Metal-poor Clusters Research Articles

Reddening, colour and metallicity of the M31 globular cluster system

Using metallicities from the literature, combined with the Revised Bologna Catalogue of photometric data for M31 clusters and cluster candidates [the latter of which is the most comprehensive catalogue of M31 clusters currently available, including 337 confirmed globular clusters (GCs) and 688 GC candidates], we determine 443 reddening values and intrinsic colours, and 209 metallicities for individual clusters without spectroscopic observations. This, the largest sample of M31 GCs presently available, is then used to analyse the metallicity distribution of M31 GCs, which is bimodal with peaks at [Fe/H] ≈− 1.7 and −0.7 dex. An exploration of metallicities as a function of radius from the M31 centre shows a metallicity gradient for the metal-poor GCs, but no such gradient for the metal-rich GCs. Our results show that the metal-rich clusters appear as a centrally concentrated spatial distribution; however, the metal-poor clusters tend to be less spatially concentrated. There is no correlation between luminosity and metallicity among the M31 sample clusters, which indicates that self-enrichment is indeed unimportant for cluster formation in M31. The reddening distribution shows that slightly more than half of the GCs are affected by a reddening of E(B − V) � 0.2 mag; the mean reddening value is E(B − V) = 0.28 +0.23 −0.14 mag. The spatial distribution of the reddening values indicates that the reddening on the north-western side of the M31 disc is more significant than that on the south-eastern side, which is consistent with the conclusion that the north-western side is nearer to us.

Two newly identified, relatively old star clusters in the Small Magellanic Cloud

Abstract We present the results on the age and metallicity estimates of the Small Magellanic Cloud (SMC) clusters L110, L112 and L113 obtained from CCD Washington CT1 photometry. We confirm L113 as a relatively old and metal-poor cluster, and report for the first time that L110 and 112 are also relatively old clusters (t∼ 6.5 Gyr). Their derived ages and metallicities reinforce previous suggestions that the farther a cluster is from the centre of the galaxy, the older and more metal-poor it is, with some dispersion. In addition, the bursting star formation model still appears to be the most probable paradigm to describe the SMC's star formation history. We call attention to a second possible burst at ∼6–7 Gyr, besides the known burst at ∼3 Gyr.

Red-giant branch morphology of metal-poor globular clusters in the Galactic bulge

AbstractUsing the (J − K, K) color-magnitude diagrams (CMDs) of 16 metal-poor globular clusters in the Galactic bulge, we investigate the morphological properties of their red-giant branch (RGB), comparing with those of metal-rich clusters in the Galactic bulge and metal-poor clusters in the Galactic halo. The RGB morphological parameters, such as colors at fixed magnitudes, magnitudes at a fixed color, the RGB slope, and a difference of color indices at two fixed magnitudes have been derived from the near-IR CMDs for each cluster. Metal-poor Galactic bulge clusters follow the previous empirical relations between colors at fixed magnitudes and magnitudes at a fixed color of the RGB and the cluster's metallicity. However, the RGB slope and the color difference parameters of some bulge clusters deviate slightly from the previous empirical linear relations for the other globular clusters, implying that the metal-poor bulge clusters may have different formation origin from the other globular clusters in the Galaxy.

Probing the Formation of Low‐Mass X‐Ray Binaries in Globular Clusters and the Field

