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Ýlgili Makaleler
The oxygen abundances in HII regions of the spiral galaxy M101 from the Sloan Digital Sky Survey spectra.
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| 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
| Hα line profiles for a sample of supergiant HII regions. II. Broad, low intensity components
We analyze the broad, low intensity, high velocity components that areseen in the Hα line profiles for a sample of HII regions. TheseHII regions are chosen from among the brightest and most isolated in asample of spiral galaxies for which we have photometric andspectroscopic data: NGC 157, NGC 3631, NGC 6764, NGC 3344, NGC 4321, NGC5364, NGC 5055, NGC 5985, and NGC 7479. We confirm that the lineprofiles of most of these bright, giant extragalactic HII regionscontain broad kinematic components of low intensity, but high velocity,that we denote as wings. We analyze these components, deriving emissionmeasures, central velocities, and velocity dispersions of the blue andred features, which are similar. We interpret these components asexpanding shells within the HII regions and produced by the stellarwinds from the ionizing stars. We compare the kinetic energies of theseexpanding shells with the kinetic energy available from the stellarwinds. If we allow for the hypothesis that the brightest HII regions aredensity bounded, we show that, for these HII regions, the stellar windmechanism can explain the observed shell kinetic energies.
| Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.
| An XMM-Newton view of M101 - II. Global X-ray source properties
We present the global X-ray properties of the point source population inthe grand-design spiral galaxy M101, as seen with XMM-Newton. 108 X-raysources are detected within the D25 ellipse of M101, of which~24 are estimated to be background galaxies. Multiwavelengthcross-correlations show that 20 sources are coincident with HII regionsand/or supernova remnants (SNRs), seven have identified/candidatebackground galaxy counterparts, six are coincident with foreground starsand one has a radio counterpart. While the spectral and timingproperties of the brightest sources were presented by Jenkins et al.,here we apply an X-ray colour classification scheme to split the entiresource population into different types, i.e. X-ray binaries (XRBs),SNRs, absorbed sources, background sources and supersoft sources (SSSs).Approximately 60 per cent of the population can be classified as XRBs,although there is source contamination from background active galacticnuclei (AGN) in this category as they have similar spectral shapes inthe X-ray regime. 15 sources have X-ray colours consistent with SNRs,three of which correlate with known SNR/HII radio sources. Another twoare promising new candidates for SNRs, one is unidentified, and theremainder are a mixture of foreground stars, bright soft XRBs and AGNcandidates. We also detect 14 candidate SSSs, with significantdetections in the softest X-ray band (0.3-1keV) only. 16 sources displayshort-term variability during the XMM-Newton observation, twelve ofwhich fall into the XRB category, giving additional evidence of theiraccreting nature. Using archival Chandra and ROSAT High ResolutionImager data, we find that ~40 per cent of the XMM sources show long-termvariability over a baseline of up to ~10 yr, and eight sources displaypotential transient behaviour between observations. Sources withsignificant flux variations between the XMM and Chandra observationsshow a mixture of softening and hardening with increasing luminosity.The spectral and timing properties of the sources coincident with M101confirm that its X-ray source population is dominated by accreting XRBs.
