Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
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.
| The period-luminosity relation for type II Cepheids in globular clusters
We report the result of our near-infrared observations (JHKs)for type II Cepheids (including possible RV Tau stars) in galacticglobular clusters. We detected variations of 46 variables in 26 clusters(10 new discoveries in seven clusters) and present their light curves.Their periods range from 1.2 d to over 80 d. They show a well-definedperiod-luminosity relation at each wavelength. Two type II Cepheids inNGC 6441 also obey the relation if we assume the horizontal branch starsin NGC 6441 are as bright as those in metal-poor globular clusters inspite of the high metallicity of the cluster. This result supports thehigh luminosity which has been suggested for the RR Lyr variables inthis cluster. The period-luminosity relation can be reproduced using thepulsation equation assuming that all the stars have the same mass.Cluster RR Lyr variables were found to lie on an extrapolation of theperiod-luminosity relation. These results provide important constraintson the parameters of the variable stars.Using Two Micron All-Sky Survey (2MASS) data, we show that the type IICepheids in the Large Magellanic Cloud (LMC) fit our period-luminosityrelation within the expected scatter at the shorter periods. However, atlong periods (P > 40 d, i.e. in the RV Tau star range) the LMC fieldvariables are brighter by about one magnitude than those of similarperiods in galactic globular clusters. The long-period cluster starsalso differ from both these LMC stars and galactic field RV Tau stars ina colour-colour diagram. The reasons for these differences arediscussed.
| 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.
| 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.
| Infrared Echelle Spectroscopy of Palomar 6 and M71
We present high-resolution infrared echelle spectroscopy for theglobular clusters Palomar 6 and M71. Our mean heliocentric radialvelocity of Pal 6 is +180.6+/-3.2 km s-1 and is 20 kms-1 lower than that found by Minniti in 1995. Contrary to theprevious metallicity estimates using low-resolution spectroscopy, ourresults show that Pal 6 has an intermediate metallicity, with[Fe/H]=-1.0+/-0.1, and is slightly more metal poor than M71. Reasonablechanges in the surface temperature or the microturbulent velocity of themodel atmospheres do not affect [Fe/H] at more than +/-0.2 dex. In spiteof its high metallicity, on the basis of the spectrum of a singlecluster member the [Si/Fe] and [Ti/Fe] ratios of Pal 6 appear to beenhanced by 0.4 and 0.5 dex, respectively, suggesting that the Galacticinner halo may have experienced a very rapid chemical enrichmenthistory.Based on observations made with the Infrared Telescope Facility, whichis operated by the University of Hawaii under contract to the NationalAeronautics and Space Administration.
| An Infrared Spectroscopic Study of Eight Galactic Globular Clusters
We have obtained medium-resolution infrared K-band spectra of 44 giantsin seven heavily reddened clusters in the Galactic bulge, as well as 12giants in ω Centauri. We measure the equivalent widths of the Nadoublet, the Ca triplet, and the CO band head, and then apply the newtechnique of Frogel et al. to determine the metallicity of each star.Averaging these values, we estimate the metallicity for each cluster andcompare our new [Fe/H] values with previous determinations from theliterature. Our estimates for each cluster are NGC 6256 (-1.35), NGC6539 (-0.79), HP 1 (-1.30), Liller 1 (-0.36), Palomar 6 (-0.52), Terzan2 (-0.87), and Terzan 4 (-1.62). We briefly discuss differences betweenthe various [Fe/H] scales on which it was possible to base ourcalibration, which is found to be the largest uncertainty in using thistechnique to determine metallicities.
| 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.
| Search for star clusters close to the Galactic plane with DENIS
An automated search for star clusters close to the Galactic plane (|b|< 5o) was carried out on the Point Source Catalogue of theDENIS survey. 44% of the Galactic plane have been observed andcalibrated. The method allowed to retrieve 22 known star clusters and toidentify two new ones, not published yet although previously presentedin the 2MASS web site as embedded clusters in HII regions. Extinction inthe field and in front of the clusters are estimated using a model ofpopulation synthesis. We present the method and give the properties ofthese clusters. Based on observations collected at ESO La Silla.
| The Galactic Bulge
Data for Bulge field and globular clusters are described and discussedin the light of current theories of bulge formation.
