Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  




Upload your image

DSS Images   Other Images

Related articles

Deciphering the globular-cluster code.
Not Available

The planetary nebula population of the Sagittarius dwarf spheroidal galaxy
The identification of two new planetary nebulae (PNe) in the Sagittariusdwarf spheroidal galaxy (Sgr) is presented. This brings the total numberto four. Both new PNe were previously classified as Galactic objects.The first, StWr 2-21, belongs to the main body of Sgr, from its velocityand location. The second, the halo PN BoBn1, has a location, distanceand velocity in agreement with the leading tidal tail of Sgr. Weestimate that 10 per cent of the Galactic halo consists of Sgr debris.The specific frequency of PNe indicates a total luminosity of Sgr,including its tidal tails, of MV = -14.1. StWr 2-21 shows ahigh abundance of [O/H]= -0.23, which confirms the high-metallicitypopulation in Sgr uncovered by Bonifacio et al. The steepmetallicity-age gradient in Sgr is due to interstellar medium (ISM)removal during the Galactic plane passages, ISM reformation due tostellar mass-loss, and possibly accretion of metal-enriched gas from ourGalaxy. The ISM re-formation rate of Sgr, from stellar mass-loss, is 5× 10-4Msolaryr-1, amounting to~106Msolar per orbital period.Hubble Space Telescope images of three of the PNe reveal well-developedbipolar morphologies, and provide clear detections of the central stars.All three stars with deep spectra show WR lines, suggesting that theprogenitor mass and metallicity determines whether a PN central stardevelops a WR spectrum. One Sgr PN belongs to the class of IR-[WC]stars. Expansion velocities are determined for three nebulae. Comparisonwith hydrodynamical models indicates an initial density profile of ρ~ r-3. This is evidence for increasing mass-loss rates on theasymptotic giant branch. Peak mass-loss rates are indicated of~10-4Msolaryr-1.The IR-[WC] PN, He2-436, provides the sole direct detection of dust in adwarf spheroidal galaxy, to date.

A `super' star cluster grown old: the most massive star cluster in the Local Group
We independently redetermine the reddening and age of the globularcluster (GC) 037-B327 in M31 by comparing independently obtainedmulticolour photometry with theoretical stellar population synthesismodels. 037-B327 has long been known to have a very large reddeningvalue, which we confirm to be E(B - V) = 1.360 +/- 0.013, in goodagreement with the previous results. We redetermine its most likely ageat 12.4 +/- 3.2 Gyr.037-B327 is a prime example of an unusually bright early counterpart tothe ubiquitous `super' star clusters presently observed in mosthigh-intensity star-forming regions in the local Universe. In order tohave survived for a Hubble time, we conclude that its stellar initialmass function (IMF) cannot have been top-heavy. Using this constraint,and a variety of simple stellar population (SSP) models, we determine aphotometric mass of , somewhat depending on the SSP models used, themetallicity and age adopted and the IMF representation. This mass, andits relatively small uncertainties, makes this object the most massivestar cluster of any age in the Local Group. Assuming that thephotometric mass estimate thus derived is fairly close to its dynamicalmass, we predict that this GC has a (one-dimensional) velocitydispersion of the order of (72 +/- 13) km s-1. As a surviving`super' star cluster, this object is of prime importance for theoriesaimed at describing massive star cluster evolution.

Stellar mass loss and the intracluster medium in Galactic globular clusters: a deep radio survey for HI and OH
We present the results of a survey, the deepest to date, for HI emissionat 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720MHz) in the direction towards the Galactic globular clusters M15, M2,NGC6934, NGC7006 and Pal13. The aim is to measure the amount of hydrogenin the intracluster medium, and to find OH masers in the circumstellarenvelopes of globular cluster red giants. We present a tentativedetection of 0.3Msolar of neutral hydrogen in M15 andpossible detections of neutral hydrogen in M2 and Pal13. We derive upperlimits to the neutral hydrogen content of NGC6934 and 7006. No OHemission is detected. We also present deep HI data of the northern tipof the Magellanic Stream behind Pal13.

