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The Infrared Ca II Triplet as Metallicity Indicator
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
| NGC 2298: a globular cluster on its way to disruption
We have studied the stellar main sequence (MS) of the globular clusterNGC 2298 using deep HST/ACS observations in the F606W and F814W bandscovering an area of 3.4 arcmin × 3.4 arcmin around the clustercentre or about twice the cluster's half-mass radius. Thecolour-magnitude diagram that we derive in this way reveals a narrow andwell defined MS extending down to the 10σ detection limit at m_606≃ 26.5, m_814 ≃ 25, corresponding to stars of 0.2 {M}_ȯ.The luminosity function (LF) obtained with these data, once correctedfor the limited effects of photometric incompleteness, reveals aremarkable deficiency of low-mass stars as well as a radial gradient, inthat the LF becomes progressively steeper with radius. Using themass-luminosity relation appropriate for the metallicity of NGC 2298, wederive the cluster's global mass function (GMF) by using a multi-massMichie-King model. Over the range 0.8-0.2 {M}_ȯ, the number ofstars per unit mass decreases following a power-law distribution of thetype dN/dm ∝ m0.5, where, for comparison, typical haloclusters have dN/dm ∝ m-1.5. If the IMF of NGC 2298 wassimilar to that of other metal poor halo clusters, like e.g. NGC 6397,the present GMF that we obtain implies that this object must have lostof the order of 85% of its original mass, at a rate much higher thanthat suggested by current models based on the available cluster orbit.The latter may, therefore, need revision.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555.
| The Magellanic Bridge: The Nearest Purely Tidal Stellar Population
We report on observations of the stellar populations in 12 fieldsspanning the region between the Magellanic Clouds, made with the MosaicII camera on the 4 m telescope at the Cerro-Tololo Inter-AmericanObservatory. The two main goals of the observations are to characterizethe young stellar population (which presumably formed in situ in theBridge and therefore represents the nearest stellar population formedfrom tidal debris), and to search for an older stellar component (whichwould have been stripped from either Cloud as stars, by the same tidalforces that formed the gaseous Bridge). We determine the star formationhistory of the young inter-Cloud population, which provides a constrainton the timing of the gravitational interaction that formed the Bridge.We do not detect an older stellar population belonging to the Bridge inany of our fields, implying that the material that was stripped from theClouds to form the Magellanic Bridge was very nearly a pure gas.
| Why Haven't Loose Globular Clusters Collapsed Yet?
We report on the discovery of a surprising observed correlation betweenthe slope of the low-mass stellar global mass function (GMF) of globularclusters (GCs) and their central concentration parameterc=log(rt/rc), i.e., the logarithmic ratio of tidaland core radii. This result is based on the analysis of a sample of 20Galactic GCs with solid GMF measurements from deep HST or VLT data. Allthe high-concentration clusters in the sample have a steep GMF, mostlikely reflecting their initial mass function. Conversely,low-concentration clusters tend to have a flatter GMF, implying thatthey have lost many stars via evaporation or tidal stripping. No GCs arefound with a flat GMF and high central concentration. This findingappears counterintuitive, since the same two-body relaxation mechanismthat causes stars to evaporate and the cluster to eventually dissolveshould also lead to higher central density and possibly core collapse.Therefore, more concentrated clusters should have lost proportionatelymore stars and have a shallower GMF than low-concentration clusters,contrary to what is observed. It is possible that severely depleted GCshave also undergone core collapse and have already recovered a normalradial density profile. It is, however, more likely that GCs with a flatGMF have a much denser and smaller core than that suggested by theirsurface brightness profile and may well be undergoing collapse atpresent. In either case, we may have so far seriously underestimated thenumber of post-core collapse clusters, and many may be lurking in theMilky Way.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| CCD photometry of the globular cluster M2: RR Lyrae physical parameters and new variables
We report the results of CCD V and R photometry of the RR Lyrae stars inM2. The periodicities of most variables are revised and new ephemeridesare calculated. Light-curve decomposition of the RR Lyrae stars wascarried out and the corresponding mean physical parameters [Fe/H] =-1.47, Teff = 6276K, logL = 1.63Lsolar andMV = 0.71 from nine RRab and [Fe/H] = -1.61, M =0.54Msolar, Teff = 7215K, logL =1.74Lsolar and MV = 0.71 from two RRc stars werecalculated. A comparison of the radii obtained from the above luminosityand temperature with predicted radii from non-linear convective modelsis discussed. The estimated mean distance to the cluster is 10.49 +/-0.15kpc. These results place M2 correctly in the general globularcluster sequences for Oosterhoff type, mass, luminosity and temperature,all as a function of the metallicity. Mean relationships for M,logL/Lsolar, Teff and MV as a functionof [Fe/H] for a family of globular clusters are offered. These trendsare consistent with evolutionary and structural notions on thehorizontal branch. Eight new variables are reported.
| Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Hot Populations in M87 Globular Clusters
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.
| 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.
| A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies
We have compiled a sample of globular clusters with high-quality stellarabundances from the literature to compare to the chemistries of stars inthe Galaxy and in dwarf spheroidal galaxies. Of the 45 globular clustersexamined, 29 also have kinematic information. Most of the globularclusters belong to the Galactic halo; however, a significant number havedisk kinematics or belong to the bulge. Focusing on the [α/Fe] andlight r-process element ratios, we find that most globular cluster starsmimic field stars of similar metallicities, and neither clearlyresembles the currently available stellar abundances in dwarf galaxies(including globular clusters in the Large Magellanic Cloud). Theexceptions to these general elemental ratio comparisons are alreadyknown in the literature, e.g., ω Centauri, Palomar 12, and Terzan7 associated with the Sagittarius remnant and Ruprecht 106, which has ahigh radial velocity and low [α/Fe] ratio. A few other globularclusters show more marginal peculiarities. The most notable one is thehalo cluster M68, which has a high galactocentric rotational velocity, aslightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratiosdecrease with increasing [Fe/H] at intermediate metallicities, which isconsistent with very massive stars playing a larger role in the earlychemical evolution of the Galaxy. The chemical similarities betweenglobular clusters and field stars with [Fe/H]<=-1.0 suggests a sharedchemical history in a well-mixed early Galaxy. The differences in thepublished chemistries of stars in the dwarf spheroidal galaxies suggestthat neither the globular clusters, halo stars, nor thick disk stars hadtheir origins in small isolated systems like the present-day Milky Waydwarf satellites.
| A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
| Detection of a Young Stellar Population in the Background of Open Clusters in the Third Galactic Quadrant
We report the detection of a young stellar population (<=100 Myr) inthe background of nine young open clusters belonging to a homogenoussample of 30 star clusters in the third Galactic quadrant (at217deg<=l<=260deg). Deep and accurate UBVRIphotometry allows us to measure model-independent age and distance forthe clusters and the background population with high confidence. Thispopulation is exactly the same population (the blue plume) recentlydetected in three intermediate-age open clusters and suggested to be a<=1-2 Gyr old population belonging to the Canis Major (CMa)overdensity (Bellazzini et al.; Martínez-Delgado et al.).However, we find that the young population in those three clusters andin six clusters of our sample follows the pattern of the Norma-Cygnusspiral arm as defined by CO clouds remarkably well, while in the otherthree program clusters it lies in the Perseus arm. We finally provideone example (out of 21) of a cluster that does not show any backgroundpopulation, demonstrating that this population is not ubiquitous towardCMa.
