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Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies
We find that the globular cluster (GC) systems of the Milky Way and ofour neighboring spiral galaxy, M31, comprise two distinct entities,differing in three respects. First, M31 has a set of young GCs, rangingin age from a few times 102 Myr to 5 Gyr old, as well as oldGCs. No such very young GCs are known in the Milky Way. Second, weconfirm that the oldest M31 GCs have much higher nitrogen abundancesthan do Galactic GCs at equivalent metallicities, while carbonabundances appear normal for the GCs in both galaxies. Third, Morrisonand coworkers have shown that M31 has a subcomponent of GCs that followclosely the disk rotation curve of that galaxy. Such a GC system in ourown Galaxy has yet to be found. The only plausible scenario for theexistence of the young M31 GC comes from thehierarchical-clustering-merging (HCM) paradigm for galaxy formation. Weinfer that M31 has absorbed more of its contingent of dwarf systems inthe recent past than has the Milky Way. This inference has threeimplications: First, all spiral galaxies could differ in their GCproperties, depending on how many companions each galaxy has and whenthe parent galaxy absorbs them. In this spectrum of possibilities,apparently the Milky Way ties down one end, in which almost all of itsGCs were absorbed 10-12 Gyr ago. Second, it suggests that young GCs arepreferentially formed in the dwarf companions of parent galaxies andthen absorbed by the parent galaxy during mergers. Third, the young GCsseen in tidally interacting galaxies might come from the dwarfcompanions of these galaxies, rather than be made anew in the tidalinteraction. However, there is no ready explanation for the markeddifference in nitrogen abundance for the stars in the old M31 GCsrelative to those in the oldest Galactic GCs, especially the mostmetal-poor GCs in both galaxies. The predictions made by Li &Burstein regarding the origin of nitrogen abundance in GCs areconsistent with what is found for the old M31 GCs compared to that forthe two 5 Gyr old M31 GCs.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.
| 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.
| 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.
| Feedback Heating in Cluster and Galactic Cooling Flows
Cluster cooling flow models that include both active galactic nuclei(AGN) heating and thermal conduction can reduce the overall mass coolingrate and simultaneously sustain density and temperature profiles similarto those observed. These computed flows have no ad hoc mass dropout. Toachieve this agreement, the thermal conductivity must be about0.35+/-0.10 times the Spitzer value, similar to that advocated byNarayan & Medvedev. However, when applied to galaxy/group scales,the synergistic combination of AGN heating and conduction is lesssatisfactory. When the computed density profile and the global coolingrate are lowered by AGN heating to match observations of these smallerscale flows, the gas temperatures within ~10 kpc are too large. Inaddition, best-fitting flows in galaxy/groups with AGN heating andthermal conduction require conductivities much closer to the Spitzervalue, ~0.5-1. Another difficulty with galaxy/group flows that combineAGN heating and conduction is that the iron enrichment by Type Iasupernovae is more effective when the gas density is lowered by heatingto match the observations. The hot-gas iron abundance in galactic flowswith heating and conduction greatly exceeds observed values throughoutmost of the galaxy. Galactic/group flows with central heating andconduction, therefore, require an additional process that removes theiron: failure of Type Ia supernovae ejecta to go into the hot phase,selective cooling, etc.
| 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.
| A catalogue of Mg_2 indices of galaxies and globular clusters
We present a catalogue of published absorption-line Mg_2 indices ofgalaxies and globular clusters. The catalogue is maintained up-to-datein the HYPERCAT database. The measurements are listed together with thereferences to the articles where the data were published. A codeddescription of the observations is provided. The catalogue gathers 3541measurements for 1491 objects (galaxies or globular clusters) from 55datasets. Compiled raw data for 1060 galaxies are zero-point correctedand transformed to a homogeneous system. Tables 1, 3, and 4 areavailable in electronic form only at the CDS, Strasbourg, via anonymousftp 130.79.128.5. Table 2 is available both in text and electronic form.
