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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.
| Imprints of spiral arms in the oxygen distribution over the galactic disc
A theory for the oxygen abundance radial distribution formation in thegalactic disc of a spiral galaxy is developed. We take into account thatthe main sources of oxygen are Type II supernovae (SN II), theprogenitors of which are massive short-lived stars strongly concentratedin the spiral arms. Hence oxygen is the most sensitive indicator ofspiral arms' influence on galactic disc enrichment by heavy elements.Various models for the spiral density waves were analysed. We predictthat the imprints in the oxygen radial distribution will enable us todistinguish between different models for spiral patterns. Among otherparameters, the corotation radius happens to be one of the mostimportant.
| An XMM-Newton view of M101 - I. The luminous X-ray source population
We present the first results of an XMM-Newton EPIC observation of theluminous X-ray source population in the face-on supergiant spiral galaxyM101. We have studied the spectral and temporal properties of the 14most luminous sources, all of which have intrinsic X-ray luminositiesexceeding the Eddington limit for a 1.4-Msolar neutron star,with a subset in the ultraluminous X-ray source (ULX) regime(LX>= 1039 erg s-1). Eleven sourcesshow evidence of short-term variability, and most vary by a factor of~2-4 over a baseline of 11-24 yr, providing strong evidence that thesesources are accreting X-ray binary (XRB) systems. Our resultsdemonstrate that these sources are a heterogeneous population, showing avariety of spectral shapes. Interestingly, there is no apparent spectraldistinction between those sources above and below the ULX luminositythreshold. Nine sources are well fitted with either simple absorbed discblackbody or power-law models. However, in three of the four sourcesbest fitted with power-law models, we cannot exclude the disc blackbodyfits and therefore conclude that, coupled with their high luminosities,eight out of nine single-component sources are possibly high-state XRBs.The nuclear source (XMM-10) has the only unambiguous power-law spectrum(Γ~ 2.3), which may be evidence for the presence of alow-luminosity active galactic nucleus (LLAGN). The remaining fivesources require at least two-component spectral fits, with an underlyinghard component that can be modelled by a power-law continuum or, inthree cases, a hot disc blackbody (Tin= 0.9-1.5 keV), plus asoft component modelled as a cool blackbody/disc blackbody/thermalplasma. We have compared the spectral shapes of nine sources covered byboth this observation and an archival 100-ks Chandra observation ofM101; eight show behaviour typical of Galactic XRBs (i.e. softening withincreasing luminosity), the only exception being a transient source(XMM-2) which shows little change in spectral hardness despite a factorof ~30 increase in luminosity. We find no definitive spectral signaturesto indicate that these sources contain neutron star primaries, andconclude that they are likely to be stellar-mass black hole XRBs(BHXBs), with black hole masses of ~2-23 Msolar if accretingat the Eddington limit.
| Highly Ionized Gas Surrounding High-Velocity Cloud Complex C
We present Far Ultraviolet Spectroscopic Explorer and Hubble SpaceTelescope observations of high-, intermediate-, and low-ion absorptionin high-velocity cloud (HVC) Complex C along the lines of sight towardfive active galaxies. Our purpose is to investigate the idea thatComplex C is surrounded by an envelope of highly ionized material,arising from the interaction between the cloud and a hot surroundingmedium. We measure column densities of high-velocity high-ion absorptionand compare the kinematics of low-, intermediate-, and high-ionizationgas along the five sight lines. We find that in all five cases, the H Iand O VI high-velocity components are centered within 20 kms-1 of one another, with an average displacement ofOVI-vHI>=3+/-12 km s-1. In thosedirections where the H I emission extends to more negative velocities(the so-called high-velocity ridge), so does the O VI absorption. Thekinematics of Si II is also similar to that of O VI, withOVI-vSiII>=0+/-15 km s-1. We compareour high-ion column density ratios to the predictions of various models,adjusted to account for both recent updates to the solar elementalabundances and relative elemental abundance ratios in Complex C. Alongthe PG 1259+593 sight line, we measure N(SiIV)/N(OVI)=0.10+/-0.02,N(CIV)/N(OVI)=0.35+0.05-0.06, andN(NV)/N(OVI)<0.07 (3 σ). These ratios are inconsistent withcollisional ionization equilibrium at one kinetic temperature.Photoionization by