Group. We identify 84high-velocity O VI features at >=3 σ confidence at velocitiesof -500=40+/-13 kms-1, and an average O VI column density=13.95+/-0.34 with a median value of 13.97. Values of bgreater than the 17.6 km s-1 thermal width expected for O VIat T~3×105 K indicate that additional nonthermalbroadening mechanisms are common. The O VI λ1031.926 absorptionis detected at >=3 σ confidence along 59 of the 102 sight linessurveyed. The high-velocity O VI detections indicate that ~60% of thesky (and perhaps as much as ~85%, depending on data qualityconsiderations) is covered by high-velocity H+ associatedwith the O VI. We find that N(H+)>~1018cm-2 if the high-velocity hot gas has a metallicity similarto that of the Magellanic Stream; this detection rate is considerablyhigher than that of high-velocity warm H I traced through its 21 cmemission at a comparable column density level. Some of the high-velocityO VI is associated with known H I structures (the Magellanic Stream,Complex A, Complex C, the Outer Spiral Arm, and several discrete H IHVCs). Some of the high-velocity O VI features have no counterpart in HI 21 cm emission, including discrete absorption features and positivevelocity absorption wings extending from ~100 to ~300 km s-1that blend with lower velocity absorption produced by the Galactic thickdisk/halo. The discrete features may typify clouds located in the LocalGroup, while the O VI absorption wings may be tidal debris or materialexpelled from the Galactic disk. Most of the O VI features havevelocities incompatible with those of the Galactic halo, even if thehalo has decoupled from the underlying Galactic disk. The reduction inthe dispersion about the mean of the high-velocity O VI centroids whenthe velocities are converted from the LSR to the GSR and LGSR referenceframes is necessary (but not conclusive) evidence that some of theclouds are located outside the Galaxy. Most of the O VI cannot beproduced by photoionization, even if the gas is irradiated byextragalactic ultraviolet background radiation. Several observationalquantities indicate that collisions in hot gas are the primaryionization mechanism responsible for the production of the O VI. Theseinclude the ratios of O VI column densities to those of other highlyionized species (C IV, N V) and the strong correlation between N(O VI)and O VI line width. Consideration of the possible sources ofcollisional ionization favors production of some of the O VI at theboundaries between cool/warm clouds of gas and a highly extended(R>~70 kpc), hot (T>106 K), low-density(n<~10-4-10-5 cm-3) Galactic coronaor Local Group medium. The existence of a hot, highly extended Galacticcorona or Local Group medium and the prevalence of high-velocity O VIare consistent with predictions of current galaxy formation scenarios.Distinguishing between the various phenomena producing high-velocity OVI in and near the Galaxy will require continuing studies of thedistances, kinematics, elemental abundances, and physical states of thedifferent types of high-velocity O VI found in this study. Descriptionsof galaxy evolution will need to account for the highly ionized gas, andfuture X-ray studies of hot gas in the Local Group will need to considercarefully the relationship of the X-ray absorption/emission to thecomplex high-velocity absorption observed in O VI.
| The Contribution of H I-rich Galaxies to the Damped Lyα Absorber Population at z = 0
We present a study of the expected properties of the low-redshift dampedLyα absorber population determined from a sample of H I-selectedgalaxies in the local universe. Because of a tight correlation betweenthe H I mass and H I cross section, which we demonstrate spans allgalaxy types, we can use our H I-selected sample to predict theproperties of the absorption-line systems. We use measurements of thenumber density and H I cross section of galaxies to show that the totalH I cross section at column densities sufficient to produce dampedLyα absorption is consistent with no evolution of the absorberpopulation. We also find that the dN/dz distribution is dominated bygalaxies with H I masses near 109 Msolar. However,because of the large dispersion in the correlation between H I mass andstellar luminosity, we find that the distribution of dN/dz as a functionof LJ is fairly flat. In addition, we examine the line widthsof the H I-selected galaxies and show that there may be evolution in thekinematics of H I-rich galaxies, but it is not necessary for the higherredshift population to contain a greater proportion of high-massgalaxies than we find locally.
