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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.
| Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| 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.
| On the Determination of N and O Abundances in Low-Metallicity Systems
We show that in order to minimize the uncertainties in the N and Oabundances of low-mass, low-metallicity (O/H<=1/5 solar)emission-line galaxies, it is necessary to employ separateparameterizations for inferring Te(N+) andTe(O+) from Te(O+2). Inaddition, we show that for the above systems, the ionization correctionfactor (ICF) for obtaining N/O from N+/O+, wherethe latter is derived from optical emission-line flux ratios, is=1.08+/-0.09. These findings are based on state-of-the-art single-star HII region simulations, employing our own modeled stellar spectra asinput. Our models offer the advantage of having matching stellar andnebular abundances. In addition, they have O/H as low as 1/50 solar(lower than any past work), as well as log(N/O) and log(C/O) fixed atcharacteristic values of -1.46 and -0.7, respectively. The above resultswere used to rederive N and O abundances for a sample of 68 systems with12+log(O/H)<=8.1, whose dereddened emission-line strengths werecollected from the literature. The analysis of the log(N/O) versus12+log(O/H) diagram of the above systems shows that (1) the largestgroup of objects forms the well-known N/O plateau with a value for themean (and its statistical error) of-1.43+0.0084-0.0085, (2) the objects aredistributed within a range in log(N/O) of -1.54 to -1.27 in Gaussianfashion around the mean with a standard deviation ofσ=+0.071-0.084, and (3) a χ2analysis suggests that only a small amount of the observed scatter inlog(N/O) is intrinsic.
| Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies
We have used the Balmer and Paschen jumps to determine the temperaturesof the H+ zones of a total sample of 47 H II regions. TheBalmer jump was used on MMT spectrophotometric data of 22low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies andof one H II region in the spiral galaxy M101. The Paschen jump was usedon spectra of 24 H II emission-line galaxies selected from the DataRelease 3 of the Sloan Digital Sky Survey (SDSS). To derive thetemperatures, we have used a Monte Carlo technique varying the electrontemperature in the H+ zone, the extinction of the ionized gasand that of the stellar population, the relative contribution of theionized gas to the total emission, and the star formation history to fitthe spectral energy distribution of the galaxies. For the MMT spectra,the fit was done in the wavelength range 3200-5200 Å, whichincludes the Balmer discontinuity, and for the SDSS spectra, in thewavelength range 3900-9200 Å, which includes the Paschendiscontinuity. We find for our sample of H II regions that thetemperatures of the O2+ zones determined from thenebular-to-auroral line intensity ratio of doubly ionized oxygen [O III]λλ(4959+5007)/λ4363 do not differ, in a statisticalsense, from the temperatures of the H+ zones determined fromfitting the Balmer and Paschen jumps and the spectral energydistributions (SEDs). We cannot rule out small temperature differencesof the order of 3%-5%.
| Ultraviolet-to-Far-Infrared Properties of Local Star-forming Galaxies
We present the results of a multiwavelength study of nearby galaxiesaimed at understanding the relation between the ultraviolet andfar-infrared emission in star-forming galaxies. The data set comprisesnew ultraviolet (from HST STIS), ground-based Hα, and radiocontinuum observations, together with archival infrared data (from IRASand ISO). The local galaxies are used as benchmarks for comparison ofthe infrared-to-ultraviolet properties with two populations ofhigh-redshift galaxies: the submillimeter star-forming galaxies detectedby SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). Inaddition, the long wavelength baseline covered by the present dataenables us to compare the star formation rates (SFRs) derived from theobserved ultraviolet, Hα, infrared, and radio luminosities and togauge the impact of dust opacity in the local galaxies. We also derive anew calibration for the nonthermal part of the radio SFR estimator,based on the comparison of 1.4 GHz measurements with a new estimator ofthe bolometric luminosity of the star-forming regions. We find that moreactively star-forming galaxies show higher dust opacities, which is inline with previous results. We find that the local star-forming galaxieshave a lower Fλ(205 μm)/Fλ(UV)ratio by 2-3 orders of magnitude than the submillimeter-selectedgalaxies and may have a similar or somewhat higherFλ(205 μm)/Fλ(UV) ratio thanLBGs. The Fλ(205 μm)/Fλ(UV) ratioof the local galaxy population may be influenced by the cool dustemission in the far-infrared heated by nonionizing stellar populations,which may be reduced or absent in the LBGs.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources
Aims.We search for ultra luminous X-ray source (ULXs) radio counterpartslocated in nearby galaxies in order to constrain their physicalnature. Methods: .Our work is based on a systematiccross-identification of the most recent and extensive available ULXcatalogues and archival radio data. Results: .A catalogue of 70positional coincidences is reported. Most of them are located within thegalaxy nucleus. Among them, we find 11 new cases of non-nuclear ULXsources with possibly associated radio emission.
