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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.
| Near-infrared surface brightness fluctuations and optical colours of Magellanic star clusters
This work continues our efforts to calibrate model surface brightnessfluctuation luminosities for the study of unresolved stellarpopulations, through a comparison with the data of Magellanic Cloud starclusters. We present here the relation between absoluteKs-band fluctuation magnitude and (V-I) integrated colour,using data from the Two-Micron All-Sky Survey (2MASS) and the DeepNear-Infrared Southern Sky Survey (DENIS), and from the literature. Wecompare the star cluster sample with the sample of early-type galaxiesand spiral bulges studied by Liu et al. We find that intermediate-age toold star clusters lie along a linear correlation with the same slope,within the errors, of that defined by the galaxies in the versus (V-I)diagram. While the calibration by Liu et al. was determined in thecolour range 1.05 < (V-IC)0 < 1.25, oursholds in the interval . This implies, according to Bruzual-Charlot andMouhcine-Lançon models, that the star clusters and the lateststar formation bursts in the galaxies and bulges constitute an agesequence. At the same time, a slight offset between the galaxies and thestar clusters [the latter are ~0.7 mag fainter than the former at agiven value of (V-I)], caused by the difference in metallicity ofroughly a factor of 2, confirms that the versus (V-I) plane maycontribute to break the age-metallicity degeneracy in intermediate-ageand old stellar populations. The confrontation between models and galaxydata also suggests that galaxies with Ks fluctuationmagnitudes that are brighter than predicted, given their (V-I) colour,might be explained in part by longer lifetimes of thermally pulsingasymptotic giant branch stars. A preliminary comparison between the H2MASS data of the Magellanic star clusters and the sample of 47early-type galaxies and spiral bulges observed by Jensen et al. throughthe F160WHubble Space Telescope filter leads to the same basicconclusions: galaxies and star clusters lie along correlations with thesame slope, and there is a slight offset between the star cluster sampleand the galaxies, caused by their different metallicities. Magellanicstar clusters are single populations, while galaxies are compositestellar systems; moreover, the objects analysed live in differentenvironments. Therefore, our findings mean that the relationship betweenfluctuation magnitudes in the near-infrared, and (V-I) might be a fairlyrobust tool for the study of stellar population ages and metallicities,could provide additional constraints on star formation histories, andaid in the calibration of near-infrared surface brightness fluctuationsfor cosmological distance measurements.
| 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.
| Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects
The properties of ~939 star clusters in the Large and Small MagellanicClouds were determined from ground-based CCD images in UBVR passbands.The areal coverage was extensive, corresponding to 11.0 kpc2in the LMC and 8.3 kpc2 in the SMC. After corrections forreddening, the colors and magnitudes of the clusters were converted toages and masses, and the resulting mass distributions were searched forthe effects of fading, evaporation, and size-of-sample bias. The datashow a clear signature of cluster fading below the detection threshold.The initial cluster mass function (ICMF) was determined by fitting themass and age distributions with cluster population models. These modelssuggest a new method to determine the ICMF that is nearly independent offading or disruption and is based on the slope of a correlation betweenage and the maximum cluster mass in equally spaced intervals of log age.For a nearly uniform star formation rate, this correlation has a slopeequal to 1/(α-1) for an ICMF of dn(M)/dM~M-α. Wedetermine that α is between 2 and 2.4 for the LMC and SMC usingthis method plus another method in which models are fitted to the massdistribution integrated over age and to the age distribution integratedover mass. The maximum mass method also suggests that the clusterformation rate in the LMC age gap between 3 and 13 Gyr is about a factorof 10 below that in the period from 0.1 to 1 Gyr. The oldest clusterscorrespond in age and mass to halo globular clusters in the Milky Way.They do not fit the trends for lower mass clusters but appear to be aseparate population that either had a very high star formation rate andbecame depleted by evaporation or formed with only high masses.
