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The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog
The first catalog of the RR Lyrae stars (RRLSs) in the Galactic halo bythe Quasar Equatorial Survey Team (QUEST) has been searched forsignificant overdensities that may be debris from disrupted dwarfgalaxies or globular clusters. These RRLSs are contained in a band ~2.3dwide in declination that spans ~165° in right ascension and lie ~4to ~60 kpc from the Sun. Away from the major overdensities, thedistribution of these stars is adequately fitted by a smooth halo model,in which the flattening of the halo decreases with increasinggalactocentric distance (as reported by Preston et al.). This model wasused to estimate the ``background'' of RRLSs on which the halooverdensities are overlaid. A procedure was developed for recognizinggroups of stars that constitute significant overdensities with respectto this background. To test this procedure, a Monte Carlo routine wasused to make artificial RRLS surveys that follow the smooth halo modelbut with Poisson-distributed noise in the numbers of RRLSs and, withinlimits, random variations in the positions and magnitudes of theartificial stars. The 104 artificial surveys created by thisroutine were examined for significant groups in exactly the same way asthe QUEST survey. These calculations provided estimates of thefrequencies with which random fluctuations produce significant groups.In the QUEST survey there are six significant overdensities that containsix or more stars and several smaller ones. The small ones and possiblyone or two of the larger ones may be artifacts of statisticalfluctuations, and they need to be confirmed by measurements of radialvelocity and/or proper motion. The most prominent groups are thenorthern stream from the Sagittarius dwarf spheroidal galaxy and a largegroup in Virgo, formerly known as the ``12.4 hr clump,'' which Duffauand coworkers have recently shown to contain a stellar stream (the Virgostellar stream). Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Anothergroup is related to the globular cluster Palomar 5.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| 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.
| 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.
| Galaxy Evolution Explorer Ultraviolet Photometry of Globular Clusters in M31
We present ultraviolet photometry for globular clusters (GCs) in M31from 15 deg2 of imaging using the Galaxy Evolution Explorer(GALEX). We detect 200 and 94 GCs with certainty in the near-ultraviolet(NUV; 1750-2750 Å) and far-ultraviolet (FUV; 1350-1750 Å)bandpasses, respectively. Our rate of detection is about 50% in the NUVand 23% in the FUV, to an approximate limiting V magnitude of 19. Out ofsix clusters with [Fe/H]>-1 seen in the NUV, none is detected in theFUV bandpass. Furthermore, we find no candidate metal-rich clusters withsignificant FUV flux because of the contribution of bluehorizontal-branch (HB) stars, such as NGC 6388 and NGC 6441, which aremetal-rich Galactic GCs with hot HB stars. We show that our GALEXphotometry follows the general color trends established in previous UVstudies of GCs in M31 and the Galaxy. Comparing our data with GalacticGCs in the UV and with population synthesis models, we suggest that theage ranges of M31 and Galactic halo GCs are similar.
| The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars
We discuss a 175 deg2 spectroscopic survey for bluehorizontal branch (BHB) stars in the Galactic halo. We use the TwoMicron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) toselect BHB candidates, and we find that the 2MASS and SDSS colorselection is 38% and 50% efficient, respectively, for BHB stars. Oursamples include one likely runaway B7 star 6 kpc below the Galacticplane. The global properties of the BHB samples are consistent withmembership in the halo population: the median metallicity is[Fe/H]=-1.7, the velocity dispersion is 108 km s-1, and themean Galactic rotation of the BHB stars 3 kpc<|z|<15 kpc is-4+/-30 km s-1. We discuss the theoretical basis of thePreston, Shectman, and Beers MV-color relation for BHB starsand conclude that the intrinsic shape of the BHB MV-colorrelation results from the physics of stars on the horizontal branch. Wecalculate the luminosity function for the field BHB star samples usingthe maximum likelihood method of Efstathiou and coworkers, which isunbiased by density variations. The field BHB luminosity functionexhibits a steep rise at bright luminosities, a peak between0.8
| Galactic Globular Cluster Relative Ages
We present accurate relative ages for a sample of 55 Galactic globularclusters. The ages have been obtained by measuring the differencebetween the horizontal branch and the turnoff in two internallyphotometrically homogeneous databases. The mutual consistency of the twodata sets has been assessed by comparing the ages of 16 globularclusters in common between the two databases. We have also investigatedthe consistency of our relative age determination within the recentstellar model framework. All clusters with [Fe/H]<-1.7 are found tobe old and coeval, with the possible exception of two objects, which aremarginally younger. The age dispersion for the metal-poor clusters is0.6 Gyr (rms), consistent with a null age dispersion.Intermediate-metallicity clusters (-1.7<[Fe/H]<-0.8) are onaverage 1.5 Gyr younger than the metal-poor ones, with an age dispersionof 1.0 Gyr (rms) and a total age range of ~3 Gyr. About 15% of theintermediate-metallicity clusters are coeval with the oldest clusters.All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than the mostmetal-poor ones, with a relatively small age dispersion, although themetal-rich sample is still too small to allow firmer conclusions. Thereis no correlation of the cluster age with the galactocentric distance.We briefly discuss the implication of these observational results forthe formation history of the Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555, and on observations made at the European SouthernObservatory, La Silla, Chile, and with the Isaac Newton GroupTelescopes.
