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|Near-infrared observations of the Fornax dwarf galaxy. I. The red giant branch|
Aims.We present a study of the evolved stellar populations in the dwarfspheroidal galaxy Fornax based on wide-area near-infrared observations,aimed at obtaining new independent estimates of its distance andmetallicity distribution. Assessing the reliability of near-infraredmethods is most important in view of future space- and ground-based deepnear-infrared imaging of resolved stellar systems. Methods: We haveobtained JHK imaging photometry of the stellar populations in Fornax.The observations cover an 18.5×18.5 arcmin2 centralarea with a mosaic of SOFI images at the ESO NTT. Our data sample allthe red giant branch (RGB) for the whole area. Deeeper observationsreaching the red clump of helium-burning stars have also been obtainedfor a 4.5×4.5 arcmin2 region. Results: Near-infraredphotometry led to measurements of the distance to Fornax based on theK-band location of the RGB tip and the red clump. Once corrected for themean age of the stellar populations in the galaxy, the derived distancemodulus is (m-M)0 = 20.74±0.11, corresponding to adistance of 141 Kpc, in good agreement with estimates from optical data.We have obtained a photometric estimate of the mean metallicity of redgiant stars in Fornax from their (J-K) and (V-K) colors, using severalmethods. The effect of the age-metallicity degeneracy on the combinedoptical-infrared colors is shown to be less important than for opticalor infrared colors alone. By taking age effects into account, we havederived a distribution function of global metallicity [M/H] fromoptical-infrared colors of individual stars. Our photometric MetallicityDistribution Function covers the range -2.0 < [M/H] < -0.6, with amain peak at [M/H]≃ -0.9 and a long tail of metal-poor stars, andless metal-rich stars than derived by recent spectroscopy. Ifmetallicities from Ca II triplet lines are correct, this result confirmsa scenario of enhanced metal enrichment in the last 1-4 Gyr.Based on data collected at the European Southern Observatory, La Silla,Chile, Proposals No. 65.N-0167, 66.B-0247. Full Tables 2 and 3 are onlyavailable in electronic format at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (220.127.116.11) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/467/1025
|Formation and evolution of the Galactic bulge: constraints from stellar abundances|
Aims.We compute the chemical evolution of the Galactic bulge in thecontext of an inside-out model for the formation of the Milky Way. Themodel contains updated stellar yields from massive stars. The mainpurpose of the paper is to compare the predictions of this model withnew observations of chemical abundance ratios and metallicitydistributions in order to put constraints on the formation and evolutionof the bulge. Methods: We computed the evolution of severalα-elements and Fe and performed several tests by varying differentparameters such as star formation efficiency, slope of the initial massfunction and infall timescale. We also tested the effect of adopting aprimary nitrogen contribution from massive stars. Results: The [α/Fe] abundance ratios in the Bulge are predicted to be supersolarfor a very large range in [Fe/H], each element having a different slope.These predictions are in very good agreement with most recent accurateabundance determinations. We also find a good fit of the most recentBulge stellar metallicity distributions. Conclusions: We conclude thatthe Bulge formed on a very short timescale (even though timescales muchshorter than ~0.1 Gyr are excluded) with a quite high star formationefficiency of ν ≃ 20 Gyr-1 and with an initial massfunction more skewed toward high masses (i.e. x ≤ 0.95) than thesolar neighbourhood and rest of the disk. The results obtained here aremore robust than previous ones since they are based on very accurateabundance measurements.
|Andromeda X, a New Dwarf Spheroidal Satellite of M31: Photometry|
We report the discovery of Andromeda X, a new dwarf spheroidal satelliteof M31, based on stellar photometry from the Sloan Digital Sky Survey.Using follow-up imaging data we have estimated its distance and otherphysical properties. We find that Andromeda X has a dereddened centralsurface brightness of μV,0~26.7 mag arcsec-2and a total apparent magnitude of Vtot~16.1, which at thederived distance modulus, (m-M)0~24.12-24.34, yields anabsolute magnitude of MV~-8.1+/-0.5 these values are quitecomparable to those of Andromeda IX, a previously discoveredlow-luminosity M31 satellite. The discoveries of Andromeda X and ofnumerous other extremely faint satellites around M31 and the Milky Wayin the past few years suggest that such objects may be plentiful in theLocal Group.
|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.
