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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.

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.

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.

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 .

Lyman Continuum Extinction by Dust in H II Regions of Galaxies
We examine Lyman continuum extinction (LCE) in H II regions by comparinginfrared fluxes of 49 H II regions in the Galaxy, M31, M33, and theLarge Megellanic Cloud with estimated production rates of Lymancontinuum photons. A typical fraction of Lyman continuum photons thatcontribute to hydrogen ionization in the H II regions of three spiralgalaxies is <~50%. The fraction may become smaller as the metallicity(or dust-to-gas ratio) increases. We examine the LCE effect on estimatedstar formation rates of galaxies. The correction factor for the Galacticdust-to-gas ratio is 2-5.

Wolf-Rayet galaxies
The modern state of the investigations of the Wolf-Rayet galaxies isreviewed. The basic characteristics of WR stars, up-to-date quantitativeschemes of spectral classification of different subtypes of WR stars,the evolution of single and binary massive stars, distribution of WRstars in the galaxies are also discussed. Survey of later evolutionarysynthesis models, including the WR stage with enhancement of mass lossrate, is made. On the basis of up-to-date models we analyze the galaxyobservations obtained with the largest telescopes, the methods andresults of determinations of the number of O and WR stars in WRgalaxies, relative number of different subtypes of WR stars, themetallicity effects on the determination of N(WR)/N(O+WR) andN(WC)/N(WN), constraints on the initial mass function, the age andextension of star formation bursts, the chemical enrichment ofinterstellar matter, the origin of He II λ 468.6 nm emission linein WR galaxies.

An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.

A ROSAT survey of Wolf-Rayet galaxies - II. The extended sample
We present results from an ongoing X-ray survey of Wolf-Rayet (WR)galaxies, a class of objects believed to be very young starbursts. Thispaper extends the first X-ray survey of WR galaxies by Stevens &Strickland by studying WR galaxies identified subsequent to the originalWR galaxy catalogue of Conti. Out of a sample of 40 new WR galaxies atotal of 10 have been observed with the ROSAT PSPC, and of these sevenhave been detected (NGC 1365, NGC 1569, I Zw 18, NGC 3353, NGC 4449, NGC5408 and a marginal detection of NGC 2366). Of these, all are dwarfstarbursts except for NGC 1365, which is a barred spiral galaxy possiblywith an active nucleus. We also report on observations of the relatedemission-line galaxy IRAS 0833+6517. The X-ray properties of thesegalaxies are broadly in line with those found for the original sample;they are X-ray overluminous compared with their blue luminosity and havethermal spectra with typically kT~0.4-1.0keV. There are some oddities:NGC 5408 is very overluminous in X-rays, even compared with other WRgalaxies; I Zw 18 has a harder X-ray spectrum; NGC 1365, althoughthought to contain an active nucleus, has X-ray properties that arebroadly similar to other WR galaxies, and we suggest that the X-rayemission from NGC 1365 is due to starburst activity. A good correlationbetween X-ray and blue luminosity is found for the WR galaxy sample as awhole. However, when just dwarf galaxies are considered there is littleevidence of correlation. We discuss the implications of these results onour understanding of the X-ray emission from WR galaxies and suggestthat the best explanation for the X-ray activity is starburst activityfrom a young starburst region.

H(alpha), Far-Infrared and Thermal Radio Continuum Emission Within the Late-Type Spiral Galaxy M33
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113..236D

A Deep X-Ray Image of M33
A 50.4 ks ROSAT Position Sensitive Proportional Counter image of thenearby spiral galaxy M33 reveals 37 sources within 15' of the nucleusbrighter than 7 x 10^35^ ergs s^-1^. There are at least 13 additionalsources farther from the nucleus, most of which are likely to beassociated with M33 as well. Most of the bright sources that had beendetected with Einstein in the same region are still visible. The bulk ofthe sources in the galaxy are associated with Population I tracers.Several of the sources are time variable. There are 10 sources in theimage that lie within 20" of optically identified supernova remnants(SNRs) in M33. The spectra of these sources are soft compared with mostother sources of comparable brightness, and therefore it is likely thatmost of these X-ray sources are SNRs. Based on the identification ofsources in M33, it appears likely that ROSAT hardness ratios of thistype can be used to separate SNRs and compact sources in other nearbynormal galaxies as well. The northern and the southern spiral arms ofM33 appear as diffuse features in the X-ray image. There is additionaldiffuse (or unresolved source) emission throughout the inner portions ofM33. The diffuse emission is softer than the faint point sources and theSNRs in the survey, and is well fitted in terms of a bremsstrahlungspectrum with kT ~ 0.4 keV and log N_H_ ~ 20.6.

