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The M33 Variable Star Population Revealed by Spitzer
We analyze five epochs of Spitzer Space Telescope Infrared Array Camera(IRAC) observations of the nearby spiral galaxy M33. Each epoch coverednearly a square degree at 3.6, 4.5, and 8.0 μm. The point-sourcecatalog from the full data set contains 37,650 stars. The stars haveluminosities characteristic of the asymptotic giant branch and can beseparated into oxygen-rich and carbon-rich populations by their[3.6]-[4.5] colors. The [3.6]-[8.0] colors indicate that over 80% of thestars detected at 8.0 μm have dust shells. Photometric comparison ofepochs using conservative criteria yields a catalog of 2923 variablestars. These variables are most likely long-period variables amid anevolved stellar population. At least one-third of the identified carbonstars are variable.
| Arclike Distribution of High CO (J=3-2)/CO (J=1-0) Ratio Gas Surrounding the Central Star Cluster of the Supergiant H II Region NGC 604
We report the discovery of a high CO (J=3-2)/CO (J=1-0) ratio gas withan arclike distribution (``high-ratio gas arc'') surrounding the centralstar cluster of the supergiant H II region NGC 604 in the nearby spiralgalaxy M33, based on multi-J CO observations of a5'×5' region of NGC 604 conducted using theASTE 10 m and NRO 45 m telescopes. The discovered ``high-ratio gas arc''extends to the southeast-northwest direction with a size of ~200 pc. Thewestern part of the high-ratio gas arc closely coincides with the shellsof the H II regions traced by Hα and radio continuum peaks. The CO(J=3-2)/CO (J=1-0) ratio, R3-2/1-0, ranges between 0.3 and1.2 in the observed region, and the R3-2/1-0 values of thehigh-ratio gas arc are around or higher than unity, indicating very warm(Tkin>=60 K) and dense(nH2>=103-104cm-3) conditions of the high-ratio gas arc. We suggest thatthe dense gas formation and second-generation star formation occur inthe surrounding gas compressed by the stellar wind and/or supernova ofthe first-generation stars of NGC 604, i.e., the central star cluster ofNGC 604.
| The Calibration of the O/H Abundance Indicators for Extragalactic H II Regions based on O II Recombination Lines
Based on O II recombination lines we present a new calibration (called OII_{RL}) of Pagel's O_{23} indicator to determine the O/H abundanceratio in extragalactic H II regions and emission line galaxies. The OII_{RL} calibration produces O/H abundances about a factor of two higherthan those derived from the T(4363) method with t^2 = 0.00. The OII_{RL} calibration has implications for the study of differentproperties of emission line galaxies such as their metallicity, starformation rate, and initial mass function. The O II_{RL} calibrationalso affects the abundance determinations based on other O/H indicators,that include collisionally excited lines, like those known as O_3N_2,N_2, S_{23], Ar_3O_3, and S_3O_3. We argue that the controversy betweenthe T(4363) method and the photoionization models method to derive O/Hvalues is mainly due to temperature variations inside the observed H IIregions.
| An evolutionary sequence of expanding hydrogen shells in galaxy discs
Aims.Large HI shells, with diameters of hundreds of pc and expansionvelocities of 10-20 km s-1 have been detected in theirhundreds in the Milky Way and are well observed features of local gasrich galaxies. These shells could well be predicted as a result of theimpact of OB associations on the ISM, but doubt has been cast on thisscenario by the apparent absence of OB stars close to the centres of alarge fraction of these shells in recent observations of the SMC. Herewe present observational evidence within an energetically consistentframework which strongly supports the scenario in which OB associationsdo produce the giant HI shells. Methods: Using Fabry-Perot scannedHα emission line mapping of nearby galaxy discs, we have detected,in all the H ii regions where the observations yield sufficient angularresolution and S:N ratio, dominant Hα shells with radii a few tensof pc, expanding at velocities of 50-100 km s-1, and with gasmasses of 10^4-105 M_ȯ. In previous studies, we foundthat stellar winds alone can account for the energetics of most of theHα shells, which form initially before the stars explode as SNe.We have applied a simple dynamically consistent framework in which wecan extrapolate the properties of the observed Hα shells to a few107 yr after the formation of the OB stars. The frameworkincludes the dynamical inputs of both winds and SNe on the surroundingISM. The results give quantitative statistical support to the hypothesisthat the Hα emitting shells are generic progenitors of the HIshells. Results: The results are in good agreement with the ranges ofmasses (~106 M_ȯ), velocities (up to ~20kms-1), and diameters (up to ~500 pc) of representative HIshells observed in nearby galaxies. The combined effects of stellarwinds, acting during the first few 106 yr, and SN explosions,“switching on” subsequently, are required to yield theobserved parameters.