| Clusters in the Luminous Giant H II Regions in M101
We have obtained HST WFPC2 observations of three very luminous butmorphologically different giant H II regions (GHRs) in M101, NGC 5461,NGC 5462, and NGC 5471, in order to study cluster formation in GHRs.Images obtained in the F547M and F675W bands are used to identifycluster candidates and for photometric measurements, and images in theF656N band are used to show ionized interstellar gas. The measuredcolors and magnitudes are compared with the evolutionary tracksgenerated by the Starburst99 and Bruzual & Charlot populationsynthesis models to determine the ages and masses of the clustercandidates that are more luminous than MF547M=-9.0. Thebrightest clusters detected in the PC images are measured and found tohave effective radii of 0.7-2.9 pc. NGC 5461 is dominated by a veryluminous core and has been suggested to host a super-star cluster (SSC).Our observations show that it contains three R136-class clusterssuperposed on a bright stellar background in a small region. This tightgroup of clusters may dynamically evolve into an SSC in the future, andmay appear unresolved and be identified as an SSC at large distances,but at present NGC 5461 contains no SSCs. NGC 5462 consists of looselydistributed H II regions and clusters without a prominent core. It hasthe largest number of cluster candidates among the three GHRs studied,but most of them are faint and older than 10 Myr. NGC 5471 has multiplebright H II regions and contains a large number of faint clustersyounger than 5 Myr. Two of the clusters in NGC 5471 are older than R136,but just as luminous; they may be the most massive clusters in the threeGHRs studied. The fraction of stars formed in massive clusters has beenestimated from the clusters' contribution to the total stellar continuumemission and from a comparison between the ionizing power of theclusters and the ionizing requirement of the associated H II regions.Both estimates show that <~50% of massive stars are formed in massiveclusters; consequently, the Hα luminosity of an H II region doesnot provide a sufficient condition for the existence of SSCs. Thecluster luminosity functions (LFs) of the three GHRs show differentslopes. NGC 5462 has the steepest cluster LF and the most looselydistributed interstellar gas, qualitatively consistent with thehypothesis that massive clusters are formed in high-pressureinterstellar environments. The combined cluster LF of the three GHRs hasa slope similar to the universal cluster LFs seen in starburst galaxiesand nonstarburst spiral galaxies.
| Clusters in the Luminous Giant HII Regions in M101
Giant HII regions (GHRs), with high concentrations of massive stars, arean excellent laboratory to study modes of massive star formation and apotential host of super-star clusters (SSCs). We have used HST WFPC2images in the F547M and F675W bands to study cluster formation in threevery luminous but morphologically different GHRs in M101 - NGC5461,NGC5462, and NGC5471.NGC5461 has been suggested to host a SSC in its luminous core. Ourobservations show that it contains three R136-class clusters superposedon a small, bright stellar background. This tight group of clusters maydynamically evolve into an SSC in the future, and may appear unresolvedat large distances and be identified as an SSC. NGC5462 has looselydistributed HII regions and clusters without a prominent core. It hasthe largest number of clusters among the three GHRs, but most of themare faint and older than 10 Myr. NGC5471 has multiple bright HII regionsand many faint clusters younger than 5 Myr. It has two clusters olderthan R136, but just as luminous; they may be the most massive clustersin the three GHRs.The fraction of stars formed in massive clusters is estimated from theclusters' contribution to the total stellar continuum emission and acomparison of the clusters' ionizing power to the ionization requirementof the associated HII regions. Both estimates show that < ˜ 50%of stars are formed in massive clusters. The cluster luminosityfunctions (CLFs)of the three GHRs show different slopes. NGC5462 has thesteepest CLF and most loosely distributed interstellar gas,qualitatively consistent with the hypothesis that massive clusters areformed in high-pressure interstellar environments. The combined CLF ofthese three GHRs is similar to those in starburst galaxies and normalspiral galaxies.This research is supported by grants STScI GO-6829.01-95A andGO-9934.01-A.
| Hot Interstellar Gas and Stellar Energy Feedback in the Antennae Galaxies
We have analyzed Chandra archival observations of the Antennae galaxiesto study the distribution and physical properties of its hotinterstellar gas. Eleven distinct diffuse X-ray emission regions areselected according to their underlying interstellar structures and starformation activity. The X-ray spectra of these regions are used todetermine their thermal energy content and cooling timescales. Youngstar clusters in these regions are also identified and their photometricmeasurements are compared to evolutionary stellar population synthesismodels to assess their masses and ages. The cluster properties are thenused to determine the stellar wind and supernova energies injected intothe interstellar medium (ISM). Comparisons between the thermal energy inthe hot ISM and the expected stellar energy input show that young starclusters are sufficient to power the X-ray-emitting gas in some, but notall, active star formation regions. Super star clusters, with masses>=1×105 Msolar, heat the ISM, but theyield of hot interstellar gas is not directly proportional to thecluster mass. Finally, there exist diffuse X-ray emission regions thatdo not show active star formation or massive young star clusters. Theseregions may be powered by field stars or low-mass clusters formed withinthe last ~100 Myr.
| 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.
