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Chandra ACIS Survey of M33 (ChASeM33): X-Ray Imaging Spectroscopy of M33SNR 21, the Brightest X-Ray Supernova Remnant in M33
We present and interpret new X-ray data for M33SNR 21, the brightestX-ray supernova remnant (SNR) in M33. The SNR is in seen projectionagainst (and appears to be interacting with) the bright H II region NGC592. Data for this source were obtained as part of the Chandra ACISSurvey of M33 (ChASeM33) Very Large Project. The nearly on-axis Chandradata resolve the SNR into a ~5" diameter (20 pc at our assumed M33distance of 817+/-58 kpc) slightly elliptical shell. The shell isbrighter in the east, which suggests that it is encountering higherdensity material in that direction. The optical emission is coextensivewith the X-ray shell in the north, but extends well beyond the X-ray rimin the southwest. Modeling the X-ray spectrum with an absorbed sedovmodel yields a shock temperature of 0.46+0.01-0.02keV, with an ionization timescale ofnet=2.1+0.2-0.3×1012cm-3 s and half-solar abundances(0.45+0.12-0.09). Assuming Sedov dynamics gives anaverage preshock H density of 1.7+/-0.3 cm-3. The dynamicalage estimate is 6500+/-600 yr, while the best-fit net valueand derived ne gives 8200+/-1700 yr; the weighted mean of theage estimates is 6700+/-600 yr. We estimate an X-ray luminosity(0.25-4.5 keV) of (1.2+/-0.2)×1037 ergs s-1(absorbed), and (1.7+/-0.3)×1037 ergs s-1(unabsorbed), in good agreement with the recent XMM-Newton determination. No significant excess hard emission was detected; theluminosity <~1.2×1035 ergs s-1 (2-8 keV)for any hard point source.
| A Search for Chandra-detected X-Ray Counterparts to Optically Identified and Candidate Radio Supernova Remnants in Five Nearby Face-on Spiral Galaxies
We present a search for X-ray counterparts to optically identified andcandidate radio supernova remnants (SNRs) in five nearby galaxies, M81,M101, NGC 2403, NGC 4736 (M94), and NGC 6946, using observations madewith the Chandra X-Ray Observatory. A total of 138 optically identifiedSNRs and 50 candidate radio SNRs in these galaxies were sampled by theseobservations. Nine optically identified SNRs and 12 candidate radio SNRsare positionally coincident with Chandra-detected X-ray sources thatwere not already known to be time-variable or associated with X-raybinaries. We used survival statistics to determine if the properties ofthe optically identified SNRs with and without Chandra-detectedcounterparts (referred to as group A and group NotA, respectively), aswell as the candidate radio SNRs with and without Chandra-detectedcounterparts (referred to as group B and group NotB, respectively)differ in a statistically significant manner. We find that for the SNRsin groups A and NotA, only the mean value of the diameter d differssignificantly between the two groups (26+/-4 pc compared to 62+/-6 pc).In addition, for the SNRs in groups B and NotB, we find that only thespectral index α differs significantly between the two groups(0.6+/-0.1 compared to 0.9+/-0.1). We find no correlation betweenunabsorbed X-ray and optical luminosities for the group A SNRs and nocorrelation between unabsorbed X-ray and radio luminosities for thegroup B SNRs: this result indicates that the interstellar mediumsurrounding these SNRs is inhomogeneous rather than uniform. We claimthat the higher incidence of Chandra-detected counterparts for candidateradio SNRs compared to the optically identified SNRs (as noticed inprevious works) illustrates the role played by ambient density inaffecting searches for SNRs in nearby galaxies at multiple wavelengths.We argue that deep systematic X-ray, optical, and radio observations ofother galaxies are necessary to examine the multiwavelength propertiesof SNRs, to explore wavelength-dependent selection effects in moredetail, and to search for time variability in the emission from X-raycounterparts to optically identified SNRs and candidate radio SNRs.
