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Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters
We use Hubble Space Telescope archival images to measure central surfacebrightness profiles of globular clusters around satellite galaxies ofthe Milky Way. We report results for 21 clusters around the LMC, fivearound the SMC, and four around the Fornax dwarf galaxy. The profileswere obtained using a recently developed technique based on measuringintegrated light, which is tested on an extensive simulated data set.Our results show that for 70% of the sample, the central photometricpoints of our profiles are brighter than previous measurements usingstar counts with deviations as large as 2 mag arcsec-2. About40% of the objects have central profiles deviating from a flat centralcore, with central logarithmic slopes continuously distributed between-0.2 and -1.2. These results are compared with those found for a sampleof Galactic clusters using the same method. We confirm the knowncorrelation in which younger clusters tend to have smaller core radii,and we find that they also have brighter central surface brightnessvalues. This seems to indicate that globular clusters might be bornrelatively concentrated, and that a profile with an extended flat coremight not be the ideal choice for initial profiles in theoreticalmodels.
| A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| New Optical and Near-Infrared Surface Brightness Fluctuation Models. II. Young and Intermediate-Age Stellar Populations
We present theoretical surface brightness fluctuation (SBF) amplitudesfor single-burst stellar populations of young and intermediate age (25Myr<=t<=5 Gyr) and metallicities Z=0.0003, 0.001, 0.004, 0.008,0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expectedin the Johnson-Cousins (UBVRIJHK) photometric system are provided. Wepay attention to the contribution of thermally pulsating asymptoticgiant branch (TP-AGB) stars. The sensitivity of the predicted SBF tochanges in the mass-loss scenario along the TP-AGB phase is examined.Below 0.6-1 Gyr both optical and near-IR SBF models exhibit a strongdependence on age and mass loss. We also evaluate SBF amplitudes usingMonte Carlo techniques to reproduce the random variation in the numberof stars experiencing bright and fast evolutionary phases (red giantbranch, AGB, TP-AGB). On these grounds we provide constraints on thefaintest integrated flux of real stellar populations required to derivereliable and meaningful SBF measurements. We analyze a technique forderiving SBF amplitudes of star clusters from the photometry ofindividual stars and estimate the uncertainty due to statisticaleffects, which may impinge on the procedure. The first optical SBFmeasurements for 11 Large Magellanic Cloud (LMC) star-rich clusters-withages ranging from a few megayears to several gigayears-are derived usingHubble Space Telescope observations. The measurements are compared toour SBF predictions, providing a good agreement with models ofmetallicity Z=0.0001-0.01. Our results suggest that, for TP-AGB stars, amass loss as a power-law function of the star luminosity is required inorder to properly reproduce the optical SBF data of the LMC clusters.Finally, near-IR models have been compared to available data, thusshowing that the general trend is well fitted. We suggest how toovercome the general problem of SBF models in reproducing the details ofthe near-IR SBF measurements of the Magellanic Cloud star clusters.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| Globular clusters and the formation of the outer Galactic halo
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.
| 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.
| Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - I. The old Large Magellanic Cloud clusters NGC 1928, 1939 and Reticulum
We present the results of photometric measurements from images of theLarge Magellanic Cloud (LMC) globular clusters NGC 1928, 1939 andReticulum taken with the Advanced Camera for Surveys on the Hubble SpaceTelescope. Exposures through the F555W and F814W filters result inhigh-accuracy colour-magnitude diagrams (CMDs) for these three clusters.This is the first time that CMDs for NGC 1928 and 1939 have beenpublished. All three clusters possess CMDs with features indicating themto be >10 Gyr old, including main-sequence turn-offs at V~ 23 andwell-populated horizontal branches (HBs). We use the CMDs to obtainmetallicity and reddening estimates for each cluster. NGC 1939 is ametal-poor cluster, with [Fe/H]=-2.10 +/- 0.19, while NGC 1928 issignificantly more metal rich, with [Fe/H]=-1.27 +/- 0.14. The abundanceof Reticulum is intermediate between the two, with [Fe/H]=-1.66 +/-0.12- a measurement which matches well with previous estimates. Allthree clusters are moderately reddened, with values ranging from E(V-I)= 0.07 +/- 0.02 for Reticulum and E(V-I) = 0.08 +/- 0.02 for NGC 1928,to E(V-I) = 0.16 +/- 0.03 for NGC 1939. After correcting the CMDs forextinction we estimate the HB morphology of each cluster. NGC 1928 and1939 possess HBs consisting almost exclusively of stars to the blue ofthe instability strip, with NGC 1928 in addition showing evidence for anextended blue HB. In contrast, Reticulum has an intermediate HBmorphology, with stars across the instability strip. Using a variety ofdating techniques we show that these three clusters are coeval with eachother and the oldest Galactic and LMC globular clusters, to within ~2Gyr. The census of known old LMC globular clusters therefore now numbers15 plus the unique, younger cluster ESO121-SC03. The NGC 1939 fieldcontains another cluster in the line of sight, NGC 1938. A CMD for thisobject shows it to be less than ~400 Myr old, and it is thereforeunlikely to be physically associated with NGC 1939.
| The Optical Gravitational Lensing Experiment. Catalog of RR Lyr Stars in the Large Magellanic Cloud
We present the catalog of RR Lyr stars discovered in a 4.5 squaredegrees area in the central parts of the Large Magellanic Cloud (LMC).Presented sample contains 7612 objects, including 5455 fundamental modepulsators (RRab), 1655 first-overtone (RRc), 272 second-overtone (RRe)and 230 double-mode RR Lyr stars (RRd). Additionally we attach alist ofseveral dozen other short-period pulsating variables. The catalog datainclude astrometry, periods, BVI photometry, amplitudes, and parametersof the Fourier decomposition of the I-band light curve of each object.We present density map of RR Lyr stars in the observed fields whichshows that the variables are strongly concentrated toward the LMCcenter. The modal values of the period distribution for RRab, RRc andRRe stars are 0.573, 0.339 and 0.276 days, respectively. Theperiod-luminosity diagrams for BVI magnitudes and for extinctioninsensitive index W_I are constructed. We provide the log P-I, log P-Vand log P-W_I relations for RRab, RRc and RRe stars. The mean observedV-band magnitudes of RR Lyr stars in the LMC are 19.36 mag and 19.31 magfor ab and c types, respectively, while the extinction free values are18.91 mag and 18.89 mag.We found a large number of RR Lyr stars pulsating in two modes closelyspaced in the power spectrum. These stars are believed to exhibitnon-radial pulsating modes. We discovered three stars whichsimultaneously reveal RR Lyr-type and eclipsing-type variability. If anyof these objects were an eclipsing binary system containing RR Lyr star,then for the first time the direct determination of the mass of RR Lyrvariable would be possible.We provide a list of six LMC star clusters which contain RR Lyr stars.The richest cluster, NGC 1835, hosts 84 RR Lyr variables. The perioddistribution of these stars suggests that NGC1835 shares features ofOosterhoff type I and type II groups.All presented data, including individual BVI observations and findingcharts are available from the OGLE Internet archive.
| Spectroscopy of Globular Clusters in the Fornax Dwarf Galaxy
We present low-resolution, integrated Keck spectra of the five globularclusters (GCs) of the Fornax dwarf spheroidal galaxy. We find atentative age spread among the clusters, with the GC H5 younger by 2-3Gyr than the others. The clusters generally appear to be very metal-poor([Fe/H]~-1.8), with the cluster H4 slightly more metal-rich at[Fe/H]=-1.5. We speculate that cluster H4 is similar to the Galactic GCRuprecht 106, which lacks the [α/Fe] enhancement typical amongmetal-poor GCs in the Milky Way. High-resolution spectroscopy ofindividual cluster and field stars will be needed to sort out thesurprisingly complex history of GC formation and evolution in thisgalaxy.
| Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud
We have compiled a pseudo-snapshot data set of two-colour observationsfrom the Hubble Space Telescope archive for a sample of 53 rich LMCclusters with ages of 106-1010 yr. We presentsurface brightness profiles for the entire sample, and derive structuralparameters for each cluster, including core radii, and luminosity andmass estimates. Because we expect the results presented here to form thebasis for several further projects, we describe in detail the datareduction and surface brightness profile construction processes, andcompare our results with those of previous ground-based studies. Thesurface brightness profiles show a large amount of detail, includingirregularities in the profiles of young clusters (such as bumps, dipsand sharp shoulders), and evidence for both double clusters andpost-core-collapse (PCC) clusters. In particular, we find power-lawprofiles in the inner regions of several candidate PCC clusters, withslopes of approximately -0.7, but showing considerable variation. Weestimate that 20 +/- 7 per cent of the old cluster population of theLarge Magellanic Cloud (LMC) has entered PCC evolution, a similarfraction to that for the Galactic globular cluster system. In addition,we examine the profile of R136 in detail and show that it is probablynot a PCC cluster. We also observe a trend in core radius with age thathas been discovered and discussed in several previous publications bydifferent authors. Our diagram has better resolution, however, andappears to show a bifurcation at several hundred Myr. We argue that thisobserved relationship reflects true physical evolution in LMC clusters,with some experiencing small-scale core expansion owing to mass loss,and others large-scale expansion owing to some unidentifiedcharacteristic or physical process.
| Testing stellar population models with star clusters in the Large Magellanic Cloud
We present high signal-to-noise ratio integrated spectra of 24 starclusters in the Large Magellanic Cloud (LMC), obtained using the FLAIRspectrograph at the UK Schmidt telescope. The spectra have been placedon to the Lick/IDS system in order to test the calibration of SimpleStellar Population (SSP) models. We have compared the SSP-predictedmetallicities of the clusters with those from the literature,predominantly taken from the Ca-triplet spectroscopy of Olszewski et al.(1991). We find that there is good agreement between the metallicitiesin the range -2.10 <=[Fe/H]<= 0. However, the Mg2 index(and to a lesser degree Mg b) systematically predict highermetallicities (up to +0.5 dex higher) than . Among thepossible explanations for this are that the LMC clusters possess[α/Fe] > 0. Metallicities are presented for eleven LMC clusterswhich have no previous measurements. We compare SSP ages for theclusters, derived from the Hβ, Hγ and Hδ Lick/IDSindices, with the available literature data, and find good agreement forthe vast majority. This includes six old globular clusters in oursample, which have ages consistent with their HST colour-magnitudediagram (CMD) ages and/or integrated colours. However, two globularclusters, NGC 1754 and NGC 2005, identified as old (~15 Gyr) on thebasis of HST CMDs, have Hβ line-strengths which lead ages that aretoo low (~8 and ~6 Gyr respectively). These findings are inconsistentwith their CMD-derived values at the 3σ level. Comparison betweenthe horizontal branch morphology and the Balmer line strengths of theseclusters suggests that the presence of blue horizontal branch stars hasincreased their Balmer indices by up to ~1.0 Å. We conclude thatthe Lick/IDS indices, used in conjunction with contemporary SSP models,are able to reproduce the ages and metallicities of the LMC clustersreassuringly well. The required extrapolations of the fitting functionsand stellar libraries in the models to lower ages and low metallicitiesdo not lead to serious systematic errors. However, owing to thesignificant contribution of horizontal branch stars to Balmer indices,SSP model ages derived for metal-poor globular clusters are ambiguouswithout a priori knowledge of horizontal branch morphology.