We present an analysis of low mass X-ray binaries (LMXBs) and globular clusters (GCs) in five galaxies using Chandra, and HST data. Of the 186 LMXBs within the optical fields 71 are in GCs, confirming that LMXBs are formed particularly efficiently in GCs. However, there is no statistically significant correlation between the distance of a GC from the center of its host galaxy and its LMXB hosting probability. Metal-rich GCs are 3.4 times more likely to host LMXBs than metal-poor ones. Intriguingly, the LMXBs in NGC 1399 are preferentially in the reddest GCs of the metal-rich subsystem. This indicates that the red peak of the color distribution itself encompasses GCs with a range of enrichment histories. The strength of this effect varies from galaxy to galaxy, possibly indicating differences in their metal-enrichment histories. Field LMXBs in our program galaxies are more concentrated towards the center of their host galaxies than GC-LMXBs. This suggests that a majority of field LMXBs are formed in situ and have not escaped from current GCs. This is consistent with previous specific frequency based studies. The brightest LMXBs in GCs appear to be preferentially associated with luminous, metal-rich clusters. We show that it is probable that some of these GCs host multiple bright LMXBs, while the probability is much lower for metal-poor GCs. This implies that LMXBs in more metal-rich cluster systems should reveal a longer high luminosity X-ray tail, and show less X-ray variability than metal-poor cluster populations. (abridged)

Space Velocities of Southern Globular Clusters. V. A Low Galactic Latitude Sample

We have measured the absolute proper motions of globular clusters NGC 2808, 3201, 4372, 4833, 5927, and 5986. The proper motions are on the Hipparcos system, and they are the first determinations ever made for these low Galactic latitude clusters. The proper-motion uncertainties range from 0.3 to 0.5 mas yr-1. The inferred orbits indicate that (1) the single metal-rich cluster in our sample, NGC 5927, dynamically belongs to the thick disk; (2) the remaining metal-poor clusters have rather low-energy orbits of high eccentricity, and among these there appear to be two pairs of dynamically associated clusters; (3) the most energetic cluster in our sample, NGC 3201, is on a highly retrograde orbit?which had already been surmised from radial velocity alone?with an apocentric distance of 22 kpc; and (4) none of the metal-poor clusters appear to be associated with the recently detected SDSS streams or with the Monoceros structure. These are the first results of the Southern Proper Motion program where the second-epoch observations are taken with the recent CCD camera system installed on the double astrograph at El Leoncito, Argentina.

Photometric study of distant open clusters in the second quadrant: NGC 7245, King 9, King 13 and IC 166

We present a UBV CCD photometric study of four open clusters, NGC 7245, King 9, IC 166 and King 13, located between $l = 90^o - 135^o$. All are embedded in a rich galactic field. NGC 7245 and King 9 are close together in the sky and have similar reddenings. The distances and ages are: NGC 7245, 3.8$\pm$0.35 kpc and 400 Myr; King 9 (the most distant cluster in this quadrant) 7.9$\pm$1.1 kpc and 3.0 Gyr. King 13 is 3.1$\pm$0.3 kpc distant and 300 Myr old. King 9 and IC 166 (4.8$\pm$0.5 kpc distant & 1 Gyr old) may be metal poor clusters (Z=0.008), as estimated from isochrone fitting. The average value of the distance of young clusters from the galactic plane in the above longitude range and beyond 2 kpc ($-47\pm$16 pc, for 64 clusters), indicates that the young disk bends towards the southern latitudes.

On the origin of mass-metallicity relations, blue tilts and scaling relations for metal-poor globular cluster systems

We investigate formation processes and physical properties of globular cluster systems (GCSs) in galaxies based on high-resolution cosmological simulations with globular clusters. We focus on metal-poor clusters (MPCs) and correlations with their host galaxies by assuming that MPC formation is truncated at a high redshift (z_trun > 6). We find that the correlation between mean metallicities (Z_gc) of MPCs and their host galaxy luminosities (L) flattens from z=z_trun to z=0. We also find that the observed relation (Z_gc ~ L^0.15) in MPCs can be reproduced well in the models with Z_gc ~ L^0.5 at z=z_trun when z_trun ~ 10, if mass-to-light-ratios are assumed to be constant at z=z_trun. However, better agreement with the observed relation is found for models with different mass-to-light-ratios between z=z_trun and z=0. It is also found that the observed color-magnitude relation of luminous MPCs (i.e., ``blue tilts'') may only have a small contribution from the stripped stellar nuclei of dwarf galaxies, which have nuclei masses that correlate with their total mass at z=z_trun. The simulated blue tilts are found to be seen more clearly in more massive galaxies, which reflects the fact that more massive galaxies at z=0 are formed from a larger number of dwarfs with stellar nuclei formed at z>z_trun. The half-number radii (R_e) of GCSs, velocity dispersions of GCSs (sigma), and their host galaxy masses (M_h) are found to be correlated with one another such that R_e ~ M_h^{0.57} and sigma ~ M_h^{0.32}.