| The nature of the Wolf-Rayet galaxy Mrk 209 from photoionization models
We present a detailed photoionization model of the brightest knot ofstar formation in the blue compact dwarf galaxy Mrk 209. The modelreproduces the intensities of main lines emitted by the ionized gas,resulting in a very good agreement between observed and predicted linetemperatures and chemical abundances of the observed ionic species.The model has been calculated using the spectral energy distribution ofa massive cluster of recent formation as the ionizing source. Thefeatures of Wolf-Rayet (WR) stars observed in the spectrum of theobject, along with its ionizing properties, lead to different solutionsfor the ages and characteristics of the ionizing stellar populations.The found solutions are compatible with either a composite population oftwo ionizing clusters or a continuous star formation. In the first case,a young cluster, with an age less than 3 Myr, would be responsible formost of the ionization properties while an older cluster, with either3.2 or 4.2 Myr, would be responsible for the emission of the observed WRfeatures in the spectrum of the knot. In the second case, the durationof the star formation episode is found to be 3.6 Myr.The addition of a certain amount of dust was required in order toreproduce correctly the measured electron temperatures. This dustimplies a gas/dust ratio of 6.13 × 10-3, identical tothat in the Milky Way, and a fraction of absorbed photons of f = 0.58.The taking into account the dust grain physics combined with athick-shell geometry solves the problem of the heating in this objectand allows the fitting of the auroral lines, the line temperatures andthe ionization structure of the nebula with a negligible presence oftemperature fluctuations, in agreement with the most recent resultsfound for this type of objects from the measurement of the Balmer jumptemperature.An analysis of the ionization structure of the gas as well as thecalculation of the main ionization correction factors for the unseenionic stages is presented.
| 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.
| 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.
| Free-free absorption in the gravitational lens JVAS B0218+357
We address the issue of anomalous image flux ratios seen in thedouble-image gravitational lens JVAS B0218+357. From the multi-frequencyobservations presented in a recent study (Mittal et al. 2006, A&A,447, 515) and several previous observations made by other authors, theanomaly is well-established in that the image flux-density ratio (A/B)decreases from 3.9 to 2.0 over the observed frequency range from 15 GHzto 1.65 GHz. In Mittral et al. (2206), the authors investigated whetheran interplay between a frequency-dependent structure of the backgroundradio-source and a gradient in the relative image-magnification canexplain away the anomaly. Insufficient shifts in the image centroidswith frequency led them to discard the above effect as the cause of theanomaly. In this paper, we first take this analysis further byevaluating the combined effect of the background source extension andmagnification gradients in the lens plane in more detail. This is doneby making a direct use of the observed VLBI flux-distributions for eachimage to estimate the image flux-density ratios at different frequenciesfrom a lens-model. As a result of this investigation, this mechanismdoes not account for the anomaly. Following this, we analyze the effectsof mechanisms which are non-gravitational in nature on the image fluxratios in B0218+357. These are free-free absorption and scattering, andare assumed to occur under the hypothesis of a molecular cloud residingin the lens galaxy along the line-of-sight to image A. We show thatfree-free absorption due to an Hii region covering the entire structureof image A at 1.65 GHz can explain the image flux ratio anomaly. We alsodiscuss whether Hii regions with physical parameters as derived from ouranalysis are consistent with those observed in Galactic andextragalactic Hii regions.
