			Home About us Contact

Globular Clusters (globular + cluster)

Distribution by Scientific Domains

Physics and Astronomy	97%

Selected Abstracts

A systematic study of low-mass X-ray binaries in the M31 globular cluster system

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
Mark B. Peacock
ABSTRACT We investigate low-mass X-ray binaries (LMXBs) in the M31 globular cluster (GC) system using data from the 2XMMi catalogue. These X-ray data are based on all publicly available XMM,Newton observations of the galaxy. This new survey provides the most complete and homogeneous X-ray survey of M31's GCs to date, covering >80 per cent of the confirmed old clusters in the galaxy. We associate 41 X-ray sources with confirmed old clusters in the recent M31 cluster catalogue of Peacock et al. Comparing these data with previous surveys of M31, it is found that three of these clusters are newly identified, including a bright transient source in the cluster B128. Four additional clusters, that are not detected in these 2XMMi data, have previously been associated with X-ray sources from Chandra or ROSAT observations. Including these clusters, we identify 45 clusters in M31 which are associated with X-ray emission. By considering the latest optical GC catalogues, we identify that three of the previously proposed X-ray clusters are likely to be background galaxies and two have stellar profiles. We consider the properties of LMXB hosting clusters and confirm significant trends between the presence of an LMXB and the metallicity, luminosity and stellar collision rate of a cluster. We consider the relationship between the luminosity and stellar collision rate of a cluster and note that LMXB hosting clusters have higher than average stellar collision rates for their mass. Our findings strongly suggest that the stellar collision rate is the dominant parameter related to the presence of LMXBs. This is consistent with the formation of LMXBs in GCs through dynamical interactions. [source]

A spectroscopic survey of EC4, an extended cluster in Andromeda's halo,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
M. L. M. Collins
ABSTRACT We present a spectroscopic survey of candidate red giant branch stars in the extended star cluster, EC4, discovered in the halo of M31 from our Canada,France,Hawaii Telescope/MegaCam survey, overlapping the tidal streams, Streams ,Cp' and ,Cr'. These observations used the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope to obtain spectra around the Caii triplet region with ,1.3 Å resolution. Six stars lying on the red giant branch within two core radii of the centre of EC4 are found to have an average vr=,287.9+1.9,2.4 km s,1 and ,v,corr = 2.7+4.2,2.7 km s,1, taking instrumental errors into account. The resulting mass-to-light ratio for EC4 is M/L = 6.7+15,6.7 M,/L,, a value that is consistent with a globular cluster within the 1, errors we derive. From the summed spectra of our member stars, we find EC4 to be metal-poor, with [Fe/H]=,1.6 ± 0.15. We discuss several formation and evolution scenarios which could account for our kinematic and metallicity constraints on EC4, and conclude that EC4 is most comparable with an extended globular cluster. We also compare the kinematics and metallicity of EC4 with Streams ,Cp' and ,Cr', and find that EC4 bears a striking resemblance to Stream ,Cp' in terms of velocity, and that the two structures are identical in terms of both their spectroscopic and photometric metallicities. From this, we conclude that EC4 is likely related to Stream ,Cp'. [source]

A Subaru/Suprime-Cam wide-field survey of globular cluster populations around M87 , I. Observation, data analysis and luminosity function

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006
Naoyuki Tamura
ABSTRACT In this paper and a companion paper, we report on a wide-field imaging survey of the globular cluster (GC) populations around M87 carried out with Suprime-Cam on the 8.2-m Subaru telescope. Here, we describe the observations, data reduction and data analysis, and present luminosity functions of GC populations around M87 and NGC 4552, another luminous Virgo elliptical in our survey field. The imaging data were taken in the B, V and I bands with a sky coverage of extending from the M87 centre out to ,0.5 Mpc. GC candidates were selected by applying a colour criterion on the B,V and V,I diagram to unresolved objects, which greatly reduces contamination. The data from control fields taken with Subaru/Suprime-Cam were also analysed for subtraction of contamination in the GC sample. These control field data are compatible with those in the M87 field in terms of the filter set (BVI), limiting magnitudes and image quality, which minimize the possibility of introducing any systematic errors into the subtractive correction. We investigate GC luminosity functions (GCLFs) at distances , 10 arcmin (,45 kpc) from the host galaxy centre in detail. By fitting Gaussians to the GCLFs, the V -band turnover magnitude (VTO) is estimated to be 23.62 ± 0.06 and 23.56 ± 0.20 mag for the GC population in M87 and NGC 4552, respectively. The GCLF is found to be a function of GC colour; VTO of the red GC subpopulation (V,I > 1.1) is fainter than that of the blue GC subpopulation (V,I, 1.1) in both M87 and NGC 4552, as expected if the colour differences are primarily due to a metallicity effect, and the mass functions of the two subpopulations are similar. The radial dependence of the GCLF is also investigated for the GC population in M87. The GCLF of each subpopulation at 1 ,R, 5 arcmin is compared to that at 5 ,R, 10 arcmin, but no significant trend with distance is found in the shape of the GCLF. We also estimate GC-specific frequencies (SN) for M87 and NGC 4552. The SN of the M87 GC population is estimated to be 12.5 ± 0.8 within 25 arcmin. The SN value of the NGC 4552 GC population is estimated to be 5.0 ± 0.6 within 10 arcmin. [source]

