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On the Age and Metallicity Estimation of Spiral Galaxies Using Optical and Near-Infrared Photometry
In integrated light, some color-color diagrams that use optical andnear-infrared photometry show surprisingly orthogonal grids as age andmetallicity are varied, and they are coming into common usage forestimating the average age and metallicity of spiral galaxies. In thispaper we reconstruct these composite grids using simple stellarpopulation models from several different groups convolved with someplausible functional forms of star formation histories at fixedmetallicity. We find that the youngest populations present (t<2 Gyr)dominate the light, and because of their presence the age-metallicitydegeneracy can be partially broken with broadband colors, unlike olderpopulations. The scatter among simple stellar population models bydifferent authors is, however, large at ages t<2 Gyr. The dominantuncertainties in stellar population models arise from convective coreovershoot assumptions and the treatment of the thermally pulsingasymptotic giant branch phase and helium abundance may play asignificant role at higher metallicities. Real spiral galaxies areunlikely to have smooth, exponential star formation histories, andburstiness will cause a partial reversion to the single-burst case,which has even larger model-to-model scatter. Finally, it is emphasizedthat the current composite stellar population models need someimplementation of chemical enrichment histories for the proper analysisof the observational data.
| Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| Physical parameters of 15 intermediate-age LMC clusters from modelling of HST colour-magnitude diagrams
Aims.We analyzed HST/WFPC2 colour-magnitude diagrams (CMDs) of 15populous Large Magellanic Cloud (LMC) stellar clusters with ages between~0.3 Gyr and ~3 Gyr. These (V, B-V) CMDs are photometrically homogeneousand typically reach V ˜ 22. Accurate and self-consistent physicalparameters (age, metallicity, distance modulus and reddening) wereextracted for each cluster by comparing the observed CMDs with syntheticones. Methods: These determinations involved simultaneous statisticalcomparisons of the main-sequence fiducial line and the red clumpposition, offering objective and robust criteria to determine the bestmodels. The models explored a regular grid in the parameter spacecovered by previous results found in the literature. Control experimentswere used to test our approach and to quantify formal uncertainties. Results: In general, the best models show a satisfactory fit to thedata, constraining well the physical parameters of each cluster. Theage-metallicity relation derived by us presents a lower spread thansimilar results found in the literature for the same clusters. Ourresults are in accordance with the published ages for the oldestclusters, but reveal a possible underestimation of ages by previousauthors for the youngest clusters. Our metallicity results in generalagree with the ones based on spectroscopy of giant stars and with recentworks involving CMD analyses. The derived distance moduli implied by themost reliable solutions, correlate with the reddening values, asexpected from the non-negligible three-dimensional distribution of theclusters within the LMC. Conclusions: .The inferred spatialdistribution for these clusters is roughly aligned with the LMC disk,being also more scattered than recent numerical predictions, indicatingthat they were not formed in the LMC disk. The set of ages andmetallicities homogeneously derived here can be used to calibrateintegrated light studies applied to distant galaxies.
| Red Giant Stars in the Large Magellanic Cloud Clusters
We present deep J, H, and Ks photometry and accurate colormagnitude diagrams down to K~18.5 for a sample of 13 globular clustersin the Large Magellanic Cloud. This data set combined with the previoussample of six clusters published by our group gives the opportunity tostudy the properties of giant stars in clusters with different ages(ranging from ~80 Myr up to 3.5 Gyr). Quantitative estimates of starpopulation ratios (by number and luminosity) in the asymptotic giantbranch (AGB), the red giant branch (RGB), and the He clump have beenobtained and compared with theoretical models in the framework ofprobing the so-called phase transitions. The AGB contribution to thetotal luminosity starts to be significant at ~200 Myr and reaches itsmaximum at 500-600 Myr, when the RGB phase transition is starting. At~900 Myr the full development of an extended and well-populated RGB hasbeen completed. The occurrences of both the AGB and RGB phasetransitions are sharp events, lasting a few hundred megayears only.These empirical results agree very well with the theoretical predictionsof simple stellar population models based on canonical tracks and thefuel-consumption approach.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m NTT, within the observing programs64.N-0038 and 68.D-0287.