NEAR-IR PHOTOMETRIC PROPERTIES OF HB, MSTO, AND SGB FOR METAL POOR GALACTIC GLOBULAR CLUSTERS

We report photometric features of the HB, MSTO, and SGB for a set of metal-poor Galactic globular clusters on the near-IR CMDs. The magnitude and color of the MSTO and SGB are measured on the fiducial normal points of the CMDs by applying a polynomial fit. The near-IR luminosity functions of horizontal branch stars in the classical second parameter pair M3 and M13 indicate that HB stars in M13 are dominated by hot stars that are rotatively faint in the infrared, whereas HB stars in M3 are brighter than those in M13. The luminosity functions of HB stars in the observed bulge clusters, except for NGC 6717, show a trend that the fainter hot HB stars are dominated in the relatively metal-poor clusters while the relatively metal-rich clusters contain the brighter HB stars. It is suggestive that NGC 6717 would be an extreme example of the second-parameter phenomenon for the bulge globular clusters.

Comparison between the Globular Clusters in NGC 5128 and the Galaxy

Some of the properties of the globular clusters in NGC 5128, the nearest giant elliptical galaxy, are compared to those of Galactic globular clusters. Assuming the color-metallicity relations that hold for Galactic globular clusters, one finds that the metal-poor clusters in NGC 5128 that have [Fe/H] 10 pc are typically less luminous in NGC 5128 than are their more compact counterparts. In this respect the NGC 5128 cluster system is similar to the Galactic globular cluster system, in which large clusters also tend to be less luminous than compact ones. Finally, the present data may hint at the possibility that the NGC 5128 cluster system differs from that surrounding the Milky Way, in the sense that the NGC 5128 objects do not seem to exhibit a clear-cut gap between the regions of the FWHM-Mv plane that are occupied by globular clusters and by dwarf spheroidal galaxies.

NGC 2298: a globular cluster on its way to disruption

We have studied the stellar main sequence (MS) of the globular cluster NGC 2298 using deep HST/ACS observations in the F606W and F814W bands covering an area of 3. 4× 3. 4 around the cluster centre or about twice the cluster’s half -mass radius. The colourmagnitude diagram that we derive in this way reveals a narrow and well defined MS extending down to the 10σ detection limit at m606≃ 26.5, m814≃ 25, corresponding to stars of∼ 0.2 M⊙. The luminosity function (LF) obtained with these data, once corrected for the limited effects of photometric incompleteness, reveals a remarkable deficiency of low-mass stars as well as a radial gradient, in th at the LF becomes progressively steeper with radius. Using the mass‐luminosity relation appropriate for the metallicity of NGC 2298, we derive the cluster’s global mass function (GMF) by using a multi-mass Michie‐King model. Over the range 0.8− 0.2 M⊙, the number of stars per unit mass decreases following a power-law distribution of the type dN/dm∝ m 0.5 , where, for comparison, typical halo clusters have dN/dm∝ m −1.5 . If the IMF of NGC 2298 was similar to that of other metal poor halo clusters, like e.g. NGC 6397, the present GMF that we obtain implies that this object must have lost of order 85 % of its original mass, at a rate much higher than that suggested by current models based on the available cluster orbit. The latter may, therefore, need revision.

