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Age distribution of young clusters and field stars in the Small Magellanic Cloud
Aims.In this paper we discuss the cluster and field star formation inthe central part of the Small Magellanic Cloud. The main goal is tostudy the correlation between young objects and their interstellarenvironment. Methods: . The ages of about 164 associations and 311clusters younger than 1 Gyr are determined using isochrone fitting. Thespatial distribution of the clusters is compared with the HI maps, withthe HI velocity dispersion field, with the location of the CO clouds andwith the distribution of young field stars. Results: .The clusterage distribution supports the idea that clusters formed in the last 1Gyr of the SMC history in a roughly continuous way with periods ofenhancements. The two super-shells 37A and 304A detected in the HIdistribution are clearly visible in the age distribution of theclusters: an enhancement in the cluster formation rate has taken placefrom the epoch of the shell formation. A tight correlation between youngclusters and the HI intensity is found. The degree of correlation isdecreasing with the age of the clusters. Clusters older than 300 Myr arelocated away from the HI peaks. Clusters and associations younger than10 Myr are related to the CO clouds in the SW region of the SMC disk. Apositive correlation between the location of the young clusters and thevelocity dispersion field of the atomic gas is derived only for theshell 304A, suggesting that the cloud-cloud collision is probably notthe most important mechanism of cluster formation. Evidence ofgravitational triggered episode due to the most recent close interactionbetween SMC and LMC is found both in the cluster andfield star distribution.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. X. CO emission from star formation regions in LMC and SMC
We present J=1-0 and J=2-1 12CO maps of several star-formingregions in both the Large and the Small Magellanic Cloud, and brieflydiscuss their structure. Many of the detected molecular clouds arerelatively isolated and quite small with dimensions of typically 20 pc.Some larger complexes have been detected, but in all cases the extent ofthe molecular clouds sampled by CO emission is significantly less thanthe extent of the ionized gas of the star-formation region. Very littlediffuse extended CO emission was seen; diffuse CO in between orsurrounding the detected discrete clouds is either very weak or absent.The majority of all LMC lines of sight detected in 13CO hasan isotopic emission ratio I( 12CO)/I( 13CO) ofabout 10, i.e. twice higher than found in Galactic star-formingcomplexes. At the lowest 12CO intensities, the spread ofisotopic emission ratios rapidly increases, low ratios representingrelatively dense and cold molecular gas and high ratios marking COphoto-dissociation at cloud edges.
| The Small Magellanic Cloud in the far infrared. I. ISO's 170 mu m map and revisit of the IRAS 12-100 mu m data
The ISOPHOT experiment onboard the ISO satellite generated a completeview of the Small Magellanic Cloud (SMC) at 170 mu m with 1.5 arcminresolution. The map is analysed using an automated photometry programenabling accurate photometric characterization of the far infrared (FIR)emitting regions. An integrated FIR luminosity of 8.5x 107Lsun is obtained, leading to a star formation rate ofSFRFIR=0.015 Msun/yr. With an average dusttemperature of , the total dustmass follows to MD=3.7x105 Msun. Inthis paper, the sources detected at 170 mu m are compared with thoseobtainable from the IRAS satellite data. For this purpose, the 12 mu m,25 mu m, 60 mu m, and 100 mu m IRAS high resolution (HiRes) maps of theSMC are re-examined using the same method. In contrast to formerstudies, this provides an all-band ISO/IRAS source catalog which is nolonger based on eyeball classification, but relies on an algorithm whichis capable of automated, repeatable photometry, even for irregularsources. In the mid infrared IRAS bands numerous bright FIR emittingregions in the SMC are detected and classified: 73 sources are found at12 mu m, 135 at 25 mu m (most of them with Fnu <1.0 Jy).All three FIR bands at 170 mu m, 100 mu m, and 60 mu m reproduce theoverall morphological structure of the SMC similarly well, in contrastto the 12 mu m and 25 mu m maps which only contain a limited number ofextended sources and do not trace the main body of the SMC. 243 sourcesare detected in the ISO 170 mu m map, 155 of them with Fnu>=2.0 Jy. Comparable numbers are found for the two FIR IRASmaps at 60 mu m (384) and 100 mu m (338) with fluxes up to 450 Jy. 70 ofthe 243 170 mu m sources are assigned a general SED type (``cold'',``warm'', i.e., <30 K, >30 K) for the first time. A comparisonwith earlier IRAS results suggests that many source flux densities inthose studies have been under- or overestimated because ofnon-standardized fitting methods. Many sources with flux densities up to40 Jy listed in former catalogs cannot be identified in our data.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.The tables in Appendices A to E are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp//cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/401/873}