| Diffuse X-Ray Emission from M101
The total 0.45-1.0 keV luminosity of M101 is 3.2×1039ergs s-1, of which 2.3×1039 ergss-1 is due to diffuse emission. (Chandra observes only ~60%of the diffuse emission observed by ROSAT because of its superiorpoint-source detection.) Of the diffuse emission seen by Chandra, nomore than 6% can be due to luminous unresolved point sources, such asX-ray binaries, and ~10% is likely due to dwarf stars. The diffuseemission traces the spiral arms and is roughly correlated with theHα and far-UV emission. The radial distribution closely followsthe optical profile. The bulk of the diffuse emission is characterizedby a two-thermal component spectrum with kT=0.20 and 0.75 keV, and theratio of the emission measures of the two components is roughly constantas a function of both radius and surface brightness. The softercomponent has a sufficiently large covering factor that the bulk of theemission is likely extraplanar. We find no evidence of an extendedaxisymmetric X-ray halo, suggesting that any such halo has a strengthmuch smaller than current predictions.
| 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
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| Chandra X-Ray Sources in M101
A deep (98.2 ks) Chandra Cycle 1 observation has revealed a wealth ofdiscrete X-ray sources as well as diffuse emission in the nearby face-onspiral galaxy M101. From this rich data set we have created a catalog ofthe 110 sources from the S3 chip detected with a significance of greaterthan 3 σ. This detection threshold corresponds to a flux of~10-16 ergs cm-2 s-1 and a luminosityof ~1036 ergs s-1 for a distance to M101 of 7.2Mpc. The sources display a distinct correlation with the spiral arms ofM101 and include a variety of X-ray binaries, supersoft sources,supernova remnants, and other objects of which only ~27 are likely to bebackground sources. There are only a few sources in the interarmregions, and most of these have X-ray colors consistent with that ofbackground active galactic nuclei. The derived logN-logS relation forthe sources in M101 (background subtracted) has a slope of -0.80+/-0.05over the range of 1036-1038 ergs s-1.The nucleus is resolved into two nearly identical X-ray sources, eachwith a 0.5-2.0 keV flux of 4×1037 ergs s-1.One of these sources coincides with the optical nucleus, and the othercoincides with a cluster of stars 110 pc to the south. The fieldincludes 54 optically identified supernova remnants (SNRs), of which 12are detected by Chandra. Two of the SNR sources are variable and hencemust be compact objects. In total, eight of the X-ray sources showevidence for short-term temporal variation during this observation. Twoof these variable sources are now brighter than the ROSAT detectionthreshold, but they were not detected in the previous ROSAT observationstaken in 1992 and 1996. There are also two variable sources previouslyseen with ROSAT that apparently have faded below the Chandra detectionthreshold. The brightest source in the field shows extreme long-term andshort-term temporal variability. At its peak brightness it has asuper-Eddington luminosity greater than 1039 ergss-1. There are 10 supersoft sources (SSSs) in the field,which can be divided into two distinct subclasses: the brighter class(three objects) has a luminosity of ~1038 ergs s-1and a blackbody temperature of ~70 eV, whereas the other class (sevenobjects) is an order of magnitude fainter and has a blackbodytemperature of only ~50 eV.
| 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
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| A New Probe of the Molecular Gas in Galaxies: Application to M101
Recent studies of nearby spiral galaxies suggest that photodissociationregions (PDRs) are capable of producing much of the observed H I ingalaxy disks. In that case, measurements of the observed H I columndensity and the far-ultraviolet (FUV) photon flux responsible for thephotodissociation process provide a new probe of the volume density ofthe local underlying molecular hydrogen. We develop the method and applyit to the giant Scd spiral galaxy M101 (NGC 5457). The H I columndensity and amount of FUV emission have been measured for a sample of 35candidate PDRs located throughout the disk of M101 using the Very LargeArray and the Ultraviolet Imaging Telescope. We find that, aftercorrection for the best-estimate gradient of metallicity in theinterstellar medium (ISM) of M101 and for the extinction of theultraviolet emission, molecular gas with a narrow range of density from30-1000 cm-3 is found near star-forming regions at all radiiin the disk of M101 out to a distance of 12'~26 kpc, close tothe photometric limit of R25~13.5′. In this picture,the ISM is virtually all molecular in the inner parts of M101. Thestrong decrease of the H I column density in the inner disk of thegalaxy at RG<10 kpc is a consequence of a strong increasein the dust-to-gas ratio there, resulting in an increase of theH2 formation rate on grains and a corresponding disappearanceof hydrogen in its atomic form.