| Elemental Abundance Ratios in Stars of the Outer Galactic Disk. II. Field Red Giants
We summarize a selection process to identify red giants in the directionof the southern warp of the Galactic disk, employing VICphotometry and multiobject spectroscopy. We also present results fromfollow-up high-resolution, high signal-to-noise echelle spectroscopy ofthree field red giants, finding [Fe/H] values of about -0.5. The fieldstars, with galactocentric distances estimated at 10-15 kpc, support theconclusion of Yong and coworkers that the Galactic metallicity gradientdisappears beyond RGC values of 10-12 kpc for the older starsand clusters of the outer disk. We summarize the detailed abundancepatterns for 15 other elements for these stars and compare them withrecently obtained results for old open cluster red giants in the outerdisk. The field and cluster stars at such large distances show verysimilar abundance patterns, and, in particular, all show enhancements ofthe α-elements O, Mg, Si, Ca, and Ti and the r-process element Eu.These results suggest that Type II supernovae have been significantcontributors to star formation in the outer disk relative to Type Iasupernovae within the past few gigayears. We also compare our resultswith those available for much younger objects. The limited results forthe H II regions and B stars in the outer disk also suggest that theradial metallicity gradient in the outer disk is shallow or absent. Themuch more extensive results for Cepheids confirm these trends and thatthe change in slope of the metallicity gradient may occur at a largergalactocentric distance than for the older stars and clusters. However,the younger stars also show rising α-element enhancements withincreasing RGC, at least beyond 12 kpc. These trends areconsistent with the idea of a progressive growth in the size of theGalactic disk with time and episodic enrichment by Type II supernovae aspart of the disk's growth.This paper makes use of observations obtained at the National OpticalAstronomy Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA), Inc., under contract fromthe National Science Foundation. We also employ data products from theTwo Micron All Sky Survey, which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center,California Institute of Technology, funded by the National Aeronauticsand Space Administration and the National Science Foundation.
| The Absence of Extratidal Structure in the Sculptor Dwarf Spheroidal Galaxy
The results of a wide-field survey of the Sculptor dwarf spheroidalgalaxy are presented. Our aims were to obtain an accurate map of theouter structure of Sculptor and to determine the level of interactionbetween this system and the Galaxy. Photometry was obtained in twocolors down to the magnitude limits of V=20 and I=19, covering a3.1d×3.1d area centered on Sculptor. The resulting color-magnitudedata were used as a mask to select candidate horizontal-branch (HB) andred giant branch stars for this system. Previous work has shown that thered HB stars are more concentrated than the blue HB stars. We havedetermined the radial distributions of these two populations and showthat the overall Sculptor density profile is well described by atwo-component model based on a combination of these radialdistributions. In addition, spectra of the Ca II triplet region wereobtained for over 700 candidate red giant stars over the 10deg2 region using the 2dF instrument on the Anglo-AustralianTelescope. These spectra were used to remove foreground Galactic starsbased on radial velocity and Ca II triplet strength. The final list ofSculptor members contains 148 stars, 7 of which are located beyond thenominal tidal radius. Both the photometric and spectroscopic data setsindicate no significant extratidal structure. These results support atmost a mild level of interaction between this system and the Galaxy, andwe have measured an upper mass limit for extratidal material to be2.3%+/-0.6% of the Sculptor luminous mass. This lack of tidalinteraction indicates that previous velocity dispersion measurements(and hence the amount of dark matter detected) in this system are notstrongly influenced by the Galactic tidal field.
| Triggering and Feedback: The Relation between the H I Gas and the Starburst in the Dwarf Galaxy NGC 1569
As part of our study on the impact of violent star formation on theinterstellar medium (ISM) of dwarf galaxies, we report observations ofneutral atomic hydrogen (H I) in the starburst dwarf galaxy NGC 1569.High-resolution measurements with the Very Large Array (B, C, and Dconfiguration) are aimed at identifying morphological and kinematicalsignatures in H I caused by the starburst. Our kinematical data suggesta huge hole in the H I distribution, probably due to the large number ofsupernovae explosions in the center of the galaxy over the past 20 Myr.Investigating the large-scale H I structure, we confirm the existence ofa possible H I companion and a so-called H I bridge east of NGC 1569.Furthermore, we report the detection of additional low-intensity H Ihalo emission, which leads us to suggest a revised halo structure. Onthe basis of our new picture, we discuss the origin of the halo gas andpossible implications for the evolution of the starburst in NGC 1569.
