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A multi-scale study of infrared and radio emission from Scd galaxy M 33
Aims.We investigate the energy sources of the infrared (IR) emission andtheir relation to the radio continuum emission at various spatial scaleswithin the Scd galaxy M 33. Methods: We use the data at the Spitzerwavelengths of 24, 70, and 160 μm, as well as recent radio continuummaps at 3.6 cm and 20 cm observed with the 100-m Effelsberg telescopeand VLA, respectively. We use the wavelet transform of these maps to a)separate the diffuse emission components from compact sources, b)compare the emission at different wavelengths, and c) study the radio-IRcorrelation at various spatial scales. An Hα map serves as atracer of the star forming regions and as an indicator of the thermalradio emission. Results: The bright HII regions affect the waveletspectra causing dominant small scales or decreasing trends towards thelarger scales. The dominant scale of the 70 μm emission is largerthan that of the 24 μm emission, while the 160 μm emission shows asmooth wavelet spectrum. The radio and Hα maps are well correlatedwith all 3 MIPS maps, although their correlations with the 160 μm mapare weaker. After subtracting the bright HII regions, the 24 and 70μm maps show weaker correlations with the 20 cm map than with the 3.6cm map at most scales. We also find a strong correlation between the 3.6cm and Hα emission at all scales. Conclusions: Comparing theresults with and without the bright HII regions, we conclude that the IRemission is influenced by young, massive stars increasingly withdecreasing wavelength from 160 to 24 μm. The radio-IR correlationsindicate that the warm dust-thermal radio correlation is stronger thanthe cold dust-nonthermal radio correlation at scales smaller than 4 kpc.A perfect 3.6 cm-Hα correlation implies that extinction has nosignificant effect on Hα emitting structures.
| 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.
| 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.
| The impact of the visibility of the [OIII]λ4363 line on the general properties of HII galaxies in the Local Universe
We present a statistical study of a very large sample of HII galaxiestaken from the literature. We focus on the differences in severalproperties between galaxies that show the auroral line[OIII]λ4363 and those that do not present this feature in theirspectra. It turns out that objects without this auroral line are moreluminous, are more metal-rich and present a lower ionization degree. Theunderlying population is found to be much more important for objectswithout the [OIII]λ4363 line, and the effective temperature ofthe ionizing star clusters of galaxies not showing the auroral line isprobably lower. We also study the subsample of HII galaxies whoseproperties most closely resemble the properties of theintermediate-redshift population of luminous compact blue galaxies(LCBGs). The objects from this subsample are more similar to the objectsnot showing the [OIII]λ4363 line. It might therefore be expectedthat the intermediate-redshift population of LCBGs is powered by verymassive, yet somewhat aged, star clusters. The oxygen abundance of LCBGswould be greater than the average oxygen abundance of local HIIgalaxies.
| 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.
| 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.
| Wolf-Rayet stars in M33 - I. Optical spectroscopy using CFHT-MOS
We have obtained spectroscopy of a large sample of Wolf-Rayet stars inM33 with the Canada-France-Hawaii Telescope Multi-Object Spectrograph(CFHT-MOS), including 26 WC stars, 15 WN stars and a WN/C star. Ingeneral, spectral types are merely refined, although the spectral typeof X9 from Massey & Johnson is revised from WNL?+abs to WC4+abs,whilst their G1 and C21 candidates are not confirmed as Wolf-Rayetstars. We also re-examine the metallicity gradient of M33 from H IIregions and identify the present sample, lying in the inner disc, with8.6 <= log(O/H) <= 8.9. Spectral types are in accord with similarregions in the Milky Way. Our large sample has allowed us to examine theclaimed anticorrelation between WC linewidths and galactocentricdistance by Schild et al. We find a much larger scatter, though thereremains an absence of broad-line WC stars in the inner disc andnarrow-line WC stars in the outer galaxy.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| 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).
| The spiral galaxy M 33 mapped in the FIR by ISOPHOT. A spatially resolved study of the warm and cold dust
The Sc galaxy M 33 has been mapped with ISOPHOT in the far-infrared, at60, 100, and 170 mu m. The spatial resolution of these FIR maps allowsthe separation of spiral arms and interarm regions and the isolation ofa large number of star-forming regions. The spectral energy distributionin the FIR indicates a superposition of two components, a warm oneoriginating from dust at ~ 45 K, and a cold one, at ~ 16 K. The warmcomponent is concentrated towards the spiral arms and the star-formingregions, and is likely heated by the UV radiation from OB stars. Thecold component is more smoothly distributed over the disk, and heated bythe diffuse interstellar radiation. For the about 60 star-formingregions detected the Hα /FIR flux ratio increases significantlywith the distance from the galaxy center, probably due to decreasingextinction. An anti-correlation of FHa/ F60 withF170 suggests the intrinsic extinction to be related to thecold dust surface brightness according to AV/ S170~ 0.03 mag MJy-1 sr. For the total galaxy the star formationrate (SFR) derived from the FIR is in agreement with that derived fromthe de-extincted Hα emission. For individual star-formingregions, a consistency between SFRs derived from the optical and fromthe FIR requires only a fraction of the UV radiation to be absorbedlocally. The individual star-forming regions also show a local radio-FIRcorrelation. This local correlation is, however, due to quite differentcomponents than to those that lead to the well-known global radio-FIRcorrelation for entire galaxies.Based on observations with the Infrared Space Observatory ISO. ISO is anESA project with instruments funded by ESA Member States (especially thePI countries France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and 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).
