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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.
|On the Iron Content of NGC 1978 in the LMC: A Metal-rich, Chemically Homogeneous Cluster|
We present a detailed abundance analysis of giant stars in NGC 1978, amassive, intermediate-age stellar cluster in the Large Magellanic Cloud,characterized by a high ellipticity and suspected to have a metallicityspread. We analyzed 11 giants, all cluster members, by usinghigh-resolution spectra acquired with UVES/FLAMES at the ESO Very LargeTelescope. We find an iron content of [Fe/H] = -0.38 dex with very lowσ[Fe/H]=0.07 dex dispersion, a mean heliocentric radialvelocity vr=293.1+/-0.9 km s-1, and a velocitydispersion σvr=3.1 km s-1, thusexcluding the presence of a significant metallicity, as well asvelocity, spread within the cluster.Based on observations collected at the Very Large Telescope of theEuropean Southern Observatory (ESO), Cerro Paranal, Chile, underprograms 072.D-0342 and 074.D-0369.
|Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters|
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
|RR Lyrae stars in the inner Large Magellanic Cloud: halo-like location with a disk-like distribution|
Aims. The distribution of RR Lyrae stars (RRLS) in the inner LargeMagellanic Cloud (LMC), and the structure of the halo of the LMCdelineated by these stars are studied here. Methods.RRLS identified bythe OGLE II survey are used to estimate their number densitydistribution in the bar region of the LMC. To find their location, Iestimated the scale-height of their distribution in the LMC usingextinction corrected average magnitudes of ab type stars. Results.Thedensity is found to vary differently along and across the bar of theLMC, and the difference is found to be statistically significant. Thedensity distribution is found to be elongated like the LMC bar and theposition angle (PA) of the elongation is estimated to be 112.5 °± 15.3 °. This value of PA is found to be same as the PAmajof the bar, within the errors, estimated using red clump stars andgiants. The ellipticity of their density distribution is estimated to be~0.5, very similar to the ellipticity of the bar, estimated from giants.The above results show that majority of the population of RRLS in thecentral region of the LMC are found to have the signature of the bar.This result could mean that most of these stars are located in the disk,considering the bar as a disk feature. On the other hand, theirscale-height was found to be 3.0 ± 0.9 kpc. This indicates thatRRLS are located in the halo and not in the disk. Conclusions.Thus thesestars in the inner LMC have halo-like location and a disk-like densitydistribution. I discuss some possible formation scenarios for thispuzzling combination.
|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.
|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.
|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.
|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.
|Chemical Abundances in 12 Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy|
High-resolution infrared spectra (λ/Δλ=50,000) havebeen obtained for 12 red giant members of the Large Magellanic Cloud(LMC) with the Gemini South 8.3 m telescope and Phoenix spectrometer.Two wavelength regions, at 15540 and 23400 Å, were observed.Quantitative chemical abundances of carbon (both 12C and13C), nitrogen, and oxygen were derived from molecular linesof CO, CN, and OH, while sodium, scandium, titanium, and iron abundanceswere obtained from neutral atomic lines. The 12 LMC red giants span ametallicity range from [Fe/H]=-1.1 to [Fe/H]=-0.3. It is found thatvalues for both [Na/Fe] and [Ti/Fe] in the LMC giants fall below theircorresponding Galactic values (at these same [Fe/H] abundances) by about~0.1-0.5 dex; this effect is similar to abundance patterns found in thefew dwarf spheroidal galaxies with published abundances. The program redgiants all show evidence of first dredge-up mixing of material exposedto the CN cycle, that is, low 12C/13C ratios andlower 12C with higher 14N abundances. The carbonand nitrogen trends are similar to what is observed in samples ofGalactic red giants, although the LMC red giants seem to show smaller12C/13C ratios for a given stellar mass. Thisrelatively small difference in the carbon isotope ratios between LMC andGalactic red giants could be due to increased extra mixing in stars oflower metallicity, as suggested previously in the literature.Comparisons of the oxygen-to-iron ratios in the LMC and the Galaxyindicate that the trend of [O/Fe] versus [Fe/H] in the LMC falls about0.2 dex below the Galactic trend. Such an offset can be modeled as dueto an overall lower rate of supernovae per unit mass in the LMC relativeto the Galaxy, as well as a slightly lower ratio of supernovae of TypeII to supernovae of Type Ia. Based on observations obtained at theGemini Observatory, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under a cooperative agreement with theNSF on behalf of the Gemini partnership: the National Science Foundation(US), the Particle Physics and Astronomy Research Council (UK), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).