Caroline Herschel's catalogue of nebulae
Not Available

From Supermassive Black Holes to Dwarf Elliptical Nuclei: A Mass Continuum
Considerable evidence suggests that supermassive black holes reside atthe centers of massive galactic bulges. At a lower galactic mass range,many dwarf galaxies contain extremely compact nuclei that structurallyresemble massive globular clusters. We show that both these types ofcentral massive objects (CMOs) define a single unbroken relation betweenCMO mass and the luminosity of their host galaxy spheroid. Equivalently,MCMO is directly proportional to the host spheroid mass over4 orders of magnitude. We therefore suggest that the dE,N nuclei may bethe low-mass analogs of supermassive black holes and that these twotypes of CMOs may have both developed starting from similar initialformation processes. The overlap mass interval between the two types ofCMOs is small and suggests that for MCMO>107Msolar, the formation of a black hole was strongly favored,perhaps because the initial gas infall to the center was too rapid andviolent for star formation to occur efficiently.

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.

037-B327 in M31: Luminous Globular Cluster or Core of a Former Dwarf Spheroidal Companion to M31?
The cluster 037-B327 is of interest because it is both the most luminousand the most highly reddened cluster known in M31. Deep observationswith the Advanced Camera for Surveys on the Hubble Space Telescopeprovide photometric data in the F606W band and also show that thiscluster is crossed by a dust lane. We have determined the structuralparameters of 037-B327 by fitting the observed surface brightnessdistribution to a King model with rc=0.72" (=2.69 pc),rt=5.87" (=21.93 pc), and a concentration indexc=log(rt/rc)=0.91. The surface brightness profileappears to be essentially flat within 0.25" of the center and shows nosigns of core collapse. Although the dust lane affects the photometry,the King model fits the surface brightness profile well except for theregions badly affected by the dust lane. We also calculate thehalf-light radius, rh=1.11" (=4.15 pc). Combined withprevious photometry, we find that this object falls in the same regionof the MV versus logRh diagram as do ωCentauri, M54, and NGC 2419 in the Milky Way and the massive cluster G1in M31. All four of these objects have been claimed to be the strippedcores of former dwarf galaxies. This suggests that 037-B327 may also bethe stripped core of a former dwarf companion to M31.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withproposal 9453.

Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters
We present new (B, I) photometry for the globular cluster systems ineight brightest cluster galaxies (BCGs), obtained with the ACS/WFCcamera on the Hubble Space Telescope. In the very rich cluster systemsthat reside within these giant galaxies, we find that all have stronglybimodal color distributions that are clearly resolved by themetallicity-sensitive (B-I) index. Furthermore, the mean colors andinternal color range of the blue subpopulation are remarkably similarfrom one galaxy to the next, to well within the +/-0.02-0.03 maguncertainties in the foreground reddenings and photometric zero points.By contrast, the mean color and internal color range for the redsubpopulation differ from one galaxy to the next by twice as much as theblue population. All the BCGs show population gradients, with muchhigher relative numbers of red clusters within 5 kpc of their centers,consistent with their having formed at later times than the blue,metal-poor population. A striking new feature of the color distributionsemerging from our data is that for the brightest clusters(MI<-10.5) the color distribution becomes broad and lessobviously bimodal. This effect was first noticed by Ostrov et al. andDirsch et al. for the Fornax giant NGC 1399; our data suggest that itmay be a characteristic of many BCGs and perhaps other large galaxies.Our data indicate that the blue (metal-poor) clusters brighter thanMI~=-10 become progressively redder with increasingluminosity, following a mass/metallicity scaling relationZ~M0.55. A basically similar relation has been found for M87by Strader et al. (2005). We argue that these GCS characteristics areconsistent with a hierarchical-merging galaxy formation picture in whichthe metal-poor clusters formed in protogalactic clouds or densestarburst complexes with gas masses in the range107-1010 Msolar, but where the moremassive clusters on average formed in bigger clouds with deeperpotential wells where more preenrichment could occur.

Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.

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.

Hot Populations in M87 Globular Clusters
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.

Globular Clusters in Dwarf Galaxies
Data are currently available on the luminosities and half-light radii of101 globular clusters associated with low-luminosity parent galaxies.The luminosity distribution of globular clusters embedded in dwarfgalaxies having Mv>-16 is found to differ dramaticallyfrom that for globular clusters surrounding giant host galaxies withMv<-16. The luminosity distribution of globular clustersin giant galaxies peaks at Mv~-7.5, whereas that for dwarfgalaxies is found to increase monotonically down to the completenesslimit of the cluster data at Mv~-5.0. Unexpectedly, thepower-law distribution of the luminosities of globular clusters hostedby dwarf galaxies is seen to be much flatter than that of the bright,unevolved part of the luminosity distribution of globular clustersassociated with giant galaxies. The specific frequency of globularclusters fainter than Mv=-7.5 is found to be particularlyhigh in dwarf galaxies. The luminosity distribution of the LMC globularclusters is similar to that found in giant galaxies and differs fromthose of globular clusters in dwarf galaxies. The present data appear toshow no strong dependence of globular cluster luminosity on themorphological type of their parent galaxies. No attempt is made toexplain the unexpected discovery that the luminosity distribution ofglobular clusters is critically dependent on parent galaxy luminosity(or mass) but insensitive to the morphological type of the host galaxy.

