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Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge
We present near-IR color-magnitude diagrams and physical parameters fora sample of 24 Galactic globular clusters toward the bulge. In thispaper we discuss the properties of 12 new clusters (out of the 24) inaddition to those previously studied and published by our group. Thecompilation includes measurements of the cluster reddening, distance,photometric metallicity, horizontal branch red clump, and red giantbranch morphological (e.g., mean ridgelines) and evolutionary (e.g.,bump and tip) features. The compilation is available in electronic formon the World Wide Web, and it will be updated regularly.Based on data taken at the ESO New Technology Telescope, within theobserving programs 73.D-0313, 75.D-0372, and 77.D-0757.
| 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.
| 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.
| 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.
| 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.
| 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.
| The Red Giant Branch luminosity function bump
We present observational estimates of the magnitude difference betweenthe luminosity function red giant branch bump and the horizontal branch(Delta F555WbumpHB), and of star counts in thebump region (Rbump), for a sample of 54 Galactic globularclusters observed by the HST. The large sample of stars resolved in eachcluster, and the high photometric accuracy of the data allowed us todetect the bump also in a number of metal poor clusters. To reduce thephotometric uncertainties, empirical values are compared withtheoretical predictions obtained from a set of updated canonical stellarevolution models which have been transformed directly into the HSTflight system. We found an overall qualitative agreement between theoryand observations. Quantitative estimates of the confidence level arehampered by current uncertainties on the globular cluster metallicityscale, and by the strong dependence of DeltaF555WbumpHB on the cluster metallicity. In case ofthe Rbump parameter, which is only weakly affected by themetallicity, we find a very good quantitative agreement betweentheoretical canonical models and observations. For our full clustersample the average difference between predicted and observedRbump values is practically negligible, and ranges from-0.002 to -0.028, depending on the employed metallicity scale. Theobserved dispersion around these values is entirely consistent with theobservational errors on Rbump. As a comparison, the value ofRbump predicted by theory in case of spurious bump detectionsdue to Poisson noise in the stellar counts would be ~ 0.10 smaller thanthe observed ones. We have also tested the influence on the predictedDelta F555WbumpHB and Rbump values ofan He-enriched component in the cluster stellar population, as recentlysuggested by D'Antona et al. (\cite{d02}). We find that, underreasonable assumptions concerning the size of this He-enrichedpopulation and the degree of enrichment, the predicted DeltaF555WbumpHB and Rbump values are onlymarginally affected.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, and on observations retrieved withthe ESO ST-ECF Archive.
| B, V and I photometry of the intermediate metallicity bulge globular clusters NGC 6325 and NGC 6355
Colour-Magnitude Diagrams are presented for the globular clusters NGC6325 and NGC 6355. These clusters are embedded in crowded bulge fields.Both clusters are concentrated and have blue Horizontal Branches, andNGC 6325 shows a blue tail. We derive a reddening E(B-V) ~ 0.95 and adistance from the Sun dsun ~ 6.9 kpc for NGC 6325, and E(B-V)~ 0.78 and dsun ~ 8.8 kpc for NGC 6355. They are both at ~ 1kpc from the Galactic center, above the plane. The metallicity has beenestimated based on red giant branch morphology, being consistent with anintermediate metallicity of [Fe/H] ~ -1.3 for both clusters, at thelower end of the metallicity distribution of bulge field stars. They addto the list of blue Horizontal Branch clusters with intermediatemetallicities within the bulge volume.Observations collected at the European Southern Observatory - ESO,Chile, proposal No. 64L-0212(A).