|Oxygen, sodium, magnesium, and aluminium as tracers of the galactic bulge formation|
Aims.This paper investigates the peculiar behaviour of the light even(alpha-elements) and odd atomic number elements in red giants in thegalactic bulge, both in terms of the chemical evolution of the bulge,and in terms of possible deep-mixing mechanisms in these evolved stars. Methods: Abundances of the four light elements O, Na, Mg, and Al aremeasured in 13 core He-burning giant stars (red clump stars) and 40 redgiant branch stars in four 25 arcmin fields spanning the bulge from -3to -12° galactic latitude. Special care was taken in the abundanceanalysis, performing a differential analysis with respect to themetal-rich solar-neighbourhood giant μLeo, which best resembles ourbulge sample stars. This approach minimises systematic effects that canarise in the analysis of cool metal-rich stars due to continuumdefinition issues and blending by molecular lines (CN) and, cancels outpossible model atmosphere deficiencies. Results: We show that theresulting abundance patterns point towards a chemical enrichmentdominated by massive stars at all metallicities. Oxygen, magnesium, andaluminium ratios with respect to iron are overabundant with respect toboth galactic disks (thin and thick) for [Fe/H] > -0.5. A formationtimescale for the galactic bulge shorter than for both the thin andthick disks is therefore inferred. To isolate the massive-starcontribution to the abundances of O, Mg, Al, and Na, we use Mg as aproxy for metallicity (instead of Fe), and further show that: (i) thebulge stars [O/Mg] ratio follows and extends the decreasing trend of[O/Mg] found in the galactic disks to higher metallicities. This is achallenge for predictions of O and Mg yields in massive stars, which sofar predicted no metallicity dependence in this ratio; (ii) the [Na/Mg]ratio trend with increasing [Mg/H] is found to increase in threedistinct sequences in the thin disk, the thick disk, and the bulge. Thebulge trend is well represented by the predicted metallicity-dependentyields of massive stars, whereas the galactic disks have Na/Mg ratiosthat are too high at low metallicities, pointing to an additional sourceof Na from AGB stars; (iii) contrary to the case of the [Na/Mg] ratio,there appears to be no systematic difference in the [Al/Mg] ratiobetween bulge and disk stars, and the theoretical yields by massivestars agree with the observed ratios, leaving no space for AGBcontribution to Al.Based on ESO-VLT observations 71.B-0617, 73.B-0074, and ParisObservatory GTO 71.B-0196. Full Tables 2 to 6 are only available inelectronic form at the CDS via anonymous ftp to cdsarc-ustrasbg.fr(18.104.22.168) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/799 Tables 11 and 12are only available in electronic form at http://www.aanda.org
|Inside the whale: the structure and dynamics of the isolated Cetus dwarf spheroidal|
This paper presents a study of the Cetus dwarf, an isolated dwarf galaxywithin the Local Group. A matched-filter analysis of the INT/WFC imagingof this system reveals no evidence for significant tidal debris thatcould have been torn off the galaxy, bolstering the hypothesis thatCetus has never significantly interacted with either the Milky Way orM31. Additionally, Keck/Deimos spectroscopic observations identify thisgalaxy as a distinct kinematic population possessing a systematicvelocity of -87 +/- 2kms-1 and with a velocity dispersion of17 +/- 2kms-1 while tentative, these data also suggest thatCetus possesses a moderate rotational velocity of ~8kms-1.The population is confirmed to be relatively metal-poor, consistent with[Fe/H] ~ -1.9, and, assuming virial equilibrium, implies that the Cetusdwarf galaxy possesses a mass-to-light ratio of ~70. It appears,therefore, that Cetus may represent a primordial dwarf galaxy, retainingthe kinematic and structural properties lost by other members of thedwarf population of the Local Group in their interactions with the largegalaxies. An analysis of Cetus' orbit through the Local Group indicatesthat it is at apocentre; taken in conjunction with the general dwarfpopulation, this shows the mass of the Local Group to be >~2 ×1012Msolar.