the extragalactic background is ruled out as thesource of the high ions since the path lengths required would make HVCsunreasonably large; photoionization by radiation from the disk of theGalaxy also appears unlikely since the emerging photons are notenergetic enough to produce O VI. By themselves, ionic ratios areinsufficient to discriminate between various ionization models, but byconsidering the absorption kinematics as well, we consider the mostlikely origin for the highly ionized high-velocity gas to be at theconductive or turbulent interfaces between the neutral/warm ionizedcomponents of Complex C and a surrounding hot medium. The presence ofinterfaces on the surface of HVCs provides indirect evidence for theexistence of a hot medium in which the HVCs are immersed. This mediumcould be a hot (T>~106 K) extended Galactic corona or hotgas in the Local Group.Based on observations from the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer mission, operated by Johns Hopkins University,supported by NASA contract NAS 5-32985, and from the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale
We use high signal-to-noise ratio spectra of 20 H II regions in thegiant spiral galaxy M101 to derive electron temperatures for the H IIregions and robust metal abundances over radii R=0.19-1.25R0(6-41 kpc). We compare the consistency of electron temperatures measuredfrom the [O III] λ4363, [N II] λ5755, [S III]λ6312, and [O II] λ7325 auroral lines. Temperatures from[O III], [S III], and [N II] are correlated with relative offsets thatare consistent with expectations from nebular photoionization models.However, the temperatures derived from the [O II] λ7325 line showa large scatter and are nearly uncorrelated with temperatures derivedfrom other ions. We tentatively attribute this result to observationaland physical effects, which may introduce large random and systematicerrors into abundances derived solely from [O II] temperatures. Ourderived oxygen abundances are well fitted by an exponential distributionover six disk scale lengths, from approximately 1.3(O/H)solar in the center to 1/15 (O/H)solar in theoutermost region studied [for solar 12+log(O/H)=8.7]. We measuresignificant radial gradients in N/O and He/H abundance ratios, butrelatively constant S/O and Ar/O. Our results are in approximateagreement with previously published abundances studies of M101 based ontemperature measurements of a few H II regions. However, our abundancesare systematically lower by 0.2-0.5 dex than those derived from the mostwidely used strong-line ``empirical'' abundance indicators, againconsistent with previous studies based on smaller H II region samples.Independent measurements of the Galactic interstellar oxygen abundancefrom ultraviolet absorption lines are in good agreement with theTe-based nebular abundances. We suspect that most of thedisagreement with the strong-line abundances arises from uncertaintiesin the nebular models that are used to calibrate the ``empirical''scale, and that strong-line abundances derived for H II regions andemission-line galaxies are as much as a factor of 2 higher than theactual oxygen abundances. However, other explanations, such as theeffects of temperature fluctuations on the auroral line basedabundances, cannot be completely ruled out. These results point to theneed for direct abundance determinations of a larger sample ofextragalactic H II regions, especially for objects more metal-rich thansolar.
| 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.
| 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
| The oxygen abundance distribution in M 101
The well-observed spiral galaxy M 101 was considered. The radialdistributions of oxygen abundances determined in three different ways(with the classic Te-method, with the R23-method,and with the P-method) were compared. It was found that the parameters(the central oxygen abundance and the gradient) of the radial(O/H)P abundance distribution are close to those of the(O/H)T_e abundance distribution. The parameters of the(O/H)R_23 abundance distribution differ significantly fromthose of the (O/H)T_e abundance distribution: the central(O/H)R_23 oxygen abundance is higher by around 0.4 dex andthe gradient is steeper by a factor of around 1.5 as compared to thosevalues in the (O/H)T_e abundance distribution. The dispersionin (O/H)P abundance at fixed radius is rather small, ~ 0.08dex, and is equal to that in (O/H)T_e abundance. Thedispersion in (O/H)R_23 abundance at fixed radius isappreciably larger, ~ 0.16 dex, compared to that in (O/H)T_eabundance. It has been shown that the extra dispersion in(O/H)R_23 abundances is an artifact and reflects scatter inexcitation parameter P at fixed radius.
