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Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| Massive young stellar objects in the Large Magellanic Cloud: water masers and ESO-VLT 3-4 μm spectroscopy
We investigate the conditions of star formation in the Large MagellanicCloud (LMC). We have conducted a survey for water maser emission arisingfrom massive young stellar objects in the 30 Doradus region (N157) andseveral other HII regions in the LMC (N105A, N113 and N160A). We haveidentified a new maser source in 30Dor at the systemic velocity of theLMC. We have obtained 3-4 μm spectra, with the European SouthernObservatory (ESO)-Very Large Telescope (VLT), of two candidate youngstellar objects. N105AIRS1 shows H recombination line emission, and itsSpectral Energy Distribution (SED) and mid-infrared colours areconsistent with a massive young star ionizing the molecular cloud.N157BIRS1 is identified as an embedded young object, based on its SEDand a tentative detection of water ice. The data on these four HIIregions are combined with mid-infrared archival images from the SpitzerSpace Telescope to study the location and nature of the embedded massiveyoung stellar objects and signatures of stellar feedback. Our analysisof 30Dor, N113 and N160A confirms the picture that the feedback from themassive O- and B-type stars, which creates the HII regions, alsotriggers further star formation on the interfaces of the ionized gas andthe surrounding molecular cloud. Although in the dense cloud N105A starformation seems to occur without evidence of massive star feedback, thegeneral conditions in the LMC seem favourable for sequential starformation as a result of feedback. In an Appendix, we present watermaser observations of the galactic red giants RDoradus and WHydrae.
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| 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 Global Content, Distribution, and Kinematics of Interstellar O VI in the Large Magellanic Cloud
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofinterstellar O VI absorption toward 12 early-type stars in the LargeMagellanic Cloud (LMC). The observations have a velocity resolution of<~20 km s-1 (FWHM) and clearly show O VI 1031.926 Åabsorption at LMC velocities toward all 12 stars. From theseobservations we derive column densities of interstellar O VI in thisnearby galaxy; the observed columns are in the rangelogN(OVI)=13.9-14.6, with a mean of 14.37 and a standard deviation of+/-38% (+0.14-0.21 dex). The observations probeseveral sight lines projected onto known superbubbles in the LMC, butthese show relatively little (if any) enhancement in O VI column densitycompared to sight lines toward relatively quiescent regions of the LMC.The observed LMC O VI absorption is broad, with Gaussian dispersionsσ~30-50 km s-1. This implies temperaturesT<~(2-5)×106, indicating that much of the broadeningis nonthermal because O VI has a very low abundance at such hightemperatures. The O VI absorption is typically displaced ~-30 kms-1 from the corresponding low-ionization absorptionassociated with the bulk of the LMC gas. The general properties of theLMC O VI absorption are very similar to those of the Milky Way halo. Theaverage column density of O VI and the dispersion of the individualmeasurements about the mean are identical to those measured for the haloof the Milky Way, even though the metallicity of the LMC is a factor of~2.5 lower than the Milky Way. The velocity dispersion measured for theLMC material is also consistent with recent measurements of the Galactichalo. The striking similarities in these quantities suggest that much ofthe LMC O VI may arise in a vertically extended distribution similar tothe Galactic halo. We discuss the measurements in the context of a halocomposed of radiatively cooling hot gas and/or turbulent mixing layers.If the observed O VI absorption is tracing a radiatively coolinggalactic fountain flow, the mass flow rate from one side of the LMC diskis of the order M~1 Msolar yr-1, with a mass fluxper unit area of the disk M/Ω~2×10-2Msolar yr-1 kpc-2.
| The Effects of Dust in Simple Environments: Large Magellanic Cloud H II Regions
We investigate the effects of dust on Large Magellanic Cloud (LMC)H II region spectral energy distributions usingarcminute-resolution far-ultraviolet (FUV), Hα, far-infrared(FIR), and radio images. Widely used indicators of the amount of lightlost to dust (attenuation) at Hα and in the FUV correlate witheach other, although often with substantial scatter. There are twointeresting systematic discrepancies: First, Hα attenuationsestimated from the Balmer decrement are lower than those estimated fromthe Hα-to-thermal radio luminosity ratio. Our data, at this stage,cannot unambiguously identify the source of this discrepancy. Second,the attenuation at 1500 Å and the UV spectral slope, β,correlate, although the slope and scatter are substantially differentfrom the correlation first derived for starbursting galaxies by Calzettiet al. Combining our result with those of Meurer et al. forultraluminous infrared galaxies and Calzetti et al. for starburstinggalaxies, we conclude that no single relation between β and 1500Å attenuation is applicable to all star-forming systems.