| Near-infrared color evolution of LMC clusters
We present here the digital aperture photometry for 28 LMC clusterswhose ages are between 5 Myr and 12 Gyr. This photometry is based on ourimaging observations in JHK and contains integrated magnitudes andcolors as a function of aperture radius. In contrast to optical colors,our near-infrared colors do not show any strong dependence on clusterages.Tables 2 and 3 and Fig. 2 are only available in electronic form athttp://www.edpsciences.org
| Surface brightness profiles and structural parameters for globular clusters in the Fornax and Sagittarius dwarf spheroidal galaxies
We present radial surface brightness profiles for all five globularclusters in the Fornax dwarf spheroidal galaxy, and for the four presentmembers of the Sagittarius dwarf spheroidal galaxy. These profiles arederived from archival Hubble Space Telescope observations, and have beencalculated using the same techniques with which we measured profiles inour previous studies of Large and Small Magellanic Cloud (LMC and SMC)clusters, apart from some small modifications. From the surfacebrightness profiles, we have determined structural parameters for eachcluster, including core radii and luminosity and mass estimates. We alsoprovide a brief summary of literature measurements of other parametersfor these clusters, including their ages, metallicities and distances.Our core radius measurements are mostly in good agreement with thosefrom previous lower resolution studies, although for several clustersour new values are significantly different. The profile for Fornaxcluster 5 does not appear to be well fitted by a King-type model and wesuggest that it is a post-core-collapse candidate. We examine thedistribution of cluster core radii in each of the two dwarf galaxysystems and compare these with the distribution of core radii for oldLMC clusters. The three distributions match within the limits ofmeasurement errors and the small-sample sizes. We discuss theimplications of this in the context of the radius-age trend we havepreviously highlighted for the Magellanic Cloud clusters.
| Distance to the Large Magellanic Cloud: The RR Lyrae Stars
New photometry and spectroscopy for more than a hundred RR Lyrae starsin two fields located close to the bar of the Large Magellanic Cloud(LMC) are used to derive new accurate estimates of the averagemagnitude, the local reddening, the luminosity-metallicity relation, andthe distance to the LMC. The average apparent luminosity of the RRLyraes with complete V and B light curves is=19.412+/-0.019 (σ=0.153),=19.807+/-0.022 (σ=0.172) in our field A (62 stars)and =19.320+/-0.023 (σ=0.159),=19.680+/-0.024 (σ=0.163) in our field B (46 stars).The average V apparent luminosity of the clump stars in the same areasis 0.108 and 0.029 mag brighter than the RR Lyrae level(=19.304+/-0.002 and 19.291+/-0.003, in fieldA: 6728 stars, and B: 3851 stars, respectively). Metallicities fromlow-resolution spectra obtained with the Very Large Telescope have beenderived for 101 RR Lyrae stars, finding an average value of[Fe/H]=-1.48+/-0.03 (σ=0.29, on the Harris metallicity scale). Anestimate of the reddening within the two fields was obtained (1) fromthe Sturch method applied to the fundamental-mode pulsators (RRab's)with known metal abundance and (2) from the colors of the edges of theinstability strip defined by the full sample of RR Lyrae variable stars.We obtained E(B-V)=0.116+/-0.017 and 0.086+/-0.017 mag in fields A andB, respectively, with a clear-cut indication of a 0.03 mag differentialreddening between the two fields. We find that reddening in field A is0.028 mag smaller than derived by OGLE-II in the same area. On average,the new reddenings are also 0.035 mag larger than derived from Cepheidswith projected distances within 2° from the centers of our fields.The new metallicities were combined with the apparent averageV0 luminosities to determine the slope of theluminosity-metallicity relation for the RR Lyrae stars. We derivedΔMV(RR)/Δ [Fe/H]=0.214+/-0.047, with no clearevidence for the change in slope at [Fe/H]=-1.5, as recently suggestedby evolutionary/pulsation and horizontal-branch models.The dereddened apparent