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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
| The Chemical Properties of Milky Way and M31 Globular Clusters. I. A Comparative Study
A comparative analysis is performed between high-quality integratedspectral indices of 30 M31 globular clusters, 20 Milky Way globularclusters, and a sample of field and cluster elliptical galaxies. We findthat the Lick CN indices in the M31 and Galactic clusters are enhancedrelative to the bulges of the Milky Way, M31, and elliptical spheroids,in agreement with Burstein and coworkers. Although not particularlyevident in the Lick CN indices, the near-UV cyanogen feature(λ3883) is strongly enhanced with respect to the Galacticglobular clusters at metallicities -1.5<[Fe/H]<-0.3. Carbon showssigns of varying among these two groups. For [Fe/H]>-0.8, we observeno systematic differences in the Hδ, Hγ, or Hβ indicesbetween the M31 and Galactic globular clusters, in contrast to previousstudies. The elliptical galaxy sample lies offset from the loci of theglobular clusters in both the cyanogen-[MgFe] and Balmer-line-[MgFe]planes. Six of the M31 clusters appear young and are projected onto theM31 disk. Population synthesis models suggest that these are metal-richclusters with ages 100-800 Myr, metallicities -0.20<=[Fe/H]<=0.35,and masses 0.7-~7.0×104 Msolar. Two otheryoung clusters are Hubble V in NGC 205, observed as a template, and anolder (~3 Gyr) cluster some 7 kpc away from the plane of the disk. Thesix clusters projected onto the disk show signs of rotation similar tothe H I gas in M31, and three clusters exhibit thin disk kinematics,according to Morrison and coworkers. Dynamical mass estimates anddetailed structural parameters are required for these objects todetermine whether they are massive open clusters or globular clusters.If they are the latter, our findings suggest globular clusters may tracethe buildup of galaxy disks. In either case, we conclude that theseclusters are part of a young, metal-rich disk cluster system in M31,possibly as young as 1 Gyr old.
| 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.
| Cepheids in LMC Clusters and the Period-Age Relation
We have made a new comparison of the positions of Cepheids and clustersin the LMC and constructed a new empirical period-age relation takinginto account all available data on Cepheids in the LMC bar provided bythe OGLE project. The most probable relation is logT=8.50-0.65 logP, inreasonably good agreement with theoretical expectations. NumerousCepheids in rich clusters of the LMC provide the best data for comparingtheories of stellar evolution and pulsation and the dynamical evolutionof clusters with observations. These data suggest that stars undergoingtheir first crossing of the instability strip are first-overtonepulsators, though the converse is true of only a small fraction offirst-overtone stars. Several rich clusters with suitable ages have noCepheids—a fact that is not understood and requires verification.Differences in the concentration of Cepheids toward their clustercenters probably reflect the fact that the clusters are at differentstages of their dynamical evolution, with the Cepheids in clustercoronas being ejected from the cluster cores during dynamicalinteractions between stars.
| Star Clusters in Local Group Galaxies
Not Available
| 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.