| ROSAT X-Ray Observations of the Spiral Galaxy M81
We present results from the analysis of deep ROSAT HRI and PSPCobservations of the spiral galaxy M81. The inferred total (0.5-2 keVband) luminosity of M81 is ~3×1040 ergs s-1,excluding the contribution from identified interlopers found within theD25 ellipse. The nucleus of the galaxy alone accounts forabout 65% of this luminosity. The rest is due to 26 other X-ray sources(contributing ~10%) and to apparently diffuse emission, which is seenacross much of the galactic disk and is particularly bright in the bulgeregion around the nucleus. Spectral analysis further gives evidence fora soft component, which can be characterized by a two-temperatureoptically thin plasma with temperature at ~0.15 and 0.60 keV and anabsorption of the galactic foreground only. These components, accountingfor ~13% of the X-ray emission from the region, apparently arise in acombination of hot gas and faint discrete sources. We find interestingspatial coincidences of luminous (1037-1040 ergss-1) and variable X-ray sources with shock-heated opticalnebulae. Three of them are previously classified as supernova remnantcandidates. The other one is far off the main body of M81 but isapparently associated with a dense H I concentration produced mostlikely by the tidal interactions of the galaxy with its companions.These associations suggest that such optical nebulae may be powered byoutflows from luminous X-ray binaries, which are comparable to, or moreluminous than, Galactic ``microquasars.''
| Bidimensional Spectroscopy of Nearby Starbursts
Not Available
| An Ultradeep High-Resolution X-Ray Image of M101: The X-Ray Source Population in a Late-Type Spiral
We have studied the X-ray source population of the face-on spiral galaxyM101 (NGC 5457). Within a field of radius 17' (36 kpc at the distance of7.2 Mpc), covered by an ultradeep (229 ks) ROSAT HRI image, 51 X-raysources are detected with signal-to-noise ratios greater than 3.5. Abouthalf of these sources are associated with the galaxy. The luminosity ofthese galactic sources individually ranges from ~4x10^37 to 2x10^39 ergss^-1 in the 0.5-2 keV band. The average luminosity distribution of thesources can be characterized by a power-law function:dN/dL_X=9.5L^-1.9_X sources per 10^38 ergs s^-1. Combined with archivaldata from the ROSAT PSPC, the Einstein IPC, and the ASCA GIS, we haveexamined spatial, spectral, and timing properties of the X-ray sources.In particular, we have explored the nature of various superluminousX-ray sources with luminosities significantly greater than the Eddingtonlimit (~2x10^38 ergs s^-1) for a ~1.6 M_solar object (a neutron star).These X-ray sources, detected in various ROSAT HRI and PSPCobservations, are not transients and appear to result from recentmassive star formation in outer spiral arms. Three superluminous PSPCsources are associated with giant H II complexes and are clearlyresolved. Two other superluminous ROSAT HRI sources are likelyassociated with shell-like supernova (or more likely hypernova)remnants, which are known to be abnormally luminous in optical and/orradio. We further identify two superluminous sources, which all showhighly absorbed X-ray spectra and time variability during and/or betweenthe observations, as candidates for X-ray binary systems that containblack holes. A comparison of seven nearby spirals shows that their X-raysource luminosity distributions, normalized by total H I masses, arevery similar. But both the number of superluminous X-ray sources and thetotal X-ray luminosity appear to be correlated with the star-formingrate of a galaxy.
| ISO observations of five giant HII regions in M 101.
Not Available
| X-ray emission from NGC 4321 (M 100): detection of supernova 1979C
In a 42.8 ks ROSAT HRI X-ray observation of the spiral galaxy NGC 4321(M 100) X-ray emission from the supernova 1979C is discovered, sixteenyears after its outburst, with an (0.1-2.4 keV) X-ray luminosity of L_x= 1.0 x 10(39) erg s(-1) . No X-ray emission is observed from the threeother historical supernovae in NGC 4321 (SN 1901B, SN 1914A and SN1959E). In addition to SN 1979C, seven X-ray point sources are detectedinside the D25 ellipse of the galaxy, with luminositiesranging from 4.2 x 10(38) to 6.5 x 10(39) erg s(-1) . Apart from twobright sources in the nuclear region of NGC 4321, none of the otherpoint-like X-ray sources show any time variability over the observationperiod. An unresolved diffuse emission component fills the entireoptical extent of the galaxy. The total luminosity of the diffusecomponent is 3.5 x 10(40) erg s(-1) . Point sources contribute 1.4 x10(40) erg s(-1) to the total luminosity of 5.5 x 10(40) erg s(-1) .Three archival Einstein HRI observations of NGC 4321 were merged into asingle 41.3 ks observation. Six point-like X-ray sources are detectedinside the D25 ellipse of NGC 4321 with Einstein (0.1-4.5keV) luminosities in the range 1.1 - 5.1 x 10(39) erg s(-1) . Three ofthe sources coincide with the positions of ROSAT sources (the two bulgesources and a southern interarm source). Comparison of the ROSAT andEinstein luminosities show that the sources are variable. Einstein upperlimits are evaluated at the positions of all other ROSAT sources andhistorical supernovae in NGC 4321.