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| High-Ionization Emission in Metal-deficient Blue Compact Dwarf Galaxies
Primordial stars are expected to be very massive and hot, producingcopious amounts of hard ionizing radiation. The best place to study hardionizing radiation in the local universe is in very metal-deficient bluecompact dwarf (BCD) galaxies. We have carried out a MMT spectroscopicsearch for [Ne V] λ3426 (ionization potential of 7.1 ryd), [Fe V]λ4227 (ionization potential of 4 ryd), and He II λ4686(ionization potential of 4 ryd) emission in a sample of 18 BCDs. We haveadded data from previous work and from the Data Release 3 of the SloanDigital Sky Survey. In total, we have assembled a BCD high-ionizationsample with [Ne V] emission in four galaxies, [Fe V] emission in 15galaxies, and He II emission in 465 galaxies. With this large sample, wehave reached the following conclusions. There is a general trend ofhigher [Ne V], [Fe V], and He II emission at lower metallicities.However, metallicity is not the only factor that controls the hardnessof the radiation. High-mass X-ray binaries and main-sequence stars areprobably excluded as the main sources of the very hard ionizingradiation responsible for [Ne V] emission. The most likely source of [NeV] emission is probably fast radiative shocks moving with velocities>~450 km s-1 through a dense interstellar medium with anelectron number density of several hundreds cm-3 andassociated with supernova explosions of the most massive stars. Thesehave masses of ~50-100 Msolar and are formed in very compactsuper-star clusters (SSCs). The softer ionizing radiation required forHe II emission is likely associated with less massive evolved starsand/or radiative shocks moving through a less dense interstellar medium.The observations reported here were obtained at the MMT Observatory, ajoint facility of the Smithsonian Institution and the University ofArizona.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| 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.
| A Near-Solar Metallicity, Nitrogen-deficient Lyman Limit Absorber Associated with Two S0 Galaxies
The UV spectrum of the bright quasar PHL 1811 at zem=0.192reveals a foreground gas system at z=0.080923 withlogN(HI)=17.98+/-0.05. We have determined the abundances of variousatomic species in this system from a spectrum covering the wavelengthrange 1160-1730 Å recorded at 7 km s-1 resolution bythe E140M grating of the Space Telescope Imaging Spectrograph (STIS) onthe Hubble Space Telescope (HST), supplemented by coverage at shorterwavelengths by the Far Ultraviolet Spectroscopic Explorer (FUSE). Theabundances of C II, Si II, S II, and Fe II compared to that of O Iindicate that a considerable fraction of the gas is in locations wherethe hydrogen is ionized. An oxygen abundance [O/H]=-0.19+/-0.08 in the HI-bearing gas indicates that the chemical enrichment of the gas isunusually high for an extragalactic QSO absorption system. However, thissame material has an unusually low abundance of nitrogen,[N/O]<-0.59, indicating that there may not have been enough timeduring this enrichment for secondary nitrogen to arise from low- andintermediate-mass stars. From the convergence of high Lyman series lineswe can determine the velocity width of H I, and after correcting forturbulent broadening shown by the O I absorption feature, we derive atemperature T=7070+3860-4680 K. We determine alower bound for the electron density n(e)>10-3cm-3 by modeling the ionization by the intergalacticradiation field and an upper bound n(e)<0.07 cm-3 from theabsence of much C II in an excited fine-structure level. The thermalpressure in the range 4cm-3K
| 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.
