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Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters
We use Hubble Space Telescope archival images to measure central surfacebrightness profiles of globular clusters around satellite galaxies ofthe Milky Way. We report results for 21 clusters around the LMC, fivearound the SMC, and four around the Fornax dwarf galaxy. The profileswere obtained using a recently developed technique based on measuringintegrated light, which is tested on an extensive simulated data set.Our results show that for 70% of the sample, the central photometricpoints of our profiles are brighter than previous measurements usingstar counts with deviations as large as 2 mag arcsec-2. About40% of the objects have central profiles deviating from a flat centralcore, with central logarithmic slopes continuously distributed between-0.2 and -1.2. These results are compared with those found for a sampleof Galactic clusters using the same method. We confirm the knowncorrelation in which younger clusters tend to have smaller core radii,and we find that they also have brighter central surface brightnessvalues. This seems to indicate that globular clusters might be bornrelatively concentrated, and that a profile with an extended flat coremight not be the ideal choice for initial profiles in theoreticalmodels.
| The DART imaging and CaT survey of the Fornax dwarf spheroidal galaxy
Aims.As part of the DART project we have used the ESO/2.2m Wide FieldImager in conjunction with the VLT/FLAMES GIRAFFE spectrograph to studythe detailed properties of the resolved stellar population of the Fornaxdwarf spheroidal galaxy out to and beyond its tidal radius. Fornax dwarfspheroidal galaxy has had a complicated evolution and containssignificant numbers of young, intermediate age and old stars. Weinvestigate the relation between these different components by studyingtheir photometric, kinematic and abundance distributions. Methods: . We re-derived the structural parameters of the Fornax dwarfspheroidal using our wide field imaging covering the galaxy out to itstidal radius, and analysed the spatial distribution of the Fornax starsof different ages as selected from colour-magnitude diagram analysis. Wehave obtained accurate velocities and metallicities from spectra in theCa II triplet wavelength region for 562 Red Giant Branch stars whichhave velocities consistent with membership of the Fornax dwarfspheroidal. Results: .We have found evidence for the presence ofat least three distinct stellar components: a young population (few 100Myr old) concentrated in the centre of the galaxy, visible as a MainSequence in the colour-magnitude diagram; an intermediate age population(2-8 Gyr old); and an ancient population (>10 Gyr), which aredistinguishable from each other kinematically, from the metallicitydistribution and in the spatial distribution of stars found in thecolour-magnitude diagram. Conclusions: . From our spectroscopicanalysis we find that the "metal rich" stars ([Fe/H]> -1.3) show aless extended and more concentrated spatial distribution, and displaycolder kinematics than the "metal poor" stars ([Fe/H]<-1.3). There istentative evidence that the ancient stellar population in the centre ofFornax does not exhibit equilibrium kinematics. This could be a sign ofa relatively recent accretion of external material, such as the mergerof another galaxy or other means of gas accretion at some point in thefairly recent past, consistent with other recent evidence ofsubstructure (Coleman et al. 2004, AJ, 127, 832; 2005, AJ, 129, 1443).
| VLT/UVES spectroscopy of individual stars in three globular clusters in the Fornax dwarf spheroidal galaxy
We present a high resolution (R ~ 43 000) abundance analysis of a totalof nine stars in three of the five globular clusters associated with thenearby Fornax dwarf spheroidal galaxy. These three clusters (1, 2 and 3)trace the oldest, most metal-poor stellar populations in Fornax. Wedetermine abundances of O, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba, Nd andEu in most of these stars, and for some stars also Mn and La. Wedemonstrate that classical indirect methods (isochrone fitting andintegrated spectra) of metallicity determination lead to values of[Fe/H] which are 0.3 to 0.5 dex too high, and that this is primarily dueto the underlying reference calibration typically used by these studies.We show that Cluster 1, with [Fe /H] = -2.5, now holds the record forthe lowest metallicity globular cluster. We also measure anover-abundance of Eu in Cluster 3 stars that has only been previouslydetected in a subgroup of stars in M 15. We find that the Fornaxglobular cluster properties are a global match to what is found in theirGalactic counterparts; including deep mixing abundance patterns in twostars. We conclude that at the epoch of formation of globular clustersboth the Milky Way and the Fornax dwarf spheroidal galaxy shared thesame initial conditions, presumably pre-enriched by the same processes,with identical nucleosynthesis patterns.
| Does the Fornax dwarf spheroidal have a central cusp or core?