| N/O in spiral discs: a new algorithm for abundance determinations
We obtain nitrogen abundances in the HII regions of selected spiralgalaxies by using a new algorithm developed from grids ofphotoionization models. The new method requires only the observation ofoptical [NII], [OII] and [OIII] lines. We show the self-consistency ofthe new method, and also the agreement with more detailed modelling ofindividual HII regions. Gradients in [N/O] against effective radius and[O/H] are presented for a well-observed subset of galaxies from theVila-Costas & Edmunds sample.
| Analysis of the ISOPHOT FIR maps of M51 and M101.
The far-IR ISOPHOT maps of the spiral galaxies M51 and M101 at 60, 100,and at 175μm are analysed. For all areas in these galaxies thespectral energy distributions can be fitted with a single-temperatureemission curve. Colours and dust temperatures were estimated forinteresting sub-fields. The temperatures vary between 33K for HIIregions, and 28K for the disk, arm and interarm regimes.
| The Composition Gradient in M101 Revisited. I. H II Region Spectra and Excitation Properties
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...456..504K&db_key=AST
| Unresolved Wind-driven Shells and the Supersonic Velocity Dispersion in Giant H II Regions
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...456..264T
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| X-ray emission from giant H II regions in M101
We have examined the archival ROSAT Position Sensitive ProportionalCounter (PSPC) and the High-Resolution Imager (HRI) images of the galaxyM101 to study the X-ray properties of its giant H II regions. All fivegiant H II regions, NGC 5447, NGC 5445, NGC 5461, NGC 5462, and NGC5471, show X-ray emission. By observing their basic properties, such assize, hardness ratio, and estimated luminosity, and by comparisons withbetter known regions in the Large Magellanic Cloud (LMC), we are able tosuggest some possible mechanisms for the sources of the X-ray emission.
| Pyramid maximum entropy images of IRAS survey data
In this paper we present a new method for the construction ofhigh-resolution images from the IRAS survey data base. The method,Pyramid Maximum Entropy (PME), is a special application of multi-channelmaximum entropy image construction techniques, using pyramid images inwhich each of the (K + 1) image channels covers the same map area, butcontains 2k x 2k (k = 0, 1, ..., K) pixels. Thefinal result is a 2K x 2K-pixel image. We presentimages in all four survey wavelength bands (12, 25, 60, and 100micrometers) of three nearby galaxies with different morphologies, toillustrate the quality of the PME results. A summary of the principlesof Bayesian image (reconstruction) on which PME is based, and themathematical foundation for incorporating spatial correlations in imagesare included in two appendices.
| Parametric relations of H II galaxies
The integrated H-beta luminosities of giant H II regions and of H IIgalaxies can be predicted with accuracy comparable to the observationalerrors from the velocity widths of their emission-line profiles. Inaddition, giant H II regions also present a relation between linear sizeand their emission-line width. Based on the similarity of theserelations with those expected for virialized systems, Terlevich andMelnick (1981) interpreted the observed supersonic motions as due to thegravitational potential of a complex of gas and stars. We show that H IIgalaxies also present a relation between linear size and theiremission-line width, which supports the gravitating model. We also showthat the scatter in the luminosity-line width relation for H II galaxiesseems to be correlated with linear size and compare the results with the'fundamental plane' for elliptical galaxies from Dressier et al. (1987).
| The H II regions of the galaxy M101
The physical properties and composition of the H II region population ofthe spiral galaxy M101 (NGC 5457) were examined usingspectrophotometrically calibrated CCD imagery of two overlapping16-arcmin fields in the M101, in which individual H II regions wereidentified and mapped using an automated algorithm. The characteristicsobtained for the H II region population of M101 include the H II regionluminosity function and radial variations in extinction; the forbidden OIII line/H-beta, O/H, and forbidden S II line/H-alpha ratios; theionization parameter U; and the numbers of ionizing photons. Globalvariations of physical properties of the H II regions in M101 arecompared to model expectations derived for other galaxies. The evidentstar formation in the M101 galaxy is discussed.