| A Comprehensive Model for the Monoceros Tidal Stream
We have compiled an extensive data set on potential parts of theMonoceros tidal stream and performed an exhaustive survey of dwarfgalaxy semianalytic orbits in order to constrain its orbital properties.The best-fit orbits are subsequently realized as self-consistent N-bodysimulations in order to reproduce the spatial and velocity distributionof satellite debris. We find that all kinematic and geometricconstraints can be fit by a single stream allowing for multiple wraps.The orbital eccentricity and inclination of the progenitor are stronglyconstrained to be e=0.10+/-0.05 andi=25deg+/-5deg. Ten new estimates of propermotions from the Sloan Digital Sky Survey clearly exclude all retrogradeorbits. Particles lost by the satellite populate two nearly concentricrings, naturally explaining the detection of stream stars at both 6-8kpc (Ibata et al.; Newberg et al.) and 12-18 kpc (the Tri/And stream;Rocha-Pinto et al.) from the Sun. We have attempted to predict thepresent location of the Monoceros stream progenitor using differentinformation: (1) the kinematical and spatial distribution of detections,and (2) the different mean metallicity in the inner and the outer rings.Because of the lack of observational data in the whole range of Galacticlatitudes, the geometrical/kinematical constraints lead to a wide rangeof possible locations. By associating older parts of the model streamwith lower metallicity parts of the observed data, we argue in favor ofa current location of l~245deg, b~-18deg, with adistance to the Sun rs~=15 kpc. The mass of the progenitorhas been poorly constrained because of the slow orbital decay. Similarfits have been obtained for masses(3-9)×108Msolar. We have analyzed thepossible common origin of the Canis Major dwarf and the Monocerosstream. The Canis Major dwarf moves on a prograde, nearly circular orbit(e~=0.16) in the Milky Way disk (i~=4+14-4 deg).This orbital inclination is too low to account for the large verticaldispersion of stream stars. However, the bimodal distribution of radialvelocities in the central region found by Martin et al. probablyindicates that their selection criteria for identifying dwarf stars leadto a contamination of background stars. In that case, the kinematicaldata outlined above might result in an underestimate of the orbitalinclination. Finally, the distance estimation to Canis Major dwarf isaround a factor of 2 smaller than that obtained from our model.Unfortunately, the possible identification of the Monoceros streamprogenitor in Canis Major remains unclear.
| On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
| Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.
| Spectroscopy of Red Giants in the Large Magellanic Cloud Bar: Abundances, Kinematics, and the Age-Metallicity Relation
We report metallicities and radial velocities derived from spectra atthe near-infrared calcium triplet for 373 red giants in a 200arcmin2 area at the optical center of the LMC bar. These arethe first spectroscopic abundance measurements of intermediate-age andold field stars in the high surface brightness heart of the LMC. Themetallicity distribution is sharply peaked at the median value[Fe/H]=-0.40, with a small tail of stars extending down to[Fe/H]<=-2.1 10% of the red giants are observed to have[Fe/H]<=-0.7. The relative lack of metal-poor stars indicates thatthe LMC has a ``G dwarf'' problem, similar to the Milky Way. Theabundance distribution can be closely approximated by two Gaussianscontaining 89% and 11% of the stars, respectively: the first componentis centered at [Fe/H]=-0.37 with σ=0.15, and the second at[Fe/H]=-1.08 with σ=0.46. The dominant population has ametallicity distribution similar to that of the LMC's intermediate-agestar clusters. The mean heliocentric radial velocity of the sample is257 km s-1, corresponding to the same center-of-mass velocityas the disk (measured at larger radii). Because of the central locationof our field, kinematic constraints are not strong, but there is noevidence that the bar deviates from the general motion of the LMC disk.The velocity dispersion of the whole sample isσv=24.7+/-0.4 km s-1. When cut bymetallicity, the most metal-poor 5% of stars ([Fe/H]<-1.15) showσv=40.8+/-1.7 km s-1, more than twice thevalue for the most metal-rich 5%; this suggests that an old, thickerdisk or halo population is present. The age-metallicity relation (AMR)is almost flat during the period from 5 to 10 Gyr ago, with an apparentscatter of +/-0.15 dex about the mean metallicity for a given age.Comparing this to chemical evolution models from the literature, we findthat a burst of star formation 3 Gyr ago does not reproduce the observedAMR more closely than a steadily declining star formation rate. The AMRsuggests that the epoch of enhanced star formation, if any, must havecommenced earlier, ~6 Gyr ago-the exact time is model dependent. Wecompare the properties of the LMC and the Milky Way, and discuss ourresults in the context of models that attempt to use tidal interactionswith the Milky Way and Small Magellanic Cloud to explain the star andcluster formation histories of the LMC.