A Subaru/Suprime-Cam wide-field survey of globular cluster populations around M87 , II.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006
Colour, spatial distribution
ABSTRACT We have performed a wide-field imaging survey of the globular cluster (GC) populations around M87 with Suprime-Cam on the 8.2-m Subaru Telescope. A field extending from the centre of M87 out to a radius of ,0.5 Mpc was observed through the BVI filters. By investigating the GC colour distribution as a function of distance from M87 and NGC 4552, another luminous Virgo elliptical in our survey field, it is found that clear bimodality [(V,I)peak, 1.0 and 1.2] is seen only in the inner regions (,10 kpc) of the host galaxies and that it becomes weaker with radius due to the decreasing contribution of the red GC (V,I > 1.1) subpopulation. It is also found (both around M87 and around NGC 4552) that while the spatial distribution of the red GCs is as centrally concentrated as the host galaxy halo light distribution, the distribution of the blue GCs (V,I, 1.1) tends to be more extended. However, the blue GC distribution around M87 is not as extended as the Virgo cluster mass density profile. Based on these facts, we argue that the contribution of GCs associated with the Virgo cluster [e.g. intergalactic GCs (i-GCs)] is not significant around M87 and is not the primary reason for the high SN value of M87. Instead, most of the blue GCs around luminous ellipticals, as well as the red GCs, are presumed to be associated with the host galaxy. We model the radial profile of GC surface densities out to ,0.5 Mpc from M87 by a superposition of the GC populations associated with M87 and with NGC 4552. It is found that there are some regions where the GC surface densities are larger than that which is expected from this model, suggesting the existence of an additional i-GC population independent of the luminous ellipticals. By comparing the estimated i-GC surface density with the luminosity density of the intracluster stellar population inferred from the intergalactic planetary nebulae in the Virgo cluster, we obtain a crude estimate of i-GC specific frequency SN= 2.9+4.2,1.5. If this SN value represents the stellar population tidally stripped by a massive central galaxy from other (less-luminous) galaxies, the contribution of tidally captured GCs in the GC population of M87 would need to be low to be consistent with the high SN value of M87. [source]

Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
N. F. Martin
ABSTRACT We present the discovery of three faint dwarf galaxies and a globular cluster in the halo of the Andromeda galaxy (M31), found in our MegaCam survey that spans the southern quadrant of M31, from a projected distance of ,50 to ,150 kpc. Though the survey covers 57 deg2, the four satellites lie within 2° of one another. From the tip of the red giant branch (RGB), we estimate that the globular cluster lies at a distance of 631 ± 58 kpc from the Milky Way and along with a ,100 kpc projected distance from M31 we derive a total distance of 175 ± 55 kpc from its host, making it the farthest M31 globular cluster known. It also shows the typical characteristics of a bright globular cluster, with a half-light radius of 2.3 ± 0.2 pc and an absolute magnitude in the V band of MV,0=,8.5 ± 0.3. Isochrone fitting reveals that it is dominated by a very old population with a metallicity of [Fe/H],,1.3. The three dwarf galaxies are revealed as overdensities of stars that are aligned along the RGB tracks in their colour,magnitude diagrams. These satellites are all very faint, with absolute magnitudes in the range ,7.3 ,MV,0,,6.4, and show strikingly similar characteristics with metallicities of [Fe/H],,1.4 and half-light radii of ,120 ± 45 pc, making these dwarf galaxies two to three times smaller than the smallest previously known satellites of M31. Given their faintness, their distance is difficult to constrain, but we estimate them to be between 740 and 955 kpc which places them well within the virial radius of the host galaxy. The panoramic view of the MegaCam survey can provide an unbiased view of the satellite distribution of the Andromeda galaxy and, extrapolating from its coverage of the halo, we estimate that up to 45 ± 20 satellites brighter than MV,,6.5 should be orbiting M31. Hence faint dwarf galaxies cannot alone account for the missing satellites that are predicted by , cold dark matter models, unless they reside in dark matter minihaloes that are more massive than the typical masses of 107 M, currently inferred from their central radial velocity dispersion. [source]

Gemini/GMOS imaging of globular clusters in the Virgo galaxy NGC 4649 (M60)