| A robust method for the analysis of integrated spectra from globular clusters using Lick indices
We define a method for the analysis of the integrated spectra ofextragalactic globular clusters that provides more reliable measures ofthe age, metallicity and α-element abundance ratio than have sofar been achieved. The method involves the simultaneous fitting of up to25 Lick indices in a χ2 fitting technique that maximizesthe use of the available data. Here we compare three sets of singlestellar population (SSP) models of Lick indices to the highsignal-to-noise, integrated spectra of 20 Galactic globular clusters.The ages, [Fe/H] and α-element abundance ratios derived from theSSP models are compared with the results of resolved stellar populationstudies from the literature. We find good consistency with the publishedvalues, with an agreement of better than 0.1 dex in all three derivedparameters. The technique allows the identification of abundance ratioanomalies, such as the known nitrogen overabundance in Galactic globularclusters, and the presence of anomalous horizontal branch morphologies.It also minimizes the impact on the derived parameters of imperfectcalibration to the Lick system, and reduction errors in general. Themethod defined in this paper is therefore robust with respect to many ofthe difficulties that plague the application of SSP models in general.Consequently, it is well suited to the study of extragalactic globularcluster systems.
| Integrated spectral energy distributions and absorption-feature indices of single stellar populations
Using evolutionary population synthesis, we present integrated spectralenergy distributions and absorption-line indices defined by the LickObservatory image dissector scanner (referred to as Lick/IDS) system,for an extensive set of instantaneous burst single stellar populations(SSPs). The ages of the SSPs are in the range 1 Gyr <=τ<= 19Gyr and the metallicities are in the range -2.3 <=[Fe/H]<=+0.2.Our models use the rapid single stellar evolution algorithm of Hurley,Pols and Tout for the stellar evolutionary tracks, the empirical andsemi-empirical calibrated BaSeL-2.0 model of Lejeune, Cuisinier andBuser for the library of stellar spectra and the empirical fittingfunctions of Worthey, Faber, Gonzalez and Burstein for the Lick/IDSspectral absorption-feature indices.Applying our synthetic Lick/IDS absorption-line indices to the meritfunction, we obtain the age and the metallicity of the central region ofM32, which can be well explained by an instantaneous SSP with an age of~6.5 Gyr and a metallicity similar to solar. Applying the derived ageand the metallicity from the merit function to a number of index-indexdiagrams, we find that the plots of Hβ-Fe5015 andHβ-Fe5782 are the best index-index diagrams from whichwe can directly obtain reasonable age and metallicity.