| Temperature fluctuations in H II regions: t{^2} for the two-phase model
Aims.We investigate temperature fluctuations in H II regions in terms ofa two-phase model, which assumes that the nebular gas consists of a hotand a cold phase. Methods: We derive general formulae for T([O III]),the [O III] forbidden line temperature, and T(H I), the hydrogen Balmerjump temperature, in terms of the temperatures of the hot and coldphases, Th and T_c. Results: For large temperaturedifferences, the values of t2 required to account for theobserved difference between T([O III]) and T(H I) are much lower thanthose deduced using the classical formulae that assume random and smallamplitude temperature fluctuations. One should therefore be cautiouswhen using a two-phase model to account for empirically derivedt2 values. We present a correction of a recent work byGiammanco & Beckman, who use a two-phase model to estimate theionization rate of H II regions by cosmic rays. We show that a verysmall amount of cold gas is sufficient to account for t2values typically inferred for H ii regions.
| The Localized Chemical Pollution in NGC 5253 Revisited: Results from Deep Echelle Spectrophotometry
We present echelle spectrophotometry of the blue compact dwarf galaxyNGC 5253 obtained with the VLT UVES. We have measured the intensities ofa large number of permitted and forbidden emission lines in four zonesof the central part of the galaxy. We detect faint C II and O IIrecombination lines, the first time that these are unambiguouslydetected in a dwarf starburst galaxy. The physical conditions of theionized gas have been derived using a large number of different lineintensity ratios. Chemical abundances of He, N, O, Ne, S, Cl, Ar, and Fehave been determined following standard methods. C++ andO++ abundances have been derived from pure recombinationlines and are larger than those obtained from collisionally excitedlines (from 0.30 to 0.40 dex for C++ and from 0.19 to 0.28dex for O++). This result is consistent with a temperaturefluctuation parameter (t2) between 0.050 and 0.072. Weconfirm previous results that indicate the presence of a localized Nenrichment in certain zones of NGC 5253 and detect a possible slight Heoverabundance in the same zones. The enrichment pattern agrees with thatexpected for the pollution by the ejecta of Wolf-Rayet (W-R) stars. Theamount of enriched material needed to produce the observed overabundanceis consistent with the mass lost by the number of W-R stars estimated inthe starbursts. We discuss the possible origin of the difference betweenabundances derived from recombination and collisionally excited lines(the so-called abundance discrepancy problem) in H II regions, findingthat a recent hypothesis based on the delayed enrichment by SN ejectainclusions seems not to explain the observed features.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 70.C-0008(A).
| Interstellar abundances in the neutral and ionized gas of NGC 604
Aims.We present FUSE spectra of the giant H ii region NGC 604 in thespiral galaxy M 33. Chemical abundances are tentatively derived fromfar-UV absorption lines and compared to those derived from opticalemission lines. Methods: .Absorption lines from neutral hydrogenand heavy elements were observed against the continuum provided by theyoung massive stars embedded in the H ii region. We derived the columndensities of H i, N i, O i, Si ii, P ii, Ar i, and Fe ii, fitting theline profiles with either a single component or several components. Weused CLOUDY to correct for contamination from the ionized gas. ArchivalHST/STIS spectra across NGC 604 allowed us to investigate howinhomogeneities affect the final H i column density. Results:.Kinematics show that the neutral gas is physically related to the H iiregion. The STIS spectra reveal H i column density fluctuations up to 1dex across NCG 604. Nevertheless, we find that the H i column densitydetermined from the global STIS spectrum does not differ significantlyfrom the average over the individual sightlines. Our net results usingthe column densities derived with FUSE, assuming a single component,show that N, O, Si, and Ar are apparently underabundant in the neutralphase by a factor of 10 or more with respect to the ionized phase,while Fe is the same. However, we discuss the possibility that theabsorption lines are made of individual unresolved components, and findthat only P ii, Ar i, and Fe ii lines should not be affected by thepresence of hidden saturated components, while N i, O i, and Si ii mightbe much more affected. Conclusions: .If N, O, and Si are actuallyunderabundant in the neutral gas of NGC 604 with respect to the ionizedgas, this would confirm earlier results obtained for the blue compactdwarfs, and their interpretations. However, a deeper analysis focused onP, Ar, and Fe mitigates the above conclusion and indicates that theneutral gas and ionized gas could have similar abundances.