|Na-O anticorrelation and horizontal branches. VI. The chemical composition of the peculiar bulge globular cluster NGC 6388|
We present the LTE abundance analysis of high resolution spectra for redgiant stars in the peculiar bulge globular cluster NGC 6388. Spectra ofseven members were taken using the UVES spectrograph at the ESO VLT2 andthe multiobject FLAMES facility. We exclude any intrinsic metallicityspread in this cluster: on average, [Fe/H]=-0.44±0.01±0.03dex on the scale of the present series of papers, where the first errorbar refers to individual star-to-star errors and the second issystematic, relative to the cluster. Elements involved in H-burning athigh temperatures show large spreads, exceeding the estimated errors inthe analysis. In particular, the pairs Na and O, Al and Mg areanticorrelated and Na and Al are correlated among the giants in NGC6388, the typical pattern observed in all galactic globular clustersstudied so far. Stars in NGC 6388 shows an excess of α-processelements, similar to the one found in the twin bulge cluster NGC 6441.Mn is found underabundant in NGC 6388, in agreement with the averageabundance ratio shown by clusters of any metallicity. Abundances ofneutron-capture elements are homogeneously distributed within NGC 6388;the [Eu/Fe] ratio stands above the value found in field stars of similarmetallicity.Based on observations collected at ESO telescopes under programme073.D-0211. Full Table 3 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/967
|New catalogue of blue stragglers in open clusters|
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
|Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters|
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
|The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy|
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
|VLT-UVES abundance analysis of four giants in NGC 6553|
Context: .Metal-rich globular clusters trace the formation of bulges.Abundance ratios in the metal-rich globular clusters such as NGC 6553can constrain the formation timescale of the Galactic bulge. Aims: .The purpose of this study is to determine the metallicity andelemental ratios in individual stars of the metal-rich bulge globularcluster NGC 6553. Methods: .A detailed abundance analysis of fourgiants in NGC 6553 is carried out, based on optical high-resolutionéchelle spectra obtained with UVES at the ESO VLT-UT2 Kueyentelescope. Results: .A metallicity [Fe/H] = -0.20 dex is derived,together with α-element enhancement of Mg and Si ([Mg/Fe] = +0.28,[Si/Fe] = +0.21), solar Ca and Ti ([Ca/Fe] = +0.05, [Ti/Fe] = -0.01),and a mild enhancement of the r-process element Eu with [Eu/Fe] = +0.10.A mean heliocentric radial velocity of -1.86 km s-1 ismeasured. We compare our results with previous investigations of thecluster.
|An Empirical Tool to Derive Metallicity, Reddening, and Distance for Old Stellar Populations from Near-Infrared Color-Magnitude Diagrams|
We present an empirical method to derive photometric metallicity,reddening, and distance to old stellar populations by using a few majorfeatures of the red giant branch (RGB) in near-IR color-magnitudediagrams. We combine the observed RGB features with a set of equationslinking the global metallicity [M/H] to suitable RGB parameters (colors,magnitudes, and slope), as calibrated from a homogeneous sample ofGalactic globular clusters with different metallicities. This techniquecan be applied to efficiently derive the main population parameters ofold stellar systems, with the goal of using ground-based adaptive opticsand space facilities to probe the stellar content of remote galaxies.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG), operated on the island La Palma bythe Fundacion Galileo Galilei of INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
|AL 3 (BH 261): A New Globular Cluster in the Galaxy|
AL 3 (BH 261), previously classified as a faint open cluster candidate,is shown to be a new globular cluster in the Milky Way, by means of B,V, and I color-magnitude diagrams. The main feature of AL 3 is aprominent blue extended horizontal branch. Its color-magnitude diagramsmatch those of the intermediate-metallicity cluster M5. The cluster isprojected in a rich bulge field, also contaminated by the disk mainsequence. The globular cluster is located in the Galactic bulge at adistance from the Sun dsolar=6.0+/-0.5 kpc. The reddening isE(B-V)=0.36+/-0.03, and the metallicity is estimated to be[Fe/H]~-1.3+/-0.25. AL 3 is probably one of the least massive globularclusters of the Galaxy.The observations were carried out at the European Southern Observatory,La Silla, Chile [proposal 64L-0212(A)].