ACS Photometry of Newly Discovered Globular Clusters in the Outer Halo of M31

We report the first results from deep ACS imaging of 10 classical globular clusters in the far outer regions (15 kpc Rp 100 kpc) of M31. Eight of the clusters, including two of the most remote M31 globular clusters presently known, are described for the first time. Our F606W, F814W color-magnitude diagrams extend ~3 mag below the horizontal branch and clearly demonstrate that the majority of these objects are old (10 Gyr), metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional four have -1.9 [Fe/H] -1.5. The remaining object is more metal-rich, with [Fe/H] ~ -0.70. Several clusters exhibit the second-parameter effect. Using aperture photometry, we estimate integrated luminosities and structural parameters for all clusters. Many, including all four clusters with projected radii greater than 45 kpc, are compact and very luminous, with -8.9 MV -8.3. These four outermost clusters are thus quite unlike their Milky Way counterparts, which are typically diffuse, subluminous (-6.0 MV -4.7), and more metal-rich (-1.8 [Fe/H] -1.3).

Stellar Populations and Mass Loss in M15: ASpitzer Space TelescopeDetection of Dust in the Intracluster Medium

We present Spitzer Space Telescope IRAC and MIPS observations of the Galactic globular cluster M15 (NGC 7078), one of the most metal-poor clusters, with a [Fe/H] = -2.4. Our Spitzer images reveal a population of dusty red giants near the cluster center, a previously detected planetary nebula designated K648, and a possible detection of the intracluster medium arising from mass-loss episodes from the evolved stellar population. Our analysis suggests that (9 ± 2) × 10-4 M⊙ of dust is present in the core of M15, and that this material has accumulated over a period of ≈106 yr, a timescale 10 times shorter than the last Galactic plane crossing event. We also present Spitzer IRS follow-up observations of K648, including the detection of the [Ne II] 12.81 μm line, and discuss abundances derived from infrared fine-structure lines.

Abundance anomalies in hot horizontal branch stars of the Galactic globular cluster NGC 2808

the date of receipt and acceptance should be inserted later Abstract. We present metallicity measurements of 25 stars in the blue horizontal branch of the Galactic globular cluster NGC 2808. Our measurements are based on moderate-resolution spectra taken with the multi-object fiber facility FLAMES- UVES, mounted on Kueyen at the Very Large Telescope. We confirm that stars hotter than a threshold temperature have super-solar abundance, while the cooler ones respect the nominal metallicity of the cluster, i.e. (Fe/H)≃-1.1. The threshold temperature is estimated to be about 12 000 K, corresponding to the so called u-jump, and coincides with the sudden departure of the cluster horizontal branch from the models. The metallicity increases with temperature for star hotter than the ju mp, confirming the hypothesis that the process responsible for t his abrupt metallic enhancement is the levitation due to the strong radiation field in absence of a significative convective enve lope. A metallicity dependence of the abundance enhancement is also suggested, with more metal poor clusters having a higher increase in metal content. The slope in the temperature vs. abundance diagram is higher than the errors involved, and the metal content of the cluster plays possibly a role in determining the a mplitude of the jump (more metal poor clusters show more enhancement after the jump), although other parameters, such as clusters ' characteristics and even the atomic species involved, may also someway contribute.

VLT-UVES analysis of two giants in the bulge metal-poor globular cluster HP-1

Context.Metal-poor globular clusters in the bulge are important tracers of early chemical evolution. HP-1 is among the six metal-poor clusters within 5° of the Galactic center, and could be the one closest to the centerAims. The main purpose of this study is the determination of metallicity and elemental ratios. Methods.High resolution spectra of two giants of the bulge globular cluster HP-1 were obtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph. This is the second metal-poor globular cluster in the bulge for which a detailed abundance analysis is presented. Multiband photometry was used to derive effective temperatures.Results. The present analysis provides a metallicity [Fe/H] = . The α-elements oxygen and silicon show [ α/Fe] ≈ +0.3, whereas magnesium, calcium and titanium show solar ratios. A proper motion analysis indicates that the two stars are cluster members. Conclusions.The metallicity is unexpected for a blue Horizontal Branch (BHB) cluster. HP-1 is the first known cluster with such a high metallicity combined with a BHB and a steep Red Giant Branch (RGB). Together with NGC 6388 and NGC 6441 of it would be third with such characteristics, but it differs from them, since these two other clusters have also a populous Red HB, and a normal slope of the RGB for their metallicity, which is not the case of HP-1.