| 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
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| Hα line profiles for a sample of supergiant HII regions. II. Broad, low intensity components
We analyze the broad, low intensity, high velocity components that areseen in the Hα line profiles for a sample of HII regions. TheseHII regions are chosen from among the brightest and most isolated in asample of spiral galaxies for which we have photometric andspectroscopic data: NGC 157, NGC 3631, NGC 6764, NGC 3344, NGC 4321, NGC5364, NGC 5055, NGC 5985, and NGC 7479. We confirm that the lineprofiles of most of these bright, giant extragalactic HII regionscontain broad kinematic components of low intensity, but high velocity,that we denote as wings. We analyze these components, deriving emissionmeasures, central velocities, and velocity dispersions of the blue andred features, which are similar. We interpret these components asexpanding shells within the HII regions and produced by the stellarwinds from the ionizing stars. We compare the kinetic energies of theseexpanding shells with the kinetic energy available from the stellarwinds. If we allow for the hypothesis that the brightest HII regions aredensity bounded, we show that, for these HII regions, the stellar windmechanism can explain the observed shell kinetic energies.
| Detection of [O I] λ6300 and Other Diagnostic Emission Lines in the Diffuse Ionized Gas of M33 with Gemini-North
We present spectroscopic observations of diffuse ionized gas (DIG) inM33 near the H II region NGC 604. We present the first detection of [OI] λ6300 in the DIG of M33, one of the critical lines fordistinguishing photoionization from shock ionization models. We measure[O I]/Hα in the range of 0.04-0.10 and an increase in this ratiowith decreasing emission measure. Our measurements of [S II]/Hαand [N II]/Hα also rise with decreasing emission measure, whileour [O III]/Hβ measurements remain fairly constant. We have onetentative detection of He I in the region of brightest emission measure,with a ratio of He I/Hα = 0.033 +/- 0.019, indicating that thehelium is at least partially ionized. We compare our observedemission-line ratios to photoionization models and find that field starionization models do not fit our data well. Leaky H II region models areconsistent with our data, without the need to invoke additionalionization mechanisms to fit our [O I] or [O III] measurements. Theclosest large H II region is NGC 604 and is therefore a likely candidatefor the source of the ionizing photons for the gas in this region.
| 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.
| On the Feedback from Super Stellar Clusters. I. The Structure of Giant H II Regions and H II Galaxies
We review the structural properties of giant extragalactic H II regionsand H II galaxies based on two-dimensional hydrodynamic calculations andpropose an evolutionary sequence that accounts for their observeddetailed structure. The model assumes a massive and young stellarcluster surrounded by a large collection of clouds. These are thusexposed to the most important star formation feedback mechanisms:photoionization and the cluster wind. The models show how the twofeedback mechanisms compete with each other in the disruption of cloudsand lead to two different hydrodynamic solutions: the storage of cloudsinto a long-lasting ragged shell that inhibits the expansion of thethermalized wind and the steady filtering of the shocked wind gasthrough channels carved within the cloud stratum. Both solutions arehere claimed to be concurrently at work in giant H II regions and H IIgalaxies, causing their detailed inner structure. This includes multiplelarge-scale shells, filled with an X-ray-emitting gas, that evolve tofinally merge with each other, giving the appearance of shells withinshells. The models also show how the inner filamentary structure of thegiant superbubbles is largely enhanced with matter ablated from cloudsand how cloud ablation proceeds within the original cloud stratum. Thecalculations point at the initial contrast density between the cloud andthe intercloud media as the factor that defines which of the twofeedback mechanisms becomes dominant throughout the evolution. Animatedversions of the models presented can be found athttp://www.iaa.csic.es/~eperez/ssc/ssc.html.