| Companions of Bright Barred Shapley-Ames Galaxies
Companion galaxy environment for a subset of 78 bright and nearby barredgalaxies from the Shapley-Ames Catalog is presented. Among the spiralbarred galaxies, there are Seyfert galaxies, galaxies with circumnuclearstructures, galaxies not associated with any large-scale galaxy cloudstructure, galaxies with peculiar disk morphology (crooked arms), andgalaxies with normal disk morphology; the list includes all Hubbletypes. The companion galaxy list includes the number of companiongalaxies within 20 diameters, their Hubble type, and projectedseparation distance. In addition, the companion environment was searchedfor four known active spiral galaxies, three of them are Seyfertgalaxies, namely, NGC 1068, NGC 1097, and NGC 5548, and one is astarburst galaxy, M82. Among the results obtained, it is noted that theonly spiral barred galaxy classified as Seyfert 1 in our list has nocompanions within a projected distance of 20 diameters; six out of 10Seyfert 2 bar galaxies have no companions within 10 diameters, six outof 10 Seyfert 2 galaxies have one or more companions at projectedseparation distances between 10 and 20 diameters; six out of 12 galaxieswith circumnuclear structures have two or more companions within 20diameters.
| SCUBA observations of galaxies with metallicity measurements: a new method for determining the relation between submillimetre luminosity and dust mass
Using a new technique, we have determined a value for the constant ofproportionality between submillimetre emission and dust mass, the dustmass-absorption coefficient (κd) at 850μm. Ourmethod has an advantage over previous methods in that we avoidassumptions about the properties of dust in the interstellar medium. Ouronly assumption is that the fraction of metals incorporated in the dust(ɛ) in galaxies is a universal constant. To implement ourmethod, we require objects that have submillimetre and far-infrared fluxmeasurements as well as gas mass and metallicity estimates. We presentdata for all the galaxies with suitable measurements, including newsubmillimetre maps for five galaxies. We find κ850=0.07 +/- 0.02 m2 kg-1. We have also been able touse our sample to investigate our assumption that ɛ is auniversal constant. We find no evidence that ɛ is different fordwarf and giant galaxies, and show that the scatter in ɛ fromgalaxy to galaxy is apparently quite small.
| Hα, SCUBA and MERLIN imaging of NGC 4490
We describe Hα, SCUBA and MERLIN imaging of the interacting galaxypair NGC 4490 and 4485. We detect an Hα filament emerging from thedisc of NGC 4490 to a projected distance of 3kpc which has counterpartsin both the radio continuum and HI. The HI counterpart extends to aprojected distance of ~30kpc from NGC 4490 and we argue that this isevidence that the giant HI envelope in this system has its origins instar formation. We use SCUBA and radio continuum data to attempt toplace constraints on the distribution of dust with respect to the starforming regions. This analysis is limited by the lack of an independentestimate of the dust temperature, something that both `SIRTF' and`SOFIA' will be able to provide, however we find some evidence that mostobscuring dust is not located within HII regions themselves.
| A New Database of Observed Spectral Energy Distributions of Nearby Starburst Galaxies from the Ultraviolet to the Far-Infrared
We present a database of UV-to-FIR data of 83 nearby starburst galaxies.The galaxies are selected based upon the availability of IUE data. Wehave recalibrated the IUE UV spectra for these galaxies by incorporatingthe most recent improvements. For 45 of these galaxies we useobservations by Storchi-Bergmann et al. and McQuade et al. for thespectra in the optical range. The NIR data are from new observationsobtained at the NASA/IRTF and the Mount Laguna Observatory, combinedwith the published results from observations at the Kitt Peak NationalObservatory. In addition, published calibrated ISO data are included toprovide mid-IR flux densities for some of the galaxies. Theoptical-to-IR data are matched as closely as possible to the IUE largeaperture. In conjunction with IRAS and ISO FIR flux densities, all thesedata form a set of observed spectral energy distributions (SEDs) of thenuclear regions of nearby starburst galaxies. The SEDs should be usefulin studying star formation and dust/gas attenuation in galaxies. We alsopresent the magnitudes in the standard BVRI and various HST/WFPC2bandpasses synthesized from the UV and optical wavelength ranges ofthese SEDs. For some of the galaxies, the HST/WFPC2 magnitudessynthesized from the SEDs are checked with those directly measured fromWFPC2 images to test the photometric errors of the optical data andtheir effective matching of apertures with the UV data. The implicationsof the new SEDs on the star formation rates and dust/gas attenuation arebriefly discussed.
| Bar Galaxies and Their Environments
The 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.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| Dust-to-gas ratio and star formation history of blue compact dwarf galaxies
This paper investigates the origin of the observed large variety indust-to-gas ratio, D, among blue compact dwarf galaxies (BCDs). Byapplying our chemical evolution model, we find that the dust destructioncan largely suppress the dust-to-gas ratio when the metallicity of a BCDreaches 12+log (O/H) ~ 8, i.e., a typical metallicity level of BCDs. Wealso show that dust-to-gas ratio is largely varied owing to the changeof dust destruction efficiency that has two effects: (i) a significantcontribution of Type Ia supernovae to total supernova rate; (ii)variation of gas mass contained in a star-forming region. While massloss from BCDs was previously thought to be the major cause for thevariance of D, we suggest that the other two effects are also important.We finally discuss the intermittent star formation history, whichnaturally explains the large dispersion of dust-to-gas ratio among BCDs.