Outer structure of the Galactic warp and flare: explaining the Canis Major over-density
Aims.In this paper we derive the structure of the Galactic stellar warpand flare. Methods: .We use 2MASS red clump and red giant stars,selected at mean and fixed heliocentric distances ofRȯ≃3, 7 and 17 kpc. Results: .Our resultscan be summarized as follows: (i) a clear stellar warp signature isderived for the 3 selected rings, proving that the warp starts alreadywithin the solar circle; (ii) the derived stellar warp is consistent(both in amplitude and phase-angle) with that for the Galacticinterstellar dust and neutral atomic hydrogen; (iii) the consistency andregularity of the stellar-gaseous warp is traced out to aboutR_GC˜20 kpc; (iv) the Sun seems not to fall on the line of nodes.The stellar warp phase-angle orientation (φ˜15°) is closeto the orientation angle of the Galactic bar and this, most importantly,produces an asymmetric warp for the inner Rȯ≃3 and7 kpc rings; (v) a Northern/Southern warp symmetry is observed only forthe ring at Rȯ≃17 kpc, at which the dependency onφ is weakened; (vi) treating a mixture of thin and thick diskstellar populations, we trace the variation with R_GC of the diskthickness (flaring) and derive an almost constant scale-height (~0.65kpc) within R_GC˜15 kpc. Further out, the disk flaring increasegradually reaching a mean scale-height of ~1.5 kpc at R_GC˜23 kpc;(vii) the derived outer disk warping and flaring provide further robustevidence that there is no disk radial truncation at R_GC˜14 kpc. Conclusions: .In the particular case of the Canis Major (CMa)over-density we confirm its coincidence with the Southern stellarmaximum warp occurring near l˜240° (forRȯ≃7 kpc) which brings down the Milky Waymid-plane by ~3° in this direction. The regularity and consistencyof the stellar, gaseous and dust warp argues strongly against a recentmerger scenario for Canis Major. We present evidence to conclude thatall observed parameters (e.g. number density, radial velocities, propermotion etc) of CMa are consistent with it being a normal Milky Wayouter-disk population, thereby leaving no justification for more complexinterpretations of its origin. The present analysis or outer diskstructure does not provide a conclusive test of the structure or originof the Monoceros Ring. Nevertheless, we show that a warped flared MilkyWay contributes significantly at the locations of the Monoceros Ring.Comparison of outer Milky Way H I and CO properties with those of othergalaxies favors the suggestion that complex structures close to planarin outer disks are common, and are a natural aspect of warped andflaring disks.

Na-O anticorrelation and HB. I. The Na-O anticorrelation in NGC 2808
We derived the atmospheric parameters and elemental abundances of Fe, O,and Na for about 120 red giant stars in the Galactic globular clusterNGC 2808. Our results are based on the analysis of medium-highresolution (R=22 000{-}24 000) GIRAFFE spectra acquired with the FLAMESspectrograph at VLT-UT2 as a part of a project aimed at studying theNa-O anticorrelation as a function of physical parameters in globularclusters. We present the anticorrelation of Na and O abundances in NGC2808 here, and discuss the distribution function of stars along thisrelation. Besides a bulk of O-normal stars with the typical compositionof field halo stars, NGC 2808 seems to host two other groups of O-poorand super O-poor stars. In this regard, NGC 2808 is similar to M 13, thetemplate cluster for the Na-O anticorrelation. However, in contrast to M13, most stars in NGC 2808 are O-rich. This might be related to thehorizontal branch morphologies that are very different in these twoclusters. The average metallicity we found for NGC 2808 is [Fe/H]=-1.10(rms = 0.065 dex, from 123 stars). We also found some evidence of asmall intrinsic spread in metallicity, but more definitive conclusionsare hampered by the presence of a small differential reddening.