| Ages and metallicities of five intermediate-age star clusters projected towards the Small Magellanic Cloud
Colour-magnitude diagrams are presented for the first time for L32, L38,K28 (L43), K44 (L68) and L116, which are clusters projected on to theouter parts of the Small Magellanic Cloud (SMC). The photometry wascarried out in the Washington system C and T1 filters,allowing the determination of ages by means of the magnitude differencebetween the red giant clump and the main-sequence turn-off, andmetallicities from the red giant branch locus. The clusters have ages inthe range 2-6Gyr, and metallicities in the range-1.65<[Fe/H]<-1.10, increasing the sample of intermediate-ageclusters in the SMC. L116, the outermost cluster projected on to theSMC, is a foreground cluster, and somewhat closer to us than the LargeMagellanic Cloud. Our results, combined with those for other clusters inthe literature, show epochs of sudden chemical enrichment in theage-metallicity plane, which favour a bursting star formation history asopposed to a continuous one for the SMC.
| Age and metallicity for six LMC clusters and their surrounding field population
We investigate, on the basis of CCD Strömgren photometry, the agesand metallicities of six LMC clusters together with their surroundingfield population. The clusters and metallicities are: NGC 1651 (in therange [Fe/H] = -0.65 dex to -0.41 dex), NGC 1711 (-0.57 ∓ 0.17dex), NGC 1806 (-0.71 ∓ 0.23 dex), NGC 2031 (-0.52 ∓ 0.21dex) and NGC 2136/37 (-0.55 ∓ 0.23 dex) and NGC 2257 (-1.63∓ 0.21 dex). The metallicities for NGC 1651, NGC 1711, NGC 1806and NGC 2031 have been determined for the first time (NGC 2031 and NGC2136/37 are interesting for the Cepheid distance scale). In the clustersurroundings, we found about 650 field stars that were suitable to beused for a determination of an age-metallicity relation (AMR). Ourmethod is to estimate ages for individual stars on the basis ofStrömgren isochrones with individually measured metallicities. Withthis method we are able to sample the AMR of the field population up to8 Gyr. Our metallicity data are incompatible with models predicting manymetal-poor stars (G-dwarf problem). The metallicity of the fieldpopulation increased by a factor of six, starting around 2 Gyr ago. Theproposed AMR is consistent with the AMR of the LMC cluster system(including ESO 121 SC03 and three clusters with an age of 4 Gyr). Theproposed AMR is incompatible with the recently proposed AMR by Pagel& Tautvaisiene.
| The elliptical galaxy formerly known as the Local Group: merging the globular cluster systems
Prompted by a new catalogue of M31 globular clusters, we have collectedtogether individual metallicity values for globular clusters in theLocal Group. Although we briefly describe the globular cluster systemsof the individual Local Group galaxies, the main thrust of our paper isto examine the collective properties. In this way we are simulating thedissipationless merger of the Local Group, into presumably an ellipticalgalaxy. Such a merger is dominated by the Milky Way and M31, whichappear to be fairly typical examples of globular cluster systems ofspiral galaxies. The Local Group `Elliptical' has about 700 +/- 125globular clusters, with a luminosity function resembling the `universal'one. The metallicity distribution has peaks at [Fe/H] ~ -1.55 and -0.64with a metal-poor to metal-rich ratio of 2.5:1. The specific frequencyof the Local Group Elliptical is initially about 1 but rises to about 3,when the young stellar populations fade and the galaxy resembles an oldelliptical. The metallicity distribution and stellar populationcorrected specific frequency are similar to that of some known earlytype galaxies. Based on our results, we briefly speculate on the originof globular cluster systems in galaxies.
| The Optical Gravitational Lensing Experiment. Cepheids in Star Clusters from the Magellanic Clouds
We present Cepheids located in the close neighborhood of star clustersfrom the Magellanic Clouds. 204 and 132 such stars were found in the LMCand SMC, respectively. The lists of objects were constructed based oncatalogs of Cepheids and star clusters, recently published by theOGLE-II collaboration. Location of selected Cepheids on the skyindicates that many of them are very likely cluster members. Photometricdata of Cepheids and clusters are available from the OGLE Internetarchive.