| Luminosity profiles of advanced mergers of galaxies using 2MASS data
A sample of 27 disturbed galaxies that show signs of interaction buthave a single nucleus were selected from the Arp and the Arp-Madorecatalogues. For these, the Ks band images from the Two MicronAll Sky Survey (2MASS) are analysed to obtain their radial luminosityprofiles and other structural parameters. We find that in spite of theirsimilar optical appearance, the sample galaxies vary in their dynamicalproperties, and fall into two distinct classes. The first class consistsof galaxies which can be described by a single r1/4 law andthe second class consists of galaxies that show an outer exponentialdisk. A few galaxies that have disturbed profiles cannot be fit intoeither of the above classes. However, all the galaxies are similar inall other parameters such as the far-infrared colours, the molecularhydrogen content and the central velocity dispersion. Thus, thedynamical parameters of these sets seem to be determined by the ratio ofthe initial masses of the colliding galaxies. We propose that thegalaxies in the first class result from a merger of spiral galaxies ofequal masses whereas the second class of galaxies results from a mergerof unequal mass galaxies. The few objects that do not fall into eithercategory show a disturbed luminosity profile and a wandering centre,which is indicative of these being unrelaxed mergers. Of the 27 galaxiesin our sample, 9 show elliptical-like profiles and 13 show an outerexponential. Interestingly, Arp 224, the second oldest merger remnant ofthe Toomre sequence shows an exponential disk in the outer parts.
| Recovering physical parameters from galaxy spectra using MOPED
We derive physical parameters of galaxies from their observed spectrausing MOPED, the optimized data compression algorithm of Heavens,Jimenez & Lahav. Here we concentrate on parametrizing galaxyproperties, and apply the method to the NGC galaxies in Kennicutt'sspectral atlas. We focus on deriving the star formation history,metallicity and dust content of galaxies. The method is very fast,taking a few seconds of CPU time to estimate ~17 parameters, and istherefore specially suited to studying large data sets, such as theAnglo-Australian two-degree-field (2dF) galaxy survey and the SloanDigital Sky Survey (SDSS). Without the power of MOPED, the recovery ofstar formation histories in these surveys would be impractical. InKennicutt's atlas, we find that for the spheroidals a small recent burstof star formation is required to provide the best fit to the spectrum.There is clearly a need for theoretical stellar atmospheric models withspectral resolution better than 1Å if we are to extract all therich information that large redshift surveys contain in their galaxyspectra.
| Nebular emission from star-forming galaxies
We present a new model for computing consistently the line and continuumemission from galaxies, based on a combination of recent populationsynthesis and photoionization codes. We use effective parameters todescribe the Hii regions and the diffuse gas ionized by single stellargenerations in a galaxy, among which the most important ones are thezero-age effective ionization parameter, the effective gas metallicityand the effective dust-to-heavy element ratio. We calibrate the nebularproperties of our model using the observed [Oiii]/Hβ, [Oii]/[Oiii],[Sii]/Hα and [Nii]/[Sii] ratios of a representative sample ofnearby spiral and irregular, starburst and Hii galaxies. To computewhole (line plus continuum) spectral energy distributions, we includethe absorption by dust in the neutral interstellar medium (ISM) using arecent simple prescription, which is consistent with observations ofnearby starburst galaxies. Our model enables us to interpretquantitatively the observed optical spectra of galaxies in terms ofstars, gas and dust parameters. We find that the range of ionized-gasproperties spanned by nearby galaxies implies factors of 3.5 and 14variations in the Hα and [Oii] luminosities produced per unit starformation rate (SFR). When accounting for stellar Hα absorptionand absorption by dust in the neutral ISM, the actual uncertainties inSFR estimates based on the emergent Hα and [Oii] luminosities areas high as several decades. We derive new estimators of the SFR, thegas-phase oxygen abundance and the effective absorption optical depth ofthe dust in galaxies. We show that, with the help of other lines such as[Oii], Hβ, [Oiii], [Nii] or [Sii], the uncertainties in SFRestimates based on Hα can be reduced to a factor of only 2-3, evenif the Hα line is blended with the adjacent [Nii] lines. WithoutHα, however, the SFR is difficult to estimate from the [Oii],Hβ and [Oiii] lines. The reason for this is that the absorption bydust in the neutral ISM and the ionized-gas parameters are thendifficult to constrain independently. This suggests that, whileinsufficient by itself, the Hα line is essential for estimatingthe star formation rate from the optical emission of a galaxy.
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