| A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream
We have obtained moderate resolution (~6 km s-1) spectroscopyof several hundred M giant candidates selected from Two Micron All SkySurvey photometry. Radial velocities are presented for stars mainly inthe southern Galactic hemisphere, and the primary targets have Galacticpositions consistent with association to the tidal tail system of theSagittarius (Sgr) dwarf galaxy. M giant stars selected from the apparenttrailing debris arm of Sgr have velocities showing a clear trend withorbital longitude, as expected from models of the orbit and destructionof Sgr. A minimum 8 kpc width of the trailing stream about the Sgrorbital midplane is implied by verified radial velocity members. Thecoldness of this stream (σv~10 km s-1)provides upper limits on the combined contributions of stream heating bya lumpy Galactic halo and the intrinsic dispersion of released stars,which is a function of the Sgr core mass. We find that the Sgr trailingarm is consistent with a Galactic halo that contains one dominant,LMC-like lump; however, some lumpier halos are not ruled out. An upperlimit to the total mass-to-light ratio of the Sgr core is 21 in solarunits. Evidence for other velocity structures is found among the moredistant (>13 kpc) M giants. A second structure that roughly mimicsexpectations for wrapped, leading Sgr arm debris crosses the trailingarm in the southern hemisphere; however, this may also be an unrelatedtidal feature. Among the bright, nearby (<13 kpc) M giants toward thesouth Galactic pole are a number with large velocities that identifythem as halo stars; these too may trace halo substructure, perhaps partof the Sgr leading arm near the Sun. The positions and velocities ofsouthern hemisphere M giants are compared with those of southernhemisphere globular clusters potentially stripped from the Sgr system.Support for association of the globular clusters Pal 2 and Pal 12 withSgr debris is found, based on positional and radial velocity matches.Our discussion includes description of a masked-filteredcross-correlation methodology that achieves better than 1/20 of aresolution element velocities in moderate-resolution spectra. Theimproved velocity resolution achieved allows tighter constraints to beplaced on the coldness of the Sgr stream than previously established.
| A Globular Cluster Metallicity Scale Based on the Abundance of Fe II
Assuming that in the atmospheres of low-mass, metal-poor red giantstars, one-dimensional models based on local thermodynamic equilibriumaccurately predict the abundance of iron from Fe II, we derive aglobular cluster metallicity scale based on the equivalent widths of FeII lines measured from high-resolution spectra of giants in 16 keyclusters lying in the abundance range-2.4<[Fe/H]II<-0.7. We base the scale largely on theanalysis of spectra of 149 giant stars in 11 clusters by the Lick-Texasgroup supplemented by high-resolution studies of giants in five otherclusters. We also derive ab initio the true distance moduli for certainkey clusters (M5, M3, M13, M92, and M15) as a means of setting stellarsurface gravities. Allowances are made for changes in the abundancescale if one employs (1) Kurucz models with and without convectiveovershooting to represent giant star atmospheres in place of MARCSmodels and (2) the Houdashelt et al. color-temperature scale in place ofthe Alonso et al. scale.We find that [Fe/H]II is correlated linearly withW', the reduced strength of the near-infrared Ca II tripletdefined by Rutledge et al., although the actual correlation coefficientsdepend on the atmospheric model employed. The correlations, limited tothe range -2.4<[Fe/H]II<-0.7, are as follows:1.[Fe/H]II=0.531W'-3.279(MARCS),2.[Fe/H]II=0.537W'-3.225 (Kurucz withconvective overshooting),3.[Fe/H]II=0.562W'-3.329 (Kurucz withoutconvective overshooting).We also discuss how to estimate [X/Fe] ratios. We suggest that C, N, andO, as well as elements appearing in the spectrum in the singly ionizedstate, e.g., Ti, Sc, Ba, La, and Eu, should be normalized to theabundance of Fe II. Other elements, which appear mostly in the neutralstate, but for which the dominant species is nevertheless the ionizedstate, are probably best normalized to Fe I, but uncertainties remain.