The dark matter dominated Fornax dwarf spheroidal has five globularclusters orbiting at ~1kpc from its centre. In a cuspy cold dark matterhalo the globulars would sink to the centre from their current positionswithin a few Gyr, presenting a puzzle as to why they survive undigestedat the present epoch. We show that a solution to this timing problem isto adopt a cored dark matter halo. We use numerical simulations andanalytic calculations to show that, under these conditions, the sinkingtime becomes many Hubble times; the globulars effectively stall at thedark matter core radius. We conclude that the Fornax dwarf spheroidalhas a shallow inner density profile with a core radius constrained bythe observed positions of its globular clusters. If the phase spacedensity of the core is primordial then it implies a warm dark matterparticle and gives an upper limit to its mass of ~0.5keV, consistentwith that required to significantly alleviate the substructure problem.
| The Fornax Project .
The Fornax Project aims at mapping the classical instability strip ofthe Fornax dwarf spheroidal galaxy, from the faint Dwarf Cepheids (V˜ 24-25 mag) to the bright Anomalous Cepheids (V ˜ 19 mag).To achieve this goal, deep B,V time-series photometry of the galaxy hasbeen obtained with the Wide Field Imagers (WFIs) of the ESO 2.2 m andCTIO 4 m telescopes, and the Clay camera at the Magellan 6.5 mtelescope. Preliminary results are presented on the Oosterhoffclassification of the RR Lyrae stars identified in a northern portion ofFornax field and in three of its globular clusters.
| 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.
| Wide field imaging from space: Galaxy formation from nearby stellar populations [review article]
A wide field, high resolution imaging facility in space would enablebreakthrough science in the study of stellar populations. Optical highresolution imaging at the diffraction limit of a 2 m telescope is aminimum requirement. In the Galaxy and Local Group, proper motionstudies could settle the contribution of cool white dwarfs to the halodark matter mass and define the dynamics of the Galactic bulge. Thewhite dwarf cooling age method could be applied to a larger number ofglobular clusters and could include measurements covering the whole of agiven cluster. This powerful method can constrain the age of theUniverse as well as the early star formation history of globularclusters. A wide field imager could derive main sequence turnoff agesfor the interaction streams and multiple globular clusters in the M31halo, as well as in other Local Group galaxies. Imaging in the halos ofnearby galaxies to 10 Mpc, including the Virgo cluster, could map outtidal streams and debris tail and would help to define the stellarpopulations (and therefore the assembly history and nature of) Galactichalos.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| Globular clusters and the formation of the outer Galactic halo
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.
| Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic and Hubble Space Telescope WFPC2 Data
We have measured the absolute proper motion of the Fornax dwarfspheroidal galaxy from a combination of photographic plate material andHubble Space Telescope Wide Field Planetary Camera 2 data that provide atime baseline of up to 50 yr. The extragalactic reference frame consistsof eight QSO images and 48 galaxies. The absolute proper motion isμαcosδ=0.59+/-0.16 mas yr-1 andμδ=-0.15+/-0.16 mas yr-1. Thecorresponding orbit of Fornax is polar, with an eccentricity of 0.27 anda radial period of 4.5 Gyr. Fornax's current location is nearpericenter. The direction of the motion of Fornax supports the notionthat Fornax belongs to the Fornax-Leo I-Leo II-Sculptor-Sextans streamas hypothesized by both Lynden-Bell and Majewski. According to our orbitdetermination, Fornax crossed the Magellanic plane ~190 Myr ago, a timethat coincides with the termination of the star formation process inFornax. We propose that ram pressure stripping due to the passage ofFornax through a gaseous medium denser than the typical intragalacticmedium left behind from the LMC may have caused the end of starformation in Fornax. The excess, anomalous clouds within the southGalactic pole region of the Magellanic Stream, whose origin has longbeen debated in the literature as constituents of either the MagellanicStream or of the extragalactic Sculptor group, are found to lie alongthe orbit of Fornax. We speculate that these clouds are strippedmaterial from Fornax as the dwarf crossed the orbit of the MagellanicClouds.