| The evolution of theV-Kcolours of single stellar populations
Models of evolutionary population synthesis of galaxies rely on theproperties of the so-called single stellar populations (SSP). In thispaper, we discuss how the integrated near-infrared colours, andespecially V-K, of SSPs evolve with age and metallicity. Some of theuncertainties associated with the properties of the underlying stellarmodels are thoroughly discussed. Our models include all the relevantstellar evolutionary phases, with particular attention being dedicatedto the asymptotic giant branch (AGB), which plays a fundamental role inthe evolution of the near-infrared part of the spectrum. First, wepresent the effects that different formulations for the mass-loss ratesproduce on the final remnant mass (i.e., on the initial-final massrelation), and hence on the AGB-termination luminosity and the relativecontribution of these stars to the integrated light. The results for theevolution of the V-K colour are very different depending on the choiceof the mass-loss prescription; the same is true also for the B-V colourin the case of low-metallicity SSPs. Secondly, we describe the changesoccurring in the integrated colours at the onset of the AGB and redgiant (RGB) branches. According to the classical formalism for the AGBevolution, the onset of this evolutionary phase is marked by a colourjump to the red, the amplitude of which is shown here to be highlydependent on the metallicity and mass-loss rates adopted in the models.We then consider the effect of the overluminosity with respect to thestandard core mass-luminosity relation that occurs in the most massiveAGB stars. Different simplified formulations for this effect are testedin the models; they cause a smoothing of the colour evolution in the agerange at which the AGB starts to develop, rather than a splitting of thecolour jump into two separate events. On the other hand, we find that atemporary red phase takes place ~1.5x10^8 yr after the RGB develops.Thanks to the transient nature of this feature, the onset of the RGB isprobably not able to cause marked features in the spectral evolution ofgalaxies. We then discuss the possible reasons for the transition of V-Kcolours (from ~1.5 to 3) that takes place in LMC clusters of SWB typeIV. A revision of the ages attributed to the single clusters revealsthat the transition may not be as fast as originally suggested. Thecomparison of the data with the models indicates that the transitionresults mainly from the development of the AGB. A gradual (or delayed)transition of the colours, as predicted by models which include theoverluminosity of the most massive AGB stars, seems to describe the databetter than the sudden colour jump predicted by classical models.
| Triggered star formation in the LMC4/Constellation III region of the Large Magellanic Cloud
The origin of a regular, 600-pc-long arc of young stars and clusters inthe Constellation III region of the Large Magellanic Cloud isconsidered. The circular form of this arc suggests that the pre-stellargas was uniformly swept up by a central source of pressure. In thecentre of the arc are six ~30-Myr-old A-type supergiant stars and aCepheid variable of similar age, which may be related to the source ofthis pressure. We calculate the expansion of a bubble around a clusterof this age, and show that it could have triggered the formation of thearc at the right time and place. Surrounding the central old stars andextending well outside the young arc is the LMC4 superbubble and giantHI shell. We show how this superbubble and shell could have formed bythe continued expansion of the 15-Myr-old cavity, following starformation in the arc and the associated new pressures. The age sequenceproposed here was not evident in the recent observations by Olsen et al.and Braun et al. because the first generation stars in the centre of theLMC superbubble are relatively faint and scarce compared to the moresubstantial population of stars less than 15 Myr old that formedthroughout the region in a second generation. These considerations leadto an examination of the origin of the LMC4/Constellation III region andother large rings in the LMC and other galaxies. Their size andcircularity could be the result of low galactic shear and a thick disc,with several generations of star formation in their interiors now toofaint to be seen.
| Ultraviolet ages of young clusters in the Magellanic Clouds.
Following a previous investigation on the integrated UV colours ofstellar clusters (Barbero et al. 1990), we study the calibration of theultraviolet colour index C(15-31) in terms of cluster age, usingobservations by the International Ultraviolet Explorer of 29 young andpopulous clusters of the Large Magellanic Cloud (LMC), and of the SmallMagellanic Cloud (SMC). The study is limited to the range of ages5x10^6^ to 8x10^8^yr, which is free from contamination by HorizontalBranch stars. It is shown that in this range of ages the theoreticalsequence C(15-31) vs. age agrees well with the one derived by combiningthe observed colour index C(15-31) with the ages determined viaisochrone fitting to the colour-magnitude diagrams while systematicdifferences, which are discussed on here, exist with respect to the agecalibration by Meurer, Cacciari and Freeman (1990). The present agecalibration C(15-31) vs. log(t), provided in an analytical form, isfinally used to determine the ages of the 29 clusters in our sample,including 13 objects for which no determination was available via theisochrone fitting method.
| 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.
| 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.
| Stellar Photometry Software
We describe the Stellar Photometry Software (SPS), which combines in asingle program procedures for locating stars, computing a mean stellarpoint-spread function (PSF), and performing aperture and/or multiplePSF-fitting photometry, along with related bookkeeping functions. Thesoftware can be run either interactively or in batch mode on computersusing the UNIX operating system. The performance of SPS is compared tothat of the photometry programs DoPHOT and IRAF/DAOPHOT, using both realand simulated CCD observations. A direct comparison of the instrumentalmagnitudes shows that all three programs produce comparable results.