| Lyman Continuum Extinction by Dust in H II Regions of Galaxies
We examine Lyman continuum extinction (LCE) in H II regions by comparinginfrared fluxes of 49 H II regions in the Galaxy, M31, M33, and theLarge Megellanic Cloud with estimated production rates of Lymancontinuum photons. A typical fraction of Lyman continuum photons thatcontribute to hydrogen ionization in the H II regions of three spiralgalaxies is <~50%. The fraction may become smaller as the metallicity(or dust-to-gas ratio) increases. We examine the LCE effect on estimatedstar formation rates of galaxies. The correction factor for the Galacticdust-to-gas ratio is 2-5.
| An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.
| HST observations of the very young SMC ``blob'' N 88A
High-resolution Hubble Space Telescope images have allowed us for thefirst time to resolve the compact SMC ionized ``blob'' N 88A (diameter ~,3''.5 or 1 pc). This very young H ii,region, which is hatching from itsnatal molecular cloud, is heavily affected by absorbing dust associatedwith the cloud. The interstellar reddening towards N 88A is on averageAV ~ ,1.5 mag and strikingly rises to more than 3.5 mag in anarrow dust band crossing the core of the H ii,region. Such a highextinction is unprecedented for an H ii,region in the metal-poor SMC. Wepresent the photometry of some 60 stars lying towards the OB associationat the center of which lies N 88A. The exciting star(s) of N 88A is notdetected, due to the heavy extinction. The chronology of star formationis discussed for the whole region. Based on observations with theNASA/ESA Hubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555. Based onobservations obtained at the European Southern Observatory, La Silla,Chile. Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Ultraviolet observations of stars in the Magellanic Clouds: a historical bridge from the Sun to R 136 and beyond.
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| 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.
| Extinction of H II regions in the Large Magellanic Cloud
The extinction properties of H II regions in the Large Magellanic Cloudare investigated using radio continuum data obtained from the MolongloObservatory Synthesis Telescope, digitized and calibrated H-alpha data,and published Balmer decrement measurements. The resultingextinction-color excess diagram suggests that (1) most H II regions inthe Magellanic Clouds have similar extinction properties to the Galacticones, (2) all imaginable gas/dust configurations are possible, and (3)the extinction of some highly reddened H II region cores originatesexternally in cocoon shells. The puzzle of different extinction-colorexcess ratios of Galactic and extragalactic H II regions is explained asbeing due to the different populations of observed samples rather thanany intrinsic differences. The extinction of the observed Galactic H IIregions produced by foreground dust overwhelms the internal extinction,while the situation in the observed extragalactic H II regions is justthe opposite.
| Kinematics of the very young nebula N59 at the edge of the supershell LMC4
The dynamics of the nebula N59 (B053540-6736), located at the boundaryof the supershell LMC4 in the Large Magellanic Cloud, is studied using ascanning Fabry-Perot interferometer. It is shown that the nebulae NGC2032 and 2035, which form the bright core of the H II region N59A(B053530-6736), belong to a single H II region which looks divided dueto the presence of a heavy dust lane. This bright core presents anexpansion motion of 24 km s(-1) . The kinetic energy involved in thismotion is of about 1.5 x 10(49) erg. This value is compatible eitherwith a supernova explosion origin or with a formation by the winds ofinterior massive stars. Since no clear traces of a SN explosion havebeen found in this nebula and since the stellar content of N59A(B053530-6736) is rich in blue stars, we conclude that these stars,mainly the very massive star HDE 269810 (R122), and probably other starshidden by the dust lane, are sufficient in providing the wind power todrive the expansion motion. The dust lane seems to be mixed in with thenebular gas and the stars, suggesting a site where star formation maystill take place. An extended shell, probably ionized by the star R122,has been detected at the same velocities as the slab, at blueshiftedvelocity, seen in the foreground absorption. The star R122 contributesto the high excitation of the faint diffuse gas, and perhaps of somefarther nebulae. To the East, the SNR 0536-676 remains as a trace of theexistence of another massive star which had already exploded. Thekinematics of N59B (B053610-6736) which contains the SNR 0536-676, isalso studied, corroborating the results of previous studies. Based onobservations collected at the European Southern Observatory
| A catalogue of compact radio sources in and behind the Large Magellanic Cloud
We present the results of a continuum snapshot survey of a 3 deg X 4 degregion of the Large Magellanic Cloud including the area of the giantmolecular cloud and the 30 Doradus nebula. The observations have beencarried out with the Australia Telescope Compact Array (ATCA) at 1.4 and2.4 GHz. Most fields are complete to about 6 mJy peak flux density at1.4 GHz and to about 3 mJy at 2.4 GHz. The positions, peak and integralflux densities of 113 compact (< 54") sources detected at 1.4 GHz andof 70 sources (<34") detected at 2.4 GHz are presented. Positions areaccurate to about 3" and peak flux densities are accurate to about 10%or better, depending on the source position relative to the pointingcenters. 32 of the sources detected at 1.4 GHz are coincident withHα objects in the catalogue of Davies et al.; these are possiblyintrinsic to the LMC. However, we suppose that most are backgroundobjects, since the number vs. flux agrees with predictions ofextragalactic source counts from other surveys. Tables 3 and 4 are alsoavailable electronically at the CDS via ftp cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html TheAustralia Telescope is funded by the Commonwealth of Australia foroperation as a National Facility managed by CSIRO.