| Image-Subtraction Photometry of the Globular Cluster M3: Identification of New Double-Mode RR Lyrae Stars
We have applied the image subtraction method of Alard and Lupton to theextensive M3 data set previously analyzed by Corwin and Carney usingDAOPHOT and ALLSTAR. This new analysis has produced light curves andperiods for 15 variables not found in the previous study but alreadyknown to be variables, and it has also resulted in improved periods forseveral other variables. The additional variables recovered with theimage subtraction analysis are in the very central region of M3, wherecrowding is severe and the photometry was not of sufficient quality thatit could be put on the standard system. The present study brings to 222the total number of RR Lyrae variables in Corwin and Carney's M3 dataset for which light curves and periods are available. Among them we haveidentified three new candidate double-mode pulsating variables (V13,V200, and V251), reported here for the first time. This brings to eightthe total number of double-mode RR Lyrae (RRd's) identified in M3. Ofthe newly discovered RRd's V13 is unusual in that it has the fundamentalas the dominant pulsation mode. M3 is unique among the globular clustersin having RRd variables with a dominant fundamental mode. Two of the newcandidate RRd's (V13 and V200) have period ratios as low as 0.738-0.739.They lie well separated from all previously known double-mode variablestars in the Petersen diagram in positions implying a large spread inmass and/or, less likely, in heavy element mass fraction, among the M3horizontal-branch (HB) stars. We explore mass transfer and heliumenhancement as possible explanations for the apparent spread in HBmasses. We also note that the masses derived from the double-modeanalyses now favor little mass loss on the red giant branch. We findthat V200 has changed its dominant pulsation mode from fundamental tofirst overtone, while V251 has changed its dominant mode from firstovertone to fundamental in the interval 1992 to 1993. Together withM3-V166 this is the first time that double-mode variables are observedto switch their dominant pulsation modes while remaining RRd's. Thephenomenon is found to occur in a 1 yr time span, thus suggesting thatthese stars are undergoing a rapid evolutionary phase and that bothredward and blueward evolution may take place among the HB stars in theOosterhoff type I cluster M3. The unusual behavior of the M3 RRd's isdiscussed in detail and compared with that of the double-mode RR Lyraeidentified so far in globular clusters and in the field of our and otherLocal Group galaxies. We find a lack of correlation between the presenceof RRd variables and any of the cluster structural parameters.
| Distance scale and variable stars in Local Group Galaxies: LMC and Fornax
We briefly review our photometric and spectroscopic study of RR Lyraevariable stars in the bar of the Large Magellanic Cloud (LMC), thatallowed us to reconcile the so-called short and long distance moduli ofthe LMC on the value mu LMC=18.51 ± 0.085 mag. Then wepresent preliminary results from the photometric study of a33'×34' area in the Fornax dwarf spheroidal galaxy containing thestellar clusters Fornax 3 (NGC 1049) and 6. We identified about 1000candidate variables in this field of Fornax, and report the firstdetection and measure of about 60 RR Lyrae variable stars in theglobular cluster Fornax 3.
| RR Lyrae stars in four globular clusters in the Fornax dwarf galaxy
We have surveyed four of the globular clusters in the Fornax dwarfgalaxy (clusters 1, 2, 3 and 5) for RR Lyrae stars, using archival F555Wand F814W Hubble Space Telescope observations. We identify 197 new RRLyrae stars in these four clusters, and 13 additional candidatehorizontal-branch variable stars. Although somewhat restricted by ourshort observational baseline, we derive periods and light curves for allof the stars in the sample, and calculate photometric parameters such asmean magnitudes and colours. This is the first time that RR Lyrae starsin the Fornax globular clusters have been quantitatively identified andmeasured. We find that the Fornax clusters have exceptionally largespecific frequencies of RR Lyrae stars, in comparison with the galacticglobular clusters. It is likely that Fornax 1 has the largest specificfrequency measured in any globular cluster. In addition, the Fornaxclusters are unusual in that their RR Lyrae populations possess meancharacteristics intermediate between the two Oosterhoff groups definedby the galactic globular clusters. In this respect the RR Lyraepopulations in the Fornax clusters most closely resemble the fieldpopulations in several dwarf galaxies. Fornax 5 has an unusually largefraction of RRc stars, and also possesses several strong RRe (secondovertone pulsator) candidates.With a large sample of horizontal-branch variable stars available to us,we revise previous measurements of the horizontal-branch morphology ineach cluster. The Fornax clusters most closely resemble the `young'galactic halo population defined by Zinn in that their horizontal-branchmorphologies are systematically redder than many galactic clusters ofsimilar metallicity. We also confirm the existence of thesecond-parameter effect among the Fornax clusters, most markedly betweenclusters 1 and 3. The edges of the instability strip are well defined inseveral of the Fornax clusters, and we are able to make measurements ofthe intrinsic V-I colours of these edges. Finally, we determineforeground reddening and distance estimates for each cluster. We find amean distance modulus to the Fornax dwarf of (m-M)0= 20.66+/- 0.03 (random) +/-0.15 (systematic). Our measurements are consistentwith a line-of-sight depth of ~8-10 kpc for this galaxy, which is inaccordance with its dimensions as measured in the plane of the sky. Thisapproximately spherical shape for Fornax is incompatible with tidalmodel explanations for the observed high internal stellar velocitydispersions in many dwarf spheroidal galaxies. Dark matter dominance issuggested.