| Stellar population models of Lick indices with variable element abundance ratios
We provide the whole set of Lick indices from CN1 toTiO2 in the wavelength range 4000 <~<~λ<~6500 Å of simple stellar population models with, for the firsttime, variable element abundance ratios, [α/Fe ]= 0.0, 0.3, 0.5,[α/Ca ]=-0.1, 0.0, 0.2, 0.5 and [α/N]=-0.5, 0.0. The modelscover ages between 1 and 15 Gyr, metallicities between 1/200 and 3.5solar. The impact from the element abundance changes on theabsorption-line indices is taken from Tripicco & Bell, using anextension of the method introduced by Trager et al. Our models are freefrom the intrinsic α/Fe bias that was imposed by the Milky Waytemplate stars up to now, hence they reflect well-defined α/Feratios at all metallicities. The models are calibrated with Milky Wayglobular clusters for which metallicities and α/Fe ratios areknown from independent spectroscopy of individual stars. Themetallicities that we derive from the Lick indices Mgb and Fe5270 are inexcellent agreement with the metallicity scale by Zinn & West, andwe show that the latter provides total metallicity rather than ironabundance. We can reproduce the relatively strong CN-absorption featuresCN1 and CN2 of galactic globular clusters withmodels in which nitrogen is enhanced by a factor of 3. An enhancement ofcarbon, instead, would lead to serious inconsistencies with the indicesMg1 and C24668. The calcium sensitive index Ca4227of globular clusters is well matched by our models with [Ca/Fe]= 0.3,including the metal-rich bulge clusters NGC 6528 and 6553. From ourα/Fe-enhanced models we infer that the index [MgFe] defined byGonzález is quite independent of α/Fe but still slightlydecreases with increasing α/Fe. We find that the index , instead,is completely independent of α/Fe and serves best as a tracer oftotal metallicity. Searching for blue indices that give similarinformation as Mg b and , we find that CN1 andFe4383 may be best suited to estimating α/Fe ratios of objects atredshifts z~ 1.
| 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.
| 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.
| Integrated spectroscopy of bulge globular clusters and fields. II. Implications for population synthesis models and elliptical galaxies
An empirical calibration is presented for the synthetic Lick indices(e.g. Mg2, , Hβ, etc.) of Simple StellarPopulation (SSP) models that for the first time extends up to solarmetallicity. This is accomplished by means of a sample of Milky Wayglobular clusters (GCs) whose metallicities range from ~Zsun/30 to Z ~ Zsun, thanks to the inclusion ofseveral metal rich clusters belonging to the Galactic bulge (e.g., NGC6553 and NGC 6528). This metallicity range approaches the regime that isrelevant for the interpretation of the integrated spectra of ellipticalgalaxies. It is shown that the spectra of both the globular clusters andthe Galactic bulge follow the same correlation between magnesium andiron indices that extends to elliptical galaxies, showing weaker ironindices at given magnesium indices with respect to the predictions ofmodels that assume solar-scaled abundances. This similarity providesrobust empirical evidence for enhanced [α/Fe] ratios in thestellar populations of elliptical galaxies, since the globular clustersare independently known to have enhanced [α/Fe] ratios fromspectroscopy of individual stars. We check the uniqueness of this alpha-overabundance solution by exploring the whole range of modelingredients and parameters, i.e. fitting functions, stellar tracks, andthe initial mass function (IMF). We argue that the standard models(meant for solar abundance ratios) succeed in reproducing the Mg-Fecorrelation at low metallicities ([Z/H]lapprox-0.7) because the stellartemplates used in the synthesis are Galactic halo stars that actuallyare alpha -enhanced. The same models, however, fail to predict theobserved Mg-Fe pattern at higher metallicities ([Z/H] >~-0.7) (i.e.,for bulge clusters and ellipticals alike) because the high-metallicitytemplates are disk stars that are not alpha -enhanced. We show that thenew set of SSP models which incorporates the dependence on the[α/Fe] ratio (Thomas et al. \cite{Thomas02b}) is able toreproduce the Mg and Fe indices of GCs at all metallicities, with analpha -enhancement [α/Fe]=+0.3, in agreement with the availablespectroscopic determinations. The Hβ index and the higher-orderBalmer indices are well calibrated, provided the appropriate morphologyof the Horizontal Branch is taken into account. In particular, theBalmer line indices of the two metal rich clusters NGC 6388 and NGC6441, which are known to exhibit a tail of warm Horizontal Branch stars,are well reproduced. Finally, we note that the Mg indices of verymetal-poor ([Z/H] <~-1.8) populations are dominated by thecontribution of the lower Main Sequence, hence are strongly affected bythe present-day mass function of individual globular clusters, which isknown to vary from cluster to cluster due to dynamical effects.