| A Chandra ACIS Study of 30 Doradus. I. Superbubbles and Supernova Remnants
We present an X-ray tour of diffuse emission in the 30 Doradusstar-forming complex in the Large Magellanic Cloud using high spatialresolution X-ray images and spatially resolved spectra obtained with theAdvanced CCD Imaging Spectrometer on board the Chandra X-RayObservatory. The dominant X-ray feature of the 30 Doradus nebula is theintricate network of diffuse emission generated by interacting stellarwinds and supernovae working together to create vast superbubbles filledwith hot plasma. We construct maps of the region showing variations inplasma temperature (T=3-9 million degrees), absorption[NH=(1-6)×1021 cm-2], andabsorption-corrected X-ray surface brightness[SX=(3-126)×1031 ergs s-1pc-2]. Enhanced images reveal the pulsar wind nebula in thecomposite supernova remnant N157B, and the Chandra data show spectralevolution from nonthermal synchrotron emission in the N157B core to athermal plasma in its outer regions. In a companion paper we show thatR136, the central massive star cluster, is resolved at the arcsecondlevel into almost 100 X-ray sources. Through X-ray studies of 30 Doradusthe complete life cycle of such a massive stellar cluster can berevealed.
| Reduced Wolf-Rayet line luminosities at low metallicity
New NTT/EMMI spectrophotometry of single WN2-5 stars in the Small andLarge Magellanic Clouds are presented, from which He ii λ4686line luminosities have been derived, and compared with observations ofother Magellanic Cloud Wolf-Rayet stars. SMC WN3-4 stars possess lineluminosities which are a factor of 4 times lower than LMC counterparts,incorporating several binary SMC WN3-4 stars from the literature.Similar results are found for WN5-6 stars, despite reduced statistics,incorporating observations of single LMC WN5-9 stars from theliterature. C iv λ5808 line luminosities of carbon sequence WRstars in the SMC and IC 1613 (both WO subtypes) from the recentliterature are a factor of 3 lower than LMC WC stars from Mt Stromlo/DBSspectrophotometry, although similar results are also obtained for thesole LMC WO star. We demonstrate how reduced line luminosities at lowmetallicity follow naturally if WR winds are metallicity-dependent, asrecent empirical and theoretical results suggest. We apply massloss-metallicity scalings to atmospheric non-LTE models of Milky Way andLMC WR stars to predict the wind signatures of WR stars in themetal-poor star forming WR galaxy I Zw 18. WN He ii λ4686 lineluminosities are 7-20 times lower than in metal-rich counterparts ofidentical bolometric luminosity, whilst WC C iv λ5808 lineluminosities are 3-6 times lower. Significant He+ Lymancontinuum fluxes are predicted for metal-poor early-type WR stars.Consequently, our results suggest a larger population of WR stars in IZw 18 than is presently assumed, particularly for WN stars, potentiallyposing a severe challenge to evolutionary models at very lowmetallicity. Finally, reduced wind strengths from WR stars at lowmetallicities impacts upon the immediate circumstellar environment oflong duration GRB afterglows, particularly since the host galaxies ofhigh-redshift GRBs tend to be metal-poor.
| 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.
| The infrared Hourglass cluster in M8*†
A detailed study of the Hourglass nebula in the M8 star-forming regionis presented. The study is mainly based on recent subarcsec-resolutionJHKs images taken at Las Campanas Observatory andcomplemented with archival Hubble Space Telescope (HST) images andlong-slit spectroscopy retrieved from the European Southern ObservatoryArchive Facility. Using the new numerical code CHORIZOS, we estimate thedistance to the earliest stars in the region to be 1.25 kpc. Infraredphotometry of all the sources detected in the field is given. Fromanalysis of the JHKs colour-colour diagrams, we find that animportant fraction of these sources exhibit significant infrared excess.These objects are candidates to be low- and intermediate-masspre-main-sequence stars. Based on HST observations, the spatialdistribution of gas, dust and stars in the region is analysed. Amorphological analysis of these images also reveals a rich variety ofstructures related to star formation (proplyds, jets, bow shocks),similar to those observed in M16 and M42, along with the detection ofthe first four Herbig-Haro objects in the region. Furthermore, along-slit spectrum obtained with the New Technology Telescope confirmsthe identification of one of them (HH 870) in the core of the Hourglassnebula, providing the first direct evidence of active star formation byaccretion in M8.