| Water masers in the Local Group of galaxies
We compare the number of detected 22 GHz H2O masers in the Local Groupgalaxies M 31, M 33, NGC 6822, IC 10, IC 1613, DDO 187, GR8, NGC 185,and the Magellanic Clouds with the water maser population of the MilkyWay. To accomplish this we searched for water maser emission in the twoLocal Group galaxies M 33 and NGC 6822 using the Very Large Array (VLA)and incorporated results from previous studies. We observed 62 Hiiregions in M 33 and 36 regions with Hα emission in NGC 6822.Detection limits are 0.0015 and 0.0008 L_ȯ for M 33 and NGC 6822,respectively (corresponding to 47 and 50 mJy in three channels with 0.7km s-1 width). M 33 hosts three water masers above ourdetection limit, while in NGC 6822 no maser source was detected. We findthat the water maser detection rates in the Local Group galaxies M 31, M33, NGC 6822, IC 1613, DDO 187, GR8, NGC 185, and the Magellanic Cloudsare consistent with expectations from the Galactic water masers if oneconsiders the different star formation rates of the galaxies. However,the galaxy IC 10 exhibits an overabundance of masers, which may resultfrom a compact central starburst.
| The massive star population in the giant HII region Tol89 in NGC5398
We present new high spectral resolution Very Large Telescope(VLT)/UV-Visual Echelle Spectrograph (UVES) spectroscopy and archivalHubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS)imaging and spectroscopy of the giant HII region Tol89 in NGC5398. Fromoptical and ultraviolet (UV) HST images, we find that the star-formingcomplex as a whole contains at least seven young compact massiveclusters. We resolve the two brightest optical knots, A and B, into fiveindividual young massive clusters along our slit, A1-4 and B1,respectively. From UV spectral modelling using the STARBURST99 code ofLeitherer et al., and nebular Hβ equivalent widths in the optical,we derive ages that are consistent with the formation of two separateburst events, of ~4 +/- 1 Myr and <3 Myr for knots A (A1-4) and B(B1), respectively. A Large Magellanic Cloud (LMC) metallicity ismeasured for both knots from a nebular line analysis, while nebular HeII4686 is observed in knot B and perhaps in knot A. We detect underlyingbroad wings on the strongest nebular emission lines indicatingvelocities up to 600 km s-1. From UV and opticalspectroscopy, we estimate that there are ~95 early WN stars and ~35early WC stars in Tol89-A, using empirical template spectra of LMCWolf-Rayet (WR) stars from Crowther & Hadfield, with the WCpopulation confined to cluster A2. Remarkably, we also detect a smallnumber of approximately three mid WNs in the smallest (mass) cluster inTol89-A, A4, whose spectral energy output in the UV is entirelydominated by the WN stars. From the strength of nebular Hβ, weobtain N(O) ~ 690 and 2800 for knots A and B, respectively, whichimplies N(WR)/N(O) ~ 0.2 for knot A. We also employ a complementaryapproach using STARBURST99 models, in which the O star content isinferred from the stellar continuum, and the WR population is obtainedfrom spectral synthesis of optical WR features using the grids fromSmith et al. We find reasonable agreement between the two methods forthe O star content and the N(WR)/N(O) ratio but find that the WR subtypedistribution is in error in the STARBURST99 models, with far too few WNstars being predicted. We attribute this failure to the neglect ofrotational mixing in evolutionary models. Our various modellingapproaches allow us to measure the cluster masses. We identify A1 as asuper star cluster (SSC) candidate with a mass of ~1-2 ×105 Msolar. A total mass of ~6 ×105 Msolar is inferred for the ionizing sourceswithin Tol89-B.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 73.B-0238(A) and with the NASA/ESA HST, obtainedfrom the ESO/ST-ECF Science Archive Facility.E-mail: fs@star.ucl.ac.uk
| Stellar Abundances and Molecular Hydrogen in High-Redshift Galaxies: The Far-Ultraviolet View
FUSE spectra of star-forming regions in nearby galaxies are compared tocomposite spectra of Lyman break galaxies (LBGs), binned by strength ofLyα emission and by mid-UV luminosity. Several far-UV spectralfeatures, including lines dominated by stellar wind and by photosphericcomponents, are very sensitive to stellar abundances. Their measurementin LBGs is compromised by the strong interstellar absorption features,allowing in some cases only upper limits to be determined. The derived Cand N abundances in the LBGs are no higher than half solar (scaled tooxygen abundance for comparison with emission-line analyses),independent of the strength of Lyα emission. P V absorptionindicates abundances as low as 0.1 solar, with an upper limit near 0.4solar in the reddest and weakest emission galaxies. Unresolvedinterstellar absorption components would further lower the derivedabundances. Trends of line strength and derived abundances are strongerwith mid-UV luminosity than with Lyα strength. H2absorption in the Lyman and Werner bands is very weak in the LBGs.Template H2 absorption spectra convolved to the appropriateresolution show that strict upper limitsN(H2)<1018 cm-2 apply in all cases,with more stringent values appropriate for the stronger emissioncomposites and for mixes of H2 level populations like thoseon Milky Way sight lines. Since the UV-bright regions are likely to bewidespread in these galaxies, these results rule out massive diffusereservoirs of primordial H2 and suggest that the dust-to-gasratio is already fairly large at z~3.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by The JohnsHopkins University under NASA contract NAS5-32985.