| Urban Astronomy: Observing the Messier Objects from the City
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| On the reliability of the semi-empirical RR Lyrae period-V-band luminosity-blue amplitude relation
We investigate the accuracy and reliability of the semi-empiricalperiod-V-band luminosity-blue amplitude (PLA) relationship for ab-typeRR Lyrae stars originally obtained by Castellani and De Santis in themid-1990s. We infer that the zero-point of this relationship depends onthe metallicity, by studying a sample of both field and clustervariables. We also show that the use of this relationship can still beuseful for those stellar systems showing an intrinsic metallicityspread, since in this case the metallicity has a negligible effect onthe final distance modulus estimate. We compare the adoptedsemi-empirical relationship with the fully empirical one recentlyprovided by Kovács & Walker. When the zero-point of thelatter relation is fixed consistently with the former one, the twoequations are equivalent. By applying the semi-empirical period-V-bandluminosity-blue amplitude relation, as well as the technique proposedrecently by Cassisi, De Santis & Piersimoni, to the globular clusterω Cen, we show that the empirical slope of the relationshipbetween the mass of the fundamental RR Lyrae pulsators and theirmetallicity is in fair agreement with the one predicted by updatedevolutionary models for horizontal branch stars.
| On the Distribution of Orbital Poles of Milky Way Satellites
In numerous studies of the outer Galactic halo some evidence foraccretion has been found. If the outer halo did form in part or whollythrough merger events, we might expect to find coherent streams of starsand globular clusters following orbits similar to those of their parentobjects, which are assumed to be present or former Milky Way dwarfsatellite galaxies. We present a study of this phenomenon by assessingthe likelihood of potential descendant ``dynamical families'' in theouter halo. We conduct two analyses: one that involves a statisticalanalysis of the spatial distribution of all known Galactic dwarfsatellite galaxies (DSGs) and globular clusters, and a second, morespecific analysis of those globular clusters and DSGs for which fullphase space dynamical data exist. In both cases our methodology isappropriate only to members of descendant dynamical families that retainnearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) isconsidered a paradigm for the type of merger/tidal interaction event forwhich we are searching, we also undertake a case study of the Sgr systemand identify several globular clusters that may be members of itsextended dynamical family. In our first analysis, the distribution ofpossible orbital poles for the entire sample of outer(Rgc>8 kpc) halo globular clusters is tested forstatistically significant associations among globular clusters and DSGs.Our methodology for identifying possible associations is similar to thatused by Lynden-Bell & Lynden-Bell, but we put the associations on amore statistical foundation. Moreover, we study the degree of possibledynamical clustering among various interesting ensembles of globularclusters and satellite galaxies. Among the ensembles studied, we findthe globular cluster subpopulation with the highest statisticallikelihood of association with one or more of the Galactic DSGs to bethe distant, outer halo (Rgc>25 kpc), second-parameterglobular clusters. The results of our orbital pole analysis aresupported by the great circle cell count methodology of Johnston,Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC6229, NGC 7006, and Pyxis are predicted to be among those most likely toshow the clusters to be following stream orbits, since these clustersare responsible for the majority of the statistical significance of theassociation between outer halo, second-parameter globular clusters andthe Milky Way DSGs. In our second analysis, we study the orbits of the41 globular clusters and six Milky Way-bound DSGs having measured propermotions to look for objects with both coplanar orbits and similarangular momenta. Unfortunately, the majority of globular clusters withmeasured proper motions are inner halo clusters that are less likely toretain memory of their original orbit. Although four potential globularcluster/DSG associations are found, we believe three of theseassociations involving inner halo clusters to be coincidental. While thepresent sample of objects with complete dynamical data is small and doesnot include many of the globular clusters that are more likely to havebeen captured by the Milky Way, the methodology we adopt will becomeincreasingly powerful as more proper motions are measured for distantGalactic satellites and globular clusters, and especially as resultsfrom the Space Interferometry Mission (SIM) become available.