| 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.
| Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds
We present accurate integrated-light photometry in Johnson/Cousins V, Rand I for a sample of 28 globular clusters in the Magellanic Clouds. Themajority of the clusters in our sample have reliable age and metallicityestimates available in the literature. The sample encompasses agesbetween 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between -1.5 and0.0 dex. The sample is dominated by clusters of ages between roughly 0.5and 2 Gyr, an age range during which the bolometric luminosity of simplestellar populations is dominated by evolved red giant branch stars andthermally pulsing asymptotic giant branch (TP-AGB) stars whosetheoretical colours are rather uncertain. The VRI colours presented inthis paper have been used to calibrate stellar population synthesismodel predictions.
| Mass segregation in rich LMC clusters from modelling of deep HST colour-magnitude diagrams
Aims.We used the deep colour-magnitude diagrams (CMDs) of five rich LMCclusters (NGC 1805, NGC 1818, NGC 1831, NGC 1868, and Hodge 14) observedwith HST/WFPC2 to derive their present day mass function (PDMF) and itsvariation with position within the cluster. Methods: .The PDMFwas parameterized as a power law in the available main-sequence massrange of each cluster, typically 0.9 ⪉ m/Mȯ ⪉2.5; its slope was determined at different positions spanning from thevery centre out to several core radii. The CMDs in the central regionsof the clusters were carefully studied earlier, resulting in accurateage, metallicity, distance modulus, and reddening values. The slopeα (where Salpeter is 2.35) was determined in annuli by followingtwo distinct methods: 1) a power law fit to the PDMF obtained from thesystemic luminosity function (LF); 2) a statistical comparison betweenobserved and model CMDs. In the second case, α is a free inputparameter in the CMD modelling process where we incorporate photometricerrors and the effect of binarity as a fraction of unresolved binaries(f{bin}=100%) with random pairing of masses from the samePDMF. Results: .In all clusters, significant mass segregation isfound from the positional dependence of the PDMF slope: α ⪉1.8 for R ≤ 1.0 R{core} and α ˜ Salpeterinside R=2 ˜ 3 R{core} (except for Hodge 14, whereα ˜ Salpeter for R ˜ 4 R{core}). Theresults are robust in the sense that they hold true for both methodsused. The CMD method reveals that unresolved binaries flatten the PDMFobtained form the systemic LF, but this effect is smaller than theuncertainties in the α determination. For each cluster weestimated dynamical ages inside the core and for the entire system. Inboth cases we found a trend in the sense that older clusters haveflatter PDMF, consistent with a dynamical mass segregation and stellarevaporation.