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
| HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands
We present the complete photometric database and the color-magnitudediagrams for 74 Galactic globular clusters observed with the HST/WFPC2camera in the F439W and F555W bands. A detailed discussion of thevarious reduction steps is also presented, and of the procedures totransform instrumental magnitudes into both the HST F439W and F555Wflight system and the standard Johnson ( B ) and ( V ) systems. We alsodescribe the artificial star experiments which have been performed toderive the star count completeness in all the relevant branches of thecolor magnitude diagram. The entire photometric database and thecompleteness function will be made available on the Web immediatelyafter the publication of the present paper. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contractNAS5-26555, and on observations retrieved from the ESO ST-ECF Archive.
| Near-Infrared Imaging of the Central Regions of Metal-Poor Inner Spheroid Globular Clusters
JHK images obtained with the Canada-France-Hawaii Telescope adaptiveoptics bonnette are used to investigate the near-infrared photometricproperties of red giant branch (RGB) and horizontal-branch (HB) stars ineight metal-poor globular clusters with RGC<=2 kpc. Theslope of the RGB on the (K, J-K) CMDs confirms the metal-poor nature ofthese clusters, four of which (NGC 6287, 6293, 6333, and 6355) are foundto have metallicities that are comparable to M92. The luminosityfunctions of RGB stars in inner spheroid and outer halo clusters havesimilar slopes, although there is a tendency for core-collapsed clustersto have slightly flatter luminosity functions than noncollapsedclusters. The distribution of red HB stars on the (K, J-K) CMDs of innerspheroid clusters with [Fe/H]~-1.5 is very different from that ofclusters with [Fe/H]~-2.2, suggesting that metallicity is the mainparameter defining HB content among these objects. The RGB bump isdetected in four of the inner spheroid clusters, and this feature isused to compute distances to these objects. Finally, the specificfrequency of globular clusters in the inner Galaxy is discussed in thecontext of the early evolution of the bulge. Based on the ratio ofmetal-poor to metal-rich clusters in the inner Galaxy it is suggestedthat the metal-poor clusters formed during an early intense burst ofstar formation. It is also demonstrated that if the globular clusterformation efficiency for the inner Galaxy is similar to that measured inother spheroidal systems, then the main body of the bulge could haveformed from gas that was chemically enriched in situ; hence, materialfrom a separate pre-enriched reservoir, such as the disk or outer halo,may not be required to form the bulge.
| Globular Cluster Subsystems in the Galaxy
Data from the literature are used to construct a homogeneous catalog offundamental astrophysical parameters for 145 globular clusters of theMilky Way Galaxy. The catalog is used to analyze the relationshipsbetween chemical composition, horizontal-branch morphology, spatiallocation, orbital elements, age, and other physical parameters of theclusters. The overall globular-cluster population is divided by a gap inthe metallicity function at [Fe/H]=-1.0 into two discrete groups withwell-defined maxima at [Fe/H]=-1.60±0.03 and -0.60±0.04.The mean spatial-kinematic parameters and their dispersions changeabruptly when the metallicity crosses this boundary. Metal-poor clustersoccupy a more or less spherical region and are concentrated toward theGalactic center. Metal-rich clusters (the thick disk subsystem), whichare far fewer in number, are concentrated toward both the Galacticcenter and the Galactic plane. This subsystem rotates with an averagevelocity of V rot=165±28 km/s and has a very steep negativevertical metallicity gradient and a negligible radial gradient. It is,on average, the youngest group, and consists exclusively of clusterswith extremely red horizontal branches. The population ofspherical-subsystem clusters is also inhomogeneous and, in turn, breaksup into at least two groups according to horizontal-branch morphology.Clusters with extremely blue horizontal branches occupy a sphericalvolume of radius 9 kpc, have high rotational velocities (Vrot=77±33 km/s), have substantial and equal negative radial andvertical metallicity gradients, and are, on average, the oldest group(the old-halo subsystem). The vast majority of clusters withintermediate-type horizontal branches occupy a more or less sphericalvolume ≈18 kpc in radius, which is slightly flattened perpendicularto the Z direction and makes an angle of ≈30° to the X-axis. Onaverage, this population is somewhat younger than the old-halo clusters(the young-halo subsystem), and exhibits approximately the samemetallicity gradients as the old halo. As a result, since theirGalactocentric distance and distance from the Galactic plane are thesame, the young-halo clusters have metallicities that are, on average,Δ[Fe/H] ≈0.3 higher than those for old-halo clusters. Theyoung-halo subsystem, which apparently consists of objects captured bythe Galaxy at various times, contains many clusters with retrogradeorbits, so that its rotational velocity is low and has large errors, Vrot=-23±54 km/s. Typical parameters are derived for all thesubsystems, and the mean characteristics of their member globularclusters are determined. The thick disk has a different nature than boththe old and young halos. A scenario for Galactic evolution is proposedbased on the assumption that only the thick-disk and old-halo subsystemsare genetically associated with the Galaxy. The age distributions ofthese two subsystems do not overlap. It is argued that heavy-elementenrichment and the collapse of the proto-Galactic medium occurred mainlyin the period between the formation of the old-halo and thick-disksubsystems.