| Age and metallicity of star clusters in the Magellanic Clouds
Spectral indices as defined by the Lick group (Hβ , Mg_2, Fe5270,Fe5335 and NaD) were measured for a sample of 14 clusters in theMagellanic Clouds. Iron and hydrogen indices, together with theintegrated colours (B-V) and (V-K) for these clusters, were used toestimate their ages and metallicities, using calibrations based onsingle stellar population models. The resulting ages and metallicitiesare in agreement with ages estimated from colour-magnitude diagrams, andmetallicities derived from spectroscopic analises or narrow bandphotometry. Based on observations collected at the European SouthernObservatory, La Silla, Chile and the Laboratório Nacional deAstrof\'{\i
| A Search for Old Star Clusters in the Large Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| Spectroscopy of giants in LMC clusters. II - Kinematics of the cluster sample
Velocities for 83 star clusters in the LMC are analyzed, based onindividual stellar velocities measured at the Calcium triplet. One-halfof the clusters are objects in the outer parts of the LMC which had noprevious velocity determinations. Published velocities for intermediateand old clusters are shown to have had systematic errors. These newvelocities with various rotation curve analyses of the LMC, and testaspects of the twisted disk model proposed by Freeman et al. (1983).When the transverse motion of the LMC is taken into account, a singlerotating disk solution fits the old and intermediate-aged clusters andother tracers (i.e., there is no need for an additional 'tilted disk'system).
| The evolution of carbon stars in the Magellanic Clouds
This study presents JHK photometric data for over 100 field stars in theSMC and for 10 in the Large Cloud together with spectroscopic resultsfor about half of them. In the Small Cloud carbon stars were found athigher temperatures and lower luminosities than previously observed. Thefaintest are below the top of the red giant branch. The medium- andlow-luminosity C stars in the M-C transition zone have a low C2 content.At these luminosities, most of the J-type stars are found close to theC2-poor stars in the HR diagram. Their C2 content is about as high as inthe coolest, most evolved C stars. The present observations of carbonstars in the SMC show that they cover a range in M(bo) from -3 to 5.9mag. The transitions from M to C via S appear to occur in both Clouds ata rather well-defined range in M(bol) for SWB and classes IV and V.
| Spectroscopy of giants in LMC clusters. I - Velocities, abundances, and the age-metallicity relation
Velocities and equivalent widths are presented for a large sample of LMCclusters. The calcium abundance is found to be a sensitive abundanceindicator over a very wide range of (Fe/H) between 0.0 and -2.2. Theage-metallicity relation is constructed for the inner and outer parts ofthe LMC. This relationsip can be characterized by a simple one-zoneenrichment model. The abundances for the inner and outer clusters at anage of 2 Gyr are nearly identical, so that little radial abundancegradient is evident in the cluster system.
| The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
| The Magellanic Clouds - Their evolution, structure and composition
Recent data related to the history of the Magellanic Clouds as galaxiesare described, and attempts to determine accurate distances to theMagellanic Clouds are discussed, with special attention given to thegeometry of the Magellanic Clouds and different methods of distancedeterminations. Consideration is given to the various star generationspresent in the Clouds (i.e., the oldest generation, of greater than 10Gyr; the intermediate-age generations, between 7 and 0.2 Gyr, and theyoungest generation, the formation of which started only about 50 Myrago) and to their occurrences in the LMC and SMC populations, as well asto the interstellar medium in the Magellanic Clouds. The structure ofthe Magellanic System, which comprises the Magellanic Clouds, theIntercloud Region, and the Magellanic Stream is described, withparticualr consideration given to the complex structure of the LMC andSMC and the kinematics of their populations.