| The Optical Gravitational Lensing Experiment. Catalog of Star Clusters from the Large Magellanic Cloud
We present the catalog of star clusters found in the area of about 5.8square degree in the central regions of the Large Magellanic Cloud. Itcontains data for 745 clusters. 126 of them are new objects. For eachcluster equatorial coordinates, radius, approximate number of membersand cross-identification are provided. Photometric data for all clusterspresented in the catalog and Atlas consisting of finding charts andcolor-magnitude diagrams are available electronically from the OGLEInternet archive.
| Star Formation Histories from Hubble Space Telescope Color-Magnitude Diagrams of Six Fields of the Large Magellanic Cloud
We present results on the analysis of background field stars found inHubble Space Telescope Wide Field Planetary Camera 2 observations of sixof the old globular clusters of the Large Magellanic Cloud. Treated ascontaminants by the globular cluster analysts, we produce (V-I,V)color-magnitude diagrams (CMDs) of the field stars and use them toexplore the LMC's star formation history. The photometry approachesV~26, well below the turnoff of an ancient (~14 Gyr) LMC population ofstars. The field star CMDs are generally characterized by an upper mainsequence broadened by stellar evolution, an old red giant branch, aprominent red clump, and an unevolved lower main sequence. The CMDs alsocontain a few visual differences, the most obvious of which is thesmeared appearance of the NGC 1916 field caused by heavy differentialreddening. More subtly, the base of the subgiant branch near the oldturnoff appears extended in V, and the red giant branch appears broad inV-I in four of the fields but not in the NGC 1754 field. We use amaximum likelihood technique to fit model CMDs drawn from Bertelli etal. isochrones to the observed CMDs. We constrain the models by theage-metallicity relation derived from LMC clusters, test four initialmass function (IMF) slopes, and fit for the reddening, distance modulus,and star formation rate. We find that we can just resolve structure inSFR(t) with time steps of ~0.15 in log(age), implying a resolution of~4x10^8 yr at an age of 1 Gyr. For a Salpeter IMF, our derived starformation history for the NGC 1754 field is characterized by an enhancedstar formation rate over the past 4 Gyr, qualitatively resembling thatderived by others for a variety of LMC fields. The remaining fourfields, however, appear to have had high levels of star formationactivity as long as 5-8 Gyr ago; these fields lie in the LMC bar whereasthe NGC 1754 field lies in the disk, suggesting that the inner regionsof the LMC contain significantly more older stars than the outerregions. Examining the residuals of the models and observations, we findthat the old red giant branches of the models provide a poor fit to theobservations, which suggests an error in the model isochrones. Theeffect of the disagreement appears to be to underestimate thecontribution of the old population.
| Spectroscopic analysis of the candidate globular clusters NGC 1928 and 1939 in the Large Magellanic Cloud
The integrated spectral properties in the range 3600-6700 A of thecandidate old clusters NGC 1928 and 1939 in the LMC bar are comparedwith those of old- and intermediate-age reference LMC clusters, theproperties of which are better established. It has been possible toinfer the age of the sample clusters by means of absorption features andthe continuum distribution, in particular in the plane W_M x W_B (whereW_B is the average of Hdelta, Hγ and H beta equivalent widths, andW_M that of Ca II K, G band and Mg i). The results indicate that NGC1928 and 1939 are compatible with old clusters. The metallicity isderived with respect to galactic globular cluster templates: [Fe/H]~-1.2 and -2.0 for NGC 1928 and 1939, respectively. We also discuss thecensus of Population II clusters in the LMC, their spatial distributionand the possibility of a LMC core and a transient morphologicalclassification for interacting late-type disc galaxies.