| A Near-Infrared Photometric Survey of Metal-poor Inner Spheroid Globular Clusters and Nearby Bulge Fields
Images recorded through J, H, K, 2.2 μm continuum, and CO filtershave been obtained of a sample of metal-poor ([Fe/H]<=-1.3) globularclusters in the inner spheroid of the Galaxy. The shape and color of theupper giant branch on the (K, J-K) color-magnitude diagram (CMD),combined with the K brightness of the giant branch tip, are used toestimate the metallicity, reddening, and distance of each cluster. COindices are used to identify bulge stars, which will bias metallicityand distance estimates if not culled from the data. The distances andreddenings derived from these data are consistent with published values,although there are exceptions. The reddening-corrected distance modulusof the Galactic center, based on the Carney et al. horizontal-branch(HB) brightness calibration, is estimated to be 14.9+/-0.1. The meanupper giant branch CO index shows cluster-to-cluster scatter that (1) islarger than expected from the uncertainties in the photometriccalibration and (2) is consistent with a dispersion in CNO abundancescomparable to that measured among halo stars. The luminosity functions(LFs) of upper giant branch stars in the program clusters tend to besteeper than those in the halo clusters NGC 288, NGC 362, and NGC 7089.The majority of inner spheroid clusters fall along the integrated J-Kversus metallicity relation defined by halo clusters; however, many ofthe inner spheroid clusters do not follow the relation betweenintegrated CO index and metallicity measured for halo clusters, in thatthey have CO indices that are too small. Bulge fields were also observednear most clusters. The slope of the giant branch LF does not varysignificantly between most fields, although the LFs in Baade's windowand near NGC 6273 are significantly shallower than average.Metallicities estimated from the slope of the upper giant branch on the(K, J-K) CMDs of fields within 6° of the Galactic center areconsistent with previous studies. Finally, the data suggest that the HBcontent may not be uniform throughout the bulge, in the sense that alarger than average number of red HB stars may occur in fields closestto the Galactic center.
| Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
| Destruction of the Galactic Globular Cluster System
We investigate the dynamical evolution of the Galactic globular clustersystem in considerably greater detail than has been done hitherto,finding that destruction rates are significantly larger than given byprevious estimates. The general scheme (but not the detailedimplementation) follows Aguilar, Hut, & Ostriker. For the evolutionof individual clusters, we use a Fokker-Planck code including the mostimportant physical processes governing the evolution: two-bodyrelaxation, tidal truncation of clusters, compressive gravitationalshocks while clusters pass through the Galactic disk, and tidal shocksdue to passage close to the bulge. Gravitational shocks are treatedcomprehensively, using a recent result by Kundic & Ostriker that the< Delta E2> shock-induced relaxation term, driving an additionaldispersion of energies, is generally more important than the usualenergy shift term < Delta E>. Various functional forms of thecorrection factor are adopted to allow for the adiabatic conservation ofstellar actions in a presence of transient gravitational perturbation.We use a recent compilation of the globular cluster positional andstructural parameters, and a collection of radial velocity measurements.Two transverse to the line-of-sight velocity components were assignedrandomly according to the two kinematic models for the cluster system(following the method of Aguilar, Hut, & Ostriker): one with anisotropic peculiar velocity distribution, corresponding to thepresent-day cluster population, and the other with the radiallypreferred peculiar velocities, similar to those of the stellar halo. Weuse the Ostriker & Caldwell and the Bahcall, Schmidt, & Soneiramodels for our Galaxy. For each cluster in our sample, we calculated itsorbits over a Hubble time, starting from the present observed positionsand assumed velocities. Medians of the resulting set of peri- andapogalactic distances and velocities are used then as an input for theFokker-Planck code. Evolution of the cluster is followed up to its totaldissolution due to a coherent action of all of the destructionmechanisms. The rate of destruction is then obtained as a median overall the cluster sample, in accord with Aguilar, Hut, & Ostriker. Wefind that the total destruction rate is much larger than that given byAguilar, Hut, & Ostriker with more than half of the present clusters(52%--58% for the Ostriker & Caldwell model, and 75%--86% for theBahcall, Schmidt, & Soneira model) destroyed in the next Hubbletime. Alternatively put, the typical time to destruction is comparableto the typical age, a result that would follow from (but is not requiredby) an initially power law distribution of destruction times. We discusssome implications for a past history of the globular cluster system andthe initial distribution of the destruction times, raising thepossibility that the current population is but a very small fraction ofthe initial population with the remnants of the destroyed clustersconstituting presently a large fraction of the spheroid (bulge + halo)stellar population.