| VLT spectroscopy of NGC 3115 globular clusters
We present results derived from VLT-FORS2 spectra of 24 differentglobular clusters associated with the lenticular galaxy NGC 3115. Asubsample of 17 globular clusters have sufficiently high signal-to-noiseto allow precision measurements of absorption line-strengths. Comparingthese indices to new stellar population models by Thomas et al. wedetermine ages, metallicities and element abundance ratios. For thefirst time these stellar population models explicitly take abundanceratio biases in the Lick/IDS stellar library into account. Our data arealso compared with the Lick/IDS observations of Milky Way and M 31globular clusters. Unpublished higher order Balmer lines(HγA ,F and HdeltaA ,F) from the Lick/IDSobservations are given in the Appendix. Our best age estimates show thatthe observed clusters which sample the bimodal colour distribution ofNGC 3115 are coeval within our observational errors (2-3 Gyr). Our bestcalibrated age/metallicity diagnostic diagram (Hβ / vs. [MgFe])indicates an absolute age of 11-12 Gyr consistent with the luminosityweighted age for the central part of NGC 3115. We confirm with ouraccurate line-strength measurements that the (V-I) colour is a goodmetallicity indicator within the probed metallicity range (-1.5 <[Fe/H] < 0.0). The abundance ratios for globular clusters in NGC 3115give an inhomogeneous picture. We find a range from solar to super-solarratios for both blue and red clusters. This is similar to the data for M31 while the Milky Way seems to harbour clusters which are mainlyconsistent with [alpha / Fe] =~ 0.3. From our accurate recessionvelocities we detect, independent of metallicity, clear rotation in thesample of globular clusters. In order to explain the metallicity andabundance ratio pattern, particularly the range in abundance ratios forthe metal rich globular clusters in NGC 3115, we favour a formationpicture with more than two distinct formation episodes. Based onobservations collected at the European Southern Observatory, CerroParanal, Chile (ESO No. 66.B-0131).
| Integrated spectroscopy of bulge globular clusters and fields. I. The data base and comparison of individual Lick indices in clusters and bulge
We present a comprehensive spectroscopic study of the integrated lightof metal-rich Galactic globular clusters and the stellar population inthe Galactic bulge. We measure line indices which are defined by theLick standard system and compare index strengths of the clusters andGalactic bulge. Both metal-rich globular clusters and the bulge aresimilar in most of the indices, except for the CN index. We find asignificant enhancement in the CN/ index ratio in metal-richglobular clusters compared with the Galactic bulge. The mean iron index of the two metal-rich globular clusters NGC 6528 and NGC 6553is comparable with the mean iron index of the bulge. Index ratios suchas Mgb/, Mg2/, Ca 4227/, andTiO/, are comparable in both stellar population indicatingsimilar enhancements in individual elements which are traced by theindices. From the globular cluster data we fully empirically calibrateseveral metallicity-sensitive indices as a function of [Fe/H] and findtightest correlations for the Mg2 index and the composite[MgFe] index. We find that all indices show a similar behavior withgalactocentric radius, except for the Balmer series, which show a largescatter at all radii. However, the scatter is entirely consistent withthe cluster-to-cluster variations in the horizontal branch morphology.Appendices B-D are only available in electronic form athttp://www.edpsciences.org
| Does the mixing length parameter depend on metallicity?. Further tests of evolutionary sequences using homogeneous databases
This paper is a further step in the investigation of the morphology ofthe color-magnitude diagram of Galactic globular clusters, and thefine-tuning of theoretical models, made possible by the recentobservational efforts to build homogeneous photometric databases. Inparticular, we examine here the calibration of the morphologicalparameter WHB vs. metallicity, originally proposed by Brocatoet al. (\cite{brocatoEtal98}; B98), which essentially measures the colorposition of the red-giant branch. We show that the parameter can be usedto have a first-order estimate of the cluster metallicity, since thedispersion around the mean trend with [Fe/H] is compatible with themeasurement errors. The tight WHB-[Fe/H] relation is thenused to show that variations in helium content or age do not affect theparameter, whereas it is strongly influenced by the mixing-lengthparameter alpha (as expected). This fact allows us, for the first time,to state that there is no trend of alpha with the metal content of acluster. A thorough examination of the interrelated questions of thealpha -elements enhancement and the color-Tefftransformations, highlights that there is an urgent need for anindependent assessment of which of the two presently acceptedmetallicity scales is the true indicator of a cluster's iron content.Whatever scenario is adopted, it also appears that a deep revision ofthe V-I-temperature relations is needed.
| 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.