| Abundances in the H I Envelope of the Extremely Low Metallicity Blue Compact Dwarf Galaxy SBS 0335-052 from Far Ultraviolet Spectroscopic Explorer Observations
We present Far Ultraviolet Spectroscopic Explorer spectroscopy of SBS0335-052, the second most metal-deficient blue compact dwarf (BCD)galaxy known [log(O/H)=-4.70]. In addition to the H I Lyman series, wedetect C II, N I, N II, O I, Si II, Ar I, and Fe II absorption lines,mainly arising from the extended H I envelope in which SBS 0335-052 isembedded. No H2 absorption lines are seen. The absence ofdiffuse H2 implies that the warm H2 detectedthrough infrared emission must be very clumpy and associated with thestar-forming regions. The clumps should be denser than ~1000cm-3 and hotter than ~1000 K and account for >=5% of thetotal H I mass. Although SBS 0335-052 is a probable young galaxy, itsneutral gas is not pristine. The metallicity of its neutral gas issimilar to that of its ionized gas and is equal to log(O/H)~-5. Thismetallicity is comparable to that found in the H I envelopes of fourother BCDs with ionized gas metallicities spanning the wide range fromlog(O/H)=-4.8 to -3.8, and in Lyα absorbers, fueling thespeculation that there may have been previous enrichment of theprimordial neutral gas to a common metallicity level of log(O/H)~-5,possibly by Population III stars.Based on observations obtained with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985.
| On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback from Momentum-driven Winds
We investigate large-scale galactic winds driven by momentum deposition.Momentum injection is provided by (1) radiation pressure produced by thecontinuum absorption and scattering of photons on dust grains and (2)supernovae (momentum injection by supernovae is important even if thesupernova energy is radiated away). Radiation can be produced by astarburst or active galactic nucleus (AGN) activity. We argue thatmomentum-driven winds are an efficient mechanism for feedback during theformation of galaxies. We show that above a limiting luminosity,momentum deposition from star formation can expel a significant fractionof the gas in a galaxy. The limiting, Eddington-like luminosity isLM~=(4fgc/G)σ4, where σ isthe galaxy velocity dispersion and fg is the gas fraction;the subscript M refers to momentum driving. A starburst that attainsLM moderates its star formation rate and its luminosity doesnot increase significantly further. We argue that elliptical galaxiesattain this limit during their growth at z>~1 and that this is theorigin of the Faber-Jackson relation. We show that Lyman break galaxiesand ultraluminous infrared galaxies have luminosities nearLM. Since these starbursting galaxies account for asignificant fraction of the star formation at z>~1, this supports ourhypothesis that much of the observed stellar mass in early-type galaxieswas formed during Eddington-limited star formation. Star formation isunlikely to efficiently remove gas from very small scales in galacticnuclei, i.e., scales much smaller than that of a nuclear starburst. Thisgas is available to fuel a central black hole (BH). We argue that a BHclears gas out of its galactic nucleus when the luminosity of the BHitself reaches ~LM. This shuts off the fuel supply to the BHand may also terminate star formation in the surrounding galaxy. As aresult, the BH mass is fixed to beMBH~=(fgκes/πG2)σ4,where κes is the electron scattering opacity. Thislimit is in accord with the observed MBH-σ relation.
| Probing the Multiphase Interstellar Medium of the Dwarf Starburst Galaxy NGC 625 with Far Ultraviolet Spectroscopic Explorer Spectroscopy
We present new Far Ultraviolet Spectroscopic Explorer (FUSE)spectroscopy of the dwarf starburst galaxy NGC 625. These observationsprobe multiple phases of the interstellar medium (ISM), including thecoronal, ionized, neutral, and molecular gas. This nearby (D=3.9+/-0.2Mpc) system shows a clear detection of outflowing coronal gas as tracedby O VI λ1032 absorption. The centroid of the O VI profile isblueshifted with respect to the galaxy systemic velocity by ~30 kms-1, suggesting a low-velocity outflow. The implied O VIvelocity extent is found to be 100+/-20 km s-1, which isfully consistent with the detected H I outflow velocity found in radiosynthesis observations. We detect multiple lines of diffuseH2 absorption from the ISM of NGC 625; this is one of only afew extragalactic systems with FUSE detections of H2 lines inthe Lyman and Werner bands. We find a potential abundance offset betweenthe neutral and nebular gas that exceeds the errors on the derivedcolumn densities. Since such an offset has been found in multiple dwarfgalaxies, we discuss the implications of a lower-metallicity halosurrounding the central star-forming regions of dwarf galaxies. Theapparent offset may be due to saturation of the observed O I line, andhigher signal-to-noise ratio (S/N) observations are required to resolvethis issue.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985.
| 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.