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.
| On Coagulation and the Stellar Mass Spectrum
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...452..652A&db_key=AST
| 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.
| A radio continuum study of the Magellanic Clouds. IV. Catalogues of radio sources in the Large Magellanic Cloud at 1.40, 2.45, 4.75, 4.85 and 8.55 GHz.
From observations with the Parkes radio telescope, we present cataloguesof radio sources in the Large Magellanic Cloud at four frequencies:1.40, 2.45, 4.75 and 8.55GHz, and an additional catalogue from a sourceanalysis of the Parkes-MIT-NRAO survey at 4.85GHz. A total of 469sources have been detected at least one of these frequencies, 132 ofwhich are reported here for the first time as radio sources.
| Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs
Strong x-ray emission detected in Large Magellanic Cloud (LMC)superbubbles has been explained as the result of interior supernovaremnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot befound using conventional criteria. We thus investigate the possibilityof using the interstellar absorption properties in the ultraviolet (UV)as a diagnostic of hidden SNR shocks. The International UltravioletExplorer (IUE) archives provide the database for this pilot study. Theycontain high-dispersion spectra of several stars in x-ray brightsuperbubbles. To distinguish the effects of SNR shocks from those oflocal stellar winds and a global hot halo around the LMC, we includedcontrol objects in different environments. We find that almost allinterstellar absorption properties can be explained by the interstellarenvironment associated with the objects. Summarizing the two mostimportant results of this study: (1) a large velocity shift between thehigh-ionization species (C IV and Si IV) and the low-ionization species(S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however,the absence of a velocity shift does not preclude the existence of SNRshocks; (2) there is no evidence that the LMC is uniformly surrounded byhot gas; hot gas is preferentially found associated with largeinterstellar structures like superbubbles and supergiant shells, whichmay extend to large distances from the plane.
| A Search for Methanol Masers in the Magellanic Clouds
We report the discovery of a second methanol maser in the LargeMagellanic Cloud and we present the results of synthesis observations ofthis and the methanol maser detected previously. The second discoverywas made using the Australia Telescope National Facility's 64-m Parkesradio telescope during an extensive maser search for 6.6-GHz maseremission from the 5_1_-6_0_ A^+^ transition in both Magellanic Clouds.Spectra were obtained towards 35 HII regions in the Large MagellanicCloud and 13 regions in the Small Magellanic Cloud, and also on a3-arcmin grid over an area 0.3^deg^ square, south of the 30 Doradusnebula. Parkes observations at 12.2 GHz towards the two maser sitesyielded no detectable emission from the 2_0_-3_-1_, E methanoltransition. The results suggest that methanol masers are less abundantin the Magellanic Clouds than in our Galaxy. Observations of the twomasers with the Australia Telescope Compact Array showed one to belocated near the continuum emission peak of the H II region MC18 (N11),while the other appeared to be centred near OH emission on thesouth-eastern boundary of MC23 (N105a).
| The massive star content of the blue dwarf galaxy IZw 36 from Faint Object Camera observations
We have observed the blue dwarf galaxy IZw 36 with the f/96 relay of theFaint Object Camera and have for the first time resolved massive stars,using the broad band filters F175W, F342W, F430W and F480LP. We havemeasured the fluxes of 143 of these objects and studied theircharacteristics in color-magnitude diagrams. A few stars may be redsupergiants but their contribution to the integrated light is less than5% in the F430W filter. The F175W-F430W color of the integrated stellarpopulation is redder than expected from the current burst of starformation, suggesting therefore the presence of an older and unresolvedunderlying population. The ultraviolet measurements combined withsynthetic photometry calculations allow us to place the massive stars ina bolometric magnitude vs. temperature diagram. In this diagram, thestars are compared to evolutionary tracks for different stellar masses.The current burst probably has an age less than 12 Myr. We infer anInitial Mass Function, with a power-law slope in the range -1.7 to -2.6for masses M>20Msun_. This is consistent with most of thevalues reported for sites of star formation in the Galaxy and theMagellanic Clouds and does not support the view of an IMF flattening atlow metallicity.