| Core Radius-Mass Evolution of Globular Clusters
Some dynamical features of present day globular clusters seem to be theresult of the effects produced at the epoch of formation, both by therate of primordial binary stars and the formation and destruction of newones. Even a mass segregation and a cluster evaporation driven by thepopulation of binary stars are possible. The spread in the core radiusamong intermediate age and old stars clusters, observed e.g. in the LMC,could be generated by these two effects. In this contribution somepreliminary results are shown.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| New Optical and Near-Infrared Surface Brightness Fluctuation Models. II. Young and Intermediate-Age Stellar Populations
We present theoretical surface brightness fluctuation (SBF) amplitudesfor single-burst stellar populations of young and intermediate age (25Myr<=t<=5 Gyr) and metallicities Z=0.0003, 0.001, 0.004, 0.008,0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expectedin the Johnson-Cousins (UBVRIJHK) photometric system are provided. Wepay attention to the contribution of thermally pulsating asymptoticgiant branch (TP-AGB) stars. The sensitivity of the predicted SBF tochanges in the mass-loss scenario along the TP-AGB phase is examined.Below 0.6-1 Gyr both optical and near-IR SBF models exhibit a strongdependence on age and mass loss. We also evaluate SBF amplitudes usingMonte Carlo techniques to reproduce the random variation in the numberof stars experiencing bright and fast evolutionary phases (red giantbranch, AGB, TP-AGB). On these grounds we provide constraints on thefaintest integrated flux of real stellar populations required to derivereliable and meaningful SBF measurements. We analyze a technique forderiving SBF amplitudes of star clusters from the photometry ofindividual stars and estimate the uncertainty due to statisticaleffects, which may impinge on the procedure. The first optical SBFmeasurements for 11 Large Magellanic Cloud (LMC) star-rich clusters-withages ranging from a few megayears to several gigayears-are derived usingHubble Space Telescope observations. The measurements are compared toour SBF predictions, providing a good agreement with models ofmetallicity Z=0.0001-0.01. Our results suggest that, for TP-AGB stars, amass loss as a power-law function of the star luminosity is required inorder to properly reproduce the optical SBF data of the LMC clusters.Finally, near-IR models have been compared to available data, thusshowing that the general trend is well fitted. We suggest how toovercome the general problem of SBF models in reproducing the details ofthe near-IR SBF measurements of the Magellanic Cloud star clusters.
| Dust-enshrouded giants in clusters in the Magellanic Clouds
We present the results of an investigation of post-Main Sequence massloss from stars in clusters in the Magellanic Clouds, based around animaging survey in the L'-band (3.8 μm) performed with the VLT at ESO.The data are complemented with JHKs (ESO and 2MASS) andmid-IR photometry (TIMMI2 at ESO, ISOCAM on-board ISO, and data fromIRAS and MSX). The goal is to determine the influence of initialmetallicity and initial mass on the mass loss and evolution during thelatest stages of stellar evolution. Dust-enshrouded giants areidentified by their reddened near-IR colours and thermal-IR dust excessemission. Most of these objects are Asymptotic Giant Branch (AGB) carbonstars in intermediate-age clusters, with progenitor masses between 1.3and ~5 M_ȯ. Red supergiants with circumstellar dust envelopes arefound in young clusters, and have progenitor masses between 13 and 20M_ȯ. Post-AGB objects (e.g., Planetary Nebulae) and massive starswith detached envelopes and/or hot central stars are found in severalclusters. We model the spectral energy distributions of the cluster IRobjects, in order to estimate their bolometric luminosities andmass-loss rates. The IR objects are the most luminous cluster objects,and have luminosities as expected for their initial mass andmetallicity. They experience mass-loss rates in the range from a few10-6 up to 10-4 M_ȯ yr-1 (ormore), with most of the spread being due to evolutionary effects andonly a weak dependence on progenitor mass and/or initial metallicity.About half of the mass lost by 1.3-3 M_ȯ stars is shed during thesuperwind phase, which lasts of order 105 yr. Objects withdetached shells are found to have experienced the highest mass-lossrates, and are therefore interpreted as post-superwind objects. We alsopropose a simple method to measure the cluster mass from L'-band images.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| Physical parameters of rich LMC clusters from modeling of deep HST colour-magnitude diagrams