A Moderate Sample Size, Multielement Analysis of the Globular Cluster M12 (NGC 6218)

We present chemical abundances of several proton-capture, α-, Fe-peak, and neutron-capture elements and radial velocities for 21 red giant branch (RGB) and asymptotic giant branch members of the Galactic globular cluster M12. Abundances are based on equivalent width measurements and synthetic spectral analyses of moderate-resolution spectra (R ~ 15,000) obtained with the 3.5 m WIYN telescope and Hydra multifiber spectrograph. The stars observed range from the RGB tip (M = -2.47) down to about 0.50 mag above the level of the horizontal branch (M = +0.11). Our spectroscopic analysis suggests that M12 is a moderately metal-poor cluster with [Fe/H] = -1.54 (σ = 0.09). While the Na abundances exhibit a range of 0.90 dex, Mg and Al abundances are enhanced by 0.37 and 0.54 dex and are nearly constant at all RGB luminosities, in contrast to the blue horizontal-branch cluster M13. The α- and Fe-peak elements indicate that M12 has undergone a similar chemical enrichment history to that of globular clusters and field stars of comparable metallicity, with [α/Fe] = +0.33 (σ = 0.11). M12 also appears to be slightly r-process-rich, with [Eu/Ba,La] = +0.22 (σ = 0.18).

RR Lyrae-based calibration of the Globular Cluster Luminosity Function

We test whether the peak absolute magnitude Mv(TO) of the Globular Cluster Luminosity Function (GCLF) can be used for reliable extragalactic distance determinations. Starting with the luminosity function of the Galactic Globular Clusters listed in Harris catalog, we determine Mv(TO) either using current calibrations of the absolute magnitude Mv(RR) of RR Lyrae stars as a function of the cluster metal content [Fe/H] and adopting selected cluster samples. We show that the peak magnitude is slightly affected by the adopted Mv(RR)-[Fe/H] relation, while it depends on the criteria to select the cluster sample. As for the GCLFs in other external galaxies, using Surface Brightness Fluctuations (SBF) measurements we give evidence that the luminosity functions of the blue (metal-poor) Globular Clusters peak at the same luminosity within ~0.2 mag, whereas for the red (metal-rich) samples the agreement is within ~0.5 mag even accounting for the theoretical metallicity correction expected for clusters with similar ages and mass distributions. Then, using the SBF absolute magnitudes provided by a Cepheid distance scale calibrated on a fiducial distance to LMC (m(LMC)=18.50 mag), we show that the Mv(TO) value of the metal-poor clusters in external galaxies(-7.67+/-0.23 mag) is in excellent agreement with the value of both Galactic (-7.66+/-0.11 mag) and M31 (-7.65+/-0.19 mag)ones.

Washington photometry of open cluster giants: two moderately metal-poor anticentre clusters

New photometric data in the Washington system are presented for red giant candidates in NGC 1817 and 2251, two open clusters located towards the Galactic anticentre direction. In the case of NGC 2251, the Washington data are supplemented with new UBV and David Dunlap Observatory (DDO) photoelectric photometry. Published radial velocities are used to separate field stars from cluster giants. The photometric data yield an effective temperature and metal abundance for each cluster member. Five independent Washington abundance indices yield mean metallicities of [Fe/H] = 0.25 ± 0.04 for NGC 1817 and 2251, respectively. From combined BV and DDO data, we also derive E(B − V ) = 0.21 ± 0.03 and [Fe/H]DDO = −0.14 ± 0.05 for NGC 2251. Both objects are then found to be on the metal-poor side of the distribution of open clusters, their metallicities being compatible with the existence of a radial abundance gradient in the disc. Using the WEBDA Open Cluster data base and the available literature, we re-examined the overall properties of a sample of 30 clusters located towards the Galactic anticentre with the distances, ages and metallicities available. This cluster sample presents no evidence of an abundance gradient perpendicular to the Galactic plane, nor is an age‐metallicity relation found. However, a radial abundance gradient of −0.093 dex kpc −1 is derived over a Galactocentric distance of 14 kpc, a gradient which is in keeping with most recent determinations. This value practically does not change when all clusters with basic parameters known up to this date are considered. Ke yw ords: methods: observational ‐ techniques: photometric ‐ open clusters and associations: individual: NGC 1817 ‐ open clusters and associations: individual: NGC 2251 ‐ Galaxy: abundances.