| Surface brightness profiles and structural parameters for globular clusters in the Fornax and Sagittarius dwarf spheroidal galaxies
We present radial surface brightness profiles for all five globularclusters in the Fornax dwarf spheroidal galaxy, and for the four presentmembers of the Sagittarius dwarf spheroidal galaxy. These profiles arederived from archival Hubble Space Telescope observations, and have beencalculated using the same techniques with which we measured profiles inour previous studies of Large and Small Magellanic Cloud (LMC and SMC)clusters, apart from some small modifications. From the surfacebrightness profiles, we have determined structural parameters for eachcluster, including core radii and luminosity and mass estimates. We alsoprovide a brief summary of literature measurements of other parametersfor these clusters, including their ages, metallicities and distances.Our core radius measurements are mostly in good agreement with thosefrom previous lower resolution studies, although for several clustersour new values are significantly different. The profile for Fornaxcluster 5 does not appear to be well fitted by a King-type model and wesuggest that it is a post-core-collapse candidate. We examine thedistribution of cluster core radii in each of the two dwarf galaxysystems and compare these with the distribution of core radii for oldLMC clusters. The three distributions match within the limits ofmeasurement errors and the small-sample sizes. We discuss theimplications of this in the context of the radius-age trend we havepreviously highlighted for the Magellanic Cloud clusters.
| Spectroscopy of Globular Clusters in the Fornax Dwarf Galaxy
We present low-resolution, integrated Keck spectra of the five globularclusters (GCs) of the Fornax dwarf spheroidal galaxy. We find atentative age spread among the clusters, with the GC H5 younger by 2-3Gyr than the others. The clusters generally appear to be very metal-poor([Fe/H]~-1.8), with the cluster H4 slightly more metal-rich at[Fe/H]=-1.5. We speculate that cluster H4 is similar to the Galactic GCRuprecht 106, which lacks the [α/Fe] enhancement typical amongmetal-poor GCs in the Milky Way. High-resolution spectroscopy ofindividual cluster and field stars will be needed to sort out thesurprisingly complex history of GC formation and evolution in thisgalaxy.
| Orbits of Globular Clusters in the Outer Galaxy: NGC 7006
We present a proper-motion study of the distant globular cluster NGC7006 based on the measurement of 25 photographic plates spanning a 40 yrinterval. The absolute proper motion determined with respect toextragalactic objects is (μαcosδ,μδ)=(-0.96, -1.14)+/-(0.35, 0.40) masyr-1. The total space velocity of NGC 7006 in aGalactocentric rest frame is 279 km s-1, placing the clusteron one of the most energetic orbits (Ra=102 kpc) known todate for clusters within 40 kpc from the Galactic center. We compare theorbits-as determined from full space velocities-of four clusters thathave apocentric radii larger than 80 kpc with those of Galacticsatellites with well-measured proper motions. These clusters are NGC5466, 6934, and 7006 and Pal 13, and the satellites are the Sagittariusdwarf spheroidal galaxy (dSph), the Large Magellanic Cloud, the UrsaMinor dSph, and the Sculptor dSph. Only NGC 5466 and 6934 seem to havesimilar orbital parameters, indicating a possible phase-spaceassociation. NGC 7006, Pal 13, and the ``pair'' NGC 5466 and 6934 do notshow any dynamical association with the Galactic satellites consideredhere. NGC 5466, 6934, and 7006 and Pal 13 have orbits that are highlyeccentric and of various inclinations with respect to the Galacticplane. In contrast, the orbits of the Galactic satellites are of low tomoderate eccentricity and highly inclined. Based on orbit types,chemical abundances, and cluster parameters, we discuss the propertiesof the hypothetical host systems of the remote globular clusters in theSearle-Zinn paradigm. It is apparent that clusters such as NGC 5466,6934, and 7006 formed in systems that more likely resemble the FornaxdSph rather than the Sagittarius dSph. We also discuss plausible causesfor the difference found so far between the orbit type of outer haloclusters and that of Galactic satellites and for the tentative, yetsuggestive, phase-space scatter found among outer halo clusters.
