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Hα line profiles for a sample of supergiant HII regions. III. Model line profiles
We carried out a series of 1D hydrodynamical computations with the ZEUSfluid solver in order to reproduce the salient features in ourhigh-resolution Hα emission line profiles of a sample of HIIregions (see Rozas et al. 2006a,b). Four models were computed. In thefirst and second cases, an ionization-bounded HII region was modeledwith and without a shell produced by stellar winds. In the third andfourth cases, a density-bounded HII region was considered, both with andwithout a shell driven by stellar winds. The resulting line profileshave been modeled using the SHAPE rendering program. We find that ourobserved high-quality profiles are well-reproduced in thedensity-bounded model that includes an expanding shell formed as aresult of the stellar winds from the ionizing stars. Finally, weconsider the effects that the finite slit widths used for theobservations have on the physical conditions deduced from the lineprofile of the ionized bubble.
| The Young Stellar Population of NGC 4214 as Observed with the Hubble Space Telescope. II. Results
We present the results of a detailed UV-optical study of the nearbydwarf starburst galaxy NGC 4214 using multifilter Hubble Space TelescopeWFPC2 and STIS photometry. The stellar extinction is found to be quitepatchy, with some areas having values of E(4405-5495)<0.1 mag andothers, associated with star-forming regions, much more heavilyobscured, a result that is consistent with previous studies of thenebular extinction. We determined the ratio of blue to red supergiantsand found it to be consistent with theoretical models for themetallicity of the Small Magellanic Cloud. The stellar initial massfunction (IMF) of the field in the range 20-100 Msolar isfound to be steeper than γ=-2.8 (γ=-2.35 for a SalpeterIMF). A number of massive clusters and associations with ages between afew and 200 Myr are detected, and their properties are discussed.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| Water masers in the Local Group of galaxies
We compare the number of detected 22 GHz H2O masers in the Local Groupgalaxies M 31, M 33, NGC 6822, IC 10, IC 1613, DDO 187, GR8, NGC 185,and the Magellanic Clouds with the water maser population of the MilkyWay. To accomplish this we searched for water maser emission in the twoLocal Group galaxies M 33 and NGC 6822 using the Very Large Array (VLA)and incorporated results from previous studies. We observed 62 Hiiregions in M 33 and 36 regions with Hα emission in NGC 6822.Detection limits are 0.0015 and 0.0008 L_ȯ for M 33 and NGC 6822,respectively (corresponding to 47 and 50 mJy in three channels with 0.7km s-1 width). M 33 hosts three water masers above ourdetection limit, while in NGC 6822 no maser source was detected. We findthat the water maser detection rates in the Local Group galaxies M 31, M33, NGC 6822, IC 1613, DDO 187, GR8, NGC 185, and the Magellanic Cloudsare consistent with expectations from the Galactic water masers if oneconsiders the different star formation rates of the galaxies. However,the galaxy IC 10 exhibits an overabundance of masers, which may resultfrom a compact central starburst.
| The massive star population in the giant HII region Tol89 in NGC5398
We present new high spectral resolution Very Large Telescope(VLT)/UV-Visual Echelle Spectrograph (UVES) spectroscopy and archivalHubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS)imaging and spectroscopy of the giant HII region Tol89 in NGC5398. Fromoptical and ultraviolet (UV) HST images, we find that the star-formingcomplex as a whole contains at least seven young compact massiveclusters. We resolve the two brightest optical knots, A and B, into fiveindividual young massive clusters along our slit, A1-4 and B1,respectively. From UV spectral modelling using the STARBURST99 code ofLeitherer et al., and nebular Hβ equivalent widths in the optical,we derive ages that are consistent with the formation of two separateburst events, of ~4 +/- 1 Myr and <3 Myr for knots A (A1-4) and B(B1), respectively. A Large Magellanic Cloud (LMC) metallicity ismeasured for both knots from a nebular line analysis, while nebular HeII4686 is observed in knot B and perhaps in knot A. We detect underlyingbroad wings on the strongest nebular emission lines indicatingvelocities up to 600 km s-1. From UV and opticalspectroscopy, we estimate that there are ~95 early WN stars and ~35early WC stars in Tol89-A, using empirical template spectra of LMCWolf-Rayet (WR) stars from Crowther & Hadfield, with the WCpopulation confined to cluster A2. Remarkably, we also detect a smallnumber of approximately three mid WNs in the smallest (mass) cluster inTol89-A, A4, whose spectral energy output in the UV is entirelydominated by the WN stars. From the strength of nebular Hβ, weobtain N(O) ~ 690 and 2800 for knots A and B, respectively, whichimplies N(WR)/N(O) ~ 0.2 for knot A. We