| Density studies on giant extragalactic H II regions
We report the results of a systematic study on the spatial variation ofthe electron density in a selected group of bright giant H II complexesin the galaxies M 101, M 51, and NGC 6822. Density values for theionized gas of the regions have been obtained from the forbidden S II6717/6731 ratio, using long-slit spectrophotometry at high spectral andspatial resolution. Our study shows evidence that some of the regionspresent gradients in the distribution of the ionized gas, the brightestzones being associated with the maximum electron density values. Forthose cases where density variation is defined, different models thatdescribe the internal gas density distribution have been considered andcompared with the observations. We conclude that more realistictheoretical models of nebulae should take into account the internaldensity gradients within the regions.
| Very high velocity gas in giant extragalactic HII regions.
Abstract image available at:http://adsabs.harvard.edu/abs/1990RMxAA..21..231C
| A multi-frequency radio continuum study of the galaxy M101
The Sc spiral galaxy M101 has been observed in the radio continuum at11.1, 6.3, 2.8, and 1.2 cm, and deep maps at the first three wavelengthsare used to explore the total radio spectrum. The brightnessdistribution of radio emission in M101 is asymmetric with higherintensities toward the SW spiral arm. The high-frequency emission isgoverned by giant H II complexes with spectra typical of opticallyfree-free emission. The distributions of the spectral index across M101at 6.3 cm and 2.8 cm shows strong flattening of the spectra at thelocation of the giant H II regions. At optical wavelengths M101 shows abehavior different from Sc galaxies: while its nucleus is not veryconspicuous in the blue, it is rather bright and dominates theappearance of the galaxy at near-IR wavelengths. The first detection oflinear radio polarization in M101 is reported; its distribution isdifficult to understand in terms of a constant strength of the uniformmagnetic field along the circumference of M101.
| The H II regions of M101. I - an atlas of 1264 emission regions
A previous survey of the H II regions in M101 (Hodge and Kennicutt) hasbeen extended by a series of CCD images and image tube plates of thegalaxy, approximately tripling the number of known emission regions.This paper provides maps and a catalog of a total of 1264 H II regionsin M101.
| Einstein X-ray observations of M101
The Einstein X-ray observations of the face-on spiral galaxy M101 arepresented. The global X-ray luminosity L(x) of M101 is about 1.2 x 10 tothe 40th ergs/s for D = 7.2 Mpc, consistent with the expected X-rayluminosity of normal spiral galaxies of its optical magnitude. The X-rayemission is mostly due to very luminous individual sources, with L(x)greater than 10 to the 38th ergs/s each, most likely very massiveaccreting binary systems. The data suggest a deficiency of sources inthe luminosity range of L(x) from about 10 to the 37th to about 10 tothe 38th ergs/s, which would indicate that the luminosity distributionof the X-ray sources in M101 might be different from that of M31 or M33.
| A maximum correlation method for image construction of IRAS survey data
An algorithm is presented for the construction of images using lineararray data with nonuniform scan coverage of object space and nonuniformdetector responses. The algorithm achieves the maximum correlationbetween adjacent pixels, i.e., the smoothest image, consistent with thedata and data uncertainties. For high spatial data density andsignal-to-noise ratio, the achievable spatial resolution can exceed thediffraction limit of the optics. The capability of the algorithm isillustrated using 60-micron data from the region centered on the galaxyM101, obtained during the all-sky survey performed by the InfraredAstronomical Satellite. The 60-micron map produced has a resolution ofabout 36 arcsec and allows the identification of many H II regions byposition and aperture photometry for the brighter ones. The achievedresolution is discussed in terms of the a priori estimate of the meancorrelation length of the data, the directly measured FWHM in the finalimage, and the results of aperture photometry of M101 H II regions NGC5447, 5455, 5461, 5462 and 5471.
| Physical conditions of H II regions in M101 and the pregalactic helium abundance
Spectrophotometry in the 3400-7400 A range is presented for eight H IIregions and the nucleus of M101. The He, N, O, Ne, S, and Ar abundancesrelative to H are derived. The O/H ratios are smaller than previouslyfound by about 0.2 dex. Negative gradients with galactocentric distanceof the O/H, N/O, and He/H ratios are found. No gradients in the S/O,Ne/O, and Ar/O ratios are found. The pregalactic helium abundance bymass has been determined from NGC 2363, NGC 5471, and the SMC H IIregions and amounts to 0.230 + or - 0.006.
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