average luminosity of the RR Lyraes defined bythe present photometry is 0=19.064+/-0.064 at[Fe/H]=-1.5. When coupled with the absolute magnitude derived from theBaade-Wesselink and the statistical parallaxes methods(MV(RR)=0.68+/-0.15 and 0.76+/-0.13 mag at [Fe/H]=-1.5), bothmethods known to favor the short distance scale, this value leads todistance moduli for the LMC of μLMC=18.38+/-0.16 andμLMC=18.30+/-0.14, respectively. If we use instead theabsolute magnitude from the new main-sequence fitting of Galacticglobular clusters from Gratton et al. [MV(RR)=0.61+/-0.07 magat [Fe/H]=-1.5], we derive μLMC=18.45+/-0.09.The average I apparent luminosity of the clump stars derived by thepresent photometry is =18.319+/-0.002 and18.307+/-0.003, in field A (σ=0.190, 6728 stars) and B(σ=0.184, 3851 stars), respectively. These values, once correctedfor our new reddening estimates, lead to0=18.12+/-0.06 mag and move the clump distancemodulus to the LMC to 18.42+/-0.07 and 18.45+/-0.07 when Udalski orPopowski metallicity-I luminosity relations for the clump stars areadopted.All these values are only 1 σ shorter than provided by thePopulation I distance indicators and make it possible to reconcile theshort- and long-distance scale on a common value for the distancemodulus of the LMC of μLMC=18.515+/-0.085 mag.Based on observations collected at the European Southern Observatory,proposals 62.N-0802, 66.A-0485, and 68.D-0466.
| 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.
| Constraining the LMC cluster age gap: Washington photometry of NGC 2155 and SL 896 (LW 480)
We carried out Washington system photometry of the intermediate-ageLarge Magellanic Cloud (LMC) star clusters NGC2155 and SL896 (LW480). Wederive ages and metallicities from the T1 versusC-T1 colour-magnitude diagrams (CMDs). For the first time anage has been obtained for SL896, 2.3+/-0.5Gyr. For NGC2155 we derive3.6+/-0.7Gyr. The two clusters basically define the lower age limit ofthe LMC age gap. In particular, NGC2155 is confirmed as the oldestintermediate-age LMC cluster so far studied. The derived metallicitiesare [Fe/H]=-0.9+/-0.2 and -0.6+/-0.2 for NGC2155 and SL896,respectively. We also studied the CMDs of the surrounding fields, whichhave a dominant turn-off comparable to that of the clusters themselves,and similar metallicity, showing that one is dealing with anintermediate-age disc where clusters and field stars have the sameorigin. We inserted the present clusters in the LMC and Small MagellanicCloud (SMC) age-metallicity relations, using a set of homogeneousdeterminations with the same method as in our previous studies, nowtotalling 15 LMC clusters and four SMC clusters, together with someadditional values from the literature. The LMC and SMC age-metallicityrelations appear to be remarkably complementary, since the SMC wasactively star-forming during the LMC quiescent age gap epoch.
| 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.
| The Ages of Globular Clusters
We examine the luminosity levels of the main-sequence turnoffs,MTOv, and horizontal branches, Mv(HB),in 16 globular clusters. An entirely new approach of inferring theluminosity levels by utilizing high-amplitude δ Scuti variables(HADS) is introduced. When the MTOv values arecompared with theoretical values inferred from models, we find all 16clusters (metal-strong to metal-poor) are coeval with an average age of~11.3 Gyr. A considerable scatter of Mv(HB) values ofclusters at similar [Fe/H] values is found. A trend for clusters withblue horizontal branches to have brighter Mv(HB) thanclusters with blue-red horizontal branches is suggested by the data. TheMv(HB) values appear to depend on another or other parametersin addition to the [Fe/H] values. In spite of this problem, we derive anequation relating Mv(HB) values of globular clusters to their[Fe/H] values. We also derive an equation relating theMTOv values of clusters to their [Fe/H] values.Both of these equations can be utilized to find cluster distances. Thedistance modulus of the LMC is found to be 18.66 from the VTOvalues of three LMC globular clusters; RR Lyrae stars in seven globularclusters yield 18.61, and RR Lyrae stars in the LMC bar yield 18.64.
