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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
| Low-excitation blobs in the Magellanic Clouds
Aims.We study an unknown, or very poorly known, interstellar H IIcomponent in the Magellanic Clouds. This is the first study ever devotedto this class of objects, which we call low-excitation blobs (LEBs). Methods: We used low-dispersion spectroscopy carried out at ESO toobtain emission line intensities of Hα, Hβ, and [O III](λλ 4959 + 5007) for 15 objects in the Large MagellanicCloud and 14 objects in the Small Magellanic Cloud. Results aredisplayed in excitation ([O III]/Hβ ratio) versus Hβluminosity diagrams. Results: We show the presence of an LEB componentin the Magellanic Clouds and study its relationship with the alreadyknown class of high-excitation blobs (HEBs). The newly found LEBs arelower excitation counterparts of HEBs and are powered by less massiveexciting stars. Further study of LEBs is expected to provide new piecesof information for a better understanding the low mass end of the upperinitial mass function in the Magellanic Clouds.
| Infrared Surface Brightness Fluctuations of Magellanic Star Clusters
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
| Principal components analysis for equivalent widths of globular clusters
The method of principal component analysis is applied to a sample of 41Galactic globular clusters and 22 younger star clusters to select someage-sensitive or metallicity-sensitive spectral lines.It is found thatsome spectral lines have great potential to determine the metallicity ofstar clusters, such as CN, CaII K, CaII H and MgI+MgH; and some otherspectral lines are good indicators of age, such as H?, H?, H? and H?.These lines can help us to solve the age-metallicity degeneracy of starclusters.
| A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
| 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.
| A digital photometric survey of the magellanic clouds: First results from one million stars.
We present the first results from, and a complete description of, ourongoing UBVI digital photometric survey of the Magellanic Clouds. Inparticular, we discuss the photometric quality and automated reductionof a CCD survey (magnitude limits, completeness, and astrometricaccuracy) that covers the central 8(deg) x 8(deg) of the LargeMagellanic Cloud (LMC) and 4(deg) x 4(deg) of the Small Magellanic Cloud(SMC). We discuss photometry of over 1 million stars from the initialsurvey observations (an area northwest of the LMC bar covering ~ 2(deg)x 1.5(deg) ) and present a deep stellar cluster catalog that containsabout 45% more clusters than previously identified within this region.Of the 68 clusters found, only 12 are also identified as concentrationsof ``old'', red clump stars. Furthermore, only three clusters areidentified solely on the basis of a concentration of red clump stars,rather than as a concentration of luminous (V < 21) main sequencestars. Extrapolating from the current data, we expect to obtain B and Vphotometry for 25 million stars, and U and I photometry for 10 and 20million stars, respectively, over the entire survey area.
| A Methanol Maser Survey of IRAS-selected Regions in the Magellanic Clouds
We report the results of an Australia telescope 6.6 GHz methanol masersurvey of 55 positions in the Large and Small Magellanic Clouds,selected on the basis of their IRAS colors to be ultracompact H IIregions. In addition, 12 regions in the LMC possessing compact Hαand radio continuum emission were observed. A new cluster of strongmasers ( 1-4 Jy) was detected from one source, IRAS 05011 -6815; noother detections are claimed above a typical 7 level of 700 mJy beam -1A comparison of the Galactic and Magellanic Cloud 6.6 GHz methanol maserpeak flux density distributions indicates a significantly lowerdetection rate in the latter. We examine the effects of the lower metaland dust abundances of the Magellanic Clouds on the production ofsaturated methanol masers, which may explain this deficiency.
| 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.
| 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.
| 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.
| The stellar-free emission component in galactic nuclei - At low-levels, evidence for shock ionization
The emission-line component in a sample of 92 galaxies was isolatedusing stellar absorption templates built from emission-free star clusterspectra and taking into account all sources of reddening extrinsic tothe line emitting regions, and the emission line properties werestatistically analyzed. Three characteristic average spectra were built,including those corresponding to nuclear H II regions, objectsidentified as LINERs, and extreme low-level emission galaxies with W(em)of not greater than 2 A. For the latter, line measurements were pusheddown to very low level. The measurements of the ratio between forbiddenS II line and forbidden S III line indicate that, in the extremelow-level emission galaxies, the shock ionization is the mechanism atwork.