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 in eight brightest cluster galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions that are clearly resolved by the metallicity-sensitive (B - I) index. Furthermore, the mean colors and internal color range of the blue subpopulation are remarkably similar from one galaxy to the next, to well within the ±0.02-0.03 mag uncertainties in the foreground reddenings and photometric zero points. By contrast, the mean color and internal color range for the red subpopulation differ from one galaxy to the next by twice as much as the blue population. All the BCGs show population gradients, with much higher 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 distributions emerging from our data is that for the brightest clusters (MI < -10.5) the color distribution becomes broad and less obviously bimodal. This effect was first noticed by Ostrov et al. and Dirsch et al. for the Fornax giant NGC 1399; our data suggest that it may be a characteristic of many BCGs and perhaps other large galaxies. Our data indicate that the blue (metal-poor) clusters brighter than MI -10 become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z ~ M0.55. A basically similar relation has been found for M87 by Strader et al. (2005). We argue that these GCS characteristics are consistent with a hierarchical-merging galaxy formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 107-1010 M☉, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more preenrichment could occur.

Extragalactic globular clusters in the near-infrared

We present the results of combined optical and near-infrared photometry for the globular cluster systems of the giant ellipticals IC 4051 and NGC 3311. We use the reduced age-metallicity degeneracy in (V - I) vs. (V - H) color-color diagrams to derive the cumulative age distribution within the red sub-population of globular clusters and to search for age sub-populations. The age distribution is then compared to the one determined for simulated globular cluster systems in order to set constraints on the relative age and size of these globular cluster sub-populations. In both galaxies we find a significant fraction of globular clusters with ages between 2-5 Gyr. We also investigate the metallicity distribution in both systems. Small number statistics prevent us from making any definite statements concerning NGC 3311, but we find that the derived metallicity distribution of the IC 4051 clusters strongly depends on the assumed age distribution. Based on our most likely result that finds a large number of young/intermediate age clusters (∼2 Gyr) within the selected globular cluster sample, we find metallicity peaks at ∼-0.2 for the old clusters and +0.8 for the young clusters. Only few very metal poor clusters are found. However, the metallicity distribution within the young/intermediate globular cluster population is significantly affected by our choice of the applied Single Stellar Population model. The mean metallicity of the second generation of globular clusters changes from the above mentioned and extremely high +0.8 dex to +0.2 dex. Note that the model dependency becomes less severe with an increasing age of the cluster population.

Radial Velocities of Newly Discovered Globular Clusters in NGC 5128

We present radial velocity measurements for 74 globular clusters (GCs) in the nearby giant elliptical galaxy NGC 5128, of which 31 are newly discovered. All the GC candidates were taken from the list of possible new clusters given in the photometric survey of Harris and coworkers. In addition to the newly confirmed clusters, we identified 24 definite foreground stars and 31 probable background galaxies. From a combined list of 299 known GCs in NGC 5128 with measured radial velocities and metallicity-sensitive (C - T1) photometric indices, we construct a new metallicity distribution function (MDF) for the cluster system. The MDF shows an approximately bimodal form, with centroids at [Fe/H] = -1.46 and -0.53 and with nearly equal numbers of metal-poor and metal-rich clusters in the two modes. However, there are many intermediate-color clusters in the distribution, and the fainter clusters tend to have a higher proportion of redness. These features of the MDF may indicate a widespread age range within the cluster system, as well as an intrinsically broad metallicity spread.