| 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.
| A catalogue of helium abundance indicators from globular cluster photometry
We present a survey of helium abundance indicators derived from acomprehensive study of globular cluster photometry in the literature.For each of the three indicators used, we conduct a thorough erroranalysis, and identify systematic errors in the computationalprocedures. For the population ratio RNHBNRGB, wefind that there is no evidence of a trend with metallicity, althoughthere appears to be real scatter in the values derived. Although thisindicator is the one best able to provide useful absolute heliumabundances, the mean value is Y~0.20, indicating the probable presenceof additional systematic error. For the magnitude difference from thehorizontal branch to the main sequence Δ and the RR Lyraemass-luminosity exponent A, it is only possible to determine relativehelium abundances reliably. This is due to continuing uncertainties inthe absolute metallicity scale for Δ, and uncertainty in the RRLyrae temperature scale for A. Both indicators imply that the heliumabundance is approximately constant as a function of [Fe/H]. Accordingto the A indicator, both Oosterhoff I and II group clusters haveconstant values independent of [Fe/H] and horizontal branch type. Inaddition, the two groups have slopes dlog/d[Fe/H]that are consistent with each other, but significantly smaller than theslope for the combined sample.
| Hubble Space Telescope Observations of the Large Magellanic Cloud Field around SN 1987A: Distance Determination with Red Clump and Tip of the Red Giant Branch Stars
We have used Hubble Space Telescope (HST) WFPC2 multiband observationsof a field around SN 1987A in the Large Magellanic Cloud to measure itsdistance from the Sun. The observations allowed us to carefullydetermine the interstellar extinction along the line of sight to a largenumber of stars and to measure the LMC distance by using two stellardistance indicators: the red clump (RC) and the tip of the red giantbranch (TRGB). From an application of the red clump method, we obtain adistance modulus of(m-M)LMC0,RC=18.59+/-0.04+/-0.08 mag (statisticalplus systematic error), in good agreement with the distance derived byusing the TRGB stars, namely,(m-M)LMC0,TRGB=18.69+/-0.25+/-0.06 mag(statistical plus systematic error). Both values agree well with thedistance to SN 1987A as determined from a study of its inner ringfluorescent echo [(m-M)SN1987A=18.55+/-0.05 mag], thusexcluding distance moduli lower than 18.43 to a 99.7% significancelevel. Differences with respect to previous results obtained using thesame distance indicators are discussed.
| Hubble Space Telescope Observations of the Oldest Star Clusters in the Large Magellanic Cloud
We present V, V-I color-magnitude diagrams (CMDs) for three old starclusters in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257, andHodge 11. Our data extend ~3 mag below the main-sequence turnoff,allowing us to determine accurate relative ages and the blue stragglerfrequencies. Based on a differential comparison of the CMDs, any agedifference between the three LMC clusters is less than 1.5 Gyr.Comparing their CMDs to those of M92 and M3, the LMC clusters, unlesstheir published metallicities are significantly in error, are the sameage as the old Galactic globulars. The similar ages to Galacticglobulars are shown to be consistent with hierarchial clustering modelsof galaxy formation. The blue straggler frequencies are also similar tothose of Galactic globular clusters. We derive a true distance modulusto the LMC of (m-M)0=18.46+/-0.09 [assuming(m-M)0=14.61 for M92] using these three LMC clusters.