| Interacting star clusters in the Large Magellanic Cloud. Overmerging problem solved by cluster group formation
We present the tidal tail distributions of a sample of candidate binaryclusters located in the bar of the Large Magellanic Cloud (LMC). Oneisolated cluster, SL 268, is presented in order to study the effect ofthe LMC tidal field. All the candidate binary clusters show tidal tails,confirming that the pairs are formed by physically linked objects. Thestellar mass in the tails covers a large range, from 1.8x 10(3) to 3x10(4) \msun. We derive a total mass estimate for SL 268 and SL 356. Atlarge radii, the projected density profiles of SL 268 and SL 356 falloff as r(-gamma ) , with gamma = 2.27 and gamma =3.44, respectively. Outof 4 pairs or multiple systems, 2 are older than the theoreticalsurvival time of binary clusters (going from a few 10(6) years to 10(8)years). A pair shows too large age difference between the components tobe consistent with classical theoretical models of binary clusterformation (Fujimoto & Kumai \cite{fujimoto97}). We refer to this asthe ``overmerging'' problem. A different scenario is proposed: theformation proceeds in large molecular complexes giving birth to groupsof clusters over a few 10(7) years. In these groups the expected clusterencounter rate is larger, and tidal capture has higher probability.Cluster pairs are not born together through the splitting of the parentcloud, but formed later by tidal capture. For 3 pairs, we tentativelyidentify the star cluster group (SCG) memberships. The SCG formation,through the recent cluster starburst triggered by the LMC-SMC encounter,in contrast with the quiescent open cluster formation in the Milky Waycan be an explanation to the paucity of binary clusters observed in ourGalaxy. Based on observations collected at the European SouthernObservatory, La Silla, Chile}
| 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.
| HSTcolour-magnitude diagrams of six old globular clusters in the LMC
We report on HST observations of six candidate old globular clusters inthe Large Magellanic Cloud (LMC): NGC 1754, 1835, 1898, 1916, 2005 and2019. Deep exposures with the F555W and F814W filters provide us withcolour-magnitude diagrams that reach to an apparent magnitude in V of~25, well below the main-sequence turn-off. These particular clustersare associated with significantly high LMC field star densities and carewas taken to subtract the field stars from the cluster colour-magnitudediagrams accurately. In two cases there is significant variablereddening across at least part of the image, but only for NGC 1916 doesthe differential reddening preclude accurate measurements of the CMDcharacteristics. The morphologies of the colour-magnitude diagrams matchwell those of Galactic globular clusters of similar metallicity. All sixhave well-developed horizontal branches, while four clearly have starson both sides of the RR Lyrae gap. The abundances obtained frommeasurements of the height of the red giant branch above the level ofthe horizontal branch are 0.3 dex higher, on average, than previouslymeasured spectroscopic abundances. Detailed comparisons with Galacticglobular cluster fiducials show that all six clusters are old objects,very similar in age to classical Galactic globulars such as M5, withlittle age spread among the clusters. This result is consistent withages derived by measuring the magnitude difference between thehorizontal branch and main-sequence turn-off. We also find a similarchronology by comparing the horizontal branch morphologies andabundances with the horizontal branch evolutionary tracks of Lee,Demarque & Zinn. Our results imply that the LMC formed at the sametime as the Milky Way Galaxy.
| Star and Cluster Formation in the Large Magellanic Cloud
A great burst of cluster formation increased the rate at which openclusters were formed in the Large Magellanic Cloud 3-5 Gyr ago by atleast an order of magnitude. On the other hand, the rate of starformation ~4 Gyr appears to have increased by a factor of only 2-4. Thisshows that the rate of cluster formation is not a good tracer of therate at which stars are formed. Normalized to the same rate of starformation, the Large Magellanic Cloud is presently forming >~600times more star clusters than the Local Group dwarf irregular IC 1613.The high rate of cluster formation in merging gas-rich galaxies suggeststhat strong shocks might favor the formation of clusters.
| 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.
| A Search for Old Star Clusters in the Large Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
| Core velocity dispersions for 25 Galactic and 10 old Magellanic globular clusters.