| Tuning the Cepheid distance scale
Ongoing observational programs (both from the ground and space) willprovide a significantly larger sample of galaxies with well-studiedCepheids both within the Local Group and in more distant galaxies.Recent efforts in the calibration of the Cepheid distance scaleutilizing Cepheids in star clusters in the Galaxy and in the MagellanicClouds are described. Some of the significant advantages of utilizingLMC Cepheids in particular are emphasized, and the current status of thefield is summarized.
| Analysis of the UV spectra of young clusters of the Large Magellanic Cloud
UV and visual spectral energy distributions of young generations havebeen synthesized from evolutionary tracks in the HR diagram and modelatmospheres. The influence of several parameters has also been analyzed.Their UV portions have been checked with the UV spectra of 24 clustersof the LMC obtained by Cassatella et al. (1987). The models generallyagree well with the observations and make it possible to derive ages forvarious assumed abundances. Ages thus derived have been compared, in anage-age diagram, with those obtained from the integrated UBV photometry.While there are no systematic differences between these two agedeterminations, a fraction of clusters displays a large scatter, largerthan what is expected from the observational errors alone. Possiblecauses for this scatter are briefly analyzed.
| Near-infrared spectral evolution of blue LMC clusters : a comparison with galactic open clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1990RMxAA..21..202B
| The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
| Blue Magellanic clusters - Near-infrared spectral evolution
New integrated spectra in the range 5600-10,000 A are presented for 28LMC and 3 SMC young star clusters. The equivalent widths (W) ofprominent features and the continuum distribution are measured. Theanalysis, supplemented by 8 additional LMC clusters from previousstudies, indicates that the red supergiant phase is indeed verytime-peaked, occuring from 7 to 12 Myr. In addition to the previous caseof NGC 2004, it is found that NGC 1805, NGC 1994, NGC 2002, NGC 2098,and NGC 2100 (as well as NGC 2011 to a lesser extent) are undergoingthis phase. The red supergiant phase is clearly denoted by strong TiObands and Ca II triplet as well as a flat continuum or (in extremecases) a continuum with positive slope above 6000 A.
| The age calibration of integrated ultraviolet colors and young stellar clusters in the Large Magellanic Cloud
Integrated colors in selected far-UV bands are presented for a largesample of Large Magellanic Cloud (LMC) clusters. Theoreticalcalculations of these integrated colors are derived and discussed. Thelocation in the two-color diagram C(18-28), C(15-31) is expected to be asensitive but smooth function of cluster age for ages in the range 5 to800 million yr. Theoretical results appear in very good agreement withthe observed colors of LMC clusters. From this comparison, the gap inthe observed colors is suggested to be caused by the lack of LMCclusters in the range of ages between 200 million to one billion yr. Thetwo-color location of old globulars is discussed, also in connectionwith available data for the M31 clusters.
| Ellipticities at R(h) of LMC star clusters
The projected ellipticities of 53 populous LMC star clusters have beenderived by means of PDS 1010A scans and a computer interactive method ofreduction implemented on an Apollo 570 workstation. Film copies of apair of J and U plates taken with the 1.2 m UK Schmidt Telescope inAustralia were used. The ellipticities derived here agree with thosefound by previous investigators, when comparisons were possible at thesame radius. Ellipticity variations within individual globular clustersare seen to be a common phenomenon, so the ellipticities e(h) at adistance corresponding to the half-mass radius R(h) from the center wereadopted to represent the cluster's flatness. Using these values for theLMC clusters, it is found that LMC clusters are more elliptical thanthose of the Galaxy. Although the young LMC globular clusters show atendency to be more elliptical than the old ones, there is no strongevidence for a significant difference among them. Finally, e(h) wasfound to increase with the total mass of the clusters, possiblyindicating that high-mass clusters have higher angular momentum, or havemore difficulty in shedding angular momentum, than do low mass clusters,and remain longer in their initial flattened shape.