| Colour Magnitude Diagrams of the moderately metal-rich globular clusters NGC 6569 and Palomar 11
For the first time Colour-Magnitude Diagrams are presented for NGC 6569and Palomar 11. NGC 6569 is a rather concentrated cluster whereasPalomar 11 is loose and sparsely populated. Cluster parameters arederived from V and I photometry. These are red Horizontal Branchglobular clusters, with metallicities comparable to that of 47 Tucanae.We derive a reddening E(B-V) ~ 0.53 and a distance from the Sundsun ~ 9.8 kpc for NGC 6569, and E(B-V) ~ 0.35 anddsun ~ 13.2 kpc for Palomar 11. NGC 6569 is located in thebulge, while Palomar 11, despite its rather high metallicity, is in theinner halo, a rare case similar to that of 47 Tucanae. Observationscollected at the European Southern Observatory - ESO, Chile, proposalno. 64L-0212(A).
| 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.
| 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.
| B and V photometry of the metal-rich bulge globular cluster NGC 6304
We present B and V photometry of the bulge globular cluster NGC 6304. Wederive a reddening E(B-V) ~ 0.5 and a distance from the Sundsun ~ 6 kpc. From the red giant branch morphology weestimate that the metallicity of NGC 6304 is intermediate between thoseof 47 Tuc and NGC 6528. The cluster is foreground to the bulk of thebulge population and the reddenings are comparable. Evidence of a hotstellar component belonging to the cluster is found. These hot starscould correspond to a blue extended horizontal branch and/or bluestragglers. Observations collected at the European Southern Observatory-- ESO, Chile, proposal no. 61.E-0335
| Blue horizontal branch globular clusters towards the bulge: Terzan 9, NGC 6139 and NGC 6453
We present V and I photometry of the globular clusters Terzan 9 and NGC6453 for the first time, and also of NGC 6139. The three clusters areprojected towards the bulge. The Colour-Magnitude Diagram morphologiesreveal that the sample clusters have blue horizontal branches. Thischaracterizes an important fraction of the clusters projected on thebulge, with implications on the early Galaxy history. From thecolour-magnitude diagrams we derive reddenings of E(B-V) = 1.95, 0.77,0.70 and distances dsun~ 4.9, 9.4, 8.5 kpc respectively forTerzan 9, NGC 6139 and NGC 6453. We conclude that these metal-poorclusters are within the bulge volume. Observations collected at theEuropean Southern Observatory - ESO, Chile, proposal No. 61.E-0335.
| The metal-rich bulge globular cluster NGC 6401
We present V and I photometry for the bulge globular cluster NGC 6401for the first time. The Colour-Magnitude Diagram reveals a redhorizontal branch, and the cluster is metal-rich ([Fe/H] ~ -0.7). NGC6401 is located at 5.3(deg) from the Galactic center, turning out to bean interesting target to trace the extent of the bulge. A reddeningE(B-V) = 0.53+/-0.15 and a distance from the Sun dsun ~12.0+/-1.0 kpc are derived. The cluster is slightly behind the bulk ofthe bulge population in that direction, but still within the bulgevolume. Since the number of clusters with Horizontal Branch informationhas increased enormously in the later years for the central 20(deg)x20(deg), we present a discussion on the distribution of red and bluehorizontal branch clusters and their possible relation to bulge and/orhalo. Observations collected at the European Southern Observatory --ESO, Chile, proposal no. 61.E-0335
| Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy
The High-Resolution Echelle Spectrometer (HIRES) on the Keck I telescopehas been used to measure the first radial velocities for stars belongingto 11 heavily reddened globular clusters in the direction of the innerGalaxy. The sample consists of the clusters Terzan 3, NGC 6256, IC 1257,NGC 6380 (=Ton 1), Ton 2 (=Pismis 26), Djorg 1, NGC 6540 (=Djorg 3), IC1276 (=Pal 7), Terzan 12, NGC 6749, and Pal 10. Candidate clustermembers were selected from a combination of previously publishedcolor-magnitude diagrams (CMDs) and new instrumental CMDs obtained withthe Palomar 1.5 m telescope. The systemic velocities of Djorg 1 and Pal10 should be considered provisional, since velocities are available foronly two stars. For the remaining nine clusters, we have measured radialvelocities for three to nine member stars. Using our HIRES spectra, weestimate metallicities of [Fe/H]~=-0.75 for both Terzan 3 and IC 1276,two clusters lacking previous metallicity estimates. The question ofkinematic substructuring among the Galactic globular clusters isinvestigated using an updated catalog of globular