| Morphologies and ages of star cluster pairs and multiplets in the Small Magellanic Cloud
An isophotal atlas of 75 star cluster pairs and multiplets in the SmallMagellanic Cloud is presented, comprising 176 objects. They areconcentrated in the SMC main body. The isophotal contours were made fromDigitized Sky Survey* images and showed relevant structuralfeatures possibly related to interactions in about 25% of the sample.Previous N-body simulations indicate that such shapes could be due totidal tails, bridges or common envelopes. The diameter ratio between themembers of a pair is preferentially in the range 1 - 2, with a peak at1. The projected separation is in the range ~ 3 - 22 pc with apronounced peak at ~ 13 pc. For 91 objects it was possible to deriveages from Colour-Magnitude Diagrams using the OGLE-II photometricsurvey. The cluster multiplets in general occur in OB stellarassociations and/or HII region complexes. This indicates a common originand suggests that multiplets coalesce into pairs or single clusters in ashort time scale. Pairs in the SMC appear to be mostly coeval andconsequently captures are a rare phenomenon. We find evidence that starcluster pairs and multiplets may have had an important role in thedynamical history of clusters presently seen as large single objects.The images in this study are based on photographic data obtained usingthe UK Schmidt Telescope, which was operated by the Royal ObservatoryEdinburgh, with funding from the UK Science and Engineering ResearchCouncil, until 1988 June, and thereafter by the Anglo-AustralianObservatory. Original plate material is copyright by the RoyalObservatory Edinburgh and the Anglo-Australian Observatory. The plateswere processed into the present compressed digital form with theirpermission. The Digitized Sky Survey was produced at the Space TelescopeScience Institute under US Government grant NAG W-2166.
| Ultraviolet Imaging Telescope Observations of the Magellanic Clouds
We present wide-field far-ultraviolet (FUV; 1300-1800 Å) images ofthe Large and Small Magellanic Clouds (LMC, SMC). These data wereobtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1(1990 December 1-10) and Astro-2 (1995 March 2-18) missions; the imagesprovide an extensive FUV mosaic of the SMC and contain numerous regionsin the LMC, covering a wide range of stellar densities and current starformation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OBassociations are completely or partially included in the observedfields. FUV data can identify the hottest OB stars more easily than canoptical photometry, and these stars dominate the ionizing flux, which iscorrelated to the observed Hα flux of the associated H ii regions.Of the H ii regions in the catalog of Davies, Elliott, & Meaburn(DEM), the UIT fields completely or partially include 102 DEM regions inthe LMC and 74 DEM regions in the SMC. We present a catalog of FUVmagnitudes derived from point-spread function photometry for 37,333stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMCwere presented recently by Cornett et al.), with a completeness limit ofm_UV ~ 15 mag and a detection limit of m_UV ~ 17.5. The averageuncertainty in the photometry is ~0.1 mag. The full catalog withastrometric positions, photometry, and other information is alsoavailable from publicly accessible astronomical data archives. We dividethe catalog into field stars and stars that are in DEM regions. Weanalyze each of these two sets of stars independently, comparing thecomposite UV luminosity function of our data with UV magnitudes derivedfrom stellar evolution and atmosphere models in order to derive theunderlying stellar formation parameters. We find a most probable initialmass function (IMF) slope for the LMC field stars of Gamma = -1.80 +/-0.09. The statistical significance of this single slope for the LMCfield stars is extremely high, though we also find some evidence for afield star IMF slope of Gamma ~ -1.4, roughly equal to the Salpeterslope. However, in the case of the stars in the DEM regions (the starsin all the regions were analyzed together as a single group), we findthree IMF slopes of roughly equal likelihood: Gamma = -1.0, -1.6, and-2.0. No typical age for the field stars is found in our data for timeperiods up to a continuous star formation age of 500 Myr, which is themaximum age consistent with the completeness limit magnitude of thecatalog's luminosity function. The best age for the collection ofcluster stars was found to be t_0 = 3.4 +/- 1.9 Myr; this is consistentwith the age expected for a collection of OB stars from many differentclusters.