Structural parameters from ground-based observations of newly discovered globular clusters in NGC 5128
We have investigated a number of globular cluster candidates from arecent wide-field study by Harris et al. (2004a, AJ, 128, 712) of thegiant elliptical galaxy NGC 5128. We used theMagellan I telescope + MagIC camera under excellent seeing conditions(0.3 arcsec-0.6 arcsec) and obtained very high resolution images for asample of 44 candidates. Of these, 15 appear to be bonafide globularclusters in NGC 5128 while the rest are eitherforeground stars or background galaxies. We also serendipitouslydiscovered 18 new cluster candidates in the same fields. Our imagesallow us to study the light profiles of the likely clusters, all ofwhich are well resolved. This is the first ground-based study ofstructural parameters for globular clusters outside the Local Group. Wecompare the psf-deconvolved profiles with King models and derivestructural parameters, ellipticities and surface brightnesses. Wecompare the derived structural properties with those of otherwell-studied globular cluster systems. In general, our clusters aresimilar in size, ellipticity, core radius and central surface brightnessto their counterparts in other galaxies, in particular those inNGC 5128 observed with HST by Harris et al. (2002,AJ, 124, 1435). However, our clusters extend to higher ellipticities andlarger half-light radii than their Galactic counterparts, as do theHarris et al. sample. Combining our results with those of Harris et al.fills in the gaps previously existing in rh - MVparameter space and indicates that any substantial difference betweenpresumed distinct cluster types in this diagram, including for examplethe Faint Fuzzies of Larsen & Brodie (2000, AJ, 120, 2938) and the"extended, luminous" M 31 clusters of Huxor et al. (2005, MNRAS, 360,1007) is now removed and that clusters form a continuum in this diagram.Indeed, this continuum now extends to the realm of the Ultra CompactDwarfs. The metal-rich clusters in our sample have half-light radii thatare almost twice as large in the mean as their metal-poor counterparts,at odds with the generally accepted trend. The possibility exists thatthis result could be due in part to contamination by backgroundgalaxies. We have carried out additional analysis to quantify thiscontamination. This shows that, although galaxies cannot be easily toldapart from clusters in some of the structural diagrams, the combinationof excellent image quality and Washington photometry should limit thecontamination to roughly 10% of the population of cluster candidates.Finally, our discovery of a substantial number of new cluster candidatesin the relatively distant regions of the NGC 5128halo suggests that current values of the total number of globularclusters may be underestimates.

Chandra observations of the globular cluster M 54
We have carried out a Chandra observation of the globular cluster M 54.We detected 7 sources located within the half-mass radius of M 54, at aflux limit of 1.5 × 10-15 erg s-1cm-2 in the 0.3-8 keV energy band. The spatial distributionand the colour/spectral properties of the 7 sources suggest that theyare likely to be cataclysmic variables or LMXBs in the globular cluster.M 54 shows the largest number of X-ray sources with luminosities greaterthan 1032 erg s-1 compared to other globularclusters observed using Chandra and XMM-Newton. We searched for acorrelation between the number of sources above this luminosity levelwith globular cluster parameters. We found evidence that the number ofsources peaks at a King concentration parameter c ~ 1.7-1.9, withglobular clusters which are core-collapsed or have low-c values having asmaller number of sources. We speculate on possible reasons for this.

Distances and Galactic Kinematics of Two Blue Metal-poor Stars
We use an SX Phe period-luminosity relation to derive distance estimatesand Galactic kinematic parameters for the short-period pulsating bluemetal-poor stars CS 29499-057 and CS 22966-043. The resulting kinematicsare quite distinctive relative to the metal-poor halo field. Even thoughthese two field blue stragglers' binarity is consistent with the idea offormation in a mass transfer system, we speculate that these stars'origin instead lies in a Galactic merger or interaction with a dwarfgalaxy satellite, or in the merger of a stellar binary system. Theformer possibility is consistent with CS 22966-043's significant[α/Fe] deficiency. A non-mass-transfer origin for both binaries isalso consistent with (though not necessarily precluded by) the lack ofany significant s-process enhancement seen in either star, and the longorbital period of CS 29499-057 that may preclude a mass transfer origin.In an effort to determine distances to additional blue metal-poor stars,we have photometrically monitored CS 22890-069, CS 22872-062, and CS22884-065. However, the light curves of these three objects indicate noshort-period photometric variability at the level of a few hundredths ofa magnitude.