| Electron densities in planetary nebulae
Electron densities for 146 planetary nebulae have been obtained foranalyzing a large sample of forbidden lines by interpolating theoreticalcurves obtained from solutions of the five-level atoms using up-to-datecollision strengths and transition probabilities. Electron temperatureswere derived from forbidden N II and/or forbidden O III lines or wereestimated from the He II 4686 A line strengths. The forbidden O IIdensities are generally lower than those from forbidden Cl III by anaverage factor of 0.65. For data sets in which forbidden O II andforbidden S II were observed in common, the forbidden O II values dropto 0.84 that of the forbidden S II, implying that the outermost parts ofthe nebulae might have elevated densities. The forbidden Cl II andforbidden Ar IV densities show the best correlation, especially wherethey have been obtained from common data sets. The data give resultswithin 30 percent of one another, assuming homogeneous nebulae.
| A new radio continuum survey of the Magellanic Clouds at 1.4 GHz. II - The radio morphology, and thermal and nonthermal emission of the LMC
The distribution of the radio continuum emission of the LMC is examined.The distribution of the spectral index and a reliable spectrum of theradio emission of the LMC are derived using radio continuum surveys ofthe LMC at 408, 1400, and 2300 MHz. The distribution of the spectralindex across the LMC shows a close positional agreement between flatspectrum and H-alpha emitting regions. The total nonthermal radioluminosity is derived, giving a total equipartition magnetic fieldstrength of about 6 microG. Accurate thermal radio flux densities of themost prominent LMC H II complexes are determined.
| A grid of star cluster properties for stellar population synthesis
Measurements of equivalent widths are presented for 70 windows (as wellas the normalized continuous distribution) in the integrated spectra of63 star clusters with known age, metallicity, and reddening. From thisdata a grid of mean star cluster properties is derived as a function ofage and metallicity. Equivalent widths are provided for the Balmer linesand for a selection of 13 metallic features, together with the continuumdistribution, at ages 16.5 Gyr, 5 Gyr, 1 Gyr, 500 Myr, 100 Myr, 50 Myr,and 10 Myr and for metallicities (Z/solar Z) = 0.6, 0.0, -0.5, -1.0,-1.5 and -2.0.
| 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.
| A base of star clusters for stellar population synthesis
Results are reported from an analysis of the integrated spectra of 63star clusters which are intended as the bases for population synthesisin galaxies. The cluster data are tabulated in terms of temperatures,ages, specific gravities and metallicities. The survey covers galacticglobular clusters, intermediate age red globular clusters and blueglobular clusters in the Magellanic Clouds, rich and compact galacticopen clusters and H II regions. The equivalent widths (W) of absorptionspectra are discussed in terms of the age and metallicity of theclusters. Attention is given to the impact of younger turn-off points onW for features in the blue wing of the spectrum. A relatively uniformmodel is found for the age/metallicity relationship for Balmer lines inH spectra. The star cluster spectra were tested and found to be validfor synthesizing about 50 percent of the nuclei in a sample of 152normal galaxies of types E to Sc and luminosities of -16.6 to -23.3.
| Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
| Observations of giant bubbles in the Large Magellanic Cloud
Forbidden line S II/H-alpha intensity line ratio mapping and radialvelocity data as well as monochromatic photographs are presented for asample of bubbles in the LMC and some classical H II regions. Thebubbles of unknown origin have line ratios greater than those of the HII regions and thus appear to fill the gap between thermal andnonthermal radio sources. All the bubbles or filamentary nebulae haveimportant internal kinematical motions while the velocity dispersion isabout 6-7 km/s for the H II regions. The large complex nebulae havevalues similar to simple H II regions in their brightest parts, whilethe faintest parts exhibit greater dispersions and conspicuoussplittings and broadenings. The ionized bubbles appear to beintermediate between classical young H II regions and supernovaremnants.
| 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.
| 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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