also employ a complementaryapproach using STARBURST99 models, in which the O star content isinferred from the stellar continuum, and the WR population is obtainedfrom spectral synthesis of optical WR features using the grids fromSmith et al. We find reasonable agreement between the two methods forthe O star content and the N(WR)/N(O) ratio but find that the WR subtypedistribution is in error in the STARBURST99 models, with far too few WNstars being predicted. We attribute this failure to the neglect ofrotational mixing in evolutionary models. Our various modellingapproaches allow us to measure the cluster masses. We identify A1 as asuper star cluster (SSC) candidate with a mass of ~1-2 ×105 Msolar. A total mass of ~6 ×105 Msolar is inferred for the ionizing sourceswithin Tol89-B.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 73.B-0238(A) and with the NASA/ESA HST, obtainedfrom the ESO/ST-ECF Science Archive Facility.E-mail: fs@star.ucl.ac.uk
| Stellar Abundances and Molecular Hydrogen in High-Redshift Galaxies: The Far-Ultraviolet View
FUSE spectra of star-forming regions in nearby galaxies are compared tocomposite spectra of Lyman break galaxies (LBGs), binned by strength ofLyα emission and by mid-UV luminosity. Several far-UV spectralfeatures, including lines dominated by stellar wind and by photosphericcomponents, are very sensitive to stellar abundances. Their measurementin LBGs is compromised by the strong interstellar absorption features,allowing in some cases only upper limits to be determined. The derived Cand N abundances in the LBGs are no higher than half solar (scaled tooxygen abundance for comparison with emission-line analyses),independent of the strength of Lyα emission. P V absorptionindicates abundances as low as 0.1 solar, with an upper limit near 0.4solar in the reddest and weakest emission galaxies. Unresolvedinterstellar absorption components would further lower the derivedabundances. Trends of line strength and derived abundances are strongerwith mid-UV luminosity than with Lyα strength. H2absorption in the Lyman and Werner bands is very weak in the LBGs.Template H2 absorption spectra convolved to the appropriateresolution show that strict upper limitsN(H2)<1018 cm-2 apply in all cases,with more stringent values appropriate for the stronger emissioncomposites and for mixes of H2 level populations like thoseon Milky Way sight lines. Since the UV-bright regions are likely to bewidespread in these galaxies, these results rule out massive diffusereservoirs of primordial H2 and suggest that the dust-to-gasratio is already fairly large at z~3.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by The JohnsHopkins University under NASA contract NAS5-32985.
| Confidence limits of evolutionary synthesis models. IV. Moving forward to a probabilistic formulation
Context: .Synthesis models predict the integrated properties of stellarpopulations. Several problems exist in this field, mostly related to thefact that integrated properties are distributed. To date, this aspecthas been either ignored (as in standard synthesis models, which areinherently deterministic) or interpreted phenomenologically (as in MonteCarlo simulations, which describe distributed properties rather thanexplain them). Aims: .This paper presents a method of populationsynthesis that accounts for the distributed nature of stellarproperties. Methods: .We approach population synthesis as aproblem in probability theory, in which stellar luminosities are randomvariables extracted from the stellar luminosity distribution function(sLDF). Results: .With standard distribution theory, we derive thepopulation LDF (pLDF) for clusters of any size from the sLDF, obtainingthe scale relations that link the sLDF to the pLDF. We recover thepredictions of standard synthesis models, which are shown to compute themean of the luminosity function. We provide diagnostic diagrams and asimplified recipe for testing the statistical richness of observedclusters, thereby assessing whether standard synthesis models can besafely used or a statistical treatment is mandatory. We also recover thepredictions of Monte Carlo simulations, with the additional bonus ofbeing able to interpret them in mathematical and physical terms. We giveexamples of problems that can be addressed through our probabilisticformalism: calibrating the SBF method, determining the luminosityfunction of globular clusters, comparing different isochrone sets,tracing the sLDF by means of resolved data, including fuzzy stellarproperties in population synthesis, among others. Additionally, thealgorithmic nature of our method makes it suitable for developinganalysis tools for the Virtual Observatory. Conclusions: .Thoughstill under development, ours is a powerful approach to populationsynthesis. In an era of resolved observations and pipelined analyses oflarge surveys, this paper is offered as a signpost in the field ofstellar populations.
| 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.