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
Duncan A. Forbes
ABSTRACT We present Sloan g and i imaging from the Gemini Multi-object Spectrograph (GMOS) instrument on the Gemini North telescope for the globular cluster (GC) system around the Virgo galaxy NGC 4649 (M60). Our three pointings, taken in good seeing conditions, cover an area of about 90 square arcmin. We detect 2151 unresolved sources. Applying colour and magnitude selection criteria to this source list gives 995 candidate GCs. Our source list is greater than 90 per cent complete to a magnitude of i= 23.6, and has little contamination from background galaxies. We find fewer than half a dozen potential ultracompact dwarf galaxies around NGC 4649. Foreground extinction from the nearby spiral NGC 4647 is limited to be AV < 0.1. We confirm the bimodality in the GC colour distribution found by earlier work using Hubble Space Telescope/WFPC2 imaging. As is commonly seen in other galaxies, the red GCs are concentrated towards the centre of the galaxy, having a steeper number density profile than the blue GC subpopulation. The varying ratio of red-to-blue GCs with radius can largely explain the overall GC system colour gradient. The underlying galaxy starlight has a similar density profile slope and colour to the red GCs. This suggests a direct connection between the galaxy field stars and the red GC subpopulation. We estimate a total GC population of 3700 ± 900, with the uncertainty dominated by the extrapolation to larger radii than observed. This total number corresponds to a specific frequency SN= 4.1 ± 1.0. Future work will present properties derived from GMOS spectra of the NGC 4649 GCs. [source]

On the reliability of the semi-empirical RR Lyrae period,V -band luminosity,blue amplitude relation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002
R. De Santis
Abstract We investigate the accuracy and reliability of the semi-empirical period,V -band luminosity,blue amplitude (PLA) relationship for ab-type RR Lyrae stars originally obtained by Castellani and De Santis in the mid-1990s. We infer that the zero-point of this relationship depends on the metallicity, by studying a sample of both field and cluster variables. We also show that the use of this relationship can still be useful for those stellar systems showing an intrinsic metallicity spread, since in this case the metallicity has a negligible effect on the final distance modulus estimate. We compare the adopted semi-empirical relationship with the fully empirical one recently provided by Kovács & Walker. When the zero-point of the latter relation is fixed consistently with the former one, the two equations are equivalent. By applying the semi-empirical period,V -band luminosity,blue amplitude relation, as well as the technique proposed recently by Cassisi, De Santis & Piersimoni, to the globular cluster , Cen, we show that the empirical slope of the relationship between the mass of the fundamental RR Lyrae pulsators and their metallicity is in fair agreement with the one predicted by updated evolutionary models for horizontal branch stars. [source]

Spectroscopy of globular cluster candidates in the Sculptor group galaxies NGC 253 and 55

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2000
M. A. Beasley
We have obtained spectra for 103 published globular cluster candidates in the Sculptor group galaxies NGC 253 and 55. On the basis of radial velocities and digitized plate images, 14 globular clusters are identified in NGC 253 and one probable globular cluster is identified in NGC 55. The majority of the objects in the sample appear to be background galaxies. We have obtained and analysed COSMOS plate scans of NGC 253 and 55 and use these along with the spectroscopically identified clusters to define new samples of globular cluster candidates in the two galaxies which should have reduced contamination. [source]

Dynamical evolution of the mass function and radial profile of the Globular Cluster systems of the Milky Way and M87

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2008
J. Shin
Abstract Evolution of the mass function (MF) and radial distribution (RD) of the globular cluster (GC) systems of the Milky Way and M87 are calculated using an advanced and realistic Fokker-Planck (FP) model that considers dynamical friction, disk/bulge shocks, and eccentric cluster orbits. We perform hundreds of FP calculations with different initial cluster conditions, and then search a wide parameter space for the best-fit initial GC MF and RD that evolves into the observed present-day GC MF and RD. By allowing both MF and RD of the initial GC system to vary, we find that in case of the Milky Way, our best-fit models have a higher peak mass for a log-normal initial MF and a higher cutoff mass for a powerlaw initial MF than previous estimates. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Use of a ,-glucan hydrocolloidal suspension in the manufacture of low-fat Cheddar cheese: manufacture, composition, yield and microstructure

INTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 1 2004
G. Konuklar
Summary Low-fat Cheddar cheese was manufactured using a , -glucan, hydrocolloidal fat replacer denoted as Nutrim. The composition, production efficiency, microstructure, and utility of replacing fat with Nutrim were examined. Cheese samples (designated as Nutrim-I, and Nutrim-II) containing Nutrim were produced with mean fat levels of 6.84 and 3.47%, respectively. A low-fat cheese was also produced as a control with a mean fat level of 11.2%. Nutrim-II cheese had significantly higher moisture, salt, and ash contents as compared with the low-fat control cheese. The low-fat control cheese had a higher yield normalized for 54% moisture and 1.5% salt content as compared with the Nutrim-II cheese. Scanning electron microscopy revealed smaller and more uniform fat droplet voids in the Nutrim cheese than the low-fat control, and a more dense, noncontinuous background protein matrix with globular clusters suggesting a physical buffering afforded by the presence of the , -glucan hydrocolloid or its associated water. [source]