Extinction characteristics of giant HII regions - Star-forming complexes in the galaxies M33, LMC, and NGC 2403
The discrepancies between the extinction of gas emission and that of thestarlight in giant HII regions, star-forming complexes in the galaxiesM33, LMC, and NGC 2403, were empirically investigated. The extinctionvalues were determined for 30 stars in eight associations in M33. Anempirical relation between the extinction of starlight and that of thegas emission in giant HII regions, star-forming complexes in thegalaxies under study, was obtained.

The ultraviolet color gradient in the late-type spiral galaxy M33
The ultraviolet surface brightness and color distributions for thelate-type spiral galaxy M33 are derived from images at 1520 and 2490 Athat were obtained by the Ultraviolet Imaging Telescope (UIT) on theAstro Spacelab mission. Although the surface brightness shows a generaldecline with radius, the dominant spiral arms cause significantdeviations from an exponential fit. Colors in elliptical annuli become0.2-0.3 mag bluer with increasing radius. Our measures of individual HII regions and sections of spiral arms follow this same trend. Interarmregions are redder than the arms. A metallicity gradient affecting onlythe colors of stars is inadequate in accounting for the observed colorgradient. A plausible explanation invokes a combination of an LMC-typereddening curve and a radial gradient in the internal reddening in M33.

Near-infrared spectra and classification diagnostics of Seyfert galaxies
Observational results of a previous spectroscopic survey of Seyfertgalaxies in the near-IR are presented, and the potential for usingemission-line ratios in this spectral region as a classificationdiagnostic tool is examined. Near-IR CCD spectra, which cover thewavelength range of 7000-10,000 at a nominal resolution of about 12 A,of 15 additional Seyferts and two starburst galaxies are obtained.Relative emission-line intensities from these observations, incombination with measurements from previous studies and measurements ofnew, signal-to-noise ratio optical spectra of many of these objects, areused to study the diagnostic diagrams involving forbidden S III 9069,9531/H-alpha, forbidden O II 7320, 7330/H-alpha, forbidden S II 6716,6731/H-alpha, and forbidden O III 5007/H-beta. Comparisons are made inthese diagrams between observational data from the active galaxies andpublished measurements of H II regionlike objects, as well as withpredictions from simple one-component models calculated for the twotypes of objects.

Ionization correction factors and compositions of H II regions
The determination of ionization correction factors (ICFs) is discussedby means of a grid of new photoionization models for H II regions,mainly using the plane-parallel model stellar atmospheres of Kurucz(1979). The ionic abundances of the models are integrated either overthe nebular volume or along a radius. The observations of the OrionNebula do not lie along a single atmospheric track, which is interpretedas the effects of good spatial resolution. Observations of lesswell-resolved nebulae (30 Dor and NGC 604) show fairly good agreementwith a Kurucz T(asterisk 3) = 42 track. Observations of NGC 5471 in M101also suggest a track that is 'hotter' than any Kurucz atmosphere. Thechemical compositions of H II regions are compatible with those found inunevolved stars.

High chemical abundances in Virgo spiral galaxies?
Evidence is presented that spiral galaxies in the Virgo Cluster havesystematically higher interstellar abundances than comparable fieldgalaxies. This conclusion is based on spectra of H II regions in fiveVirgo spirals of type Sc. A possible explanation is that abundances inthe field spirals are strongly affected by infall of metal-poor gas orby radial inflow of gas from the outer H I disk. These processes areinhibited in the cluster environment, and the Virgo spirals may haveevolved more nearly in the manner of the closed box 'simple model' ofchemical evolution.

The infrared structure of M33
IRAS spatial observations of M33 are presented and compared to UVoptical, and radio wavelength data. At 12, 25, 60, and 100 microns theemission appears as two bright spiral arms superposed on a diffuse disk,with localized sources at the position of the nucleus and the brightestH II region complexes. The global IR properties of M33 arecharacteristic of quiescent spirals. The 8-30 micron mid-IR globalemission is about 20 percent of the far-IR emission, similar to theratio of these fluxes found for the local ISM. The local IR excess ofM33 is about 2-3 times larger than the excess derived for H II regionsin the Galaxy. The radial distributions of the IR colors are flat ornearly flat, implying near-constant dust temperatures across the diffusedisk. The integrated F(IR)-to-radio continuum emission of M33 followsthe relation seen in other galaxies, but the radial distribution of thetwo emission components differ. A striking correlation is seen betweenthe IR emission structure and the distribution of H II regions.