| Neon and Oxygen Abundances in M33
We present new spectroscopic observations of 13 H II regions in theLocal Group spiral galaxy M33. The regions observed range from 1 to 7kpc in distance from the nucleus. Of the 13 H II regions observed, the[O III] λ4363 line was detected in six regions. Electrontemperatures were thus able to be determined directly from the spectrausing the [O III] λλ4959, 5007/λ4363 line ratio.Based on these temperature measurements, oxygen and neon abundances andtheir radial gradients were calculated. For neon, a gradient of-0.016+/-0.017 dex kpc-1 was computed, which agrees with theNe/H gradient derived previously from ISO spectra. A gradient of-0.012+/-0.011 dex kpc-1 was computed for O/H, much shallowerthan was derived in previous studies. The newly calculated O/H and Ne/Hgradients are in much better agreement with each other, as expected frompredictions of stellar nucleosynthesis. We examine the correlationbetween the WC/WN ratio and metallicity, and find that the new M33abundances do not impact the observed correlation significantly. We alsoidentify two new He II-emitting H II regions in M33, the first to bediscovered in a spiral galaxy other than the Milky Way. In both casesthe nebular He II emission is not associated with Wolf-Rayet stars.Therefore, caution is warranted in interpreting the relationship betweennebular He II emission and Wolf-Rayet stars when both are observed inthe integrated spectrum of an H II region.
| Massive Stellar Content of Giant H II Regions in M33 and M101
Far-ultraviolet (900-1200 Å) spectral synthesis of nine giantextragalactic H II regions in M33 and M101 is performed to study theirmassive stellar content. Several parameters are quantified, predicted,and compared to the literature: age, stellar mass, initial mass function(IMF) slope, number of O-type and Wolf-Rayet stars, and Hα and5500 Å continuum fluxes. The results of this particular techniqueare consistent with other methods and observations. This work shows thata total stellar mass of a few 103 Msolar is neededto populate the IMF bins well enough at high masses to obtain accurateresults from the spectral synthesis technique in the far-ultraviolet. Aflat IMF slope seems to characterize better the stellar line profiles ofthese objects, which is likely the first sign of a small numberstatistics effect on the IMF. Finally, the H II region NGC 5461 isidentified as a good candidate for hosting a second generation of stars,not yet seen at far-ultraviolet wavelengths.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| What is the temperature structure in the giant HII region NGC 588?
We present the results of an exhaustive study of the ionized gas inNGC 588, a giant Hii region in the nearby spiralgalaxy M 33. This analysis uses a high number ofdiagnostics in the optical and infrared ranges. Four temperaturediagnostics obtained with optical lines agree with a gas temperature of11 000 K, while the [Oiii] λ5007/λ88 μm ratio yields amuch lower temperature of ≈8000 K. This discrepancy suggests thepresence of large temperature inhomogeneities in the nebula. Weinvestigated the cause of this discrepancy by constructingphotoionization models of increasing complexity. In particular, we usedthe constraints from the Hα and Hβ surface brightnessdistributions and state-of-the-art models of the stellar ionizingspectrum. None of the successive attempts was able to reproduce thediscrepancy between the temperature diagnostics, so the thermal balanceof NGC 588 remains unexplained. We give an estimateof the effect of this failure on the O/H and Ne/O estimates and showthat O/H is known to within ±0.2 dex.
| The Hot Interstellar Medium in M31 and M33
We report measurements from Far Ultraviolet Spectroscopic Explorerspectra of OB stars in M31 and M33, of O VI and C II interstellarabsorptions. The stars are located in OB 78 in the outer region of M31and in several locations across M33. The interstellar absorptions agreewell between stars in nearby locations, and their principal radialvelocities indicate that they arise in those galaxies and follow thedisk rotation.