An HST/ACS view of the inhomogeneous outer halo of M31,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2009
J. C. Richardson
ABSTRACT We present a high precision photometric view of the stellar populations in the outer halo of M31, using data taken with the Hubble Space Telescope/Advanced Camera for Surveys. We analyse the field populations adjacent to 11 luminous globular clusters which sample the galactocentric radial range 18 ,R, 100 kpc and reach a photometric depth of ,2.5 mag below the horizontal branch (mF814W, 27 mag). The colour,magnitude diagrams are well populated out to ,60 kpc and exhibit relatively metal-rich red giant branches, with the densest fields also showing evidence for prominent red clumps. We use the Dartmouth isochrones to construct metallicity distribution functions which confirm the presence of dominant populations with ,[Fe/H],,,0.6 to ,1.0 dex and considerable metallicity dispersions of 0.2 to 0.3 dex (assuming a 10 Gyr population and scaled-solar abundances). The average metallicity over the range 30,60 kpc is [Fe/H]=,0.80 ± 0.14 dex, with no evidence for a significant radial gradient. Metal-poor stars ([Fe/H],,1.3) typically account for ,10,20 per cent of the population in each field, irrespective of radius. Assuming our fields are unbiased probes of the dominant stellar populations in these parts, we find that the M31 outer halo remains considerably more metal rich than that of the Milky Way out to at least 60 kpc. [source]

An optimized H, index for disentangling stellar population ages

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
J. L. Cervantes
ABSTRACT We have defined a new H, absorption index definition, H,o, which has been optimized as an age indicator for old and intermediate age stellar populations. Rather than using stellar spectra, we employed for this purpose a library of stellar population spectral energy distributions of different ages and metallicities at moderately high resolution. H,o provides us with improved abilities for lifting the age,metallicity degeneracy affecting the standard H, Lick index definition. The new index, which has also been optimized against photon noise and velocity dispersion, is fully characterized with wavelength shift, spectrum shape, dust extinction and [,/Fe] abundance ratio effects. H,o requires spectra of similar qualities as those commonly used for measuring the standard H, Lick index definition. Aiming at illustrating the use and capabilities of H,o as an age indicator we apply it to Milky Way globular clusters and to a well selected sample of early-type galaxies covering a wide range in mass. The results shown here are particularly useful for applying this index and understand the involved uncertainties. [source]

AGB variables and the Mira period,luminosity relation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Patricia A. Whitelock
ABSTRACT Published data for large-amplitude asymptotic giant branch variables in the Large Magellanic Cloud (LMC) are re-analysed to establish the constants for an infrared (K) period,luminosity relation of the form MK=,[log P, 2.38]+,. A slope of ,=,3.51 ± 0.20 and a zero-point of ,=,7.15 ± 0.06 are found for oxygen-rich Miras (if a distance modulus of 18.39 ± 0.05 is used for the LMC). Assuming this slope is applicable to Galactic Miras we discuss the zero-point for these stars using the revised Hipparcos parallaxes together with published very long baseline interferometry (VLBI) parallaxes for OH masers and Miras in globular clusters. These result in a mean zero-point of ,=,7.25 ± 0.07 for O-rich Galactic Miras. The zero-point for Miras in the Galactic bulge is not significantly different from this value. Carbon-rich stars are also discussed and provide results that are consistent with the above numbers, but with higher uncertainties. Within the uncertainties there is no evidence for a significant difference between the period,luminosity relation zero-points for systems with different metallicity. [source]

The mass function of , Centauri down to 0.15 M,,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
A. Sollima
ABSTRACT By means of deep FORS1/VLT and ACS/Hubble Space Telescope observations of a wide area in the stellar system , Cen we measured the luminosity function of main-sequence stars down to R= 22.6 and IF814W= 24.5. The luminosity functions obtained have been converted into mass functions and compared with analytical initial mass functions (IMFs) available in the literature. The mass function obtained, reaching M, 0.15 M,, can be well reproduced by a broken power law with indices ,=,2.3 for M > 0.5 M, and ,=,0.8 for M < 0.5 M,. Since the stellar populations of , Cen have been proved to be actually unaffected by dynamical evolution processes, the mass function measured in this stellar system should represent the best approximation of the IMF of a star cluster. The comparison with the MF measured in other Galactic globular clusters suggests that possible primordial differences in the slope of the low-mass end of their MF could exist. [source]

The dynamical formation of LMXBs in dense stellar environments: globular clusters and the inner bulge of M31