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
| Massive star populations in Wolf-Rayet galaxies
We 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.
| Metal Abundances of KISS Galaxies. III. Nebular Abundances for Fourteen Galaxies and the Luminosity-Metallicity Relationship for H II Galaxies
We report results from the third in a series of nebular abundancestudies of emission-line galaxies from the KPNO InternationalSpectroscopic Survey (KISS). Galaxies with coarse metallicity estimatesof 12+log(O/H) less than 8.2 dex were selected for observation. Spectraof 14 galaxies, which cover the full optical region from [O II]λλ3727, 3729 to beyond [S II] λλ6717, 6731,are presented, and abundance ratios of N, O, Ne, S, and Ar are computed.The auroral [O III] λ4363 line is detected in all 14 galaxies.Oxygen abundances determined through the direct electron temperature(Te) method confirm that the sample is metal-poor with7.61<=12+log(O/H)<=8.32. By using these abundances in conjunctionwith other Te-based measurements from the literature, wedemonstrate that H II galaxies and more quiescent dwarf irregulargalaxies follow similar metallicity-luminosity (L-Z) relationships. Theprimary difference is a zero-point shift between the correlations suchthat H II galaxies are brighter by an average of 0.8 B magnitudes at agiven metallicity. This offset can be used as evidence to argue thatlow-luminosity H II galaxies typically undergo factor of 2 luminosityenhancements, and starbursts that elevate the luminosities of their hostgalaxies by 2-3 mag are not as common. We also demonstrate that theinclusion of interacting galaxies can increase the scatter in the L-Zrelation and may force the observed correlation toward lowermetallicities and/or larger luminosities. This must be taken intoaccount when attempting to infer metal abundance evolution by comparinglocal L-Z relations with ones based on higher redshift samples, sincethe fraction of interacting galaxies should increase with look-backtime.
| 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.
| Missing Massive Stars in Starbursts: Stellar Temperature Diagnostics and the Initial Mass Function
Determining the properties of starbursts requires spectral diagnosticsof their ultraviolet radiation fields, to test whether very massivestars are present. We test several such diagnostics, using new models ofline ratio behavior combining CLOUDY, Starburst99, and up-to-datespectral atlases. For six galaxies we obtain new measurements of He I1.7 μm/Br10, a difficult to measure but physically simple (andtherefore reliable) diagnostic. We obtain new measurements of He I 2.06μm/Brγ in five galaxies. We find that He I 2.06 μm/Brγand [O III]/Hβ are generally unreliable diagnostics in starbursts.The heteronuclear and homonuclear mid-infrared line ratios (notably [NeIII] 15.6 μm/[Ne II] 12.8 μm) consistently agree with each otherand with He I 1.7 μm/Br10 this argues that the mid-infrared lineratios are reliable diagnostics of spectral hardness. In a sample of 27starbursts, [Ne III]/[Ne II] is significantly lower than modelpredictions for a Salpeter initial mass function (IMF) extending to 100Msolar. Plausible model alterations strengthen thisconclusion. By contrast, the low-mass and low-metallicity galaxies II Zw40 and NGC 5253 show relatively high neon line ratios, compatible with aSalpeter slope extending to at least ~40-60 Msolar. Onesolution for the low neon line ratios in the high-metallicity starburstswould be that they are deficient in >~40 Msolar starscompared to a Salpeter IMF. An alternative explanation, which we prefer,is that massive stars in high-metallicity starbursts spend much of theirlives embedded within ultracompact H II regions that prevent the near-and mid-infrared nebular lines from forming and escaping. Thishypothesis has important consequences for starburst modeling andinterpretation.
| Systematic Effects and a New Determination of the Primordial Abundance of 4He and dY/dZ from Observations of Blue Compact Galaxies
We 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.