| On the importance of the few most massive stars: The ionizing cluster of NGC 588
We present the results of a double analysis of the ionizing cluster inNGC 588, a giant HII region (GHR) in the outskirts ofthe nearby galaxy M 33. For this purpose, we obtainedground based long-slit spectroscopy and combined it with archival groundbased and space borne imaging and spectroscopy, in the wavelength range1100-9800 Å. A first modeling of the cluster was performed usingintegrated properties, such as the spectral energy distribution (SED),broad band colors, nebular emission Hβ equivalent width, the mainultraviolet resonance lines, and the presence of Wolf-Rayet starfeatures. By applying standard assumptions about the initial massfunction (IMF), we were unable to fit satisfactorily these observationaldata. This contradictory result led us to carry out a second modeling,based on a resolved photometric analysis of individual stars in HubbleSpace Telescope (HST) images, by means of finding the best fit isochronein color-magnitude diagrams (CMD), and assigning a theoretical SED toeach individual star. The overall SED of the cluster, obtained byintegrating the individual stellar SEDs, is found to fit better theobserved SED than the best solution found through the integrated firstanalysis, but at a significantly later stage of evolution of the clusterof 4.2 Myr, as obtained from the best fit to the CMD. A comparativeanalysis of both methods traces the different results to the effects ofstatistical fluctuations in the upper end of the IMF, which aresignificant in NGC 588, with a computed cluster massof 5600 Mȯ, as predicted by Cerviño et al. (2002,A&A, 381, 51). We discuss the results in terms of the stronginfluence of the few most massive stars, six in the case of NGC588, that dominate the overall SED and, in particular, theionizing far ultraviolet range beyond the Lyman limit.Tables 1, 2, 4, Figs. 1, 2, 9, 10 12, 13 and Appendix A are onlyavailable in electronic form at http://www.edpsciences.org
| Far-Ultraviolet Spectroscopy of Star-forming Regions in Nearby Galaxies: Stellar Populations and Abundance Indicators
We present Far Ultraviolet Spectroscopic Explorer spectroscopy andsupporting data for star-forming regions in nearby galaxies, to examinetheir massive-star content and explore the use of abundance andpopulation indicators in this spectral range for high-redshift galaxies.New far-ultraviolet spectra are shown for four bright H II regions inM33 (NGC 588, 592, 595, and 604), the H II region NGC 5461 in M101, andthe starburst nucleus of NGC 7714, supplemented by the very lowmetallicity galaxy I Zw 18. In each case we see strong Milky Wayabsorption systems from H2, but intrinsic absorption withineach galaxy is weak or undetectable, perhaps because of the ``UV bias''in which reddened stars that lie behind molecular-rich areas are alsoheavily reddened. We see striking changes in the stellar wind lines fromthese populations with metallicity, suggesting that C II, C III, C IV, NII, N III, and P V lines are potential tracers of stellar metallicity instar-forming galaxies. Three of these relations-involving N IV, C III,and P V-are nearly linear over the range from O/H=0.05-0.8 solar. Themajor difference in continuum shapes among these systems is that thegiant H II complex NGC 604 has a stronger continuum shortward of 950Å than any other object in this sample. Small number statisticswould likely go in the other direction; we favor this as the result of adiscrete star-forming event ~3 Myr ago, as suggested by previous studiesof its stellar population.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by JohnsHopkins University under NASA contract NAS 5-32985.
| 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).
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Infrared Space Observatory Long-Wavelength Spectrometer Spectroscopy of Star-forming Regions in M33
We present Infrared Space Observatory (ISO) Long-Wavelength Spectrometer(LWS) far-infrared (FIR) spectra of the nucleus and six giant H IIregions in M33 (NGC 595, IC 142, NGC 592, NGC 604, NGC 588, and IC 133).The seven fine-structure lines observed in the FIR are used to model theH II and photodissociation regions (PDRs). There is no observed trend inthe FIR properties, observed with the LWS, as a function of galacticradius or metallicity. The cold neutral medium (CNM) is the mainreservoir for the atomic gas, containing between 60% and 95% of the gas.The FIRLWS spectral energy distribution can be fitted with asingle-temperature graybody spectrum with a temperature in the range35K<=T<=49 K. The [C II] 158 μm line flux is 0.2%-0.7%FIRLWS, which is typical of values seen (0.1%-1% FIR) in thenuclei of star-forming galaxies. The [C II]/FIRLWS ratiopeaks at the nucleus and is fairly constant across the rest of thesample. Massive star formation is traced by the intensity of the [O III]88 μm line. The emission from the observed FIR lines that arisesolely from H II regions can be modeled as a single component with agiven oxygen and nitrogen abundance, effective temperature, density, andionizing flux. There is no need for an extended low-density component(ELDWIM). Apart from NGC 604 and NGC 595, the fractional [C II] emissionthat arises from the H II regions and/or PDRs is not well constrained,but typically 5%-50% arises in the H II regions, 10%-35% from the CNM,and the bulk of the emission (40%-90%) in the PDRs. The average PDR inthis sample has a gas density ~103.1 cm-3, anaverage incident far-ultraviolet flux (in units of the localinterstellar value) 0>=102.4, a gas temperatureT~200 K, and an AV~10 through the clouds. NGC 604 has 40% ofthe atomic gas residing in the PDRs, while the rest have a much smallerfraction, ~5%-15%. The PDRs are similar to those found in otherstar-forming galaxies such as Centaurus A. 0> is at thelower end of the range observed in samples of spiral and starburstgalaxies (2.2<=logG0<=5), and sits comfortablyin the middle of the observed range (1.8<=logn<=4.2).
| 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.