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
R. Voss
ABSTRACT The radial distribution of luminous (LX > 1036 erg s,1) X-ray point sources in the bulge of M31 is investigated using archival Chandra observations. We find a significant increase in the specific frequency of X-ray sources, per unit stellar mass, within 1 arcmin from the centre of the galaxy. The radial distribution of surplus sources in this region follows the ,2* law, suggesting that they are low-mass X-ray binaries (LMXBs) formed dynamically in the dense inner bulge. We investigate dynamical formation of LMXBs, paying particular attention to the high-velocity regime characteristic for galactic bulges, which has not been explored previously. Our calculations suggest that the majority of the surplus sources are formed in tidal captures of black holes by main-sequence stars of low mass, M*, 0.3,0.4 M,, with some contribution of neutron star (NS) systems of same type. Due to the small size of the accretion discs, a fraction of such systems may be persistent X-ray sources. Some of the sources may be ultracompact X-ray binaries with helium star/white dwarf companions. We also predict a large number of faint transients, both NS and BH systems, within ,1 arcmin from the M31 galactic centre. Finally, we consider the population of dynamically formed binaries in Galactic globular clusters, emphasizing the differences between these two types of stellar environments. [source]

The age, metallicity and ,-element abundance of Galactic globular clusters from single stellar population models

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Jon T. Mendel
ABSTRACT Establishing the reliability with which stellar population parameters can be measured is vital to extragalactic astronomy. Galactic globular clusters (GCs) provide an excellent medium in which to test the consistency of single stellar population (SSP) models as they should be our best analogue to a homogeneous (single) stellar population. Here we present age, metallicity and ,-element abundance measurements for 48 Galactic GCs as determined from integrated spectra using Lick indices and SSP models from Thomas, Maraston & Korn, Lee & Worthey and Vazdekis et al. By comparing our new measurements to independent determinations we are able to assess the ability of these SSPs to derive consistent results , a key requirement before application to heterogeneous stellar populations like galaxies. We find that metallicity determinations are extremely robust, showing good agreement for all models examined here, including a range of enhancement methods. Ages and ,-element abundances are accurate for a subset of our models, with the caveat that the range of these parameters in Galactic GCs is limited. We are able to show that the application of published Lick index response functions to models with fixed abundance ratios allows us to measure reasonable ,-element abundances from a variety of models. We also examine the age,metallicity and [,/Fe],metallicity relations predicted by SSP models, and characterize the possible effects of varied model horizontal branch morphology on our overall results. [source]

Searches for ultracompact dwarf galaxies in galaxy groups

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007
E. A. Evstigneeva
ABSTRACT We present the results of a search for ultracompact dwarf galaxies (UCDs) in six different galaxy groups: Dorado, NGC 1400, 0681, 4038, 4697 and 5084. We searched in the apparent magnitude range 17.5 ,bJ, 20.5 (except NGC 5084: 19.2 ,bJ, 21.0). We found one definite plus two possible UCD candidates in the Dorado group and two possible UCD candidates in the NGC 1400 group. No UCDs were found in the other groups. We compared these results with predicted luminosities of UCDs in the groups according to the hypothesis that UCDs are globular clusters formed in galaxies. The theoretical predictions broadly agree with the observational results, but deeper surveys are needed to fully test the predictions. [source]

Structural parameters of Mayall II = G1 in M31

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
J. Ma
ABSTRACT Mayall II = G1 is one of the most luminous globular clusters (GCs) known in M31. New deep, high-resolution observations with the Advanced Camera for Surveys on the Hubble Space Telescope are used to provide accurate photometric data to the smallest radii yet. In particular, we present the precise variation of ellipticity and position angle, and of surface brightness for the core of the object. Based on these accurate photometric data, we redetermine the structural parameters of G1 by fitting a single-mass isotropic King model. We derive a core radius, rc= 0.21 ± 0.01 arcsec (= 0.78 ± 0.04 pc), a tidal radius, rt= 21.8 ± 1.1 arcsec (= 80.7 ± 3.9 pc), and a concentration index c= log (rt/rc) = 2.01 ± 0.02. The central surface brightness is 13.510 mag arcsec,2. We also calculate the half-light radius, at rh= 1.73 ± 0.07 arcsec (= 6.5 ± 0.3 pc). The results show that, within 10 core radii, a King model fits the surface brightness distribution well. We find that this object falls in the same region of the MV versus log Rh diagram as , Centauri, M54 and NGC 2419 in the Milky Way. All three of these objects have been claimed to be the stripped cores of now defunct dwarf galaxies. We discuss in detail whether GCs, stripped cores of dwarf spheroidals and normal dwarf galaxies form a continuous distribution in the MV versus log Rh plane, or if GCs and dwarf spheroidals constitute distinct classes of objects; we present arguments in favour of this latter view. [source]