| Some Integrated Properties of Galactic Globular Clusters
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....112.2634V
| A Catalog of Parameters for Globular Clusters in the Milky Way
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.1487H&db_key=AST
| A Search for Radio Pulsars in Globular Clusters, Supernova Remnants and Transient X-Ray Sources
We present the results of a targeted search for radio pulsars inglobular clusters, supernova remnants (SNRs) and transient X-raysources. 85 globular clusters, 29 SNRs and three transient X-ray sourceswere observed in the course of the search, during which four pulsarswere found. For each object searched, the sensitivity limit is given.The four new pulsars discovered were all in globular clusters, namelyPSR B1620-26 in M4, PSR B1718-19 in NGC6342, and PSR B1820-30A andB1820-30B in NGC6624. While PSR B1620-26 provides strong support for thebinary evolution hypothesis for the formation of millisecond pulsars,PSR B1718-19 and B1820-30A are somewhat anomalous as they are apparentlyrelatively young, the former pulsar and PSR B1820-30B have high magneticfields, and all are located in old globular clusters (ages ~10^10yr).Not surprisingly, the well-known pulsar PSR B0531+21 was detected in theCrab nebula SNR. The number of pulsars detected was probably limited bythe finite size of pulsar emission beams, as in any survey. Otherfactors such as large distance, low pulsar luminosity and orbital motionmay have inhibited the detection of other pulsars in globular clusters.Pulsars in SNRs and transient X-ray sources are probably less affectedby these factors. Our inability to detect the former is probably due tothe relatively high level of emission from the SNRs, the limited areathat we observed within each target and possibly the high birth velocityof pulsars which carries them away from the centres of their remnants.Processes concerning the accretion discs of transient X-ray sourcesprobably thwarted the detection of any potential radio pulsars in thesesystems.
| Comparative classification of globular clusters in the galaxy and M31 using cluster analysis
The distributions of 100 globular clusters in the Galaxy and 167globular clusters in M31 in a three-dimensional space with coordinatesMV, (B-V)0, and (U-B)O are considered. A centroid method of clusteranalysis is used. Classes are identified consisting of clusters that aresimilar in their parameters. The classes are not isolated andcontinually cross each other. This may indicate continuity in theprocess of formation of the aggregate of clusters for each galaxy.Comparison of the characteristics of the clusters in both galaxies, aswell as cluster analysis of the joint distribution for both samples,showed that certain combinations of parameters are characteristic forclusters in one galaxy but atypical for those in the other. Therefore, asignificant number of clusters do not have 'twins' in the other galaxy.This indicates a possible connection between the properties of globularclusters and the properties of the parent galaxy.
| Perigalactic Distances of Globular Clusters
Abstract image available at:http://adsabs.harvard.edu/abs/1995AJ....110.1171V
| The ROSAT XRT Sky Survey of X-ray sources in globular clusters.