| Distribution and Kinematics of O VI in the Galactic Halo
Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of 100extragalactic objects and two distant halo stars are analyzed to obtainmeasures of O VI λλ1031.93, 1037.62 absorption along pathsthrough the Milky Way thick disk/halo. Strong O VI absorption over thevelocity range from -100 to 100 km s-1 reveals a widespreadbut highly irregular distribution of O VI, implying the existence ofsubstantial amounts of hot gas with T~3×105 K in theMilky Way thick disk/halo. The integrated column density, log[N(O VI)cm-2], ranges from 13.85 to 14.78 with an average value of14.38 and a standard deviation of 0.18. Large irregularities in the gasdistribution are found to be similar over angular scales extending from<1° to 180°, implying a considerable amount of small- andlarge-scale structure in the absorbing gas. The overall distribution ofO VI is not well described by a symmetrical plane-parallel layer ofpatchy O VI absorption. The simplest departure from such a model thatprovides a reasonable fit to the observations is a plane-parallel patchyabsorbing layer with an average O VI midplane density of n0(OVI)=1.7×10-8 cm-3, a scale height of ~2.3kpc, and a ~0.25 dex excess of O VI in the northern Galactic polarregion. The distribution of O VI over the sky is poorly correlated withother tracers of gas in the halo, including low- andintermediate-velocity H I, Hα emission from the warm ionized gasat ~104 K, and hot X-ray-emitting gas at ~106 K.The O VI has an average velocity dispersion, b~60 km s-1, andstandard deviation of 15 km s-1. Thermal broadening alonecannot explain the large observed profile widths. The average O VIabsorption velocities toward high-latitude objects(|b|>45deg) range from -46 to 82 km s-1, with ahigh-latitude sample average of 0 km s-1 and a standarddeviation of 21 km s-1. High positive velocity O VI absorbingwings extending from ~100 to ~250 km s-1 observed along 21lines of sight may be tracing the flow of O VI into the halo. Acombination of models involving the radiative cooling of hot fountaingas, the cooling of supernova bubbles in the halo, and the turbulentmixing of warm and hot halo gases is required to explain the presence ofO VI and other highly ionized atoms found in the halo. The preferentialventing of hot gas from local bubbles and superbubbles into the northernGalactic polar region may explain the enhancement of O VI in the north.If a fountain flow dominates, a mass flow rate of approximately 1.4Msolar yr-1 of cooling hot gas to each side of theGalactic plane with an average density of 10-3cm-3 is required to explain the average value of log[N(OVI)sin|b|] observed in the southern Galactic hemisphere. Such a flowrate is comparable to that estimated for the Galacticintermediate-velocity clouds.
| The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in and near the Galaxy
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofthe O VI λλ1031.926, 1037.617 absorption lines associatedwith gas in and near the Milky Way, as detected in the spectra of asample of 100 extragalactic targets and two distant halo stars. Wecombine data from several FUSE Science Team programs with guest observerdata that were public before 2002 May 1. The sight lines cover most ofthe sky above Galactic latitude |b|>25deg-at lowerlatitude the ultraviolet extinction is usually too large forextragalactic observations. We describe the details of the calibration,alignment in velocity, continuum fitting, and manner in which severalcontaminants were removed-Galactic H2, absorption intrinsicto the background target and intergalactic Lyβ lines. Thisdecontamination was done very carefully, and in several sight lines verysubtle problems were found. We searched for O VI absorption in thevelocity range -1200 to 1200 km s-1. With a few exceptions,we only find O VI in the velocity range -400 to 400 km s-1the exceptions may be intergalactic O VI. In this paper we analyze the OVI associated with the Milky Way (and possibly with the Local Group). Wediscuss the separation of the observed O VI absorption into componentsassociated with the Milky Way halo and components at high velocity,which are probably located in the neighborhood of the Milky Way. Wedescribe the measurements of equivalent width and column density, and weanalyze the different contributions to the errors. We conclude thatlow-velocity Galactic O VI absorption occurs along all sight lines-thefew nondetections only occur in noisy spectra. We further show thathigh-velocity O VI is very common, having equivalent width >65 mÅin 50% of the sight lines and equivalent width >30 mÅ in 70% ofthe high-quality sight lines. The central velocities of high-velocity OVI components range from |vLSR|=100 to 330 km s-1there is no correlation between velocity and absorption strength. Wediscuss the possibilities for studying O VI absorption associated withLocal Group galaxies and conclude that O VI is probably detected in M31and M33. We limit the extent of an O VI halo around M33 to be <100kpc [at a 3 σ detection limit of log N(O VI)~14.0]. Using themeasured column densities, we present 50 km s-1 wide O VIchannel maps. These show evidence for the imprint of Galactic rotation.They also highlight two known H I high-velocity clouds (complex C andthe Magellanic Stream). The channel maps further show that O VI atvelocities <-200 km s-1 occurs along all sight lines inthe region l=20deg-150deg, b<-30deg,while O VI at velocities >200 km s-1 occurs along all sightlines in the region l=180deg-300deg,b>20deg.