Kinematic structure in the Galactic halo at the North Galactic Pole: RR Lyrae and blue horizontal branch stars show different kinematics

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
T. D. Kinman
ABSTRACT Radial velocities and proper motions (derived from the GSC-II data base) are given for 38 RR Lyrae (RRL) stars and 79 blue horizontal branch (BHB) stars in a ,200 deg2 area around the North Galactic Pole (NGP). Both heliocentric (UVW) and galactocentric (VR, V,, Vz) space motions are derived for these stars using a homogeneous distance scale consistent with (m,M)0= 18.52 for the Large Magellanic Cloud (LMC). An analysis of the 26 RRL and 52 BHB stars whose height (Z) above the plane is less than 8 kpc shows that this halo sample is not homogeneous. Our BHB sample (like that of Sirko et al.) has a zero galactic rotation (V,) and roughly isotropic velocity dispersions. The RRL sample shows a definite retrograde rotation (V,=,95 ± 29 km s,1) and non-isotropic velocity dispersions. The combined BHB and RRL sample has a retrograde galactic rotation (V) that is similar to that found by Majewski for his sample of subdwarfs in Selected Area (SA) 57. The velocity dispersion of the RRL stars that have a positive W motion is significantly smaller than the dispersion of those ,streaming down' with a negative W. Also, the ratio of RRL to BHB stars is smaller for the sample that has positive W. Our halo sample occupies 10.4 kpc3 at a mean height of 5 kpc above the Galactic plane. In this volume, one component (rich in RRL stars) shows retrograde rotation and the streaming motion that we associate with the accretion process. The other component (traced by the BHB stars) shows essentially no rotation and less evidence of streaming. These two components have horizontal branch (HB) morphologies that suggest that they may be the field star equivalents of the young and old halo globular clusters, respectively. Clearly, it is quite desirable to use more than one tracer in any kinematic analysis of the halo. [source]

A systematic survey for infrared star clusters with |b| < 20° using 2MASS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
D. Froebrich
ABSTRACT We used star density maps obtained from the Two-Micron All-Sky Survey (2MASS) to obtain a sample of star clusters in the entire Galactic Plane with |b| < 20°. A total of 1788 star cluster candidates are identified in this survey. Among those are 681 previously known open clusters and 86 globular clusters. A statistical analysis indicates that our sample of 1021 new cluster candidates has a contamination of about 50 per cent. Star cluster parameters are obtained by fitting a King profile to the star density. These parameters are used to statistically identify probable new globular cluster candidates in our sample. A detailed investigation of the projected distribution of star clusters in the Galaxy demonstrates that they show a clear tendency to cluster on spatial scales in the order of 12,25 pc, a typical size for molecular clouds. [source]

An extensive study of dynamical friction in dwarf galaxies: the role of stars, dark matter, halo profiles and MOND

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
F. J. Sánchez-Salcedo
ABSTRACT We investigate the in-spiralling time-scales of globular clusters (GCs) in dwarf spheroidal (dSph) and dwarf elliptical (dE) galaxies, due to dynamical friction (DF). We address the problem of these time-scales having been variously estimated in the literature as much shorter than a Hubble time. Using self-consistent two-component (dark matter and stars) models, we explore mechanisms which may yield extended DF time-scales in such systems in order to explain why dwarf galaxies often show GC systems. As a general rule, dark matter and stars both give a comparable contribution to the dynamical drag. By exploring various possibilities for their gravitational make-up, it is shown that these studies help to constrain the parameters of the dark matter haloes in these galaxies, as well as to test alternatives to dark matter. Under the assumption of a dark halo having a central density core with a typical King core radius somewhat larger than the observed stellar core radius, DF time-scales are naturally extended upwards of a Hubble time. Cuspy dark haloes yield time-scales ,4.5 Gyr, for any dark halo parameters in accordance with observations of stellar line-of-sight velocity dispersion in dSph galaxies. We confirm, after a detailed formulation of the DF problem under the alternative hypothesis of modified Newtonian dynamics (MOND) and in the lack of any dark matter, that due to the enhanced dynamical drag of the stars, the DF time-scales in MOND would be extremely short. Taking the well-measured structural parameters of the Fornax dSph and its GC system as a case study, we conclude that requiring DF time-scales comparable to the Hubble time strongly favours dark haloes with a central core. [source]

Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006
Paul Goudfrooij
ABSTRACT We present accurate integrated-light photometry in Johnson/Cousins V, R and I for a sample of 28 globular clusters in the Magellanic Clouds. The majority of the clusters in our sample have reliable age and metallicity estimates available in the literature. The sample encompasses ages between 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between ,1.5 and 0.0 dex. The sample is dominated by clusters of ages between roughly 0.5 and 2 Gyr, an age range during which the bolometric luminosity of simple stellar populations is dominated by evolved red giant branch stars and thermally pulsing asymptotic giant branch (TP-AGB) stars whose theoretical colours are rather uncertain. The VRI colours presented in this paper have been used to calibrate stellar population synthesis model predictions. [source]