| The H I Kinematics and Distribution of Four Blue Compact Dwarf Galaxies
We present VLA H I observations of the blue compact dwarf (BCD) galaxiesNGC 2366, NGC 4861, VII Zw 403, and Haro 2. These galaxies span a rangeof BCD morphological types. The cometary-like BCDs NGC 2366 and NGC 4861have regular rotational kinematics with a V/σ of 8.7 and 6.4,respectively. On the other hand, the velocity fields of the iE BCD VIIZw 403 and of the nE BCD Haro 2 lack regularity, and their rotationalmotion is around the major, not the minor, axis. The H I distribution iscentrally peaked in VII Zw 403 and Haro 2, a general feature of all iEand nE-type BCDs, the most common ones. In contrast, cometary-type BCDshave multiple H I peaks that are scattered over the disk. The activeregions of star formation are associated with regions of high H I columndensities, with slight displacements between the H I and stellar peaks.NGC 2366 shows many H I minima, resulting from the disruptive influenceof massive star formation and supernovae on the interstellar medium(ISM). In NGC 2366 and NGC 4861, there is a tendency for H I gas with ahigher velocity dispersion to be associated with regions of lower H Icolumn density. This anticorrelation can be understood in the context ofa two-phase model of the ISM. In all BCDs, the radio continuum emissionis associated with the star-forming regions and is predominantly thermalin nature. H I clouds with no optical counterparts have been found inthe vicinity of NGC 4861 and Haro 2.Based on observations obtained at the National Radio AstronomyObservatory, a facility of the National Science Foundation, operatedunder cooperative agreement by Associated Universities, Inc.
| A New Nonparametric Approach to Galaxy Morphological Classification
We present two new nonparametric methods for quantifying galaxymorphology: the relative distribution of the galaxy pixel flux values(the Gini coefficient or G) and the second-order moment of the brightest20% of the galaxy's flux (M20). We test the robustness of Gand M20 to decreasing signal-to-noise ratio (S/N) and spatialresolution and find that both measures are reliable to within 10% forimages with average S/N per pixel greater than 2 and resolutions betterthan 1000 and 500 pc, respectively. We have measured G andM20, as well as concentration (C), asymmetry (A), andclumpiness (S) in the rest-frame near-ultraviolet/optical wavelengthsfor 148 bright local ``normal'' Hubble-type galaxies (E-Sd) galaxies, 22dwarf irregulars, and 73 0.05
| Abundance differences between the neutral and the ionized gas of the dwarf galaxy IZw 36
We present a FUSE spectroscopic study of the nearby gas-rich,metal-deficient blue compact dwarf (BCD) galaxy IZw 36. Atomic hydrogenand many metal lines are observed in absorption against the stellarcontinuum of young, massive stars embedded in the ionized region.Profile fitting of absorption lines allowed us to determine abundancesand investigate the chemical composition of the neutral gas. This studypresents strong evidence that nitrogen is 16+7-8times less abundant in the neutral gas than in the ionized gas (alluncertainties are 2\sigma). Similarly, the oxygen abundance estimatedusing phosphorus as a tracer is lower in the neutral gas by a factor of8+17-5. We also find that argon is underabundantby a factor of 32+8-7 and that log (Ar I/O I) <-3.0 which is inconsistent with the Ar/O ratio -2.1 ± 0.1(Lodders \cite{Lod03}), implying that argon is likely ionized into Ar IIin the neutral medium.
| The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxies
We 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/
| FUSE observations of the H I interstellar gas of I Zw 18
We present the analysis of FUSE observations of the metal-deficientdwarf galaxy I Zw 18 . We measured column densities of H I, N I, O I, ArI, Si II, and Fe II. The O I/H I ratio (\log(O I/HI)=-4.7+0.8-0.6) is consistent with the O/H ratioobserved in the H II regions (all uncertainties are 2-σ). If theoxygen is depleted in the H I region compared to the H II regions, thedepletion is at most 0.5 dex. This is also consistent with the \log(O/H)ratios -5 measured with FUSE in the H I regions of other bluecompact dwarf galaxies. With \log(N I/OI)=-2.4+0.6-0.8, the measured N I/O I ratio islower than expected for primary nitrogen. The determination of the N IIcolumn density is needed to discriminate between a large ionization of NI or a possible nitrogen deficiency. The neutral argon is alsoapparently underabundant, indicating that ionization into Ar II islikely important. The column densities of the other α-chainelements Si II and Ar I favor the lower edge of the permitted range of OI column density, \log (Ncm-2(O I)) 16.3.
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