| 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 Hot Interstellar Medium in M31 and M33
We report measurements from Far Ultraviolet Spectroscopic Explorerspectra of OB stars in M31 and M33, of O VI and C II interstellarabsorptions. The stars are located in OB 78 in the outer region of M31and in several locations across M33. The interstellar absorptions agreewell between stars in nearby locations, and their principal radialvelocities indicate that they arise in those galaxies and follow thedisk rotation.
| Far-Ultraviolet Spectroscopy of Star-forming Regions in Nearby Galaxies: Stellar Populations and Abundance Indicators
We present Far Ultraviolet Spectroscopic Explorer spectroscopy andsupporting data for star-forming regions in nearby galaxies, to examinetheir massive-star content and explore the use of abundance andpopulation indicators in this spectral range for high-redshift galaxies.New far-ultraviolet spectra are shown for four bright H II regions inM33 (NGC 588, 592, 595, and 604), the H II region NGC 5461 in M101, andthe starburst nucleus of NGC 7714, supplemented by the very lowmetallicity galaxy I Zw 18. In each case we see strong Milky Wayabsorption systems from H2, but intrinsic absorption withineach galaxy is weak or undetectable, perhaps because of the ``UV bias''in which reddened stars that lie behind molecular-rich areas are alsoheavily reddened. We see striking changes in the stellar wind lines fromthese populations with metallicity, suggesting that C II, C III, C IV, NII, N III, and P V lines are potential tracers of stellar metallicity instar-forming galaxies. Three of these relations-involving N IV, C III,and P V-are nearly linear over the range from O/H=0.05-0.8 solar. Themajor difference in continuum shapes among these systems is that thegiant H II complex NGC 604 has a stronger continuum shortward of 950Å than any other object in this sample. Small number statisticswould likely go in the other direction; we favor this as the result of adiscrete star-forming event ~3 Myr ago, as suggested by previous studiesof its stellar population.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by JohnsHopkins University under NASA contract NAS 5-32985.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Infrared Space Observatory Long-Wavelength Spectrometer Spectroscopy of Star-forming Regions in M33
We present Infrared Space Observatory (ISO) Long-Wavelength Spectrometer(LWS) far-infrared (FIR) spectra of the nucleus and six giant H IIregions in M33 (NGC 595, IC 142, NGC 592, NGC 604, NGC 588, and IC 133).The seven fine-structure lines observed in the FIR are used to model theH II and photodissociation regions (PDRs). There is no observed trend inthe FIR properties, observed with the LWS, as a function of galacticradius or metallicity. The cold neutral medium (CNM) is the mainreservoir for the atomic gas, containing between 60% and 95% of the gas.The FIRLWS spectral energy distribution can be fitted with asingle-temperature graybody spectrum with a temperature in the range35K<=T<=49 K. The [C II] 158 μm line flux is 0.2%-0.7%FIRLWS, which is typical of values seen (0.1%-1% FIR) in thenuclei of star-forming galaxies. The [C II]/FIRLWS ratiopeaks at the nucleus and is fairly constant across the rest of thesample. Massive star formation is traced by the intensity of the [O III]88 μm line. The emission from the observed FIR lines that arisesolely from H II regions can be modeled as a single component with agiven oxygen and nitrogen abundance, effective temperature, density, andionizing flux. There is no need for an extended low-density component(ELDWIM). Apart from NGC 604 and NGC 595, the fractional [C II] emissionthat arises from the H II regions and/or PDRs is not well constrained,but typically 5%-50% arises in the H II regions, 10%-35% from the CNM,and the bulk of the emission (40%-90%) in the PDRs. The average PDR inthis sample has a gas density ~103.1 cm-3, anaverage incident far-ultraviolet flux (in units of the localinterstellar value) 0>=102.4, a gas temperatureT~200 K, and an AV~10 through the clouds. NGC 604 has 40% ofthe atomic gas residing in the PDRs, while the rest have a much smallerfraction, ~5%-15%. The PDRs are similar to those found in otherstar-forming galaxies such as Centaurus A. 0> is at thelower end of the range observed in samples of spiral and starburstgalaxies (2.2<=logG0<=5), and sits comfortablyin the middle of the observed range (1.8<=logn<=4.2).