| A Survey of 526 Soutern Flat Spectrum Radio Sources with the Parkes / Tidbinbilla Interferometer
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| The stellar content of the Large Magellanic Cloud II region N 59 A
We present UBV photometry of the stellar cluster associated with N59A, adusty H II region in the LMC. N59A's main detected source of ionizationis an O5V (or possibly earlier type) star with a visual extinction of1.2 mag. N59A also contains fifteen O-B3 stars that may contribute tothe ionization; these stars are affected by greatly differing amounts ofextinction. However, the observed stellar content of N59A cannotcompletely account for the ionization of the gas and the heating of theassociated dust. Some early massive star(s) still probably lieundetected in the core of (or behind) the central absorbing cloud. Inaddition to this young population associated with the H II region, wedetect an underlying older population of giant stars. We have alsodetected one Galactic star, and a few supergiant candidates. Theseresults are discussed in terms of the initial mass function.
| A comparison of far infrared and H-alpha emission of H-II regions in the Magellanic Clouds
From a comparison of the IRAS and smoothed H-alpha maps of theMagellanic Clouds, it was found that H-II regions with core-halostructure usually have higher F(60 microns)/F(H-alpha) ratios andprobably emit more in the far infrared than do extended low-density H-IIregions. This is consistent with the idea that the far infrared emissionis mainly produced by dust within H-II regions.
| Near-infrared spectra and classification diagnostics of Seyfert galaxies
Observational results of a previous spectroscopic survey of Seyfertgalaxies in the near-IR are presented, and the potential for usingemission-line ratios in this spectral region as a classificationdiagnostic tool is examined. Near-IR CCD spectra, which cover thewavelength range of 7000-10,000 at a nominal resolution of about 12 A,of 15 additional Seyferts and two starburst galaxies are obtained.Relative emission-line intensities from these observations, incombination with measurements from previous studies and measurements ofnew, signal-to-noise ratio optical spectra of many of these objects, areused to study the diagnostic diagrams involving forbidden S III 9069,9531/H-alpha, forbidden O II 7320, 7330/H-alpha, forbidden S II 6716,6731/H-alpha, and forbidden O III 5007/H-beta. Comparisons are made inthese diagrams between observational data from the active galaxies andpublished measurements of H II regionlike objects, as well as withpredictions from simple one-component models calculated for the twotypes of objects.
| A radio continuum study of the Magellanic Clouds. II - The far-infrared/radio correlation in the Large Magellanic Cloud
The correlation between the far-infrared (FIR) and the 6.3 cm radiocontinuum of the Large Magellanic Cloud (LMC) was studied. Radiocontinuum maps and infrared maps are presented. Statistical studiesindicated that the local point-by-point correlation between the two mapsis strong and appears to be due to the correlation between warm FIR andthermal radio emissions both of which are associated with young ionizingstars. The cool FIR/nonthermal radio correlation appears to be stronglylinked to the global FIR/radio correlation. A systematic difference wasobserved between the eastern and western half of the LMC. This asymmetrymay be due to different ages of massive stars or different gas-to dustratios in the two halves.
| A Comparison of Far Infrared and Hα Emission of HII Regions in the Magellanic Clouds
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| Flux densities at 8400 MHz for a large sample of radio sources
This paper presents 8400-MHz flux densities for 1194 southern radiosources. The sources were selected from the Parkes 2700-MHz Survey toinclude all those stronger than 0.5 Jy at that survey's findingfrequency of 2700 MHz. The new fluxes have an accuracy of about 8percent, corresponding to 0.05 Jy for a typical source. It isanticipated that the data will be useful in defining the high-frequencyradio spectra of many sources as well as in pinpointing objects withwhich to improve the southern, astrometric absolute reference frame.
| Radio positions and optical identifications for a sample of southern flat-spectrum radio sources. II
Presented here are optical identifications for southern radio sourcesusing new arcsecond positions. These sources are all flat radio-spectrumsources stronger than 0.25 Jy at 2700 MHz. All lie between rightascension (RA) (B1950.0) 18h00m and O6h00m declination (Dec.) -80 degand -50 deg (B1950.0). The radio positions have standard deviations ofabout 1.0 arcsec in both RA and Dec. The optical positions are withrespect to the FK4 reference system as approximated by the Perthcatalogs and have position uncertainties of 0.5 arcsec. Magnitudeestimates are on the IIIa-J scale and are accurate to 0.4 mag for QSOsand 0.5 mag for galaxies. The sample is complete to the 22.5-mag limitof the SERC IIIa-J sky survey. There are 198 sources in the completesample. Thirty-one sources (16 percent) show some radio structure withthe 20-arcsec beam. For the 175 unresolved sources, there are 124 QSOs(71 percent), 26 galaxies (15 percent), and 25 empty fields (14 percent)suggested.
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