| Interstellar H2 in M 33 detected with FUSE
FUSE spectra of the four brightest H Ii regions in M33 show absorption by interstellar gas in the Galaxy and inM 33. On three lines of sight molecular hydrogen inM 33 is detected. This is the first measurement ofdiffuse H2 in absorption in a Local Group galaxy other thanthe Magellanic Clouds. A quantitative analysis is difficult because ofthe low signal to noise ratio and the systematic effects produced byhaving multiple objects in the FUSE aperture. We use the M33 FUSE data to demonstrate in a more general manner thecomplexity of interpreting interstellar absorption line spectra towardsmulti-object background sources. We derive H2 columndensities of ~1016 to 1017 cm-2 along 3sight lines (NGC 588, NGC 592,NGC 595). Because of the systematic effects, thesevalues most likely represent upper limits and the non-detection ofH2 towards NGC 604 does not exclude theexistence of significant amounts of molecular gas along this sight line.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, available in the public archive. FUSE isoperated for NASA by the Johns Hopkins University under NASA contractNAS5-32985.
| Multicolor Photometry of 145 of the H II Regions in M33
This paper is the first in a series presenting CCD multicolor photometryfor 145 H II regions, selected from 369 candidate regions fromBoulesteix et al., in the nearby spiral galaxy M33. The observations,which covered the whole area of M33, were carried out with the BeijingAstronomical Observatory 60/90 cm Schmidt telescope, in 13intermediate-band filters, covering a range of wavelengths from 3800 to10000 Å. This procedure provides a series of maps that can beconverted into a multicolor map of M33, in pixels of 1.7"×1.7".Using aperture photometry we obtain the spectral energy distributions(SEDs) for these H II regions. We also give their identification charts.Using the relationship between the Beijing-Arizona-Taiwan-Connecticutintermediate-band system used for the observations and the UBVRIbroadband system, the magnitudes in the B and V bands are then derived.Histograms of the magnitudes in V and in B-V are plotted, and thecolor-magnitude diagram is also given. The distribution of magnitudes inthe V band shows that the apparent magnitude of almost all the regionsis brighter than 18, corresponding to an absolute magnitude of -6.62 foran assumed distance modulus of 24.62, which corresponds to a singlemain-sequence O5 star, while the distribution of color shows that thesample is blue, with a mode close to -0.05, as would be expected from arange of typical young clusters.
| Neon Abundances in the H II Regions of M33
We present neon abundances for 25 H II regions of M33, measured fromline profiles of the mid-infrared transitions of [Ne II] and [Ne III]taken with the Infrared Space Observatory Short-Wavelength Spectrometer.The distribution of neon abundances as a function of galactocentricradius is best described as a step, -0.15 dex relative to the solar neonabundance from 0.7 to 4.0 kpc and -0.35 dex from 4.0 to 6.7 kpc, withestimated intrinsic scatter of 0.07 dex. The nearly flat neon abundancedistribution differs from the steep oxygen abundance gradient found byprevious investigators. Unless the oxygen abundance determinations arewrong, the chemical evolution of the galaxy has been radially dependent.
| New light on the search for low-metallicity galaxies - I. The N2 calibrator
We present a simple metallicity estimator based on the logarithmic [Nii]ratio, hereafter N2, which we envisage will become very useful forranking galaxies in a metallicity sequence from redshift survey-qualitydata even for moderately low spectral resolution. We have calibrated theN2 estimator using a compilation of Hii galaxies having accurate oxygenabundances, plus photoionization models covering a wide range ofabundances. The comparison of models and observations indicates thatboth primary and secondary nitrogen are important for the relevant rangeof metallicities. The N2 estimator follows a linear relation withlog(O/H) that holds for the whole abundance range covered by the sample,from approximately to twice the Solar value . We suggest that the ([Sii]ratio (hereafter S2) can also be used as a rough metallicity indicator.Because of its large scatter the S2 estimator will be useful only insystems with very low metallicity, where [Nii] λ 6584 is notdetected or in low-resolution spectra where [Nii] λ 6584 isblended with Hα .