| The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in and near the Galaxy
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofthe O VI λλ1031.926, 1037.617 absorption lines associatedwith gas in and near the Milky Way, as detected in the spectra of asample of 100 extragalactic targets and two distant halo stars. Wecombine data from several FUSE Science Team programs with guest observerdata that were public before 2002 May 1. The sight lines cover most ofthe sky above Galactic latitude |b|>25deg-at lowerlatitude the ultraviolet extinction is usually too large forextragalactic observations. We describe the details of the calibration,alignment in velocity, continuum fitting, and manner in which severalcontaminants were removed-Galactic H2, absorption intrinsicto the background target and intergalactic Lyβ lines. Thisdecontamination was done very carefully, and in several sight lines verysubtle problems were found. We searched for O VI absorption in thevelocity range -1200 to 1200 km s-1. With a few exceptions,we only find O VI in the velocity range -400 to 400 km s-1the exceptions may be intergalactic O VI. In this paper we analyze the OVI associated with the Milky Way (and possibly with the Local Group). Wediscuss the separation of the observed O VI absorption into componentsassociated with the Milky Way halo and components at high velocity,which are probably located in the neighborhood of the Milky Way. Wedescribe the measurements of equivalent width and column density, and weanalyze the different contributions to the errors. We conclude thatlow-velocity Galactic O VI absorption occurs along all sight lines-thefew nondetections only occur in noisy spectra. We further show thathigh-velocity O VI is very common, having equivalent width >65 mÅin 50% of the sight lines and equivalent width >30 mÅ in 70% ofthe high-quality sight lines. The central velocities of high-velocity OVI components range from |vLSR|=100 to 330 km s-1there is no correlation between velocity and absorption strength. Wediscuss the possibilities for studying O VI absorption associated withLocal Group galaxies and conclude that O VI is probably detected in M31and M33. We limit the extent of an O VI halo around M33 to be <100kpc [at a 3 σ detection limit of log N(O VI)~14.0]. Using themeasured column densities, we present 50 km s-1 wide O VIchannel maps. These show evidence for the imprint of Galactic rotation.They also highlight two known H I high-velocity clouds (complex C andthe Magellanic Stream). The channel maps further show that O VI atvelocities <-200 km s-1 occurs along all sight lines inthe region l=20deg-150deg, b<-30deg,while O VI at velocities >200 km s-1 occurs along all sightlines in the region l=180deg-300deg,b>20deg.
| Interstellar H2 in M 33 detected with FUSE
FUSE spectra of the four brightest H Ii regions in M33 show absorption by interstellar gas in the Galaxy and inM 33. On three lines of sight molecular hydrogen inM 33 is detected. This is the first measurement ofdiffuse H2 in absorption in a Local Group galaxy other thanthe Magellanic Clouds. A quantitative analysis is difficult because ofthe low signal to noise ratio and the systematic effects produced byhaving multiple objects in the FUSE aperture. We use the M33 FUSE data to demonstrate in a more general manner thecomplexity of interpreting interstellar absorption line spectra towardsmulti-object background sources. We derive H2 columndensities of ~1016 to 1017 cm-2 along 3sight lines (NGC 588, NGC 592,NGC 595). Because of the systematic effects, thesevalues most likely represent upper limits and the non-detection ofH2 towards NGC 604 does not exclude theexistence of significant amounts of molecular gas along this sight line.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, available in the public archive. FUSE isoperated for NASA by the Johns Hopkins University under NASA contractNAS5-32985.
| Wolf-Rayet stars in Local Group starbursts
I describe recent spectroscopic observations of Wolf-Rayet stars inthree giant HII regions of M 33 and in the Local Group starburst galaxyIC 10.