| The merger time of binary globular clusters
In a binary globular cluster the tidal forces increase the internalenergy of the clusters and may lead to either merger or disruption of acluster depending upon whether the two clusters are !of equal mass orvary much in mass. Since the physical process of merger is the same forboth galaxy and cluster pairs, we have estimated the merger time anddisruption time for two globular clusters by using the simple formulaeused for merger time of binary spherical galaxies derived by integratingthe tidal force over one period of the binary. The results thus obtainedare compared with those of n-body simulations and the agreement is foundto be fairly good.
| The elliptical galaxy formerly known as the Local Group: merging the globular cluster systems
Prompted by a new catalogue of M31 globular clusters, we have collectedtogether individual metallicity values for globular clusters in theLocal Group. Although we briefly describe the globular cluster systemsof the individual Local Group galaxies, the main thrust of our paper isto examine the collective properties. In this way we are simulating thedissipationless merger of the Local Group, into presumably an ellipticalgalaxy. Such a merger is dominated by the Milky Way and M31, whichappear to be fairly typical examples of globular cluster systems ofspiral galaxies. The Local Group `Elliptical' has about 700 +/- 125globular clusters, with a luminosity function resembling the `universal'one. The metallicity distribution has peaks at [Fe/H] ~ -1.55 and -0.64with a metal-poor to metal-rich ratio of 2.5:1. The specific frequencyof the Local Group Elliptical is initially about 1 but rises to about 3,when the young stellar populations fade and the galaxy resembles an oldelliptical. The metallicity distribution and stellar populationcorrected specific frequency are similar to that of some known earlytype galaxies. Based on our results, we briefly speculate on the originof globular cluster systems in galaxies.
| Updated Information on the Local Group
The present note updates the information published in my recentmonograph on The Galaxies of the Local Group. Highlights include (1) theaddition of the newly discovered Cetus dwarf spheroidal as a certainmember of the Local Group; (2) an improved distance for the Sagittariusdwarf irregular galaxy (SagDIG), which now places this object very closeto the edge of the Local Group zero-velocity surface; (3) moreinformation on the evolutionary histories of some individual Local Groupmembers; and (4) improved distance determinations to, and luminositiesfor, a number of Local Group members. These data increase the number ofcertain (or probable) Local Group members to 36. The spatialdistribution of these galaxies supports Hubble's claim that the LocalGroup ``is isolated in the general field.'' Currently available evidencesuggests that star formation continued much longer in many dwarfspheroidals than it did in the main body of the Galactic halo. It issuggested that ``young'' globular clusters, such as Ruprecht 106, mighthave formed in now defunct dwarf spheroidals. Assuming SagDIG, which isthe most remote Local Group galaxy, to lie on, or just inside, thezero-velocity surface of the Local Group yields a dynamical age>~17.9+/-2.7 Gyr. However, this value is meaningful only if the outerregions of the local Group are in virial equilibrium.
| The stellar populations of the Fornax dwarf spheroidal galaxy
We present B,V,I CCD photometry of about 40000 stars in four regions ofthe Fornax dwarf spheroidal galaxy down to V ~ 23.5, the largestthree-color data set obtained for this galaxy until now. The resultantcolor-magnitude diagrams, based on a wide color baseline, show a varietyof features tracing the history of star formation of this dwarf galaxy.One of the most distinctive features in our diagrams is the conspicuousyoung main sequence, indicating recent star formation untilapproximately 2*E8 yr ago. A plume of stars brighter than thered HB clump, with (B- I) ~ 0.5, trace the helium-burning phase of theyoung population. A comparison of the color and extension of thisfeature with model isochrones suggests a relatively metal-richpopulation ([Fe/H] ~ -0.7) with age 300-400 Myr. This represents animportant constraint for understanding the chemical enrichment historyof Fornax. An extended upper AGB tail and a prominent red HB clump signthe presence of the well-known dominant intermediate-age population withan age range 2-10 Gyr, for which we have estimated a mean age 5.4+/-1.7.About 0.2 mag below the red clump, an extended HB is indicative of anold population. We show that blue HB stars may be present in the outerregions. Together with previous detection of RR Lyrae, this providesevidence for a minority field population that is as old and metal-pooras that in the Fornax globular clusters. We have identified the AGBbump, a clustering of stars that occurs at the beginning of heliumshell-burning evolution, at a luminosity M_V =~ -0.4. This is an exampleof the short-lived evolutionary phases that can be revealed in stellarpopulations