Homogeneous Photometry. IV. On the Standard Sequence in the Globular Cluster NGC 2419
I present a new analysis of CCD-based BVRI broadband photometry for theglobular cluster NGC 2419, based on 340 CCD images either donated bycolleagues or retrieved from public archives. The calibrated resultshave been made available through my Web site. I compare the results ofmy analysis with those of an independent analysis of a subset of thesedata by Saha et al. (2005, PASP, 117, 37), who have found acolor-dependent discrepancy of up to 0.05 mag between their I-bandphotometry and mine for stars in this cluster. I conclude that a majorpart of the discrepancy appears to be associated with small shuttertiming errors (a few hundredths of a second) in the Mini-Mosaic (MIMO)camera on the WIYN 3.5 m telescope. Smaller contributions to the anomalylikely come from (1) a color-scale error with a maximum amplitude of~+/-0.02 mag in my secondary standard list as of 2004 September, and (2)statistical effects arising from the previous study's use of arelatively small number of standard-star observations to determine acomparatively large number of fitting parameters in the photometrictransformations.Based in part on observations obtained at the 3.5 m and 0.9 m WIYNTelescopes. The WIYN Observatory is a joint facility of the Universityof Wisconsin-Madison, Indiana University, Yale University, and theNational Optical Astronomy Observatory (NOAO).

Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.

Deepsky delights.
Not Available

New light on the initial mass function of the Galactic halo globular clusters
We present new constraints on the initial mass spectrum of the GalacticOld Halo globular clusters. This has remained poorly known so far, asboth an initial power-law mass spectrum and an initial lognormal massfunction could have evolved into the presently observed globular clustermass distribution, making the initial contribution of now lost low-massobjects ill-determined. Our approach consists of comparing the evolutionwith time of both the radial mass density profile and the number densityprofile of the globular cluster system. Using the analytical expressionestablished by Vesperini & Heggie for the temporal evolution of themass of a globular cluster on a circular orbit in a stable Galacticpotential, we evolve the mass and number density profiles of manyputative globular cluster systems, each starting with a differentinitial cluster mass spectrum and initial cluster space density. We thencompare the modelled profiles with those of the Old Halo cluster systemin order to investigate which system(s) provide(s) the best consistencywith the data. Specifically, we build on the following points: (i) thepresently observed mass density profile and number density profile ofthe Old Halo cluster system show the same shape; (ii) assuming thatglobular clusters were initially distributed the same way in mass at allgalactocentric distances, the mass and number density profiles had alsothe same shape initially; and (iii) according to our simulations, themass density profile remains well preserved, irrespective of the initialcluster mass spectrum, while the temporal evolution of its numbercounterpart depends sensitively on initial conditions. We show that toobtain a mass density profile and a number density profile that areidentical in shape, both initially and after a Hubble time of evolution,the globular cluster system must have been depleted in low-mass objectsfrom the beginning. We deduce that the initial distribution in mass ofthe globular clusters was either a lognormal mass function, similar tothat today, or a power-law mass spectrum with a slope ~=-2 and truncatedat large mass, say, around 105Msolar. In contrast,a power-law mass spectrum with a similar slope but extending down to lowcluster mass (i.e. a few thousand solar masses) seems to be ruled out.

A new population of extended, luminous star clusters in the halo of M31
We present three new clusters discovered in the halo of M31 which,although having globular-like colours and luminosities, have unusuallylarge half-light radii, ~30 pc. They lie at projected galactocentricdistances of ~15 to ~35 kpc. These objects begin to fill the gap inparameter space between globular clusters and dwarf spheroidals, and areunlike any clusters found in the Milky Way, or elsewhere to date.Colour-magnitude diagrams, integrated photometric properties and derivedKing profile fit parameters are given, and we discuss possible originsof these clusters and their relationships to other populations.