| H_2 and its relation to CO in the LMC and other magellanic irregular galaxies
H_2 column densities towards CO clouds in the LMC and SMC are estimatedfrom their far-infrared surface brightness and HI column density. Thenewly derived H_2 column densities imply N(H_{2)}/I(CO) conversionfactors (in units of 10(21) mol cm(-2) (K km s(-1) )(-1) ) X_LMC =1.3+/-0.2 and X_SMC = 12+/-2. LMC and SMC contain total (warm) H_2masses of 1.0+/-0.3 x 10(8) Msun and 0.75+/-0.25 x 10(8)Msun respectively. Local H_2/HI mass ratios similar to thosein LMC and SMC are found in the magellanic irregulars NGC 55, 1569,4214, 4449 and 6822 and in the extragalactic HII region complexes NGC604, 595 and 5461 in M 33 and M 101 respectively. In these HII regionsand in NGC 4449, we find X = 1-2; in NGC 55, 4214 and 6822 X = 3-6 againin units of 10(21) mol cm(-2) (K km s(-1) )(-1) . The post-starburstgalaxy NGC 1569 has a very high value similar to that of the SMC. TheCO-H_2 conversion factor X is found to depend on both the ambientradiation field intensity per nucleon {sigma _FIR/{N_H and metallicity[O]/[H]: log X ~ 0.9+/-0.1 logfrac {sigma _FIR{N_H - 3.5+/-0.2log([O])/([H]). Neglecting dependency on radiation field, a reasonableapproximation is also provided by log X ~ -2.7+/-0.3 log([O])/([H]).Milky Way values are consistent with these relations. This result isinterpreted as the consequence of selective photodissociation of COsubjected to high radiation field energy densities and poor(self)shielding in low-metallicity environments, and especially thepreferential destruction of diffuse CO in `interclump' gas. Althoughlocally H_2 may be the dominant ISM-component, the average global H_2/HImass ratio is 0.2+/-0.04 and the average H_2 gas mass fraction is0.12+/-0.02. Magellanic irregulars have warm molecular gas fractionsvery similar to those of our Galaxy, whereas other global properties(mass, luminosity, metallicity, CO luminosity) are very different.
| A radio continuum study of the Magellanic Clouds V. Catalogues of radio sources in the Small Magellanic Cloud at 1.42, 2.45, 4.75, 4.85 and 8.55 GHz
We present catalogues of radio sources in the Small Magellanic Cloudfrom observations with the Parkes radio telescope at 1.42, 2.45, 4.75and 8.55 GHz, and an additional catalogue from the Parkes-MIT-NRAOsurvey at 4.85 GHz. A total of 224 sources were detected at at least oneof these frequencies, 60 of which are reported here for the first timeas radio sources. We compare positions and flux densities of thesesources with previously published results and find no significantpositional displacement or flux discrepancies. Tables 2-7 are onlyavailable electronically at the CDS via ftp 130.79.128.5 or athttp://cdsweb.u-strasbg.fr/Abstract.html
| Extinction and reddening of HII regions in the Small Magellanic Cloud.
We present absolute Hα and Hβ fluxes, obtained with aFabry-Perot spectrophotometer, of 24 bright HII regions in the SmallMagellanic Cloud. The photographic Hα maps of Kennicutt &Hodge (1986) are re-calibrated using these new Hα fluxes; the newcalibration gives fluxes 25% smaller than those previously published.These photographic and photoelectric Hα data are used inconjunction with radio continuum observations at 843 MHz from theMolonglo Observatory Synthesis Telescope to study the dust associatedwith SMC HII regions. For most regions the derived reddenings andextinctions are compatible with the standard Galactic extinction law anduniform interstellar extinction. A few regions display relatively highreddening and extinction; these are bright compact sources, such asN13AB, N27 and the cores of N81 and N88, all of which probably haveclosely associated dust. Low resolution Hi observations do not detectthese high concentrations of dust.