using adequately large star data samples, whose measurementsprovide powerful tests of theoretical models. Based on precise detectionof the tip of the RGB in a selected RGB sample, we measure a correcteddistance modulus (m-M)_0 = 20.70 +/- 0.12. An independent estimate ofthe distance to Fornax was also obtained from the mean magnitude of oldhorizontal branch stars, yielding a distance modulus(m-M)_0=20.76+/-0.04, in good agreement with the distance estimated fromthe red giant branch tip and previous results. The large baseline of the(B-I) colors together with the size of the stellar sample allowed us toanalyze in detail the color distribution of the red giant stars. We findthat it can be approximately described as the superposition of twopopulations. The dominant component, comprising ~ 70% of the red giantstars, consists of relatively metal-enriched intermediate-age stars. Itsmean metallicity is [Fe/H]=-1.39 +/- 0.15, based on a comparison of thefiducial locus of the bulk of the Fornax red giants with the homogeneousGalactic globular cluster set of Da Costa & Armandroff(\cite{daco+arma90}). Once the younger mean age of Fornax is taken intoaccount, our best estimate for the mean abundance of the bulk of thegalaxy is [Fe/H]~ -1.0 +/- 0.15. The dominant intermediate-age componenthas an intrinsic color dispersion sigma_0 (B-I) = 0.06 +/- 0.01 mag,corresponding to a relatively low abundance dispersion, sigma_ [Fe/H] =0.12 +/- 0.02 dex. Further, there is a distinct small population of redgiants on the blue side of the RGB. While these stars could be eitherold or young red giants, we show that their spatial distribution isconsistent with the radial gradient of old horizontal branch stars, andcompletely different from that of the younger population. Thisunambiguously qualifies them as old and metal-poor. This resultclarifies the nature of the red giant branch of Fornax, suggesting thatits exceptional color width is due to the presence of two mainpopulations yielding a large abundance range (-2.0 < [Fe/H] <-0.7). This evidence suggests a scenario in which the Fornax dSphstarted forming a stellar halo and its surrounding clusters togetherabout 10-13 Gyr ago, followed by a major star formation epoch (probablywith a discontinuous rate) after several Gyr. Based on data collected atthe European Southern Observatory, La Silla, Chile, Proposal N.56.A-0538
| Hubble Space Telescope Photometry of the Fornax Dwarf Spheroidal Galaxy: Cluster 4 and Its Field
Using observations from the Hubble Space Telescope archive,color-magnitude diagrams (CMDs) have been constructed for globularcluster 4 in the Fornax dwarf spheroidal galaxy and its surroundingfield. These diagrams extend below the main-sequence turnoffs and haveyielded measurements of the ages of the populations. The most prominentfeatures of the CMD of the Fornax field population are a heavilypopulated red clump of horizontal-branch (HB) stars, a broad red giantbranch (RGB), and a main sequence that spans a large range inluminosity. In this CMD, there are very few stars at the positions ofthe HBs of the five globular clusters in Fornax, which suggests thatonly a very small fraction of the field population resembles theclusters in age and chemical composition. The large span in luminosityof the main sequence suggests that star formation began in the field~=12 Gyr ago and continued to ~=0.5 Gyr ago. There are separate subgiantbranches in the CMD, which indicates that the star formation was notcontinuous but occurred in bursts. The CMD of cluster 4 has a steep RGB,from which we estimate [Fe/H]~=-2.0. This is considerably lower thanestimates from the integrated light of the cluster, and the origins ofthis discrepancy are discussed. Cluster 4 has a very red HB and is,therefore, a prime example of the second-parameter effect. Comparisonsof cluster 4 with the other Fornax clusters and with M68, a verymetal-poor globular cluster of the Galactic halo, reveal that cluster 4is ~=3 Gyr younger than these other clusters, which have much bluer HBs.This age difference is consistent with the prediction that age is thesecond parameter to within the uncertainties. The CMD of cluster 4 isvirtually identical to that of the unusual globular cluster of theGalactic halo Ruprecht 106, which suggests that they have very similarages and chemical compositions. We discuss the possibility that cluster4 also resembles R106 in having a higher [Fe/H] than is indicated by itssteep RGB and also a lower [α/Fe] ratio than is usual for aglobular cluster, as indicated by some recent observations of R106. TheCMDs of the five Fornax clusters indicate that cluster age is amajor-but probably not the sole-second parameter. Buonanno et al.recently concluded that cluster density probably influenced the HBmorphologies of clusters 1, 2, 3, and 5. Despite a very large differencein central density, the HBs of cluster 4 and R106 are very similar. Thissuggests that density may act as a second parameter in clusters thathave HBs that are on the verge of moving toward the blue or are alreadyblue for another reason, such as very old age.