| Ellipticities at R(h) of LMC star clusters
The projected ellipticities of 53 populous LMC star clusters have beenderived by means of PDS 1010A scans and a computer interactive method ofreduction implemented on an Apollo 570 workstation. Film copies of apair of J and U plates taken with the 1.2 m UK Schmidt Telescope inAustralia were used. The ellipticities derived here agree with thosefound by previous investigators, when comparisons were possible at thesame radius. Ellipticity variations within individual globular clustersare seen to be a common phenomenon, so the ellipticities e(h) at adistance corresponding to the half-mass radius R(h) from the center wereadopted to represent the cluster's flatness. Using these values for theLMC clusters, it is found that LMC clusters are more elliptical thanthose of the Galaxy. Although the young LMC globular clusters show atendency to be more elliptical than the old ones, there is no strongevidence for a significant difference among them. Finally, e(h) wasfound to increase with the total mass of the clusters, possiblyindicating that high-mass clusters have higher angular momentum, or havemore difficulty in shedding angular momentum, than do low mass clusters,and remain longer in their initial flattened shape.
| The evolution of the Magellanic Clouds. I - The ages of globular clusters
Theoretical and observed maximum luminosities of AGB stars in theMagellanic Cloud clusters are compared in order to obtain cluster ageestimations. The ages of 10 clusters in the SMC and 25 in the LMC areconsidered for the cases of several rates of mass loss by AGB stars. Itis demonstrated that discrepancies between ages derived from AGB peakluminosities and from the Main-Sequence turn off and maximum luminositycan be accounted for by the intensive mass loss during the AGBevolutionary phase.
| The continuity of cluster formation in the Large Magellanic Cloud
Four LMC star clusters are examined whose colors suggest ages between 2and 10 Gyr. Three of these, NGC 1754, NGC 1795 and SL 506, have ageswithin the well-populated range from 0.8 to 3 Gyr. No conclusion can bereached about the age of NGC 2005, which is located in the Bar and verycrowded. No cluster has yet been found in the LMC with a main-sequenceturnoff age between 4 and 10 Gyr.
| Ages and metallicities of LMC and SMC red clusters through H-beta and G band photometry
Narrow band integrated photometry of the H-beta and G band absorptionfeatures for 41 LMC and 10 SMC red star clusters is presented. Anage-metallicity calibration is provided for the color-color diagram. SWBtypes between IV and VII are derived for 23 unclassified clusters, andtheir distribution in the age versus metallicity plane is discussed. Astudy of chemical evolution of the Magellanic Clouds has shown that theLMC presents a steeper chemical enrichment slope. An intrinsicmetallicity dispersion is found in the LMC chemical evolution,indicating that the gas has been inhomogeneous at any time, with localenrichment prevailing over a global one. One zone model describes theevolution of both clouds, the efficiency of star cluster formation beinglarger in the LMC. The LMC presents a burst of star cluster formation att = 4.5 x 10 to the 9th yr. New B - V data for fainter SMC clusters arealso presented, providing an essentially complete color histogram forclusters with globular cluster appearance.
| Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
| The extended giant branches of intermediate age globular clusters in the Magellanic Clouds. II
In order to obtain a complete sample of upper asymptotic giant branch(AGB) stars in the red globular clusters of the Magellanic Clouds, aphotographic near-infrared survey of the clusters was conducted. Theresults are compared with previous photometry and the problem of errorarising from variability of carbon stars is addressed. Stars withoutspectra are tentatively classified based on their JHK colors. Apparentand absolute bolometric magnitudes and effective temperatures werecalculated from the IR colors, allowing for the location of the redstars and of the cluster giant branches in the physical H-R diagram tobe determined. Stellar evolution on the AGB is discussed, leading toimproved estimates of the extent of the upper AGB. A carbon star censusis presented and the ages of the clusters is derived with suitablycomplete photometry. On this basis, the chemical enrichment history ofthe Clouds is discussed.
| Brightness and polarization structure of four supernova remnants 3C 58, IC 443, W28, and W44 at 2.8 centimeter wavelength.
Abstract image available at:http://adsabs.harvard.edu/abs/1972A&A....20..237K
| A Catalogue of Clusters in The LMC
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