The distribution and kinematics of early high-, peaks in present-day haloes: implications for rare objects and old stellar populations

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005
Jürg Diemand
ABSTRACT We show that the hierarchical assembly of cold dark matter haloes preserves the memory of the initial conditions. Using N -body cosmological simulations, we demonstrate that the present-day spatial distribution and kinematics of objects that formed within early(z, 10) protogalactic systems (old stars, satellite galaxies, globular clusters, massive black holes, etc.) depends mostly on the rarity of the peak of the primordial density field to which they originally belonged. Only for objects forming at lower redshifts does the exact formation site within the progenitor halo (e.g. whether near the centre or in an extended disc) become important. In present-day haloes, material from the rarer early peaks is more centrally concentrated and falls off more steeply with radius compared to the overall mass distribution, has a lower velocity dispersion, moves on more radial orbits, and has a more elongated shape. Population II stars that formed within protogalactic haloes collapsing from ,2.5, fluctuations would follow today an r,3.5 density profile with a half-light radius of 17 kpc and a velocity anisotropy that increases from isotropic in the inner regions to nearly radial at the halo edge. This agrees well with the radial velocity dispersion profile of Galaxy halo stars from the recent work of Battaglia et al. and with the anisotropic orbits of nearby halo stars. [source]

Gemini/GMOS imaging of globular clusters in the Virgo galaxy NGC 4649 (M60)

Dynamical limits on galactic winds, halo MACHOs and intergalactic globular clusters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002
HongSheng Zhao
Abstract We argue that any violent galactic winds following the early epoch of starbursts will significantly weaken the potentials of galaxies, and leave lasting signatures such as a lowered dark halo density and preferentially radial/escaping orbits for halo tracers such as globular clusters. A galaxy is disintegrated if more than half of its dynamical mass is blown off. The presence of dense haloes in galaxies and the absence of intergalactic/escaping globulars should imply an upper limit on the amount of baryons lost in galactic winds of around 4 per cent of the total mass of the galaxy. This translates to limits on the baryons participating in the early starbursts and baryons locked in stellar remnants, such as white dwarfs. The numbers of halo white dwarfs claimed in recent proper motion searches and microlensing observations in the Galactic halo are too high to be consistent with our dynamical upper limits. Similar arguments also imply upper limits for the number of neutron stars and stellar black holes in galaxy haloes. Nevertheless, a milder outflow is desirable, especially in dwarf galaxies, both to lower their cold dark matter central density and to inject metals into the intergalactic medium. [source]

Testing stellar population models with star clusters in the Large Magellanic Cloud

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002
Michael A. Beasley
Abstract We present high signal-to-noise ratio integrated spectra of 24 star clusters in the Large Magellanic Cloud (LMC), obtained using the FLAIR spectrograph at the UK Schmidt telescope. The spectra have been placed on to the Lick/IDS system in order to test the calibration of Simple Stellar Population (SSP) models. We have compared the SSP-predicted metallicities of the clusters with those from the literature, predominantly taken from the Ca-triplet spectroscopy of Olszewski et al. (1991). We find that there is good agreement between the metallicities in the range ,2.10 ,[Fe/H], 0. However, the Mg2 index (and to a lesser degree Mg b) systematically predict higher metallicities (up to +0.5 dex higher) than ,Fe,. Among the possible explanations for this are that the LMC clusters possess [,/Fe] > 0. Metallicities are presented for eleven LMC clusters which have no previous measurements. We compare SSP ages for the clusters, derived from the H,, H, and H, Lick/IDS indices, with the available literature data, and find good agreement for the vast majority. This includes six old globular clusters in our sample, which have ages consistent with their HST colour-magnitude diagram (CMD) ages and/or integrated colours. However, two globular clusters, NGC 1754 and NGC 2005, identified as old (,15 Gyr) on the basis of HST CMDs, have H, line-strengths which lead ages that are too low (,8 and ,6 Gyr respectively). These findings are inconsistent with their CMD-derived values at the 3, level. Comparison between the horizontal branch morphology and the Balmer line strengths of these clusters suggests that the presence of blue horizontal branch stars has increased their Balmer indices by up to ,1.0 Å. We conclude that the Lick/IDS indices, used in conjunction with contemporary SSP models, are able to reproduce the ages and metallicities of the LMC clusters reassuringly well. The required extrapolations of the fitting functions and stellar libraries in the models to lower ages and low metallicities do not lead to serious systematic errors. However, owing to the significant contribution of horizontal branch stars to Balmer indices, SSP model ages derived for metal-poor globular clusters are ambiguous without a priori knowledge of horizontal branch morphology. [source]