| The structure of our stellar system.
Not Available
| 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.
| CCD Photometry of the RR LYRAE Stars in NGC147
We report the results of CCD V and R photometry of the RRstars known inNGC147. The periodicities of most variables are revised and newephemerides are calculated. The Blazhko effect has been detected in V2and V6. Three previously reported variables; V5, V9, and V15 are foundto be non-variable. A new variable V18 was discovered with a period of0.49205 days and an amplitude of 0.15. Using the approach of Fourierdecomposition of the light curves, the physical parameters of the RRaband RRc variables were estimated. The cluster is of the Oosterhoff type.With the newly values [Fe/H]= -1.22Â+/-0.31 and 16.8Â+/-1.3,the cluster fits very well into the Oosterhoff type-metallicity andmetallicity-temperature sequence found in globular clusters. Acomparison with ZAHB models indicates that the RRab stars have not yetevolved off the horizontal branch, a result consistent with the meanperiod of the RRab variables and with the metallicity of the clusterderived in this work.
| Abundance Variations Within Globular Clusters
Abundance variations within globular clusters (GCs), and of GC starswith respect to field stars, are important diagnostics of a variety ofphysical phenomena, related to the evolution of individual stars, masstransfer in binary systems, and chemical evolution in high densityenvironments. The broad astrophysical implications of GCs as buildingblocks of our knowledge of the Universe make a full understanding oftheir history and evolution basic in a variety of astrophysical fields.We review the current status of the research in this field, comparingthe abundances in GCs with those obtained for field stars, discussing indepth the evidence for H-burning at high temperatures in GC stars,describing the process of self-enrichment in GCs with particularreference to the case of the most massive Galactic GC ( Cen), anddiscussing various classes of cluster stars with abundance anomalies.Whereas the overall pattern might appear very complex at first sight,exciting new scenarios are opening where the interplay between GCdynamical and chemical properties are closely linked with each other.
| The Globular Cluster System of the Canis Major Dwarf Galaxy
Prompted by the discovery of the accreted Canis Major dwarf galaxy andits associated globular cluster (GC) system, we investigate thecontribution of accreted GCs to the Galactic system. The Canis MajorGCs, and those associated with the Sagittarius dwarf galaxy, exhibit arange of Galactocentric radii, prograde and retrograde motions, andhorizontal-branch morphologies, indicating that such properties are oflimited use in identifying accreted GCs. In contrast, we find that theage-metallicity relation (AMR) of these dwarf galaxies is distinct fromthat of the main Galactic GC distribution at intermediate-to-highmetallicities ([Fe/H]>~-1.3). The accretion of GCs with a distinctAMR would explain much of the apparent age spread in the Galactic GCsystem. The Canis Major and Sagittarius AMRs are similar to those ofother Local Group dwarf galaxies and are consistent with a simpleclosed-box chemical enrichment model-a further indication that these GCsformed outside of the Milky Way. The Canis Major GCs all havesmaller-than-average sizes for their Galactocentric distances, lendingfurther support to their origin outside of the Milky Way. Our findingssuggest that accretion of similar mass dwarfs does not appear to haveplayed a major role in building the stellar mass of the thick disk orbulge of the Milky Way.
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