| The absolute magnitudes of RR Lyrae stars from BT HIPPARCOS parallaxes parallaxes
Using the method of ``reduced parallaxes'' for the Halo RR Lyrae starsin the hipparcos catalogue we derive a zero point of 0.77 +/- 0.26 magfor an assumed slope of 0.18 in the M_V-[Fe/H] relation. This is 0.28magnitude brighter than the value Fernley et al. (1998a) derived byemploying the method of statistical parallax for the identical sampleand using the same slope. We point out that a similar difference existsbetween the ``reduced parallaxes'' method and the statistical parallaxmethod for the Cepheids in the hipparcos catalogue. We also determinethe zero point for the M_K-log P0 relation, and obtain avalue of -1.16 +/- 0.27 mag (for a slope of -2.33). The distance modulito the hipparcos RR Lyrae stars derived from the two relations agreewell. The derived distance scale is in good agreement with the resultsfrom the Main Sequence fitting distances of Galactic globular clustersand with the results of theoretical Horizontal Branch models, andimplies a distance modulus to the LMC of 18.61 +/- 0.28 mag. Based ondata from the ESA Hipparcos astrometry satellite.}
| 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.
| 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.
| WFPC2 Observations of Star Clusters in the Magellanic Clouds. II. The Oldest Star Clusters in the Small Magellanic Cloud
We present our analysis of archival Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) observations in F450W (~B) and F555W (~V) ofthe intermediate-age populous star clusters NGC 121, NGC 339, NGC 361,NGC 416, and Kron 3 in the Small Magellanic Cloud. We use publishedphotometry of two other SMC populous star clusters, Lindsay 1 andLindsay 113, to investigate the age sequence of these seven starclusters in order to improve our understanding of the formationchronology of the SMC. We analyzed the V versus B-V and M_V versus(B-V)_0 color-magnitude diagrams of these populous Small MagellanicCloud star clusters using a variety of techniques and determined theirages, metallicities, and reddenings. These new data enable us to improvethe age-metallicity relation of star clusters in the Small MagellanicCloud. In particular, we find that a closed-box continuous starformation model does not reproduce the age-metallicity relationadequately. However, a theoretical model punctuated by bursts of starformation is in better agreement with the observational data presentedherein.
| 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.
| Optical Gravitational Lensing Experiment. Distance to the Magellanic Clouds with the Red Clump Stars: Are the Magellanic Clouds 15% Closer than Generally Accepted?
We present a new distance determination to the Large and SmallMagellanic Clouds using the newly developed red clump stars method(Paczynski and Stanek 1998). This new, single-step, Hipparcos calibratedmethod seems to be one of the most precise techniques of distancedetermination with very small statistical error due to large number ofred clump stars usually available. The distances were determinedindependently along four lines-of-sight located at opposite sides ofeach Magellanic Cloud. The results for each line-of-sight are veryconsistent. For the SMC we obtain the distance modulus:m-M=18.56+/-0.03+/-0.06 mag (statistical and systematic errors,respectively) and for the LMC: m-M=18.08+/-0.03+/-0.12 mag wheresystematic errors are mostly due to uncertainty in reddening estimates.Both distances will be refined and systematic errors reduced whenaccurate reddening maps for our fields are available. Distance moduli toboth Magellanic Clouds are approx 0.4 mag smaller than generallyaccepted values. The modulus to the LMC is in good agreement withtherecent determinations from RR Lyrae type stars and upper limit resultingfrom the SN1987A echo. We suspect that the distance to the LMC and SMCis shorter by about 15% than previously assumed: 42 kpc and 52 kpc,respectively. Calibrations of the period-luminosity relation forCepheids which give overestimated distances to the LMC and SMC areprobably incorrect and require urgent reanalysis. We also present ourcolor-magnitude diagrams around the red clump for the LMC and SMC. Weidentify vertical red clump, first noted by Zaritsky and Lin (1997),inthe color-magnitude diagram of both Magellanic Clouds and we interpretit as an evolutionary feature rather than unknown stellar populationbetween the LMC and our Galaxy.
| 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.
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