|Na-O anticorrelation and HB. III. The abundances of NGC 6441 from FLAMES-UVES spectra|
Aims.The aim of the present work is to determine accurate metallicitiesfor a group of red giant branch stars in the field of the bulge GlobularCluster NGC 6441. This is the third paper in a seriesresulting from a large project aimed at determining the extent of theNa-O anticorrelation among Globular Cluster stars and exploring itsrelationship with HB morphology. Methods: .We present an LTEabundance analysis of these objects, based on data gathered with theFLAMES fiber facility and the UVES spectrograph at VLT2. Results:.Five of the thirteen stars observed are members of the cluster. Theaverage Fe abundance for these five stars is [Fe/H] = -0.39±0.04± 0.05 dex, where the first error bar includes theuncertainties related to star-to-star random errors, and the second onethe systematic effects related to the various assumptions made in theanalysis. The overall abundance pattern is quite typical of GlobularClusters, with an excess of the α-elements and of Eu. There isevidence that the stars of NGC 6441 are enriched inNa and Al, while they have been depleted of O and Mg by H-burning athigh temperatures, analogous with extensive observations of otherGlobular Clusters: in particular, one star is clearly Na and Al-rich andO and Mg-poor. We also obtained quite high V abundances, but it ispossible that this is an artifact of the analysis, since similar high Vabundances are also derived for the field stars. These last are all moremetal-rich than NGC 6441 and probably belong to thebulge population.
|Chemical abundances in the Galactic Bulge: evidence for a fast enrichment|
We derived oxygen abundances for 42 K giants in four fields towards theGalactic bulge. We present here the results for Oxygen, as measured fromthe forbidden line at 6300 Å. Oxygen shows a well defined trendwith [Fe/H]. With [O/Fe] being higher in bulge stars than in thick diskones, the latter already more oxygen enhanced than thin disk stars(Bensby, Feltzing, & Lundström 2004). This result supports thescenario that the bulge formed faster and probably earlier than thethick disk.
|An Empirical Calibration of the Mixing-Length Parameter α|
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
|Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters|
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
|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.
|Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452|
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
|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.
|Abundances of Baade's Window Giants from Keck HIRES Spectra. I. Stellar Parameters and [Fe/H] Values|
We present the first results of a new abundance survey of the Milky Waybulge based on Keck HIRES spectra of 27 K giants in the Baade's Window(l=1deg, b=-4deg) field. The spectral data used inthis study are of much higher resolution and signal-to-noise ratio thanprevious optical studies of Galactic bulge stars. The [Fe/H] values ofour stars, which range between -1.29 and +0.51, were used to recalibratelarge low-resolution surveys of bulge stars. Our best value for the mean[Fe/H] of the bulge is -0.10+/-0.04. This mean value is similar to themean metallicity of the local disk and indicates that there cannot be astrong metallicity gradient inside the solar circle. The metallicitydistribution of stars confirms that the bulge does not suffer from theso-called G dwarf problem. This paper also details the new abundancetechniques necessary to analyze very metal-rich K giants, including anew Fe line list and regions of low blanketing for continuumidentification.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation.
|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 Accretion Origin of a Vast Extended Stellar Disk around the Andromeda Galaxy|
We present the discovery of an inhomogenous, low surface brightness,extended disklike structure around the Andromeda galaxy (M31) based on alarge kinematic survey of more than 2800 stars with the Keck DEIMOSmultiobject spectrograph. The stellar structure spans radii from 15 kpcout to ~40 kpc, with detections out to R~70 kpc. The constituent starshave velocities close to the expected velocity of circular orbits in theplane of the M31 disk and typically have a velocity dispersion of ~30 kms-1. The color range on the upper red giant branch shows alarge spread indicative of a population with a significant range ofmetallicity. The mean metallicity of the population, measured from Ca IIequivalent widths, is [Fe/H]=-0.9+/-0.2. The morphology of the structureis irregular at large radii and shows a wealth of substructures thatmust be transitory in nature and are almost certainly tidal debris. Thepresence of these substructures indicates that the global entity wasformed by accretion. This extended disk follows smoothly on from thecentral parts of M31 disk out to ~40 kpc with an exponential density lawwith a scale length of 5.1+/-0.1 kpc, which is similar to that of thebright inner disk. However, the population possesses similar kinematicand abundance properties over the entire region where it is detected inthe survey. We estimate that the structure accounts for approximately10% of the total luminosity of the M31 disk, and given the huge scale,contains ~30% of the total disk angular momentum. This finding indicatesthat at least some galactic stellar disks are vastly larger thanpreviously thought and are formed, at least in their outer regions,primarily by accretion.