| The Excitation and Metallicity of Galactic H II Regions from Infrared Space Observatory SWS Observations of Mid-Infrared Fine-Structure Lines
We present mid-infrared Infrared Space Observatory Short-WavelengthSpectrometer (ISO-SWS) observations of the fine-structure emissionslines [Ne II] 12.8 μm, [Ne III] 15.6 μm, [Ne III] 36.0 μm, [ArII] 6.99 μm, [Ar III] 8.99 μm, [S III] 18.7 μm, [S III] 33.5μm, and [S IV] 10.5 μm and the recombination lines Brα andBrβ in a sample of 112 Galactic H II regions and 37 nearbyextra-Galactic H II regions in the LMC, SMC, and M33. We selected oursources from archival ISO-SWS data as those showing prominent [Ne II]12.8 μm or [Ne III] 15.6 μm emissions. The Galactic sources have awide range in galactocentric distance (0kpc<~Rgal<~18kpc), which enables us to study excitation and metallicity variationsover large Galactic scales. We detect a steep rise in the [Ne III] 15.6μm/[Ne II] 12.8 μm, [Ar III] 8.99 μm/[Ar II] 6.99 μm, and [SIV] 10.5 μm/[S III] 33.5 μm excitation ratios from the innerGalaxy outward, and a moderate decrease in metallicity, from ~2Zsolar in the inner Galaxy to ~1 Zsolar in theouter disk. The extra-Galactic sources in our sample show low gasdensity, low metallicity, and high excitation. We find a goodcorrelation between [Ne III] 15.6 μm/[Ne II] 12.8 μm and [Ar III]8.99 μm/[Ar II] 6.99 μm excitation ratios in our sample. Theobserved correlation is well reproduced by theoretical nebular modelsthat incorporate new-generation wind-driven non-LTE model stellaratmospheres for the photoionizing stars. In particular, the non-LTEatmospheres can account for the production of [Ne III] emission in the HII regions. We have computed self-consistent nebular and stellaratmosphere models for a range of metallicities (0.5-2Zsolar). We conclude that the increase in nebular excitationwith galactocentric radius is due to an increase in stellar effectivetemperature (as opposed to a hardening of the stellar spectral energydistributions due to the metallicity gradient). We estimate anintegrated [Ne III] 15.6 μm/[Ne II] 12.8 μm ratio for the Galaxyof 0.8, which puts it well inside the range of values for starburstgalaxies. The good fit between observations and our models support theconclusion of Thornley and coworkers that the low [Ne III] 15.6μm/[Ne II] 12.8 μm ratios observed in extra-Galactic sources aredue to global aging effects. Based on observations with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom)with the participation of ISAS and NASA.
| Narrow-band CCD photometry of giant H II regions
We have obtained accurate CCD narrow-band Hβ and Hαphotometry of giant HII regions (GEHRs) in M33, NGC 6822 and M101.Comparison with previous determinations of emission-line fluxes showslarge discrepancies; their probable origins are discussed. Combining ournew photometric data with global velocity dispersion (σ) derivedfrom emission linewidths, we review the L(Hβ)-σ relation. Are-analysis of the properties of the GEHRs included in our sample showsthat age spread and the superposition of components in multiple regionsintroduce a considerable spread in the regression. Combining theinformation available in the literature regarding ages of the associatedclusters, evolutionary footprints on the interstellar medium, andkinematical properties of the knots that build up the multiple GEHRs, wefind that a subsample - which we refer to as young and single GEHRs - dofollow a tight relation in the L-σ plane.
| On the oxygen abundance determination in HII regions. High-metallicity regions
This is our second paper devoted to the problem of line intensity -oxygen abundance calibration starting from the idea of McGaugh(\cite{mcg91}) that the strong oxygen lines ([OII] lambda lambda 3727,3729 and [OIII] lambda lambda 4959, 5007) contain the necessaryinformation to determine accurate abundances in HII regions. In theprevious study (Pilyugin 2000) the corresponding relations were obtainedfor the low-metallicity HII regions (12+log O/H <= 7.95, the lowerbranch of the O/H - R23 diagram). The high-metallicity HIIregions (12+log O/H >= 8.2, the upper branch of the O/H -R23 diagram) are considered in the present study. A relationof the type O/H=f(P, R23) between oxygen abundance and thevalue of abundance index R23, introduced by Pagel et al.(\cite{pag79}), and the excitation parameter P (which is defined here asthe contribution of the radiation in [OIII] lambda lambda 4959, 5007lines to the ``total" oxygen radiation) has been derived empiricallyusing the available oxygen abundances determined via measurement of atemperature-sensitive line ratio [OIII]4959,5007/[OIII]4363(Te-method). By comparing oxygen abundances inhigh-metallicity HII regions derived with the Te-method andthose derived with the suggested relations (P-method), it was found thatthe precision of oxygen abundance determination with the P-method isaround 0.1 dex (the mean difference for the 38 HII regions considered is~ 0.08 dex) and is comparable to that of the Te-method. Arelation of the type Te=f(P, R23) between electrontemperature and the values of abundance index R23 and theexcitation parameter P was derived empirically using the availableelectron temperatures determined via measurement oftemperature-sensitive line ratios. The maximum value of differencesbetween electron temperatures determined via measurement oftemperature-sensitive line ratios and those derived with the suggestedrelation is around 1000 K for HII regions considered here, the meanvalue of differences for 38 HII regions is ~ 500 K, which is the sameorder of magnitude as the uncertainties of electron temperaturedeterminations in high-metallicity HII regions via measuredtemperature-sensitive line ratios.