We present the analysis of deep colour-magnitude diagrams (CMDs) of fiverich LMC clusters. The data were obtained with HST/WFPC2 in the F555W(~V) and F814W (~I) filters, reaching V555 ˜ 25. Thesample of clusters is composed of NGC 1805 and NGC 1818, the youngestones (τ < 100 Myr), NGC 1831 and NGC 1868, of intermediate-age (400 < τ < 1000 Myr), and Hodge 14, the oldest (τ > 1200Myr). We discuss and apply a statistical method for correcting the CMDfor sampling incompleteness and field star contamination. Efficient useof the CMD data was made by means of direct comparisons of the observedto model CMDs. The CMD modeling process generates a synthetic MainSequence (MS), where we introduce as model inputs the information aboutage, chemical composition, present day mass function (PDMF), fraction ofunresolved binaries, distance modulus and light extinction. Thephotometric uncertainties were empirically determined from the data andincorporated into the model as well. Statistical techniques of CMDcomparisons using 1 and 2 dimensions are presented and applied as anobjective method to assess the compatibility between model and dataCMDs. By modeling the CMDs from the central region we infer themetallicity (Z), the intrinsic distance modulus ((m-M)0) andthe reddening value (E(B-V)) for each cluster. We also determined theage for the clusters with τ > 400 Myr. By means oftwo-dimensional CMD comparisons we infer the following values: for NGC1805, Z=0.007 ± 0.003, (m-M)0=18.50 ± 0.11,E(B-V)=0.03 ± 0.01; for NGC 1818, Z=0.005 ± 0.002,(m-M)0=18.49 ± 0.14, E(B-V) ˜ 0.00; for NGC1831, Z=0.012 ± 0.002, log(τ/yr)=8.70 ± 0.03,(m-M)0=18.70 ± 0.03, E(B-V) ˜ 0.00; for NGC1868, Z=0.008 ± 0.002, log(τ/yr)=8.95 ± 0.03,(m-M)0=18.70 ± 0.03, E(B-V) ˜ 0.00; for Hodge14, Z=0.008 ± 0.004, log(τ/yr)=9.23 ± 0.10,(m-M)0=18.51 ± 0.13, E(B-V)=0.02 ± 0.02. Takinginto account the uncertainties, these values are in accordance with theones obtained applying the one-dimensional CMD analysis, addingreliability to these determinations.
| Evolutionary stellar population synthesis at high spectral resolution: optical wavelengths
We present the single stellar population (SSP) synthesis results of ournew synthetic stellar atmosphere models library with a spectral samplingof 0.3 Å, covering the wavelength range from 3000 to 7000Åfor a wide range of metallicities (twice solar, solar, half solarand 1/10 solar). The stellar library is composed of 1650 spectracomputed with the latest improvements in stellar atmospheres. Inparticular, it incorporates non-local thermodynamic equilibrium (LTE)line-blanketed models for hot (Teff>= 27500 K), and LTEline-blanketed models (Phoenix) for cool (3000 <=Teff<=4500 K) stars. Because of the high spectral resolution of this library,evolutionary synthesis models can be used to predict the strength ofnumerous weak absorption lines and the evolution of the profiles of thestrongest lines over a wide range of ages. The SSP results have beencalculated for ages from 1 Myr to 17 Gyr using the stellar evolutionarytracks provided by the Geneva and Padova groups. For young stellarpopulations, our results have a very detailed coverage ofhigh-temperature stars with similar results for the Padova and Genevaisochrones. For intermediate and old stellar populations, our results,once degraded to a lower resolution, are similar to the ones obtained byother groups (limitations imposed by the stellar evolutionary physicsnotwidthstanding). The limitations and advantages of our models for theanalysis of integrated populations are described. The full set of thestellar library and the evolutionary models are available for retrievalat the websites http://www.iaa.csic.es/~rosa andhttp://www.iaa.csic.es/~mcs/sed@, or on request from the first twoauthors.