We present data obtained on globular clusters during the Rosat XRT +PSPC Sky Survey. Sources were detected in the 0.5-2.5keV range in 11globular clusters. In Ter 6 a previously unknown sources was detected byRosat ; the source H1825-331 was confirmed to be in the cluster NGC6652; the persistent flux of the known burst source in Ter 5 was alsodetected for the first time. The sources in Ter 5 and NGC 6652 aretransients, seen by Rosat in their bright state. Improved positions areobtained for the sources in NGC 6652, Ter 5, and Ter 6. From ouranalysis of the area surrounding ω Cen, we suggest that only oneof the five sources detected by Einstein is related to the globularcluster. One source was detected in the 0.07-0.5keV band only, in NGC5272. A fit of a blackbody to the spectrum of the source in NGC 5272gives a temperature of ~45eV, and a bolometric luminosity of~1.3x10^35^erg/s. Upper limits of sources in clusters for which onlyHEAO-1 observations were available have been improved by factors ten tohundred, and indicate that most clusters do not contain sources withluminosities L_x_(0.5-2.5keV)>3x10^32^erg/s.
| A new method for determining the CO to H2 conversion factor for translucent clouds
In this paper we describe a new technique for obtaining the conversionfactor between the molecular hydrogen column density and the CO(J = 1-0)integrated antenna temperature. This factor, typically known asXCO is often to be of order a few times 1020/sqcm/K km/s) for the molecular clouds in the Galaxy and is one of theprimary means of determining the molecular cloud mass from COobservations. However, for the low-extinction interstellar clouds knownas the translucent molecular clouds, estimates of XCO vary byup to a factor of 60 depending on the object and techniques employed tocalibrate XCO. Since the cloud mass is directly proportionalto XCO uncertainties in mass estimates of translucent cloudscan be more than an order of magnitude. We calibrate the H2 content intranslucent clouds by using the linear relationship between the CH andH2 column densities. The CH column density is readily determined fromobservations of the CH ground-state hyperfine main-line transition at3335 MHz. Using CH as a surrogate tracer for H2 and CO(J = 1-0)observations of a sample of translucent and dark molecular clouds, wefind a wide variation in values for XCO. For translucentclouds, XCO ranges from 0.3 to 6.8 x 1020 and fordark clouds the values range from 0.8 to 8.6. Although the averagevalues for both types of cloud are similar to the canonical valuedetermined for the Galactic molecular cloud ensemble (2-4 x1020), the scatter in individual XCO values mayindicate that XCO for a given translucent cloud cannot bedetermined a priori and must be obtained for each cloud so that areliable mass determination may be made.
| CCD photometry of globular cluster core structure. 2: U-band profiles for 15 candidate collapsed-core clusters
We present U-band CCD surface brightness profiles for 15 of the 21globular clusters that have been identified as having collapsed cores byDjorgovski & King (1986). Fourteen of the clusters were observedwith the Cerro Tololo 4 m telescope; NGC 7078 was observed with theCanada-France-Hawaii 3.6 m telescope (CFHT). We have fitted the profileswith seeing-convolved power laws, both with and without cores, to assessthe evidence for central power-law structure and to place upper limitson core radius rc. We find nine of the clusters (NGC 5946,NGC 6284, NGC 6293, NGC 6325, NGC 6342, NGC 6558, NGC 6624, NGC 6681,and NGC 7078) to have unresolved cores, with upper limits rcless than or = 1.9 arcsecs. Three of the clusters (NGC 6453, NGC 6522,and NGC 7099) have marginally resolved cores, with upper limits in therange 2.7 arcsecs less than or = rc less than or = 3.4arcsecs. The remaining three clusters (NGC 6355, NGC 6397, and NGC 6752)have resolved cores. Of the latter three clusters, NGC 6355 and NGC 6752are consistent with single-mass King model structure. The median clusterdistances are 9.2 kpc for those with unresolved cores, 7.2 kpc for thosewith marginally resolved cores, and 4.1 kpc for those with resolvedcores. The 13 clusters that do not resemble single-mass King models havecentral power-law structure with surface brightness slopes in the rangeof d ln S/d ln r = -0.6 to -0.8. These slopes are consistent with themodels of Grabhorn et al. (1992) for clusters evolving beyond corecollapse. The models include a centrally concentrated population ofnonluminous remnants with masses in the range 1.2-1.4 solar mass, thusproviding evidence for significant neutron star populations in most ofour cluster sample. This finding is consistent with the observation ofcentrally concentrated low-mass X-ray binary and millisecond pulsarpopulations in several clusters.