| Multicolor Photometry of 145 of the H II Regions in M33
This paper is the first in a series presenting CCD multicolor photometryfor 145 H II regions, selected from 369 candidate regions fromBoulesteix et al., in the nearby spiral galaxy M33. The observations,which covered the whole area of M33, were carried out with the BeijingAstronomical Observatory 60/90 cm Schmidt telescope, in 13intermediate-band filters, covering a range of wavelengths from 3800 to10000 Å. This procedure provides a series of maps that can beconverted into a multicolor map of M33, in pixels of 1.7"×1.7".Using aperture photometry we obtain the spectral energy distributions(SEDs) for these H II regions. We also give their identification charts.Using the relationship between the Beijing-Arizona-Taiwan-Connecticutintermediate-band system used for the observations and the UBVRIbroadband system, the magnitudes in the B and V bands are then derived.Histograms of the magnitudes in V and in B-V are plotted, and thecolor-magnitude diagram is also given. The distribution of magnitudes inthe V band shows that the apparent magnitude of almost all the regionsis brighter than 18, corresponding to an absolute magnitude of -6.62 foran assumed distance modulus of 24.62, which corresponds to a singlemain-sequence O5 star, while the distribution of color shows that thesample is blue, with a mode close to -0.05, as would be expected from arange of typical young clusters.
| 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.
| An X-Ray, Optical, and Radio Search for Supernova Remnants in the Nearby Sculptor Group Sd Galaxy NGC 300
We have conducted a multiwavelength (X-ray, optical, and radio) searchfor supernova remnants (SNRs) in the nearby Sculptor Group Sd galaxy NGC300. Our Very Large Array (VLA) radio observations at 6 cm and 20 cmhave been combined with previously published optical results, our ownoptical image, and archived ROSAT X-ray data to search for new SNRcandidates. Of the 28 optically identified SNRs found by Blair &Long, three exhibit some combination of X-ray and radio emission, but ingeneral X-ray and radio emission from the optically identified SNRs isundetected. A radio-selected sample of SNR candidates is constructed bysearching for positional coincidences between nonthermal radio continuumsources and sources of Hα emission with the intent of finding SNRsthat are typically overlooked using the [S II]/Hα criterion, i.e.,those SNRs that are either confused by Hα emission or which areBalmer-dominated. This search has yielded 17 SNR candidates, of which 14are new and three were already known from the optically selected sample.Four of the radio-selected candidates also possess detectable X-rayemission. A complementary analysis of the X-ray data has yielded anX-ray-selected sample of candidates consisting of six soft-spectrumsources (kT<1 keV) coincident with regions of Hα emission. Twoof these candidate X-ray SNRs are common to the radio-selected sampleand two more are common to the optically selected sample. Thus, theX-ray selection has yielded two additional candidates. Through amultiwavelength campaign, we have added sixteen new candidate SNRs tothe 28 previously known SNRs for a total of 44 SNR candidates in NGC300. The fact that these new candidates were missed in the opticalsurveys suggests a possible selection effect. SNRs identified throughoptical methods may represent only the SNRs located in regions withrelatively low confusion from Hα emission, well away fromstar-forming regions. The SNRs in or near star-forming regions are morelikely to be confused by emission from H II regions and are thereforemore likely to be missed in optical surveys. The radio-selectionprocess, on the other hand, suffers from a separate selection effect: itcan only identify candidates if they are associated with H II regions,such that our sample of radio-selected SNRs is biased towardstar-forming regions. Finally, the X-ray candidates are selected on thebasis of having soft spectra and association with H II regions, so thatthe sample is biased against X-ray-emitting SNRs with hard spectra andno optical counterparts. The fact that the optically selected,radio-selected, and X-ray-selected data sets of SNR candidates havelimited overlap is consistent with these opposing selection effects. Wepresent a simulation to investigate the effects of optical confusion ona hypothetical optical survey for SNRs in a galaxy, using ``artificial''SNRs that have been placed in a range of confused environments. We findthat the detection of SNRs becomes increasingly difficult with distance,and that the most deeply embedded SNRs are not identified even in thenearest distance investigated. We conclude that a multifrequency searchfor SNRs is required to uniformly sample the SNR population of a galaxy.