We present, for 25 Galactic and 10 old Magellanic globular clusters,projected velocity dispersion (σ_p_) measurements obtained byapplying a cross-correlation technique to integrated-light spectra. Inorder to understand and estimate the statistical errors of thesemeasurements due to small numbers of bright stars dominating theintegrated light, we provide an extensive discussion based on detailednumerical simulations. These errors are smaller if the integration areais larger and/or the cluster concentration higher. The simulations showthat measurements are reliable when the integrated light within theintegration area is brighter than a given magnitude. The statisticalerrors on the σ_p_ measurements of Magellanic globular clustersare small because of a physically large integration area, whereas theycan be important for measurements carried out over small central areasin Galactic clusters. The present observational results are used tooutline a few characteristics of the globular cluster fundamental plane.In this respect, the old Magellanic globular clusters appear similar tothe Galactic clusters.
| Internal dynamics of globular clusters
Galactic globular clusters, which are ancient building blocks of ourGalaxy, represent a very interesting family of stellar systems in whichsome fundamental dynamical processes have taken place on time scalesshorter than the age of the universe. In contrast with galaxies, theseclusters represent unique laboratories for learning about two-bodyrelaxation, mass segregation from equipartition of energy, stellarcollisions, stellar mergers, and core collapse. In the present review,we summarize the tremendous developments, as much theoretical asobservational, that have taken place during the last two decades, andwhich have led to a quantum jump in our understanding of these beautifuldynamical systems.
| Mid-infrared properties of globular clusters using the IRAS data base
We present an analysis of the mid-IR properties of 18 globular clusters(GCs) [15 in the Galaxy and three in the Large Magellanic Cloud (LMC)]using the IRAS photometric data at 12 and 25 mum. Eight of the nineGalactic GCs with central escape velocities greater than 50 km s^-1 haveIRAS sources within a radius of 60 arcsec from the centre, in agreementwith the expectation that interstellar gas and dust should indeed bepresent in the central regions of the most massive clusters owing tomass-loss processes occurring in the late stages of the stellarevolution. No other significant correlation is found between IRAS sourceincidence and any intrinsic GC parameters. Warm dust (T~300K) isdetectable mostly around unresolved giant stars, but in three massiveGCs it is also present as diffuse emission. However, most of the dustmight be cold (T<50K) and it was thus notdetected by IRAS because of its limited sensitivity at 60 and 100 mum.The inferred mass-loss rates and statistical considerations arecompatible with a non-steady mass-loss process with several episodes ofejection lasting a few times 10^5 yr.
| 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.
| Old and Intermediate-Age Stellar Populations in the Magellanic Clouds
The Magellanic Clouds have galactocentric distances of 50 and 63kiloparsecs, making it possible to probe the older populations ofclusters and stars in some detail. Although it is clear that bothgalaxies contain an old population, it is not yet certain whether thispopulation is coeval with the date of formation of the oldest globularsin the Milky Way. The kinematics of this old population in the LargeMagellanic Cloud (LMC) are surprising; no component of this oldpopulation is currently measured to be part of a hot halo supported byvelocity dispersion. Spectroscopy of field stars is beginning to showthe existence of a small population of stars with abundances [Fe/H] lessthan -1.4. These stars will help to unravel the star-formation historywhen the next generation of telescopes are commissioned. Asymptoticgiant branch stars, long-period variables, planetary nebulae, andhorizontal-branch clump stars can be used to trace the extent andkinematics of the intermediate-age population. Deep color-magnitudediagrams can be used to derive the relative proportions of stars olderthan 1 Gyr. The age distribution of populous clusters and theage-metallicity relation are used to compare the evolution of the twoMagellanic Clouds to each other. The issue of where the LMC's metalsoriginated is explored, as is the question of what triggers starformation in the Clouds.
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