| The local group of galaxies
Hubble's (1936, p. 125) view that the Local Group (LG) is ``a typical,small group of nebulae which is isolated in the general field'' isconfirmed by modern data. The total number of certain and probable Groupmembers presently stands at 35. The half-mass radius of the Local Groupis found to be R h~ 350 kpc. The zero-velocity surface, whichseparates the Local Group from the field that is expanding with theHubble flow, has a radius R o = 1.18+/- 0.15 Mpc. The totalmass of the LG is M LG = (2.3+/- 0.6)× 1012Msolar. Most of this mass appears to be concentrated in theAndromeda and Milky Way subgroups of the LG. The total luminosity of theLocal Group is found to be M V = -22.0:. This yields amass-to-light ratio (in solar units) of M/L V = 44+/- 12. Thesolar motion with respect to the LG is 306+/- 18 km s-1,directed towards an apex at l = 99o+/- 5o, and b =-4o+/- 4o. The velocity dispersion within the LGis σ r = 61+/- 8 km s-1. The galaxies NGC3109, Antlia, Sextans A and Sextans B appear to form a distinct groupingwith V r = +114+/- 12 kpc relative to the LG, that is locatedbeyond the LG zero-velocity surface at a distance of 1.7 Mpc from theLocal Group centroid. The luminosity distribution of the LG has a slopeα = -1.1+/- 0.1. This value is significantly less negative thanthat which is found in rich clusters of galaxies. The luminositydistribution of the dwarf spheroidal galaxies is steeper than that fordwarf irregulars. Furthermore the dSph galaxies are stronglyconcentrated within the Andromeda and Milky Way subclusters of the LocalGroup, whereas the majority of dIr galaxies appear to be free-floatingmembers of the LG as a whole. With the possible exception of Leo I andLeo A, most LG members appear to have started forming starssimultaneously ~ 15 Gyr ago. Many of the galaxies, for whichevolutionary data are available, appear to have shrunk with time. Thisresult is unexpected because Hubble Space Telescope observations appearto show galaxies at z ~ 3 to be smaller than they are at z = 0. In theLarge Magellanic Cloud the rate of cluster formation was low for aperiod that extended from ~ 12 Gyr to ~ 4 Gyr ago. The rate of clusterformation may have increased more rapidly 3-5 Gyr ago, than did the rateof star formation. The reason for the sudden burst of cluster formationin the LMC ~ 4 Gyr ago remains obscure. None of the dwarf galaxies inthe LG appears to have experienced a starburst strong enough to haveproduced a ``boojum''.
| A catalogue of Mg_2 indices of galaxies and globular clusters
We present a catalogue of published absorption-line Mg_2 indices ofgalaxies and globular clusters. The catalogue is maintained up-to-datein the HYPERCAT database. The measurements are listed together with thereferences to the articles where the data were published. A codeddescription of the observations is provided. The catalogue gathers 3541measurements for 1491 objects (galaxies or globular clusters) from 55datasets. Compiled raw data for 1060 galaxies are zero-point correctedand transformed to a homogeneous system. Tables 1, 3, and 4 areavailable in electronic form only at the CDS, Strasbourg, via anonymousftp 130.79.128.5. Table 2 is available both in text and electronic form.
| The Ages of the Globular Clusters in the Fornax Dwarf Galaxy
From observations with the Hubble Space Telescope, color-magnitudediagrams (CMDs) have been constructed for the four most metal-poorglobular clusters in the Fornax dSph galaxy. These diagrams confirm thatcluster 1 has a redder horizontal-branch (HB) morphology than the otherthree clusters despite its similarity in metallicity (the secondparameter effect). The near coincidence of the fiducial lines of theCMDs indicates that the ages of the Fornax clusters are identical towithin 1 Gyr. Since a larger range is predicted if age is the secondparameter, we conclude that either the sensitivity of the HB to age hasbeen underestimated or some parameter besides age is acting as a secondparameter. The Fornax clusters exhibit a correlation between HBmorphology and central density that is similar to the one among galacticglobular clusters, which suggests that some mechanism related to densityis influencing HB morphology. No significant differences in age havebeen found between the Fornax clusters and the metal-poor galacticglobular clusters M68 and M92, which suggests that cluster formationbegan nearly simultaneously over a large volume of space.