The abundance of sulfur in extragalactic H II regions
A long-slit CCD survey of forbidden S III 9069, 9532 A radiation in 13extragalactic H II regions is presented, and the data are used to studythe variation of S/O as a function of O/H. The data are consistent withthe idea that S/O remains constant as O/H varies. There is no evidencethat S/O is larger at low O/H. Photoionization models confirm thatobservations of O(+)/O(2+) and S(+)/S(2+) can be used to estimate theeffective temperature of the ionizing stars in an H II region. Thisprovides a potentially powerful new tool for studying the ionizing starsof H II regions in external galaxies. However, the models fail toreproduce the S(+)/S(2+) ratio in nebulae with large O(2+) fractions forreasonable values of T(eff). Simple chemical evolution models arecalculated to compare observations with expectations from stellarneucleosynthesis calculations.

Large-scale properties of interstellar dust and gas in M33
IRAS observations of M33 are used to study the large-scalecharacteristics of interstellar dust. At the four IRAS wavelengths, theM33 emission can be separated into a diffuse disk and a brightpoint-source component. The point-source emission is shown to beassociated with star-formation regions and to contribute half of thetotal IR luminosity of M33. The diffuse disk emission displays spectralcharacteristics similar to the Galactic cirrus. The results suggest thatlarge particles are selectively destructed by the more intenseinterstellar radiation field near the center of M33. A steep radialdecrease of the dust-to-(atomic) gas ratio is found. The results areconsistent with the grain model of Draine and Anderson (1985).

Kinematics and composition of H II regions in spiral galaxies. I - M33
Moderate-dispersion, 0l.7 A/pixel spectra of H II regions in M33 weretaken, and the velocity as well as the excitation were measured byobserving the spectral region around H-beta. Velocity measurements for55 H II regions and excitation measurements for 42 regions arepresented. The velocity data are used to measure the systemic velocityat -172 + or - 6 km/s, the inclination at 56 + or - 1 deg, the positionangle of the major axis at 23 + or - 1 deg, the rotation curve of the HII regions, and their line-of-sight velocity dispersion at 9 + or - 4km/s. These data are used to derive the M/L ratio from radii of 0.5-3.7kpc. The excitation measurements are used to derive a metallicitygradient for M33 and to quantify the dispersion in the excitation valuesat a given radius.

The chemical composition gradient across M 33
The abundance gradient in M 33 is studied on the basis of IPCS and CCDdata on emission lines in selected H II regions, using O II, O III, SII, and S III in the wavelength range 3700-9600 A to refine the oxygenabundances in the inner parts as well as to study the behavior of S/O.Spatially resolved observations in each H II region permit theionization structure to be studied and enable ionization parameters andstellar effective temperatures to be separately determined. The mainresults are: (1) the hardness of the ionizing radiation and theionization parameter increase outwards (with diminishing abundances)along the galactic radius; (2) the O/H gradient is steep in the innerregions, but much flatter in the outer regions; (3) N/O is constant overmost of the visible disk, but lower in an outer H II region; and (4) theS/H gradient is shallower than the O/H gradient exemplifying whatappears to be a universal trend for S/O to decrease with O/H in H IIregions. This latter trend is rather unexpected from the viewpoint ofnucleosynthesis theory.

Correlations between integrated parameters and H-alpha velocity widths in giant extragalactic H II regions - A new appraisal
Investigations of relationships between diameters (or luminosities) andvelocity widths of H-alpha line profiles in giant extragalactic H IIregions (GEHR) have firmly established that these parameters arecorrelated. However, three independent studies on the subject disagreeon the slopes of these relations. It is shown that all measurements ofthe velocity width of integrated H-alpha profiles of GEHRs are entirelyconsistent. Discrepancies in the relations are explained by differentsamples and by the use of different parameters such as distances togalaxies and diameters of GEHRs. Assembling all observations of H-alphavelocity widths, new values of slopes and zero-points are derived forthe relations between luminosities (or diameters) and velocity widths.

HII regions in M33. II - Radio continuum survey
A new WSRT survey of M33 in the continuum at 1.4 GHz is presented. Withan angular resolution of 25 x 49 arcsec HPBW and a rms noise of 0.2 mJyper beam area (at the field center), 112 radio sources with H-alphanebulosities have been identified. From a comparison of this survey dataand 5 GHz observations obtained with the VLA and WSRT, the spectralindices of 17 sources have been determined. Only three of these sourceshave nonthermal spectra. A compact source is discovered 1 arcmin southof the nucleus of M33. This source has a flat radio spectrum and noH-alpha counterpart. This source is either a heavily obscured compactHII region or, more likely, a source similar to the Crab Nebula.