| A New Sample of Radio-selected and Optically Confirmed Supernova Remnants in M33
Using radio data to identify and optical data to confirm, we haveestablished the largest and most complete sample of extragalacticradio-bright supernova remnants (SNRs) in the nearby spiral galaxy M33.We have identified 53 radio SNRs, doubling the size of the earliersurvey by Duric et al. The increased sample size provides more insightinto the nature of SNRs and provides an extensive informational databaseabout SNRs for future studies. The SNR sample was analyzed forstatistical trends that relate to the evolution of their radio emission.We report no significant correlation between the radio flux densitiesand diameters of SNRs in our sample. The absence of a statisticallymeaningful Sigma-D relation is consistent with the view thatenvironmental factors govern the radio evolution of SNRs. A new radioluminosity function for SNRs is presented. Although there appears to bea break in the function, the break is the result of selection effectsand not intrinsic to the sample. A comparison of the radio luminosityfunction with the integrated radio continuum properties of M33 suggeststhat SNRs may well be the primary source of the relativistic electronsin M33. A histogram of synchrotron spectral indices of the SNRs ispresented. Statistical analysis of the spectral index distributionindicates that the mean spectral index of SNRs in M33 is steeper thanthe mean in samples of SNRs in the Galaxy or the LMC and is consistentwith a particle spectrum expected from a diffusive shock accelerationprocess.
| At The Eyepiece: Galaxies - A Matter of Subtle Contrast
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| Evolved Massive Stars in the Local Group. II. A New Survey for Wolf-Rayet Stars in M33 and Its Implications for Massive Star Evolution: Evidence of the ``Conti Scenario'' in Action
We expect the evolution of massive stars to be strongly influenced bymass loss and hence to be sensitive to metallicity. It should bepossible to test this ``Conti scenario'' be comparing the populations ofevolved massive stars among the Local Group galaxies, but suchinvestigations have been hampered by incompleteness. In a previouspaper, we presented results of a new survey for red supergiants (RSGs)in selected regions of the Local Group galaxies M33, M31, and NGC 6822.In the present paper, we survey eight fields in M33 for Wolf-Rayet stars(WRs), using interference-filter imaging with a CCD to selectcandidates. Follow-up spectroscopy is used to confirm 22 newly found WRstars, 21 of WN type. We establish that our survey would readily detectWRs as weak-lined as any known, and we conclude that our survey isessentially complete. This survey confirms suspicions that the previousphotographic surveys were only 50% complete for WN-type WRs and allowsus to combine the data with equally complete samples on other LocalGroup galaxies. We find that the relative number of WC- and WN-type WRscorrelates extremely well with metallicity, varying by a factor of 3with galactocentric distance within the plane of M33, and continuing thetrend to lower and higher metallicity galaxies. The WC/WN ratio within 3kpc of the sun is slightly above this trend, and we argue that WN starsare underrepresented in this sample. The WC/WN ratio is anomalously highin IC 10, given its low metallicity, and we demonstrate that this is notbecause of selection effects but is likely caused by IC 10's currentstatus as a starburst system. We examine the spectral properties of WCstars within these galaxies, confirming the previously reported trendsthat the spectral lines are stronger and broader in regions of lowermetallicity. We suggest that the different WC spectral subclasses do notprimarily indicate different physical properties for these stars butrather are simply a reflection of the effect that the initial metalabundances has had on the stellar wind structure. Finally, we comparethe luminous RSGs with WRs in these galaxies. We find that there is avery strong correlation of the relative numbers of RSGs and WRs withmetallicity, in the sense predicted by Maeder, Lequeux, & Azzopardi:at lower metallicities the fraction of luminous (M_bol < -7) RSGs ishigher, with a factor of 6 change within the disk of M33 [Delta log(O/H) = 0.35 dex], and a factor of ~10 change from M31 (or the innerportions of M33) to NGC 6822 [Delta log (O/H) = 0.5 dex]. This is easilyexplained by the Conti scenario in terms of massive stars spendingproportionately less of their He-burning lifetimes as RSGs rather thanWRs at higher metallicities and hence higher mass-loss rates. Finally,we note that the presence of luminous RSGs and WRs stars is extremelywell correlated for the OB associations in M31 and M33: where one findsone, one finds the other. To the extent that an association is strictlycoeval, this argues that some stars of 15 M_ȯ and above indeed dogo through both RSG and WR stages. The presence of WR stars of both WNand WC types in the same associations as luminous RSGs further suggeststhat some WCs, at least, have gone through the RSG phase. We include anAppendix providing a complete catalog of confirmed WR stars in LocalGroup galaxies beyond the Magellanic Clouds. Some of the observationsreported in this paper were obtained at the Multiple Mirror TelescopeObservatory, a facility operated jointly by the University of Arizonaand the Smithsonian Institution.