| Constraints on the star formation history of the Large Magellanic Cloud
We present the analysis of deep colour-magnitude diagrams (CMDs) of 6stellar fields in the LMC. The data were obtained using HST/WFPC2 in theF814W (˜I) and F555W (˜V) filters, reaching V555˜ 26.5. We discuss and apply a method of correcting CMDs forphotometric incompleteness. A method to generate artificial CMDs basedon a model star formation history is also developed. This methodincorporates photometric error effects, unresolved binaries, reddeningand allows use of different forms of the initial mass function and ofthe SFH itself. We use the Partial Models Method, as presented byGallart and others, for CMD modelling, and include control experimentsto prove its validity in a search for constraints on the LargeMagellanic Cloud star formation history in different regions. Reliablestar formation histories for each field are recovered by this method. Inall fields, a gap in star formation with τ ˜ 700 Myr isobserved. Field-to-field variations have also been observed. The twofields near the LMC bar present some significant star forming events,having formed both young (τ ⪉ 1 Gyr) and old (τ ⪆ 10Gyr) stars, with a clear gap from 3-6 Gyr. Two other fields displayquite similar SFHs, with increased star formation having taken place atτ ≃ 2-3 Gyr and 6 ⪉ τ ⪉ 10 Gyr. The remaining twofields present star formation histories closer to uniform, with no clearevent of enhanced star formation.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| 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.
| A review of the distance and structure of the Large Magellanic Cloud
The average of 14 recent measurements of the distance to the LargeMagellanic Cloud (LMC) implies a true modulus of 18.50+/-0.02 mag, anddemonstrates a trend in the past 2 years of convergence toward astandard value. The distance indicators reviewed are the red clump, thetip of the red giant branch, Cepheid, RR Lyrae, and Mira variable stars,cluster main-sequence fitting, supernova 1987A, and eclipsing binaries.The eclipsing binaries yield a consistent distance on average; however,the internal scatter is twice as large as the average measurement error.I discuss parameters of LMC structure that pertain to distanceindicators, and speculate that warps discovered using the color of theclump are not really warps.
| 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.
| Analyzing Starbursts Using Magellanic Cloud Star Clusters as Simple Stellar Populations
Integrated spectra have been obtained of 31 star clusters in theMagellanic Clouds (MC) and of four Galactic globular clusters. Thespectra cover the wavelength range 3500-4700 Å at a resolution of3.2 Å FWHM. The MC clusters primarily cover the age range fromless than 108 to about 3 Gyr and hence are well-suited to anempirical study of aging poststarburst stellar populations. Anage-dating method is presented that relies on two spectral absorptionfeature indices, Hδ/Fe I λ4045 and Ca II, as well as anindex measuring the strength of the Balmer discontinuity. We compare thebehavior of the spectral indices in the observed integrated spectra ofthe MC clusters with that of indices generated from theoreticalevolutionary synthesis models of varying age and metal abundance. Thesynthesis models are based on those of Worthey, when coupled with thecombination of an empirical library of stellar spectra by Jones for thecooler stars and synthetic spectra, generated from Kurucz modelatmospheres, for the hotter stars. Overall, we find good agreementbetween the ages of the MC clusters derived from our integrated spectra(and the evolutionary synthesis modelling of the spectral indices) andages derived from analyses of the cluster color-magnitude diagrams, asfound in the literature. Hence, the principal conclusion of this studyis that ages of young stellar populations can be reliably measured frommodelling of their integrated spectra.
| 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
| Core radius evolution of star clusters
We use N-body simulations of star clusters to investigate the possibledynamical origins of the observed spread in core radius amongintermediate-age and old star clusters in the Large Magellanic Cloud(LMC). Two effects are considered, a time-varying external tidal fieldand variations in primordial hard binary fraction. Simulations ofclusters orbiting a point-mass galaxy show similar core radius evolutionfor clusters on both circular and elliptical orbits and we thereforeconclude that the tidal field of the LMC has not yet significantlyinfluenced the evolution of the intermediate-age clusters. The presenceof large numbers of hard primordial binaries in a cluster leads to coreradius expansion; however, the magnitude of the effect is insufficientto explain the observations. Furthermore, the range of binary fractionsrequired to produce significant core radius growth is inconsistent withthe observational evidence that all the LMC clusters have similarstellar luminosity functions.
| 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.