Turbulent gas motions in giant H II regions. II - The luminosity-velocity dispersion relation
The relation between luminosity L(H-alpha) and velocity dispersion sigmais reinvestigated in view of H-alpha and O III 5007-A forbidden linemeasurements of the velocity dispersion for 43 giant, extragalactic H IIregions. A correlation of L(H-alpha) that varies as sigma (H-alpha) tothe 6.6th power is obtained; O III-line velocity dispersions are notedto obey a similar relation, but are somewhat smaller, and the slope ofboth relations is steeper than expected for gravitationally boundsystems.

H II regions in M33. III - Physical properties
The properties of the radio H II regions in M33 are investigatedutilizing a calibrated H-alpha survey of a large number of H II regionsin the inner part of M33 (Donas, 1977) and a forbidden O III/H-betasurvey obtained by Boulesteix et al. (1981). The luminosity function ofthe H II region in the inner part of M33, the extinction in the H IIregion, and the dust content in these nebulae are determined. Theassociation of H II regions with H I concentrations is brieflydiscussed.

IUE UV spectra of extragalactic H II regions. I - The catalogue and the atlas
More than 150 spectrograms of giant H II complexes in spiral, bluecompact and irregular galaxies, excluding the Magellanic Clouds,obtained within the period March 1, 1978 and April 1, 1982 with theInternational Ultraviolet Explorer (IUE) in the wavelength range 1150 Ato 3200 A are cataloged. Many of the observations are presented ingraphic form. A comparison of repeated observations for two objects in M33 and M 101 is made to investigate the reliability of singleobservations. General considerations on the UV spectra of giantextragalactic H II regions are given and a comparison of the spectra ofH II regions at various distances from the centers of M 33 and M 101 ispresented.

The largest H II regions in M101
Coordinated millimeter, infrared, and ultraviolet observations of thefive brightest H II regions in M101 are presented. A CO complex has beenfound to be associated with NGC 5461 which is much more massive than anyMilky Way counterpart. A narrow line width has been observed whichsuggests that the molecular complex may consist of numerous fragmentswith a volume filling fraction approximately 0.01. NGC 5461 also shows a10 micron and 20 micron excess which is similar to that of galactic H IIregions. The ultraviolet observations show that the 2200 A dust featureis greatly attenuated in all of the H II regions but least of all in NGC5461. The near-infrared flux densities and B-gamma line strengths areconsistent with the hypothesis that a significant fraction of theinfrared emission is free-free.

Chemical compositions of H II regions in the Triangulum spiral, M33
Measurements of 12 H II regions secured with the Robinson-Wampler ImageTube Scanner at the Shane Telescope, Lick Observatory cover the spectralrange 3700-7600 A. The distances of these regions from the nucleus rangefrom 1 to 6 kpc. These data are analysed to establish plasma diagnosticsand chemical compositions. In a manner similar to that previouslyemployed for studies of the Magellanic Clouds, theoretical models areused as interpolation devices to establish ionization correction factorsfor S, Cl and Ar. Except for helium, the N(element)/N(H) ratios fall offwith increasing radial distance with closely similar rates.Consequently, the ratios not only of nitrogen, but also neon, sulphurand argon, with respect to oxygen, remain essentially constant. Thefollowing log N(element)/N(oxygen) are found: N = -1.25, Ne = -1.5, Cl =-3.6 and Ar = -2.4.

An optical study of M 33. I - Morphology of the gas
A general catalogue of 369 distinct H II regions has been compiled fromwide-field photographs obtained with a focal reducer and narrow-bandinterference filters. Ring-like emission nebulae have been observed asfar as 35 min of arc from the nucleus. It is proposed that these ringsrepresent a late stage in the life of expanding ionized regions. Generalhistograms related to the distribution of H II regions, intensities anddiameters are plotted. The number surface density of the faint regionsis rather constant over the whole galaxy, while for the strongerregions, it is distinctly greater in the central part. A most-probablevalue of 13 sec of arc is found for the apparent diameter of the H IIregions. A detailed spatial comparison is made with radio continuumsources, Wolf-Rayet stars, H I clouds, and clusters of hot stars. Spiralstructure is drawn based on the H II and H I observations. A very goodcorrelation is found between H II arms and the distribution of H Iclouds. Outer extensions of H emission are related to the spiralstructure.

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