Upper limits on central black hole masses of globular clusters from radio emission and a possible black hole detection in the Ursa Minor dwarf galaxy
Intermediate-mass black holes (IMBHs) have been alternatively predictedto be quite common in the centres of globular clusters or nearlyimpossible to form and retain in the centres of globular clusters. As ithas been recently shown that radio observations are currently the mostsensitive observational technique for detecting such objects, we haveobtained new deep radio observations of Omega Cen, and have reanalyzedolder observations of M 15 in the hope of constraining the masses ofpossible black holes in their centres. In both cases, upper limits ofabout 100 μJy are found atGHz frequencies. We find that if theBondi-Hoyle accretion rate truly represents the spherical accretion rateonto a black hole, then the masses of the black holes in the centres ofthese two galaxies are severely constrained - with mass limits of lessthan about 100 solar masses in both cases. If more realistic assumptionsare made based on recent work showing the Bondi rate to be a severeoverestimate, then the data for Omega Cen are marginally consistent witha black hole of about 1/1000 of the mass of the cluster (i.e. about1000Msolar). The data for M 15 are then only marginallyconsistent with previous reports of a ~2000 solar mass black hole, andwe note that there is considerable hope for either detecting the blackhole or improving this upper limit with current instrumentation.Finally, we discuss the possibility that the radio source near the coreof the Ursa Minor dwarf spheroidal galaxy is a~104-Msolar black hole.

The central density cusp of the Sagittarius dwarf spheroidal galaxy
We present an analysis of the density profile in the central region ofthe Sagittarius dwarf spheroidal galaxy. A strong density enhancement ofSgr stars is observed. The position of the peak of the detected cusp isindistinguishable from the centre of M54. The photometric properties ofthe cusp are fully compatible with those observed in the nuclei of dwarfelliptical galaxies, indicating that the Sgr dSph would appear as anucleated galaxy independently of the presence of M54 at its centre.

The structure of our stellar system.
Not Available

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.

Metal Abundances of RR Lyrae Stars in the Metal-rich Globular Cluster NGC 6441
Low-resolution spectra have been used to measure individual metalabundances of RR Lyrae stars in NGC 6441, a Galactic globular clusterknown to have very unusual horizontal-branch morphology and periods ofthe RR Lyrae stars for its high metallicity. We find an average metalabundance of [Fe/H]=-0.69+/-0.06 (rms=0.33 dex) and [Fe/H]=-0.41+/-0.06(rms=0.36 dex) on Zinn & West and Carretta & Gratton metallicityscales, respectively, consistent with the cluster metal abundancederived by Armandroff & Zinn. Most of the metallicities wereextrapolated from calibration relations defined for [Fe/H]<=-1however, they are clearly high and contrast with the rather long periodsof the NGC 6441 variables, thus confirming that the cluster does not fitin the general Oosterhoff classification scheme. The rms scatter of theaverage is larger than observational errors (0.15-0.16 dex), possiblyindicating some spread in metallicity. However, even the metal-poorvariables, if confirmed to be cluster members, are still more metal-richthan those commonly found in the Oosterhoff type II globular clusters.Based on data collected at the Very Large Telescope of the EuropeanSouthern Observatory, Paranal, Chile, program number 71.B-0621.

The ACS Virgo Cluster Survey. VII. Resolving the Connection between Globular Clusters and Ultracompact Dwarf Galaxies
NO>1Based on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.

The Unusual Abundance of Copper in the Sagittarius Dwarf Spheroidal Galaxy and Implications for the Origin of ω Centauri
We present copper abundances for 14 red giant stars in the Sagittariusdwarf spheroidal galaxy (Sgr dSph), showing that [Cu/Fe] is deficient by~0.5 dex, relative to the Galactic trend. This is most easily understoodas due to an enhanced contribution of iron peak nucleosynthesis productsfrom Type Ia supernovae (SNe Ia). The deficient [Cu/Fe] ratios mightalso be the result of a metallicity-dependent yield from SNe Ia, similarto previous conclusions for Mn, although SN Ia nucleosynthesispredictions suggest a negligible Cu yield. The enhanced SN Ia products,suggested by our low [Cu/Fe] ratios, fit a leaky box chemical evolutionscenario for the Sgr dSph, where ejecta from the old, metal-poor,population overwhelmed nucleosynthesis products from youngergenerations, resulting in young stars with uncharacteristiccompositions. The only other system known to have unusually low [Cu/Fe]is the Galactic globular cluster ω Cen, which, like theSagittarius dwarf galaxy, has strong enhancements of s-process elements.Thus, our copper abundances lend support to the idea that ω Cen isthe remaining nucleus of an accreted dwarf galaxy.

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Right ascension:18h55m06.00s
Apparent magnitude:7.7

Catalogs and designations:
Proper Names   (Edit)
MessierM 54
NGC 2000.0NGC 6715

→ Request more catalogs and designations from VizieR