Stellar populations and surface brightness fluctuations: new observations and models

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2001
John P. Blakeslee
We investigate the use of surface brightness fluctuations (SBF) measurements in optical and near-IR bandpasses for both stellar population and distance studies. New V -band SBF data are reported for five galaxies in the Fornax cluster and combined with literature data to define a V -band SBF distance indicator, calibrated against Cepheid distances to the Leo group and the Virgo and Fornax clusters. The colour dependence of the V -band SBF indicator is only ,15 per cent steeper than that found for the I band, and the mean ,fluctuation colour' of the galaxies is We use new stellar population models, based on the latest Padua isochrones transformed empirically to the observational plane, to predict optical and near-IR SBF magnitudes and integrated colours for a wide range of population ages and metallicities. We examine the sensitivity of the predicted SBF,colour relations to changes in the isochrones, stellar transformations, and initial mass function. The new models reproduce fairly well the weak dependence of V and I SBF in globular clusters on metallicity, especially if the more metal-rich globulars are younger. Below solar metallicity, the near-IR SBF magnitudes depend mainly on age, while the integrated colours depend mainly on metallicity. This could prove a powerful new approach to the age,metallicity degeneracy problem; near-IR SBF observations of globular clusters would be an important test of the models. The models also help in understanding the and fluctuation colours of elliptical galaxies, with much less need for composite stellar populations than in previous models. However, in order to obtain theoretical calibrations of the SBF distance indicators, we combine the homogeneous population models into composite models and select out those ones with fluctuation colours consistent with observations. We are able to reproduce the observed range of elliptical galaxy colours, the slopes of the V and I SBF distance indicators against (fainter SBF in redder populations), and the flattening of the I -band relation for The models also match the observed slope of I -band SBF against the Mg2 absorption index and explain the steep colour dependence found by Ajhar et al. for the HST/WFPC2 F814W-band SBF measurements. In contrast to previous models, ours predict that the near-IR SBF magnitudes will also continue to grow fainter for redder populations. The theoretical V -band SBF zero-point predicted by these models agrees well with the Cepheid-calibrated V -band empirical zero-point. However, the model zero-point is 0.15,0.27 mag too faint in the I band and 0.24,0.36 mag too faint in K. The zero-points for the I band (empirically the best determined) would come into close agreement if the Cepheid distance scale were revised to agree with the recent dynamical distance measured to NGC 4258. We note that the theoretical SBF calibrations are sensitive to the uncertain details of stellar evolution, and conclude that the empirical calibrations remain more secure. However, the sensitivity of SBF to these finer details potentially makes it a powerful, relatively unexploited, constraint for stellar evolution and population synthesis. [source]

Spectroscopy of globular cluster candidates in the Sculptor group galaxies NGC 253 and 55

The effect of stellar rotation on colour,magnitude diagrams: on the apparent presence of multiple populations in intermediate age stellar clusters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2009
N. Bastian
ABSTRACT A significant number of intermediate age clusters (1,2 Gyr) in the Magellanic Clouds appear to have multiple stellar populations within them, derived from bimodal or extended main-sequence turn-offs. If this is interpreted as an age spread, the multiple populations are separated by a few hundred million years, which would call into question the long-held notion that clusters are simple stellar populations. Here, we show that stellar rotation in stars with masses between 1.2 and 1.7 M, can mimic the effect of a double or multiple population, whereas in actuality only a single population exists. The two main causes of the spread near the turn-off are the effects of stellar rotation on the structure of the star and the inclination angle of the star relative to the observer. Both effects change the observed effective temperature, hence colour, and flux of the star. In order to match observations, the required rotation rates are 20,50 per cent of the critical rotation, which are consistent with observed rotation rates of similar mass stars in the Galaxy. We provide scaling relations which can be applied to non-rotating isochrones in order to mimic the effects of rotation. Finally, we note that rotation is unlikely to be the cause of the multiple stellar populations observed in old globular clusters, as low-mass stars (<1 M,) are not expected to be rapid rotators. [source]

High-resolution simulations of galaxy mergers: resolving globular cluster formation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
F. Bournaud
ABSTRACT Massive star clusters observed in galaxy mergers are often suggested to be progenitors of globular clusters. To study this hypothesis, we performed the highest resolution simulation of a gas-rich galaxy merger so far. The formation of massive star clusters of 105 to 107 M,, triggered by the galaxy interaction, is directly resolved in this model. We show that these clusters are tightly bound structures with little net rotation, due to evolve into compact long-lived stellar systems. Massive clusters formed in galaxy mergers are thus robust candidates for progenitors of long-lived globular clusters. The simulated cluster mass spectrum is consistent with theory and observations. Tidal dwarf galaxies of 108,9 M, can form at the same time, and appear to be part of a different class of objects, being more extended and rotating. [source]