| The distance to Galactic globular clusters through RR Lyrae pulsational properties
By adopting the same approach outlined by De Santis & Cassisi, weevaluate the absolute bolometric magnitude of the zero-age horizontalbranch (ZAHB) at the level of the RR Lyrae variable instability strip inselected Galactic globular clusters. This allows us to estimate the ZAHBabsolute visual magnitude for these clusters and to investigate itsdependence on the cluster metallicity. The derivedMV(ZAHB)-[Fe/H] relation, corrected in order to account forthe luminosity difference between the ZAHB and the mean RR Lyraemagnitude, has been compared with some of the most recent empiricaldeterminations in this field, such as the one provided byBaade-Wesselink analyses, RR Lyrae periods, Hipparcos data for fieldvariables and main-sequence fitting based on Hipparcos parallaxes forfield subdwarfs. As a result, our relation provides a clear support tothe `long' distance scale. We discuss also another method for measuringthe distance to Galactic globular clusters. This method is quite similarto the one adopted for estimating the absolute bolometric magnitude ofthe ZAHB but it relies only on the pulsational properties of the Lyraevariables in each cluster. The reliability and accuracy of this methodhave been tested by applying it to a sample of globular clusters forwhich, owing to the morphology of their horizontal branch (HB), the useof the commonly adopted ZAHB fitting is a risky procedure. We noticethat the two approaches for deriving the cluster distance modulusprovide consistent results when applied to globular clusters, the RRLyrae instability strip is well populated. As the adopted method relieson theoretical predictions on both the fundamental pulsational equationand the allowed mass range for fundamental pulsators, we give anestimate of the error affecting present results, owing to systematicuncertainties in the adopted theoretical framework.
| A new observable for timing faint red giant branch stars in globular clusters
A new observable - RBump - which is the ratio between thestar counts across the Red Giant Branch (RGB) Bump and fainter RGBstars, is presented. We use it for investigating the occurrence of adeep mixing phenomenon during these evolutionary phases. The comparisonbetween predicted and empirical RBump values, based on alarge and homogeneous set of HST data, brings out that evolutionarylifetimes predicted by canonical RGB models do account for the bulk ofGalactic Globular Clusters (GGCs) included in our sample. This evidencesuggest that Bump and fainter RGB stars do not show the occurrence of adeep mixing which significantly changes their chemical stratification.Few possible exceptions to this general rule are briefly discussed.
| Star Counts across the Red Giant Branch Bump and Below
We present a new observable-Rbump-which is the ratio betweenthe star counts across the red giant branch (RGB) bump and fainter RGBstars to investigate the occurrence of a deep-mixing phenomenon duringthese evolutionary phases. The comparison between predicted andempirical Rbump-values, based on a large and homogeneous setof Hubble Space Telescope data, brings out that evolutionary lifetimespredicted by canonical RGB models do account for the bulk of Galacticglobular clusters included in our sample (29). This evidence suggeststhat bump and fainter RGB stars do not show the occurrence of deepmixing, which significantly changes their chemical stratification. A fewpossible exceptions to this general rule are briefly discussed. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by AURA, Inc.,under NASA contract NAS5-26555, and on observations retrieved from theESO ST-ECF archive.
| Variable Stars in Galactic Globular Clusters
Based on a search of the literature up to 2001 May, the number of knownvariable stars in Galactic globular clusters is approximately 3000. Ofthese, more than 2200 have known periods and the majority (approximately1800) are of the RR Lyrae type. In addition to the RR Lyrae population,there are approximately 100 eclipsing binaries, 120 SX Phoenicisvariables, 60 Cepheids (including Population II Cepheids, anomalousCepheids and RV Tauri), and 120 SR/red variables. The mean period of thefundamental mode RR Lyrae variables is 0.585 days, for the overtonevariables it is 0.342 days (0.349 days for the first-overtone pulsatorsand 0.296 days for the second-overtone pulsators) and approximately 30%are overtone pulsators. These numbers indicate that about 65% of RRLyrae variables in Galactic globular clusters belong to Oosterhoff typeI systems. The mean period of the RR Lyrae variables in the Oosterhofftype I clusters seems to be correlated with metal abundance in the sensethat the periods are longer in the more metal poor clusters. Such acorrelation does not exist for the Oosterhoff type II clusters. Most ofthe Cepheids are in clusters with blue horizontal branches.