| Placing the Fornax and Sagittarius Dwarf Spheroidal Globular Clusters in the Horizontal-Branch Type versus Metallicity Diagram
We plot the globular clusters of the Fornax galaxy and those associatedwith the Sagittarius dwarf spheroidal galaxy in the horizontal-branchtype versus metallicity diagram. The horizontal-branch types for theFornax clusters include corrections for red horizontal-branch stars fromthe field and are based on our recent work and new results in theliterature. Fornax globular clusters continue to stand out as having redhorizontal branches for their low ([Fe/H] ~ -2) metallicities, with nocounterparts in either the outer Galactic halo or the Magellanic Clouds.The clusters associated with Sagittarius lie to the blue of the Fornaxclusters, except for the metal-rich cluster Ter 7. Although themetallicities of the three metal-poor Sagittarius globular clusters aresimilar to those of the Fornax clusters, their horizontal branches arebluer and they lie in a region also populated by the old LMC and oldhalo clusters. Neither cluster system resembles the younger Galactichalo globular clusters, often suggested to have been accreted fromdisrupted dwarf spheroidal galaxies. Except for Ter 7, both the Fornaxand Sagittarius globular clusters are metal-poor compared with theirGalactic counterparts of the same horizontal-branch type. We find nocorrelation between HB type and other cluster properties such as centralconcentration, luminosity, central surface brightness, and estimatedcollision rate.
| Homogeneous Photometry for Star Clusters and Resolved Galaxies. I. A Survey of Bright Stars in the Fornax Dwarf Spheroidal Galaxy
We present accurate photometry on the Johnson B, Kron-Cousins Rphotometric system for approximately 100,000 stars in a 1/3 deg^2 fieldcentered on the dwarf spheroidal galaxy in Fornax. We identify numerousprobable short-period variable stars, blue stars that appear to be themain sequence of a small population with an age of order 10^8 yr, andtwo distinct types of luminous red star: an extended sequence ofprimarily carbon stars and a clump of mostly M giants slightly moreluminous than the giant-branch tip. The spatial distribution of each ofthese subpopulations within the Fornax dwarf galaxy is considered.
| Fornax Globular Cluster 3: New Color-Magnitude Diagrams for Clusters and Surrounding Field.
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1471S
| Color-Magnitude Diagrams of Merged Globular Clusters: Metallicity Effects
Mergers of globular clusters (GCs) once associated with dwarf spheroidal(dSph) galaxies have recently been suggested as an explanation for thebimodal horizontal branches (HBs) of some Galactic GCs, most notably NGC1851, NGC 2808, and NGC 6229. Through analysis of the availablecolor-magnitude diagrams for the GCs in the Fornax and Sagittarius dSPHsatellites of the Milky Way, as well as their metallicity distributions,we argue that the merger of two GCs would most likely produce a bimodaldistribution in red giant branch (RGB) colors, or at least a significantbroadening of the RGB, because of the expected difference in metallicitybetween the two merging globulars. No GC with a bimodal RGB is currentlyknown, and the tightness of the RGB sequences in the above bimodal HBGCs implies that a merger origin for their HB bimodality is unlikely.
| Globular clusters 1 and 3 in the Fornax dwarf galaxy.
We have performed photometric CCD observations of the giant andhorizontal branches of the globular cluster1, and of the giant branch ofcluster3, in the Fornax dwarf spheroidal galaxy. The observations wereperformed in the V and I standard broad-band filters as well as throughtwo narrow-band filters especially designed to distinguish betweencarbon and M type stars. The AGB is richly populated with carbon-richstars, and all of them are considerably below the theoretical lowerluminosity limit for such stars. If the Fornax clusters are interpretedas resembling an earlier epoch of the Galactic globular clusters, thelow luminosities of the carbon stars therefore point to a larger role ofthe low-mass stars in the chemical enrichment of our Galaxy. The giantbranches of the Fornax clusters are much broader than canonical giantbranches in Galactic globulars, and the AGB is more well populated. Wesuggest that the morphology and stellar population of the giant branchesindicate that the dwarf galaxies are =~3Gyr younger than the Galactichalo, which in turn seems to be =~3Gyr younger than the Galacticglobular clusters.
| Mergers of Globular Clusters
Globular clusters with composite color-magnitude diagrams, such as NGC1851, NGC 2808, and Fornax 3, might have formed by mergers. It issuggested that each of these objects had two parent clusters, one with ared horizontal branch and another with a blue horizontal branch. Suchmergers could have occurred if both ancestral objects were originallymembers of a single dwarf spheroidal galaxy in which the internalvelocity dispersion was low.
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