| Mass segregation in young compact clusters in the Large Magellanic Cloud - III. Implications for the initial mass function
The distribution of core radii of rich clusters in the Large MagellanicCloud (LMC) systematically increases in both upper limit and spread withincreasing cluster age. Cluster-to-cluster variations in the stellarinitial mass function (IMF) have been suggested as an explanation. Wediscuss the implications of the observed degree of mass segregation inour sample clusters for the shape of the initial mass function. Ourresults are based on Hubble Space Telescope/WFPC2 observations of sixrich star clusters in the LMC, selected to include three pairs ofclusters of similar age, metallicity and distance from the LMC centre,and exhibiting a large spread in core radii between the clusters in eachpair. All clusters show clear evidence of mass segregation: (i) theirluminosity function slopes steepen with increasing cluster radius, and(ii) the brighter stars are characterized by smaller core radii. For allsample clusters, both the slope of the luminosity function in thecluster centres and the degree of mass segregation are similar to eachother, within observational errors of a few tenths of power-law slopefits to the data. This implies that their initial mass functions musthave been very similar, down to ~0.8-1.0 Msolar. We thereforerule out variations in the IMF of the individual sample clusters as themain driver of the increasing spread of cluster core radii with clusterage.
| On the nature of a secondary main-sequence turn-off in the rich LMC cluster NGC 1868
Evidence for a second main-sequence turn-off in a deep colour-magnitudediagram (CMD) of NGC 1868 is presented. The data were obtained withHubble Space Telescope (HST) WFPC2 and reach down to m555~=25 mag. Besides the usual τ~= 0.8 Gyr turn-off found in previousanalyses, another possible turn-off is seen at m555~= 21 mag(MV~= 2.5), which is consistent with an age of τ~= 3 Gyr.This CMD feature stands out clearly especially when contaminating fieldLarge Magellanic Cloud (LMC) stars are statistically removed. Thebackground subtracted CMD also visibly displays a red subgiant branchextending about 1.5 mag below the younger turn-off and the clump of redgiants. The significance of the secondary turn-off in NGC 1868 wasconfirmed with Monte Carlo simulations and bootstrapping techniques.Star counts in selected regions in the cluster CMD indicate a mass ratioof old population/young population in the range 5<~Mold/Myoung<~ 12 per cent, depending onthe mass-function slope. The existence of such a subpopulation in NGC1868 is significant even in the presence of uncertainties in backgroundsubtraction. The possibility that the secondary turn-off is associatedwith the field star population was examined by searching for similarfeatures in CMDs of field stars. Statistically significant excesses ofstars redwards of the main sequence were found in all such fields in therange 20 <~m555<~ 22 mag. These however are muchbroader features that do not resemble the main-sequence termination of asingle population. We also discuss other alternative explanations forthe feature at m555~= 21 mag, such as unresolved binarism,peculiar stars or CMD discontinuities associated with theBöhm-Vitense gap.
| The Dwarf Irregular/Wolf-Rayet Galaxy NGC 4214. I. A New Distance, Stellar Content, and Global Parameters
We present the results of a detailed optical and near-IR study of thenearby star-forming dwarf galaxy NGC 4214. We discuss the stellarcontent, drawing particular attention to the intermediate-age and/or oldfield stars, which are used as a distance indicator. On images obtainedwith the Hubble Space Telescope Wide Field Planetary Camera 2 andNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) instrumentsin the equivalents of the V, R, I, J and H bands, the galaxy is wellresolved into stars. We achieve limiting magnitudes of F814W~27 in theWF chips and F110W~25 in the NICMOS 2 camera. The optical andnear-infrared color-magnitude diagrams confirm a core-halo galaxymorphology: an inner, high surface brightness, young population within~1.5′ (~1 kpc) from the center of the galaxy, where the stars areconcentrated in bright complexes along a barlike structure, and arelatively low surface brightness, field star population extending outto at least 8' (7 kpc). The color-magnitude diagrams of the core regionshow evidence of blue and red supergiants, main-sequence