| Ages and Metallicities of Fornax Dwarf Elliptical Galaxies
Narrowband photometry is presented on 27 dwarf ellipticals in the Fornaxcluster. Calibrated with Galactic globular cluster data andspectrophotometric population models, the colors indicated that dwarfellipticals have a mean [Fe/H] of -1.00+/-0.28 ranging from -1.6 to-0.4. The mean age of dwarf ellipticals, also determinedphotometrically, is estimated at 10+/-1 Gyr compared with 13 Gyr forbright Fornax ellipticals. Comparison of our metallicity color andMg2 indices demonstrates that the [Mg/Fe] ratio is lower indwarf ellipticals than their more massive cousins, which is consistentwith a longer duration of initial star formation to explain theiryounger ages. There is a increase in dwarf metallicity with distancefrom the Fornax cluster center, where core galaxies are on average 0.5dex more metal-poor than halo dwarfs. In addition, we find the halodwarfs are younger in mean age compared with core dwarfs. One possibleexplanation is that the intracluster medium ram pressure strips the gasfrom dwarf ellipticals, halting star formation (old age) and stoppingenrichment (low metallicity) as they enter the core.
| Empirical relations for cluster RR Lyrae stars revisited
Our former study on the empirical relations between the Fourierparameters of the light curves of the fundamental mode RR Lyrae starsand their basic stellar parameters has been extended to considerablylarger data sets. The most significant contribution to the absolutemagnitude MV comes from the period P and from the firstFourier amplitude A1, but there are statistically significantcontributions also from additional higher order components, mostimportantly from A3 and in a lesser degree from the Fourierphase varphi51. When different colors are combined inreddening-free quantities, we obtain basically period-luminosity-colorrelations. Due to the log Teff(B-V,log g,[Fe/H]) relationfrom stellar atmosphere models, we would expect some dependence also onvarphi 31. Unfortunately, the data are still not extensiveand accurate enough to decipher clearly the small effect of this Fourierphase. However, with the aid of more accurate multicolor data on fieldvariables, we show that this Fourier phase should be present either inV-I or in B-V or in both. From the standard deviations of the variousregressions, an upper limit can be obtained on the overall inhomogeneityof the reddening in the individual clusters. This yields sigmaE(B-V)<~ 0.012 mag, which also implies an average minimumobservational error of sigmaV >~ 0.018 mag.
| A catalogue of helium abundance indicators from globular cluster photometry
We present a survey of helium abundance indicators derived from acomprehensive study of globular cluster photometry in the literature.For each of the three indicators used, we conduct a thorough erroranalysis, and identify systematic errors in the computationalprocedures. For the population ratio RNHBNRGB, wefind that there is no evidence of a trend with metallicity, althoughthere appears to be real scatter in the values derived. Although thisindicator is the one best able to provide useful absolute heliumabundances, the mean value is Y~0.20, indicating the probable presenceof additional systematic error. For the magnitude difference from thehorizontal branch to the main sequence Δ and the RR Lyraemass-luminosity exponent A, it is only possible to determine relativehelium abundances reliably. This is due to continuing uncertainties inthe absolute metallicity scale for Δ, and uncertainty in the RRLyrae temperature scale for A. Both indicators imply that the heliumabundance is approximately constant as a function of [Fe/H]. Accordingto the A indicator, both Oosterhoff I and II group clusters haveconstant values independent of [Fe/H] and horizontal branch type. Inaddition, the two groups have slopes dlog/d[Fe/H]that are consistent with each other, but significantly smaller than theslope for the combined sample.
| Spectroscopy of globular cluster candidates in the Sculptor group galaxies NGC 253 and 55
We have obtained spectra for 103 published globular cluster candidatesin the Sculptor group galaxies NGC 253 and 55. On the basis of radialvelocities and digitized plate images, 14 globular clusters areidentified in NGC 253 and one probable globular cluster is identified inNGC 55. The majority of the objects in the sample appear to bebackground galaxies. We have obtained and analysed COSMOS plate scans ofNGC 253 and 55 and use these along with the spectroscopically identifiedclusters to define new samples of globular cluster candidates in the twogalaxies which should have reduced contamination.
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