| The ultraviolet spectra of M31 globular clusters
Ultraviolet spectra of 11 of the brightest globular clusters in M31 showthat some exhibit residual flux below 3000 A, greater than that expectedfrom the bright, evolved stars in the cluster. There seems to be noapparent correlation of the strength of this ultraviolet flux withparameters such as metallicity, U-B color, visual magnitude, X-rayemission, or location within the parent galaxy. However, comparison ofthe ultraviolet colors of the M31 globular clusters with those in theGalaxy and in the Large Magellanic Cloud suggests that the M31 clustersmay contain a high percentage of blue horizontal-branch stars or thatsome clusters could be as young as about 2 x 10 to the 9th yr.
| Ultraviolet observations by the IUE of 31 clusters of the Large Magellanic Cloud
Ultraviolet observations of 31 star clusters of the Large MagellanicCloud obtained by the International Ultraviolet Explorer are presented.The clusters, mostly globular, span the range of ages from about 10 tothe 7th to 10 to the 10th yr. The sample includes several young globularclusters, which have no counterpart in the Galaxy. The paper is devotedto the study of the basic properties of the clusters in the ultraviolet,such as the interstellar reddening, the spatial extension in theultraviolet, and the ultraviolet color indices. These data are essentialfor a later study of the evolutionary stage of the clusters. Oneimportant result of this study is that a discontinuity exists in the UVtwo-color diagram, a fact which deserves further investigation, since itmight support the suggestion made by other authors that an active starformation phase took place in the Large Magellanic Cloud starting about100 million yr ago.
| 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.
| IUE observations of the clusters of the Magellanic Clouds
IUE observations of 17 clusters in the Large Magellanic Cloud arepresented. The derived ultraviolet colors become progressively redder asthe cluster's classification in the scheme of Searle, Wilkinson, andBagnuolo increases. With the aid of theoretical models of the integratedlight of coeval clusters of varying ages, it is shown that the SWBclassification is one of increasing age accompanied by decreasingmetallicity toward later SWB classes. The behavior of the UV absorptionline features in the integrated light of the LMC clusters supports thecontention.
| The kinematics of globular clusters in the Large Magellanic Cloud
Velocities for 35 globular clusters in the LMC have been combined withdata from other sources to yield velocities for a total of 59 clustersthat range in age from 100 million to 10 billion years. Clusters youngerthan one billion years are noted to have motions similar to the gas intheir vicinity and to share the rotation solution previously found onthe basis of H I velocity maps and H II region velocities. These youngclusters therefore constitute a flattened system having a lowline-of-sight velocity dispersion, consistent with that found inprevious kinematic and photometric studies. The older clusters are alsoflattened to a disk-like system, although both the systematic velocityand position angle of the line of nodes are significantly different forthese older clusters. The data presented also suggest that, unlike theMilky Way, there is no evidence for a kinematic halo population amongglobular clusters in the LMG.
| Photometric studies of composite stellar systems. V - Infrared photometry of star clusters in the Magellanic clouds
Abstract image available at:http://adsabs.harvard.edu/abs/1983ApJ...266..105P
| An ellipticity - age relation for globular clusters in the Large Magellanic Cloud. I - Measurements
It is pointed out that the rich star clusters of the Magellanic Cloudsresemble the globular clusters of the Galaxy. The present investigationhas the objective to determine the shapes of these clusters and theirdependence on age. The study has been restricted to the Large MagellanicCloud (LMC) because the Small Magellanic Cloud (SMC) does not provide alarge enough sample for the statistical analysis. The shapes of globularclusters are usually expressed in terms of ellipticities. Attention isgiven to the measurement of ellipticities with the aid of a ruler and agraduated magnifying glass, star count data on 12 LMC clusters, and ageestimates. It is found that estimates of the ellipticities of globularclusters made by eye are in excellent agreement with those based on starcounts. The ellipticity-age relation is probably explained mostnaturally by internal evolution in the structure of globular clusters.
| Photometry of bright stars in the Large Magellanic Cloud.
Abstract image available at:http://adsabs.harvard.edu/abs/1972A&A....18..271D
| A Catalogue of Clusters in The LMC
Not Available
| Studies of the Large Magellanic Cloud. V. The Young Populous Clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1961ApJ...133..413H
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