| Identification of Supernova Remnants in the Sculptor Group Galaxies NGC 300 and NGC 7793
We have obtained CCD imagery of eight fields in the Sculptor Group Scspiral galaxy NGC 300 and two fields covering the Sculptor Sd spiralgalaxy NGC 7793, using interference filters to isolate emission from Halpha + [N II], [S II], and a continuum band near 6100 A. Using theseimages, we have identified isolated nebulae that are bright in [S II]relative to H alpha , indicating probable shock heating, and that havemorphologies consistent with identification as supernova remnants(SNRs). Our lists of candidates contain 28 objects each in NGC 300 andNGC 7793. We then obtained optical (4800--6900 A) long-slit CCD spectraof many of these nebulae, as well as many adjacent photoionized regionsalong the slits for comparison. The spectra confirm that the SNRcandidates have [S II]:H alpha ratios in excess of 0.4, thus satisfyingthe usual optical criterion for being supernova remnants. Approximately80% of the objects observed spectroscopically show additional emissionlines that support their identification as supernova remnants. These arethe first large samples of supernova remnants identified in galaxiesoutside the Local Group. In general, many properties of these SNRsamples parallel the more extensive sample identified in M33. However,the dividing line between the [S II]:H alpha ratios appropriate forphotoionized and shock-heated nebulae appears to be much less distinctin these galaxies than for Local Group galaxies, causing confusion andincompleteness in these samples. This confusion effect is most prevalentin NGC 7793, where the observed [S II]:H alpha ratio in the most diffusephotoionized gas rises well above the value of 0.4 normally used todiscriminate SNRs from H II regions.
| 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.
| 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.
| High-resolution X-ray imaging of M33
The nearby spiral galaxy M33 was observed for 35 ks with thehigh-resolution imager on ROSAT in 1992 January and August. We find 27X-ray sources more luminous than 6 x 1036 ergs/s within 17.5arcmin of the nucleus, of which 12 were discovered by the EinsteinObservatory. Three of the ROSAT sources are coincident with giant H IIregions, and seven are coincident or nearly coincident with supernovaremnants. There is evidence for long-term variability between theEinstein and ROSAT observations in three sources, one of which was notdetected in the ROSAT observations. Aside from an eclipsing binary X-raysource discovered by the Einstein Observatory, there is no evidence forvariability within the ROSAT observations. Of the 120 cataloged holes inthe H I layer in the field of view, two have significant X-ray emission.One is coincident with the giant H II region IC 133 while the otherappears to be due to the chance superposition of a strong X-ray pointsource. Such holes are thought to be created by energetic stellar windsand supernovae from massive stars in young associations. We detectdiffuse X-ray emission with a luminosity of about 1039ergs/s, which may trace the spiral arms within 10 arcmin of the nucleus.
| The Discovery of a Supernova Remnant Embedded in a Giant H II Region of M33
Abstract image available at:http://adsabs.harvard.edu/abs/1993ApJ...418..743G
| Radio and Optical Properties of Supernova Remnants in M33
Although the properties and evolution of supernova remnants (SNRs) aregenerally understood, there are important questions concerning theinteraction of SNRs with the interstellar medium. These include the roleof SNRs in regulating the relativistic gas (cosmic rays) in galaxies andthe degree to which the ISM affects the evolution of SNRs. Statisticallysignificant samples of SNRs observed at several wavelengths have thepotential for yielding valuable insight into these questions. To thisend, we are carrying out a search for SNRs in the galaxy M33 at optical,radio and X-ray wavelengths. M33 is ideally suited for a study of thisnature. Remnants will all be at essentially the same distance and,because M33 is nearly face on, the effects due to interstellarabsorption are reduced. Furthermore, M33 is a spiral galaxy, allowingfor comparisons with the Milky Way. We have undertaken new radio, X-rayand optical observations of M33, and, here, we present the sample ofradio selected and optically confirmed SNRs and discuss some of theresults. We have identified ~ 100 non-thermal radio sources within20arcmin of the center of M33. Many of these sources are likely to beSNRs, and we have made followup spectroscopic observations of thesecandidates with the MMT and have found many to be associated withshock-heated gas. This radio-selected sample has the advantage overprevious optical samples of M33 in that it can detect remnants in highlyoptically confused regions. As an example, we have identified a SNRlocated in the giant HII region NGC-592 in M33 (Gordon et al., 1993 Ap.J., in press). This is a particularly interesting HII region because asoft X-ray source is located in it. We found a knot of non-thermal radioemission at the site of the X-ray source and detected shock-heated gasat optical wavelengths thus showing that the X-ray emission isassociated with an embedded SNR.