cluster distances,metallicities, and velocities. It is found that the population ofmetal-rich globular clusters shows significant rotation at allGalactocentric radii. For the metal-rich clusters within 4 kpc of theGalactic center, the measured rotation velocity and line-of-sightvelocity dispersion are similar to those of bulge field stars. Weinvestigate claims that the metal-rich clusters are associated with thecentral Galactic bar by comparing the kinematics of the innermostclusters to that of the atomic hydrogen in the inner Galaxy. Thelongitude-velocity diagram of both metal-rich and metal-poor clustersbears a remarkable similarity to that of the gas, including the samenoncircular motions that have traditionally been interpreted as evidencefor a Galactic bar, or, alternatively, a nonaxisymmetric bulge. However,uncertainties in the existing three-dimensional Galactocentric positionsfor most of the clusters do not yet allow an unambiguous discriminationbetween the competing scenarios of membership in a rigidly rotating baror in a bulge that is an oblate isotropic rotator. We conclude that themajority of metal-rich clusters within the central ~4 kpc of the Galaxyare probably associated with the bulge/bar, and not the thick disk.
| The metal-rich globular clusters of the Milky Way
We present new (V,V-I)-photometry of the metal-rich globular clustersNGC 5927, 6316, 6342, 6441 and 6760. The clustersshow differential reddening up to delta E_{V-I}=0.32 mag, for which theCMDs are corrected via extinction maps. There are hints of a variationin the extinction law. Two different ways to determine the parametersmetallicity, reddening and distance lead to consistent results. Themetallicities of the clusters range between -0.7 <= [M/H] <= 0.0dex and the absolute reddening between 0.43 <= E_{V-I} <= 0.76mag. Taking the differential reddening into account leads to slightlyincreased distances. From the resulting parameters we conclude that theusual halo-disk-distinction in the system of globular clusters seemsquestionable.
| Colour-magnitude diagrams of the post-core collapse globular clusters NGC 6256 and NGC 6717 (Palomar 9)
{ We present photometry of the globular clusters NGC 6256 in V, I, NGC6717, NGC 6256 in V, I, and NGC 6717 in B, V, obtaining clusterparameters. NGC 6256 has a post core-collapse structure, while suchfeature is suspected for NGC 6717. Both clusters are projected on thebulge. From the colour-magnitude diagrams for NGC 6256 we derivereliable cluster parameters which considerably revise its properties,with respect to previous literature. It has a blue-extended HorizontalBranch already observed in other post-core collapse clusters in thebulge, such as HP1. We derive a reddening of E(B-V) = 1.10, a distancedsun ~ 6.4 kpc, and an intermediate metallicity for NGC 6256.NGC 6717 presents a blue Horizontal Branch. We derive a reddening ofE(B-V) = 0.23 and a distance dsun ~ 7.1 kpc, locating thecluster in the bulge. The photometry is deep and we study the luminosityfunction, which presents some evidence of depletion of Main Sequencestars. This effect is not unexpected for such a poorly populated Palomarcluster in the bulge. We also estimate the cluster age from themagnitude difference between the horizontal branch and the turnoff. Weobtain Delta V() TO_HB = 3.7, which would place this intermediatemetallicity bulge globular as coeval with the halo.} Observationscollected at the European Southern Observatory - ESO, Chile.
| Calibration of the Faber--Jackson relation for M31 globular clusters using HIPPARCOS data
In this paper we present data analysis regarding globular clusters aspossible extragalactic distance indicators. For this purpose, wecollected all velocity dispersion measurements published for Galacticand M31 globular clusters. The slope and zero-point of theFaber--Jackson relation were calibrated using Hipparcos distancemeasurements, and the relation was applied to extragalactic globularclusters in M31. A distance modulus of 24.12 +/- 0.45 mag was found.This is consistent with those found by fitting the red giant branches ofglobular clusters (24.47 +/- 0.07) and from the peak of the globularcluster luminosity function (24.03 +/- 0.23), but is lower than thevalues of 24.84 +/- 0.2 mag and 24.77 +/- 0.11 mag obtained by usingHipparcos data to calibrate the Cepheid period--luminosity relation.This calibrated Faber--Jackson relation can now be used directly forother Sc galaxies with resolved globular clusters, as soon as largenumbers of spectra become available, e.g. through the Very LargeTelescope (VLT).