| A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. I. Small Magellanic Cloud and Bridge
A survey of extended objects in the Magellanic System was carried out onthe ESO/SERC R and J Sky Survey Atlases. The present work is dedicatedto the Small Magellanic Cloud and to the inter-Magellanic Cloud region("Bridge") totaling 1188 objects, of which 554 are classified as starclusters, 343 are emissionless associations, and 291 are related toemission nebulae. The survey includes cross-identifications amongcatalogs, and we present 284 new objects. We provide accurate positions,classification, homogeneous sizes, and position angles, as well asinformation on cluster pairs and hierarchical relation for superimposedobjects. Two clumps of extended objects in the Bridge and one at theSmall Magellanic Cloud wing tip might be currently forming dwarfspheroidal galaxies.
| An HI Mosaic of the Small Magellanic Cloud
Not Available
| A New Catalogue of Hα Emission Line Stars and Small Nebulae in the Small Magellanic Cloud
An objective-prism survey of the Small Magellanic Cloud has beenperformed through an Hα + [N II] interference filter, using the0.90 m Curtis Schmidt telescope of Cerro Tololo. 1898 emission-lineobjects have been detected in the main body of this galaxy, almostquadrupling the number of those found, in the same region, by theprevious objective-prism surveys. Among these objects are newlydiscovered planetary nebulae, compact HII regions and late-type stars.Continuum intensity, as well as the shape and relative strength of theHα emission-line have been estimated; coordinates, cross-identifications for the listed objects and 2.25 arcmin square findingcharts for all the objects are provided.
| Results of the ESO / SEST Key Programme on Co/ in the Magellanic Clouds - Part One - a Survey of Co/ in the Large Magellanic Cloud and the Small Magellanic Cloud
As the first part of the ESO-Swedish SEST Key Programme on CO in theMagellanic Clouds, we have observed ^12^CO J = 1-0 towards 92 positionsin the LMC and 42 positions in the SMC. In the SMC we searched foremission from H II regions, dark clouds and IRAS infrared sources. Thegenerally negative detection rate of non-IRAS sources in the SMC led toan LMC source selection based on the IRAS results. In both galaxies, COwas detected towards the majority of sources observed. We also observed^13^C0 J = 1-0 towards the brighter ^12^CO sources in the LMC (37) andSMC (9). Compared to the strength of CO lines observed in the Milky WayGalaxy with identical linear resolutions, velocity-integrated COemission is weaker by at least a factor of three in the LMC sources andan order of magnitude in the SMC sources. The mean velocity-integratedisotopic intensity ratio I_12_/I_13_ is 12.5 in the LMC and about 15 inthe SMC. Individual ratios range from 8.5 to 20. These isotopicintensity ratios are two to three times higher than those found inGalactic molecular clouds.
| Results of the ESO / SEST Key Programme - Co/ in the Magellanic Clouds - Part Two - Co/ in the Southwest Region of the Small Magellanic Cloud
We present the results of fully-sampled observations of the ^12^CO(1-0)line in the SW "Bar" region of the Small Magellanic Cloud (SMC), madewith the SEST telescope at 45" resolution. Previously, this line hadonly been observed in the SMC at the resolution of 8.8' (Rubio et al.1991). We have also observed the ^12^CO(2-1), ^13^CO(1-0) and^13^CO(2-1) lines at a number of positions. In this region of the SMC wehave detected CO only in the lower-velocity (i.e. nearer) component ofthis galaxy. The main CO features correspond to dark clouds seenoptically; some are associated with H II regions and with far-infraredsources. The spatial and velocity distribution of CO is complex withstructures on all scales and large-scale velocity gradients. This is anintrinsic property of the observed clouds: it is not due to confusionalong the line of sight. For two resolved CO clouds associated with H IIregions comparable to the Orion nebula, all four CO lines have beenobserved at a similar angular resolution. The ^12^CO(1-0)/^13^CO(1-0)line intensity ratios are between 10 and 16, the ^12^CO(2-1)/^12^CO(1-0)are slightly larger than 1 and the ^13^CO(2-1)/^13^CO(1-0) even larger.