|Halos of Spiral Galaxies. III. Metallicity Distributions|
We report results of a campaign to image the stellar populations in thehalos of highly inclined spiral galaxies, with the fields roughly 10 kpc(projected) from the nuclei. We use the F814W (I) and F606W (V) filtersin the Wide Field Planetary Camera 2 on board the Hubble SpaceTelescope. We unambiguously resolve the stellar halos 1 to 2 mag fainterthan the tip of the red giant branch. Extended halo populations aredetected in all galaxies. The color-magnitude diagrams appear to becompletely dominated by giant branch stars, with no evidence for thepresence of young stellar populations in any of the fields. Themetallicity distribution function for the galaxy sample is derived frominterpolation within an extensive grid of red giant branch loci. Theseloci are derived from theoretical sequences that are calibrated usingthe Galactic globular clusters and from empirical sequences formetal-rich stellar populations. We find that the metallicitydistribution functions are dominated by metal-rich populations, with atail extending toward the metal-poor end. To first order, the overallshapes of the metallicity distribution functions are similar to what ispredicted by a simple, single-component model of chemical evolution withthe effective yields increasing with galaxy luminosity. However,metallicity distributions significantly narrower than the simple modelare observed for a few of the most luminous galaxies in the sample. Thediscrepancies are similar to those previously observed for NGC 5128, thehalo of M31, and the Galactic bulge. Our observations can be used tohelp distinguish between models for the formation of spiral galaxies. Itappears that more luminous spiral galaxies also have more metal-richstellar halos. The increasingly significant departures from theclosed-box model for the more luminous galaxies indicate that aparameter in addition to a single yield is required to describe chemicalevolution. This parameter, which could be related to gas infall oroutflow either in situ or in progenitor dwarf galaxies that later mergeto form the stellar halo, tends to make the metallicity distributionsnarrower at high metallicity.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.
|A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies|
We have compiled a sample of globular clusters with high-quality stellarabundances from the literature to compare to the chemistries of stars inthe Galaxy and in dwarf spheroidal galaxies. Of the 45 globular clustersexamined, 29 also have kinematic information. Most of the globularclusters belong to the Galactic halo; however, a significant number havedisk kinematics or belong to the bulge. Focusing on the [α/Fe] andlight r-process element ratios, we find that most globular cluster starsmimic field stars of similar metallicities, and neither clearlyresembles the currently available stellar abundances in dwarf galaxies(including globular clusters in the Large Magellanic Cloud). Theexceptions to these general elemental ratio comparisons are alreadyknown in the literature, e.g., ω Centauri, Palomar 12, and Terzan7 associated with the Sagittarius remnant and Ruprecht 106, which has ahigh radial velocity and low [α/Fe] ratio. A few other globularclusters show more marginal peculiarities. The most notable one is thehalo cluster M68, which has a high galactocentric rotational velocity, aslightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratiosdecrease with increasing [Fe/H] at intermediate metallicities, which isconsistent with very massive stars playing a larger role in the earlychemical evolution of the Galaxy. The chemical similarities betweenglobular clusters and field stars with [Fe/H]<=-1.0 suggests a sharedchemical history in a well-mixed early Galaxy. The differences in thepublished chemistries of stars in the dwarf spheroidal galaxies suggestthat neither the globular clusters, halo stars, nor thick disk stars hadtheir origins in small isolated systems like the present-day Milky Waydwarf satellites.
|Extragalactic Globular Clusters: Old Spectroscopic Ages and New Views on Their Formation|
We present the results of a meta-analysis of Keck spectra ofextragalactic globular clusters (GCs) in a sample of eight galaxies,ranging from dwarf galaxies to massive elliptical galaxies. We inferages for the metal-poor and metal-rich GCs in these galaxies throughcomparisons to Galactic GCs. Both subpopulations appear to be no youngerthan their Galactic counterparts, with ages >~10 Gyr. This is thelargest sample of galaxies for which ages have been constrainedspectroscopically. Our results support the formation of most GCs inmassive galaxies at high redshift. We propose a scenario for theformation of GC subpopulations that synthesizes aspects of bothaccretion and in situ approaches in the context of galaxy formationthrough hierarchical merging.