| Integrated spectral study of reddened globular clusters and candidates
This paper presents integrated spectra in the range 6700 - 9500Angstroms for 20 Galactic globular clusters (and candidates) in thebulge and 5 others projected on the Galactic disk (|l|>30mbox{^{\circ}}\ and |b|<5mbox {^{\circ}}). Most of them are considerablyreddened and are among those least studied in the literature. We derivereddening and metallicity from flux-calibrated spectra, thus providingindependent information of that derived through colour-magnitudestudies. For some clusters in the sample, these parameters have beendetermined for the first time, and for others a considerable revision isgiven. We indicate the globular clusters which definitely belong to thebulge metal-rich and intermediate metallicity families. Among theobjects projected on the disk, we find that the integrated spectralproperties of Lyng\aa 7, BH 176 and Palomar 10 are compatible with thoseof metal-rich globular clusters. Finally, ESO 93-SC08 is an old opencluster, and UKS 2 is an open cluster with age ~1 Gyr. Based onobservations made at the Complejo Astronómico El Leoncito(CASLEO), Argentina, and European Southern Observatory (ESO), Chile.
| The Supermassive Black Hole of M87 and the Kinematics of Its Associated Gaseous Disk
We have obtained long-slit observations of the circumnuclear region ofM87 at three different locations, with a spatial sampling of 0."028using the Faint Object Camera f/48 spectrograph on board HST. These dataallow us to determine the rotation curve of the inner ~1" of the ionizedgas disk in [O II] lambda 3727 to a distance as close as 0."07 (~=5 pc)to the dynamic center, thereby significantly improving on both thespatial resolution and coverage of previous FOS observations. We havemodeled the kinematics of the gas under the assumption of the existenceof both a central black hole and an extended central mass distribution,taking into account the effects of the instrumental PSF, the intrinsicluminosity distribution of the line, and the finite size of the slit. Wefind that the central mass must be concentrated within a sphere whosemaximum radius is 0."05 (~=3.5 pc) and show that both the observedrotation curve and line profiles are consistent with a thin disk inKeplerian motion. We conclude that the most likely explanation for theobserved motions is the presence of a supermassive black hole and derivea value of MBH = (3.2 +/- 0.9) x 109 Mȯ for its mass.
| Core velocity dispersions for 25 Galactic and 10 old Magellanic globular clusters.
We present, for 25 Galactic and 10 old Magellanic globular clusters,projected velocity dispersion (σ_p_) measurements obtained byapplying a cross-correlation technique to integrated-light spectra. Inorder to understand and estimate the statistical errors of thesemeasurements due to small numbers of bright stars dominating theintegrated light, we provide an extensive discussion based on detailednumerical simulations. These errors are smaller if the integration areais larger and/or the cluster concentration higher. The simulations showthat measurements are reliable when the integrated light within theintegration area is brighter than a given magnitude. The statisticalerrors on the σ_p_ measurements of Magellanic globular clustersare small because of a physically large integration area, whereas theycan be important for measurements carried out over small central areasin Galactic clusters. The present observational results are used tooutline a few characteristics of the globular cluster fundamental plane.In this respect, the old Magellanic globular clusters appear similar tothe Galactic clusters.
| Gravitational mesolensing by King objects and quasar-galaxy associations
The probability of strong gravitational lensing of distant compactobjects by transparent lenses with King mass distributions iscalculated. Examples of such lenses are globular clusters, dwarfgalaxies, and clusters of hidden mass. The lens masses thus range from10^3 to 10^9 M_solar, intermediate between stellar and galactic masses.Taking account of the conic caustic of King lenses and the fractallarge-scale distribution of matter along the line of sight, it ispossible to explain the correlation between quasars and nearby galaxiesas the effect of strong gravitational lensing of the active nuclei ofdistant galaxies by globular clusters in the halos of nearby galaxies.Observational tests for verifying this mesolensing hypothesis aredescribed.
| 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
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