| Physical properties of H II regions in the Small Magellanic Cloud
Some physical properties of the H II regions and their ionizing stellarassociations, such as the Lyman continuum photon flux, the rms electrondensity, and the mass of ionized gas, were derived for 30 H II regionsin the SMC. Bright H II regions are found to be large and diffuse,whereas faint ones are compact; i.e., small and dense. The decrease ofW(H-beta) with the increase of the nebular size and decrease of rmselectron density is consistent with a trend in the sense of an H IIregion expansion with aging. However, the rms electron density tends tobe a decreasing function of the nebular ionizing flux, which isinconsistent with a single evolutionary sequence for the H II regions.
| Infrared observations of the Magellanic Clouds. I - The Small Magellanic Cloud
Results of IRAS pointed observations in four infrared wavelength bands(12, 25, 60, and 100 microns) for the SMC are presented. Maps withorthogonal scan directions are shown, and a source list containing 219infrared sources is extracted from the data. Comparison with the IRASPoint Source Catalog (PSC) shows that only three entries in this catalogare spurious. All 13 entries in the IRAS Small Scale Structure Catalog(SSS) in the SMC are confirmed. Seventy-two new infrared sources, notincluded in either the PSC or in the SSS are found. The present SMCinfrared source list is compared to other object lists. Two blueglobular clusters, 28 SAO stars, and seven planetary nebulae areidentified. No SMC stars are found. In general there is a goodcorrelation of infrared emission with the distributiion of H II regionsand dark clouds.
| Global Photometric Observations and Physical Properties of HII Regions in the Small Magellanic Cloud
Integrated photoelectric measurements of the equivalent width WH,~, the[0 III]/Hfl ratio and the Hfl emission line flux were obtained for 30 Hii regions in the SMC. Physical properties of the H H regions and theirionizing stellar associations were derived. Some aspects of the recentstar formation in the SMC and the evolution of H ii regions arediscussed
| Global photometric observations of 30 H II regions in the Small Magellanic Cloud
Integrated photoelectric measurements of the equivalent width, theforbidden O-III/H-beta ratio, and the H-beta emission line flux weremade of 30 H-II regions in the SMC. The photometric system employedconsists of a narrow H-beta filter with Delta lambda = 30 A, a wideH-beta filter with Delta lambda = 150 A, and a forbidden O-III filtercentered at 5000 A. Calibration of the system is discussed. The presentdata have been compared with previous observations.
| H II regions and star formation in the Magellanic Clouds
Photoelectrically calibrated maps of the H-alpha emission in theMagellanic Clouds have been used to measure integrated fluxes forseveral hundred H II regions and to study the properties of the H IIregion populations in the galaxies. The H II regions span a range of10,000 in luminosity, from objects on the scale of the Orion Nebula tothe 30 Doradus complex. The H-alpha luminosity function is wellrepresented over this entire range by a power law function, indicatingthat there is no characteristic luminosity scale for the H II regions.The distributions of nebular diameters, on the other hand, are fittedwell by exponential functions, with a scale length of 80 pc. Approximatefluxes for several of the extended filamentary networks in the galaxieshave also been measured. This extended component probability contributes15-25 percent of the total H-alpha luminosity of the galaxies.
| The nebular complexes of the large and small Magellanic Clouds
Long exposures of the complexes of ionized hydrogen in both the LMC andSMC have been taken with the 48-in. SRC Schmidt camera through a H-alpha+ forbidden NII interference filter of 100-A bandwidth. These plates andidentifying charts are presented in a form in which little informationis lost. A catalog of many individual emission regions in both thesegalaxies is also compiled. The relationships between the nebulositiesand OB associations as well as between 21-cm neutral hydrogen emissionand continuum radio emission are discussed, and a number ofsupernova-remnant candidates are listed for further study.
| A new catalogue of emission-line stars and planetary nebulae in the Small Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1961AJ.....66..169L
| Catalogues of Hα-EMISSION Stars and Nebulae in the Magellanic Clouds.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1956ApJS....2..315H&db_key=AST
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