| An XMM-Newton survey of the Local Group galaxy M 33 - variability of the detected sources
We present an analysis of the individual observations of a deepXMM-Newton survey of the Local Group spiral galaxy M 33. We detected atotal of 350 sources with fluxes (in the 0.2-4.5 keV energy band) in therange 6.7 × 10-16-1.5 × 10-11 ergs-1. This comprehensive study considers flux variability,spectral characteristics, and classification of the detected objects.Thirty-nine objects in our catalogue are new sources, while 311 werealready detected in a previous analysis of most of the same data usingcombined images. We present improved positions of these sources and theX-ray parameters of each source in each individual observation thatcovers the source. We then used these parameters to systematicallysearch for flux variability on time scales of hours to months or years.The long-term light-curves were generated for the 61 sources showing asignificant variability of the (0.2-4.5) keV flux, by a factor of 1.2 to144. The detected variability was then used to classify 8 new X-raybinary candidates in M 33. Together with the hardness ratio method andcross-correlation with optical, infrared, and radio data, we alsoclassify or confirm previous classification of 25 supernova remnants andcandidates, 2 X-ray binaries, and 11 super-soft source candidates (7 ofwhich are new SSS candidates). In addition, we classify 13 activegalactic nuclei and background galaxies, 6 stars, and 23 foreground starcandidates in the direction of M 33. Further 206 objects are classifiedas "hard", approximately half of which are sources intrinsic to M 33.The relative contribution of the classified XRB and SSS in M 33 is nowcomparable to M 31. The luminosity distribution of SNRs in both spiralgalaxies is almost the same, although the number of the detected SNRs inM 33 remains much higher.
| 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.
| Diagnostics of the Ionized ISM: The Importance of Density Fluctuations
It is well known that when measuring electron densities in H II regionslocally via emission line ratios the values observed are 100cm-3, butgobal values obtained via emission measure using surface brightness asthe observed parameter are ~1cm-3. This difference is attributable todensity fluctuations,and traditional models for these is the ``Fillingfactor (FF)'' model, of Osterbrock and Flather. Implicit in this modelis the optical thinness of the fluctuations. We show here that if thesefluctuations are optically thick the diagnostic diagrams which are usedto find parameters, notably abundances, will be significantly affected.We present observational arguments supporting these ``clumpy'' models,and outline some initial results of their use.
| Slit observations and empirical calculations for HII regions
When analysing HII regions, a possible source of systematic error onempirically derived quantities, such as the gas temperature and thechemical composition, is the limited size of the slit used for theobservations. In order to evaluate this type of systematic error, we usethe photoionization code AANGABA to create a virtual photoionized regionand mimic the effect of a slit observation. A grid of models was builtvarying the ionizing radiation spectrum emitted by a central stellarcluster, as well as the gas abundance. The calculated line surfacebrightness was then used to simulate slit observations and to deriveempirical parameters using the usual methods described in theliterature. Depending on the fraction of the object covered by the slit,the empirically derived physical parameters and chemical composition canbe different from those obtained from observations of the whole object.This effect is mainly dependent on the age of the ionizing stellarcluster. The low-ionization lines, which originate in the outer layersof the ionized gas, are more sensitive to the size of the area coveredby the slit than the high-ionization forbidden lines or recombinationlines, because these lines are mainly produced closer to the innerradius of the nebula. For a slit covering 50 per cent or less of thetotal area, the measured [OIII], [OII] and [OI] line intensities areless than 78, 62 and 58 per cent of the total intensity for a young HIIregion (t < 3 Myr); for older objects the effect due to the slit isless significant. Regarding the temperature indicatorT[OIII], the slit effects are small (usually less than 5 percent) because this temperature is derived from [OIII] high-ionizationlines. On the other hand, for the abundance (and temperature) indicatorR23, which depends also on the [OII] line, the slit effect isslightly higher. Therefore, the systematic error due to slitobservations on the O abundance is low, being usually less than 10 percent, except for HII regions powered by stellar clusters with a relativelow number of ionizing photons between 13.6 and 54.4 eV, which create asmaller O++ emitting volume. In this case, the systematicerror on the empirical O abundance deduced from slit observations ismore than 10 per cent when the covered area is less than 50 per cent.
| 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.
| Oxygen Recombination Line Abundances in Gaseous Nebulae
The determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines.
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