|Evolutionary population synthesis: models, analysis of the ingredients and application to high-z galaxies|
Evolutionary population synthesis models for a wide range ofmetallicities, ages, star formation histories, initial mass functionsand horizontal branch morphologies, including blue morphologies at highmetallicity, are computed. The model output comprises spectral energydistributions, colours, stellar M/L ratios, bolometric corrections andnear-infrared (IR) spectral line indices. The energetics of the postmain sequence evolutionary phases are evaluated with the fuelconsumption theorem. The impact on the models of the stellarevolutionary tracks (in particular with and without overshooting) isassessed. We find modest differences in synthetic broad-band colours asinduced by the use of different tracks in our code [e.g. Δ(V-K) ~0.08 mag, Δ(B-V) ~ 0.03 mag]. Noticeably, these differences aresubstantially smaller than the scatter among other models in theliterature, even when the latter adopt the same evolutionary tracks. Themodels are calibrated with globular cluster data from the Milky Way forold ages, and the Magellanic clouds plus the merger remnant galaxy NGC7252, both for young ages of ~0.1-2Gyr, in a large wavelength range fromthe U band to the K band. Particular emphasis is put on the contributionfrom the thermally pulsing asymptotic giant branch (TP-AGB) phase. Weshow that this evolutionary phase is crucial for the modelling of youngstellar populations by direct comparison with observed spectral energydistributions of Magellanic cloud clusters, which are characterized byrelatively high fluxes, both blueward and redward of the V band. We findthat the combination of the near-IR spectral indices C2 andH2O can be used to determine the metallicity of ~1 Gyrstellar populations. As an illustrative application, we re-analyse thespectral energy distributions of some of the high-z galaxies (2.4<~z<~ 2.9) observed with the Spitzer Space Telescope by Yan et al.Their high rest-frame near-IR fluxes is reproduced very well with themodels including TP-AGB stars for ages in the range ~0.6-1.5Gyr,suggesting formation redshifts for these objects around z~ 3-6.
|The Adequacy of Stellar Evolution Models for the Interpretation of the Color-Magnitude Diagrams of Resolved Stellar Populations|
Most of what we know about the stellar population of nearby, resolvedgalaxies comes from the interpretation of their color-magnitudediagrams, by comparison with stellar evolutionary models. We review howwell current stellar evolution models reproduce the properties of simplestellar populations. Emphasis is given to the regions of thecolor-magnitude diagram which are most useful for deriving age,metallicity, or distance of a population. Extensive comparison is madebetween the predictions of the most-used stellar evolution libraries, inorder to estimate how model dependent the results are. The presentreview, written from a user perspective, aims at emphasizing thestrengths and weaknesses of the models, and is intended both forobservers and theoreticians. We hope to encourage observers to providestronger observational constraints where they are needed, and tostimulate theoreticians to isolate the input physics responsible for thedifferent behavior between models and the reasons for the discrepancieswith data.
|α-Enhanced Integrated Lick/IDS Spectral Indices and Milky Way and M31 Globular Clusters and Early-Type Galaxies|
All 25 Lick/IDS spectral indices have been computed for the integratedlight of simple stellar populations over broad ranges of age andmetallicity and with effects from horizontal-branch stars fullyimplemented. Our models employ α-enhanced isochrones at thesub-solar metallicity regime, but solar-scaled ones at solar andsuper-solar metallicity. We have also employed the updated responsefunctions of Houdashelt et al. at the solar and super-solar metallicityregime, so that we could assess the light-element enhancement phenomenaseen from metal-rich early-type galaxies. For Balmer indices asignificant response was noted for Hγ and Hδ whenα-elements are enhanced, but Hβ is ratherα-insensitive. We also find that our 5 and 12 Gyr models ofHγ and Hδ overlap in the metal-poor regime because ofchanging populations of blue horizontal-branch stars. Furthermore, forpopulations younger than 1 Gyr, Balmer lines become weaker in themetal-poor regime because the main-sequence turnoff is hotter than10,000 K. We present models at fixed [Fe/H] (rather than fixed heavyelement mass fraction Z) and compare to Milky Way globular clusters thathave independently estimated mean [Fe/H] and [α/Fe]. Comparison ofour models with observations of Milky Way and M31 globular clusters inindex-index space are favorable, tracing the observations at a model ageof 12 Gyr without any zero-point shifts that are needed by some othermodels. The metallicity range of M31 globular clusters is similar tothat of their Galactic counterparts. We also verify Beasley et al.'srecent hypothesis of the existence of young and intermediate-age starclusters in M31. Contrary to the literature values, the Milky Wayglobular cluster NGC 6553 appears more metal-rich than NGC 6528 frommetal indices. We present Hδ and Hγ Lick/IDS indices for theLick/IDS sample of galaxies. We confirm the well-known enhancement of Mgand Na relative to Fe and Ca among early-type galaxies, and its increasewith increasing velocity dispersion. There are distinct differencesbetween globular clusters and galaxies in diagrams involvingCN1 and CN2, hinting that the globular clusterenvironment may be a special one in terms of the amount of Nincorporated into stars.
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