| STIS Spectral Imagery of the OB Stars in NGC 604. II. The Most Luminous Stars
We present results using two-dimensional spectral imagery and photometryobtained with the Hubble Space Telescope (HST) for the starburst H IIregion NGC 604, in the nearby galaxy M33. The spectral imagery wasacquired with the Space Telescope Imaging Spectrograph (STIS) using theMAMA-G140L configuration, which provided wavelength coverage spanning1170-1730 Å. From a single 1720 s STIS exposure, we have extractedspectra for 49 stars and derived individual UV spectral types for 40stars in the crowded 25"×2" stellar field sampled by the STISaperture. These stars represent a significant fraction of the young,luminous O and B stars in NGC 604. Three objects have pronounced He IIλ1640 emission, the signature of Wolf-Rayet (W-R) or luminous Ofstars. By combining UV fluxes with HST WF/PC-1 and WFPC2 photometry atvisible wavelengths, we derive the extinction curve for NGC 604. We usethis extinction curve, together with the available accurate distance forM33, derived UV spectral types, and HST photometry, to determinepositions of the luminous stars in the upper Hertzsprung-Russell diagramfor NGC 604. The revision to the O star effective temperature scale byMartins et al., based on non-LTE, line-blanketed model atmospheres, isessential in obtaining reliable positions in thelogL*-logTeff plane. These stars are quite young,with a characteristic age of ~3 Myr. The spectra and photometry indicatethat three objects are exceedingly luminous. Their inferred locations inthe H-R diagram relative to theoretical evolutionary tracks indicatestellar masses >=120 Msolar. High spatial resolution HSTimagery provides no evidence of multiple stars composing these objects.Still, we cannot eliminate the possibility that these objects are notunresolved multiple stars of lower mass, possibly W-R stars. Simpletests demonstrate that the 10 most luminous stars predominantlydetermine the UV spectral features seen in the total light of NGC 604.We conclude that the interpretation of spectral fitting of more distantstarburst galaxies, where individual stars are not resolved, must bedone with extreme care.
| 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.
| Classification of Wolf-Rayet stars in NGC 595 in M 33 through high-spatial resolution observations
NGC 595 is, after NGC 604, the second most luminous HII region in theMilky Way's neighbouring spiral galaxy M 33. Its Wolf-Rayet starcontent has mainly been unveiled by two different channels. On the onehand, the stellar population of NGC 595 has been resolved and its WRstars identified through on-line/off-line HeIIλ4686 observationsrealised with the HST. Nevertheless, due to the limited number offilters used, this did not give any information on the WR spectralsubtypes. On the other hand, spectroscopic observations of NGC 595,realised at optical and ultraviolet wavelengths, have enabled thedetermination of some spectral subtypes, but this time, the lack ofangular resolution did not allow to resolve the whole population. Thanks to our photometric technique, based on five dedicated narrow-bandfilters, we present here a determination of the spectral subtypes of NGC595 WR stars which for the first time combines high-angular resolutionand high-`spectroscopic' discrimination capabilities.
| Wolf-Rayet stars in Local Group starbursts
I describe recent spectroscopic observations of Wolf-Rayet stars inthree giant HII regions of M 33 and in the Local Group starburst galaxyIC 10.
| 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.
| Observations of Galaxies with the Midcourse Space Experiment
We have imaged eight nearby spiral galaxies with the SPIRIT III infraredtelescope on the Midcourse Space Experiment (MSX) satellite in themid-infrared at 18" resolution at 8.3, 12.1, 14.7, and 21.3 μm. Eachof the eight shows interesting structure not previously detected witholder, lower resolution infrared data sets, such as a resolved nucleusor spiral structure. The MSX data are compared with existing data setsat ultraviolet, optical, and infrared wavelengths, including recentobservations from the Infrared Space Observatory. The infraredstructures in M83 and NGC 5055 show a striking similarity to theultraviolet emission but are less similar to the optical emission.Several point sources with no identified counterparts at otherwavelengths are found near M31, NGC 4945, M83, and M101. Over 200previously known objects are also detected at 8 μm.
| 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.
| Internal Variation of Electron Density in Galactic and Extragalactic HII Regions
Not Available
| Structural Properties of Massive Young Clusters
We have retrieved multicolor WFPC2/Hubble Space Telescope data from theSTScI archive for 27 nearby massive (>~3×104Msolar) young (<20 Myr) star clusters (MYCs). The datarepresents the most complete sample of clearly resolved MYCs to date. Wehave analyzed their structural properties and have found that they canbe classified as either super-star clusters (SSCs) or as scaled OBassociations (SOBAs). SSCs have a compact core possibly surrounded by ahalo while SOBAs have no core. A morphological sequence can beestablished from SSCs with weak halos to SSCs with strong halos toSOBAs, and we propose that this is linked to the original massdistribution of the parent giant molecular clouds. Our results indicatethat a significant fraction of the stars in MYCs dissipate on timescalesof 10 Gyr or less due to the extended character of some of the clusters.Also, SSCs with ages younger than 7 Myr have smaller cores on averagethan those with ages older than 7 Myr, confirming predictions ofnumerical simulations with mass loss.
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