| An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.
| A comprehensive study of intense star formation bursts in irregular and compact galaxies
We have analyzed the properties of the star formation episodes takingplace in a sample of blue compact and irregular galaxies by comparingtheir multiwavelength observational properties with the predictions ofevolutionary population synthesis models. This method has allowed us toconstrain the age, star formation regime (instantaneous or extended) andInitial Mass Function (IMF) slope, as well as the shape and strength ofthe interstellar extinction in these regions. We find that starformation episodes are essentially short with a mean age of 3.5 Myrs.Some galaxies may be undergoing their first global episode of starformation while for the rest of the sample older stars contribute to atmost half the optical emission. The Wolf-Rayet star population (WR) iswell reproduced by the models and provides the strongest argument infavor of a short duration of the star formation episode. Supernova ratesare relatively large. The accumulation of supernova explosions withinfew Myr has contributed to a quick metal enrichment of the ISM and toits disruption by the release of huge amounts of mechanical energy. V-Kcolors agree well with the prediction that red supergiant stars are rarein low metallicity regions. A general agreement is found between thepredicted and observed far infrared emissions suggesting that thefraction of hidden stars contributing to the ionisation is minimum,except in some specific objects. A saillant result of this study is thatthe IMF slope appears to be very universal, on average very close tothat of the solar neighborhood and with no dependence on themetallicity, contrary to previous claims. We have also found nodependence whatsoever between the shape of the extinction law and themetallicity. It is likely that the strong radiation associated to thebursts destroys the dust component responsible for the 2175 Ä bump.Finally we confirm that extinction affecting the stellar continuum is insome cases significantly weaker than that derived from the Balmeremission lines. Such a discrepancy can lead to underestimations in thevalue of the {Hβ } equivalent width by a factor as large as 2,leading to an overestimation of the age of the burst. Similarly, theWolf-Rayet bump to the {Hβ } luminosities ratio can also beaffected by this differential reddening leading to an overestimation ofthe WR star population. As bursts get older they appear dustier,possibly as a result of dust ejection during the evolution of their mostmassive stars. Finally, we have found a serious general discrepancybetween the predicted and the measured radio luminosities. While part ofthis discrepancy might be attributed to aperture mismatching in somecases, it points to the presence of additional radio sources notincluded in present evolutionary models. Based on observations from theInternational Ultraviolet Explorer obtained at the ESA VILSPAobservatory, on observations taken at the Isaac Newton Telescope at theSpanish Observatorio del Roque de Los Muchachos on La Palma island andon observations with the Nançay radiotelescope.
| The Ionizing Star Clusters of Giant H II Regions in NGC 2403
We present the results of a study on the massive star population down toabout M_V~-3.1, or 12-15 M_solar, of the most luminous giant H IIregions in the nearby spiral galaxy NGC 2403, based on Hubble SpaceTelescope images and ground-based spectrograms. Particular emphasis isplaced on the distribution of the Wolf-Rayet and red supergiant starsand the information they provide about the recent star-forming historyof these large complexes. We find direct evidence for the presence ofWolf-Rayet (WR) stars in five of the six giant H II regionsinvestigated; 25-40 WR stars are inferred for the sole NGC 2403-I giantH II region. Red supergiant (RSG) stars are mainly distributed over amore extended halo, while the young blue stars and most WR stars are inor close to a compact core. One appears to be seeing young cores of Oand WR stars surrounded by older halos containing red supergiants. Wepropose a scenario in which RSG stars belonging to an early phase ofstar formation were followed by a more recent burst corresponding to avery blue mean sequence. Delayed trigger with preheating over several100 pc by the first generation of massive stars allowed the build-up ofthe required confinement for the production of parsec-scale clustercores with luminosity up to a few times 10^6 L_solar. Finally, wepresent some interesting objects found in the field of NGC 2403 outsidethe giant H II regions, such as field WR stars, globular clusters andbackground galaxies.
| Supersonic gas motion in giant extragalactic HII regions
We present high signal-to-noise ratio, long-slit spectra of the centralregion of the 30 Doradus nebula taken with unprecedented spatial (~ 0.1pc) and spectral (~ 3 km s-1) resolution. We use theseobservations to investigate the dominant line-broadening mechanisms forthe global nebula. We find no evidence for significant gravitationalbroadening of the lines. Thus, broadening appears to be dominated bystellar winds and thermal motions. However, we identify a new broadcomponent to the lines that explains the integrated profile wingsidentified in previous observations. The broad component appears at allpositions observed, and is possibly related to the disruption throughphoto-evaporation of high-density condensations within the nebula.
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