| Catalogue of Galactic globular-cluster surface-brightness profiles
We present a catalogue of surface-brightness profiles (SBPs) of 125Galactic globular clusters, the largest such collection ever gathered.The SPBs are constructed from generally inhomogeneous data, but arebased heavily on the Berkeley Global Cluster Survey of Djorgovski &King. All but four of the SBPs have photometric zero points. We derivecentral surface brightness, King-model concentrations, core radii,half-light, and other fraction-of-light radii where data permit, and webriefly discuss their use.
| Stellar OH masers toward globular clusters.
We have carried out a search for stellar maser emission from evolvedstars in globular clusters (GCs) with the NRAO 140-ft Telescope. Allknown GCs north of declination -42deg were searched in the 1667MHz mainline of the hydroxyl radical OH and its 1612MHz satellite line totypical rms sensitivity limits of 69mJy and 66mJy, respectively. Atargeted search was also done on IRAS point sources in the vicinity ofGCs, selected on the basis of their infrared colors. We detected thestellar maser emission toward 5 GCs, and found another 3 stellar masersources from a search of the literature. In 6 of these cases we findthat their cluster membership is doubtful, based on radial velocityinformation. A candidate associated with Djorg 1 is thought to be a lineof sight coincidence with a previously-unknown stellar maser located inthe galactic plane. The remaining candidate IRAS 16296-1305 isidentified with the Mira variable V720 Oph, whose membership within NGC6171 is supported by a near match between the radial velocities of thestar and the cluster, the small expansion velocity of the stellar shell,and a lack of proper motion for the star. We suggest reasons for the lowdetection rates in our survey and give directions for futureobservations.
| Space distribution and metallicities of globular clusters: The distance to the galactic centre
The system of globular clusters in the Galaxy is used in order to locatethe position of the galactic centre and, consequently, to determine thegalacto-centric distance of the Sun, R0. The spacedistribution and metallicities of the whole sample of known globularclusters are taken in to account, and the obtained results indicate that8.6 greater than R0 (kpc) greater than 6.4. Analysis of aselected sample of clusters further suggests that R0 equals7.6 +/- 0.4 kpc.
| The proto-galaxy, globular clusters, and quasars
Metallicities and horizontal-branch population gradients are used todistinguish between two populations of Galactic halo globular clusters.The beta population of clusters appears to be almost coeval and occursmainly at R less than R(solar). Clusters of the beta population wereprobably formed during a rapid Eggen, Lynden-Bell, and Sandage-typecollapse of the inner proto-Galaxy. Clusters of the alpha population alloccur at R greater than 8 kpc. Many of these objects are found to lie onplunging retrograde orbits. This suggests that clusters of the alphapopulation formed during an extended Searle-Zinn-type merger andcollapse phase. The relative ages, metallicities, and locations of thealpha and beta populations indicate that the Galaxy formed inside out,with the dense proto-Galactic core forming stars and clusters before itslower density halo. In a massive galaxy the quasar phenomenon probablytakes place after the bulk of its globular cluster system has formed. Aquasar outburst may have guillotined cluster formation in the nearby S0galaxy NGC 3115, thus preventing the formation of a thick-disk globularcluster population.
| Gamma rays from globular clusters
Globular clusters are known to contain a relatively large number ofpulsars whose individual and collective emission in the X-ray andgamma-ray energy bands may be detectable by the instruments on board theCompton Gamma-Ray Observatory (CGRO), ROSAT, and possibly SIGMA. Wediscuss the several types of high-energy emission expected from isolatedand interacting binary pulsars in globular clusters. Individual orcollective high-energy emission from isolated pulsars is expected to betoo low to be detected with current instruments. However, a class ofhigh-luminosity hidden millisecond pulsars enshrouded in the evaporatingmaterial from irradiated companion stars can produce unpulsed shockemission detectable by the high-sensitivity instruments of ROSAT andCGRO. Establishing upper limits of high-energy emission from globularclusters will be valuable in constraining models for the formation ofcluster millisecond pulsars.
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