| A Multiwavelength Study of Supernova Remnants in M33: The Radio Subsample
M33 is the current focus of a project to identify and study supernovaremnantsi (SNRs) using observations in radio, X-ray and opticalwavelengths. Here, we present the results from the radio selected sampleof SNRs in M33. The radio observations were obtained with the Very LargeArray and the Westerbork Synthesis Array at 6 and 20 cm. The flux andspectral index of all sources down to the 3sigma noise level of the mapshave been measured. From the resulting catalogue, a list of candidateSNRs has been compiled, where the sample is defined as all radio sourceswith a non-thermal spectral index. This effectively eliminates most HIIregions, but also includes background radio sources. Because of theinclusion of background sources, it is necessary to turn to otherwavelengths for confirmation. We have examined the sites of radioemission using interference filter (6100 Angstroms, [SII], Hα )observations taken with the KPNO 4m telescope. Following the method ofLong et al. (1990,Ap. J. Suppl. 72,61), we have identified a list of 37non-thermal radio sources that have a high [SII]/Hα ratio, makingthem probable optical SNR candidates. We have observed 32 of thesespectroscopically at the MMT with the Red Channel long-slitspectrograph. The results of these observations will be presented. Themultiwavelength approach allows us more than the ability to confirm theexistence of new remnants, it also gives us valuable information on thedynamics of the SNR and its interaction with the surrounding medium. Asan example, we are studying a particularily interesting remnant in M33which has been detected in X-ray, radio and optical and is located inthe HII region NGC 592. With information from all three wavelengths weare exploring the properties of the shock, the efficiency of conversionof shock energy into relativistic particles, and the state of evolutionof the remnant.
| Spatial inhomogeneities in giant extragalactic H II regions
Giant extragalactic H II regions (GEHR) have been analyzed with emphasisplaced on the spatial distribution of relevant parameters. The resultsindicate that the metallicity Z is constant across the area underconsideration of a given GEHR. The same applies to the He/H and Ne/Oabundance ratios. All the GEHR are found to have He/H = 0.075 within 30percent, without any trend with Z. Particular attention is given to thepossible causes of the point-to-point spectral variations within a GEHR.It is argued that the spatial distribution of the line ratios should beattributed to fluctuations in the ionization parameter. These variationsare not produced by inhomogeneities in the electron density, but areassociated with the aging and evolution of the ionizing stellarclusters.
| The stellar content of associations and star complexes in M33
Photographic UBV photometry is used to map stellar associations and starcomplexes in M33 to characterize the galaxy and compare the results toother nearby galaxies. The appearance of the associations is revisedaccording to the images and a cluster analysis, which show themclustering within the classical OB associations. A shock wave across thesouthern spiral arm can be tested observationally by the stellarassociations, and the central region, contrasting the outer region, isfound to be rich in WR, O, and luminous stars. The difference betweenderived association ages and expansion ages suggests the existence oftwo or more generations of star formation in the old associations.
| A multifrequency radio continuum survey of M33. III - The magnetic field
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991A&A...241...47B&db_key=AST
| The Wolf-Rayet star population in the most massive giant H II regions of M33
Narrow-band images of NGC 604, NGC 595, and NGC 592, the most massivegiant H II regions (GHRs) in M33 have been obtained, in order to studytheir Wolf-Rayet content. These images reveal the presence of ninecandidates in NGC 604 (seven WN, two WC), 10 in NGC 595 (nine WN, oneWC), and two in NGC 592 (two WN). Precise positions and estimatedmagnitudes are given for the candidates, half of which have so far beenconfirmed spectroscopically as genuine W-R stars. The flux in theemission lines of all candidates is comparable to that of normalGalactic W-R stars of similar subtype. A few of the putativesuperluminous W-R stars are shown to be close visual double or multiplestars; their newly estimated luminosities are now more compatible withthose of normal W-R stars. NGC 595 seems to be overabundant in W-R starsfor its mass compared to other GHRs, while NGC 604 is normal. Factorsinfluencing the W-R/O number ratio in GHRs are discussed: metallicityand age appear to be the most important.
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