| Vorontsov-Velyaminov Rows: Straight Segments in the Spiral Arms of Galaxies
The phenomenon of rows-straight features in the spiral patterns ofgalaxies, which was discovered by Vorontsov-Velyaminov, is investigated.The rows are not artifacts; in several cases, they outline regularspiral arms almost over their entire lengths. The galaxies M101, M51,and a number of more distant spirals are used as examples to demonstratemajor geometrical and physical properties of these structures. It isshown that the row lengths increase nearly linearly with distance fromthe disk center, and that the angle between adjacent rows is almostalways close to 2pi/3. The galaxies with rows generally belong tomoderate-luminosity Sbc-Sc systems with low rotational velocities,regular spiral patterns (Grand Design), and an HI content normal forthese types of galaxies. Two types of rows are shown to exist, whichdiffer in thickness and appear to be evolutionarily related. Theformation mechanism of the rows should probably be sought in thepeculiar behavior of the gas-compression wave in spiral density waves.
| 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.
| Star Formation in the Giant H II Regions of M101
The molecular components of three giant H II regions (NGC 5461, NGC5462, and NGC 5471) in the galaxy M101 are investigated with newobservations from single-dish telescopes (James Clerk Maxwell Telescopeand the NRAO 12 m) and from the Owens Valley Radio Observatorymillimeter array. Of the three H II regions, only NGC 5461 hadpreviously been detected in CO emission. We calculate preliminary valuesfor the molecular mass of the GMCs in NGC 5461 by assuming a CO-to-H_2factor (X factor) and then comparing these values with the virialmasses. We found that the appropriate X factor is 5 times smaller thanthe X factor in the Milky Way despite the lower metallicity of M101. Weconclude that the data in this paper demonstrate for the first time thatthe value of X may decrease in regions with intense star formation. Themolecular mass for the association of clouds in NGC 5461 isapproximately 3x10^7 M_solar and is accompanied by 1-2 times as muchatomic mass. The observed CO emission in NGC 5461 is an order ofmagnitude stronger than in NGC 5462, but it was not possible to detectmolecular gas toward NGC 5471 with the James Clerk Maxwell Telescope. Aneven larger ratio of atomic to molecular gas in NGC 5471 was observed,which might be attributed to efficient conversion of molecular to atomicgas. The masses of the individual clouds in NGC 5461, which aregravitationally bound, cover a range of (2-8)x10^5 M_solar, comparablewith the masses of Galactic giant molecular clouds. Higher star-formingefficiencies, and not massive clouds, appear to be the prerequisite forthe formation of the large number of stars whose radiation is requiredto produce the giant H II regions in M101.
| ISO observations of five giant HII regions in M 101.
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| 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.
| Vorontsov-Vel'yaminov's rows in giant spiral galaxies: geometrical properties and physical interpretation.
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| 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.
| The origin of the far-infrared luminosity within the spiral galaxy M101
High resolution 60 and 100 micron images obtained with the InfaredAstronomical Satellite (IRAS) are compared with H alpha images in orderto investigate the origin of the far-infrared luminosity within thelate-type spiral galaxy M101. There is a good correspondence between thefar-infrared and H-alpha morphology. The far-infrared and H-alphaluminosities have been measured at 129 independent locations on the starforming disk of M101. After correcting the H-alpha luminosity forextinction and extrapolating the IRAS (40-120 microns) luminosity to1000 microns we find that the far-infrared luminosity is commensuratewith that expected from the O and B stars which are required to ionizethe hydrogen gas, at all locations within M101. Additionally, the IRASHiRes 60 and 100 micron images reveal that the dust temperature peakscoincide identically with the location of H II regions. The far-infraredluminosity of M101 is radiated primarily by dust with temperatures wellin excess of that expected for cirrus, but similar to that observed forGalactic and extragalactic H II regions.
| Supersonic turbulence in giant extragalactic HII regions
I discuss in this paper the more likely physical mechanisms that couldprovide the energy input for the supersonic motions observed in giantextragalactic HII regions, together with preliminary results of anongoing observational program that aims to study, with high spatial andspectral resolution, the kinematics of the ionized gas of the regions.
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