stars,asymptotic giant branch stars, and blue loop stars. We identify somecandidate carbon stars from their extreme near-IR color. The field-starpopulation is dominated by the ``red tangle,'' which contains the redgiant branch. We use the I-band luminosity function to determine thedistance based on the tip of the red giant branch method: 2.7+/-0.3 Mpc.This is much closer than the values usually assumed in the literature,and we provide revised distance-dependent parameters such as physicalsize, luminosity, H I mass, and star formation rate. From the mean colorof the red giant branch in V and I, we estimate the mean metal abundanceof this population to be [Fe/H]~=-1.7 dex, with a large internalabundance spread characterized by σint([Fe/H])~1 dex.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| Analysis of colour-magnitude diagrams of rich LMC clusters: NGC 1831
We present the analysis of a deep colour-magnitude diagram (CMD) of NGC1831, a rich star cluster in the LMC. The data were obtained withHST/WFPC2 in the F555W ( ~ V) and F814W ( ~ I) filters, reachingm555 ~ 25. We discuss and apply a method of correcting theCMD for sampling incompleteness and field star contamination. Efficientuse of the CMD data was made by means of direct comparisons of theobserved to model CMDs. The model CMDs are built by an algorithm thatgenerates artificial stars from a single stellar population,characterized by an age, a metallicity, a distance, a reddening value, apresent day mass function and a fraction of unresolved binaries.Photometric uncertainties are empirically determined from the data andincorporated into the models as well. Statistical techniques arepresented and applied as an objective method to assess the compatibilitybetween the model and data CMDs. By modelling the CMD of the centralregion in NGC 1831 we infer a metallicity Z = 0.012, 8.75 <= log (tau/yr) <= 8.80, 18.54 <= (m-M)0 <= 18.68 and 0.00<= E(B-V) <= 0.03. For the position dependent PDMF slope ( alpha =-dlog Phi (M)/dlog M), we clearly observe the effect of mass segregationin the system: for projected distances R <= 30 arcsec, alpha =~ 1.7,whereas 2.2 <= alpha <= 2.5 in the outer regions of NGC 1831.
| A Large and Homogeneous Sample of CMDs of LMC Stellar Clusters
We present the photometric results of 21 stellar clusters of the LargeMagellanic Cloud. The WFPC2 images were retrieved from the HST archive.Simple stellar populations in a large spread of age are well representedin the sample of color-magnitude diagrams shown here.
| Deep colour-magnitude diagrams of LMC field stars imaged with HST
We present deep photometry (V<~26) in V and I bands obtained with theWide Field and Planetary Camera 2 on board the Hubble Space Telescopefor 7 fields ~5° away from the Large Magellanic Cloud centre. Thefields contain, typically, 2000 stars each. Isochrones were fitted tothe colour-magnitude diagrams in order to identify different starpopulations in these fields. An old population (τ>10Gyr) has beenfound in all fields. Some events of enhanced star formation, with agesbetween 2 and 4Gyr, were identified in the fields localized in the northto north-west regions. Luminosity functions of low-mass stars were alsoobtained for all fields. Kolmogorov Smirnov test results suggestdifferences smaller than 30 per cent in the mixture of stellarpopulations contributing to the fields. Finally, density profiles werederived for old and intermediate-age stars. The former shows a slightlysteeper decline than the latter.
| Large Magellanic Cloud stellar clusters. I. 21 HST colour magnitude diagrams
We present WFPC2 photometry of 21 stellar clusters of the LargeMagellanic Cloud obtained on images retrieved from the Hubble SpaceTelescope archive. The derived colour magnitude diagrams (CMDs) arepresented and discussed. This database provides a sample of CMDsrepresenting, with reliable statistics, simple stellar populations witha large spread of age. The stars in the core of the clusters are allresolved and measured at least down to the completeness limit; themagnitudes of the main sequence terminations and of the red giant clumpare also evaluated for each cluster, together with the radius at halfmaximum of the star density. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Institute. STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. Table 1 is only available in electronic form athttp://www.edpsciences.org
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