|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.
|Structure of the Large Magellanic Cloud from 2MASS|
We derive structural parameters and evidence for extended tidal debrisfrom star count and preliminary standard candle analyses of the LargeMagellanic Cloud based on Two-Micron All-Sky Survey (2MASS) data. Thefull-sky coverage and low extinction in Ks present an idealsample for structural analysis of the LMC. The star count surfacedensities and deprojected inclination for both young and olderpopulations are generally consistent with previous work. We fit theprojected density with an exponential disk or spherical power-law modelwith an optional two-dimensional elliptical bar. The derived disk scalelength is R=1.42+/-0.01 kpc for models without the bar and R=2.15+/-0.01kpc for models with the bar included. The median value for the bar axisratio is q=3.4 for intermediate and old LMC populations and q=4.1 foryoung populations. The radial profile shows evidence for disk truncationnear 3 kpc. We use the full areal coverage and large ``LMCdiameter''/``Galactocentric'' distance ratio to infer the diskinclination based on perspective. The values for inclination derivedfrom different populations range from 22° to 29°, with thevariance weighted average i=24.0d+/-0.3d. The inclinations derived frommodels including the bar result in a larger estimate, i=38.2d+/-0.4d. Astandard candle analysis based on a sample of carbon long-periodvariables (LPV) in a narrow color range, 1.6
|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 MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud|
We present a 9 million star color-magnitude diagram (9M CMD) of theLarge Magellanic Cloud (LMC) bar. The 9M CMD reveals a complexsuperposition of different-age and -metallicity stellar populations,with important stellar evolutionary phases occurring over 3 orders ofmagnitude in number density. First, we count the nonvariable red andblue supergiants and the associated Cepheid variables and measure thestellar effective temperatures defining the Cepheid instability strip.Lifetime predictions of stellar evolution theory are tested, withimplications for the origin of low-luminosity Cepheids. The highlyevolved asymptotic giant branch (AGB) stars in the 9M CMD have a bimodaldistribution in brightness, which we interpret as discrete oldpopulations (>~1 Gyr). The faint AGB sequence may be metal-poor andvery old. Comparing the mean properties of giant branch andhorizontal-branch (HB) stars in the 9M CMD with those of clusters, weidentify NGC 411 and M3 as templates for the admixture of old stellarpopulations in the bar. However, there are several indications that theold and metal-poor field population has a red HB morphology: the RRLyrae variables lie preferentially on the red edge of the instabilitystrip, the AGB bump is very red, and the ratio of AGB bump stars to RRLyrae variables is quite large. If the HB second parameter is age, theold and metal-poor field population in the bar likely formed after theoldest LMC clusters. Lifetime predictions of stellar evolution theorylead us to associate a significant fraction of the ~1 million red HBclump giants in the 9M CMD with the same old and metal-poor populationproducing the RR Lyrae stars and the AGB bump. In this case, comparedwith the age-dependent luminosity predictions of stellar evolutiontheory, the red HB clump is too bright relative to the RR Lyrae starsand AGB bump. Last, we show that the surface density profile of RR Lyraevariables is fitted by an exponential, favoring a disklike rather than aspheroidal distribution. We conclude that the age of the LMC disk isprobably similar to the age of the Galactic disk.
|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.
|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.
|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.
|A Search for Old Star Clusters in the Large Magellanic Cloud|
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
|The ellipticities of Galactic and Large Magellanic Cloud globular clusters|
The correlations between the ellipticity and the age and mass of LMCglobular clusters are examined, and both are found to be weak. It isconcluded that neither of these properties is mainly responsible for theobserved differences in the LMC and Galactic globular clusterellipticity distributions. Most importantly, age cannot be the primaryfactor in the LMC-Galaxy ellipticity differences, even if there is arelationship, as even the oldest LMC clusters are more elliptical thantheir Galactic counterparts. The strength of the tidal field of theparent galaxy is proposed as the dominant factor in determining theellipticities of that galaxy's globular clusters. A strong tidal fieldrapidly destroys velocity anisotropies in initially triaxial, rapidlyrotating elliptical globular clusters. A weak tidal field, however, isunable to remove these anisotropies and the clusters remain close totheir initial shapes.
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