Globular Cluster Systems Of Galaxies Research Articles

A single population of red globular clusters around the massive compact galaxy NGC 1277.

Massive galaxies are thought to form in two phases: an initial collapse of gas and giant burst of central star formation, followed by the later accretion of material that builds up their stellar and dark-matter haloes. The systems of globular clusters within such galaxies are believed to form in a similar manner. The initial central burst forms metal-rich (spectrally red) clusters, whereas more metal-poor (spectrally blue) clusters are brought in by the later accretion of less-massive satellites. This formation process is thought to result in the multimodal optical colour distributions that are seen in the globular cluster systems of massive galaxies. Here we report optical observations of the massive relic-galaxy candidate NGC 1277-a nearby, un-evolved example of a high-redshift 'red nugget' galaxy. We find that the optical colour distribution of the cluster system of NGC 1277 is unimodal and entirely red. This finding is in strong contrast to other galaxies of similar and larger stellar mass, the cluster systems of which always exhibit (and are generally dominated by) blue clusters. We argue that the colour distribution of the cluster system of NGC 1277 indicates that the galaxy has undergone little (if any) mass accretion after its initial collapse, and use simulations of possible merger histories to show that the stellar mass due to accretion is probably at most ten per cent of the total stellar mass of the galaxy. These results confirm that NGC 1277 is a genuine relic galaxy and demonstrate that blue clusters constitute an accreted population in present-day massive galaxies.

VEGAS-SSS. II. Comparing the globular cluster systems in NGC 3115 and NGC 1399 using VEGAS and FDS survey data

We analyze the globular cluster (GC) systems in two very different galaxies, NGC 3115 and NGC 1399. With the papers of this series, we aim at highlighting common and different properties in the GC systems in galaxies covering a wide range of parameter space. We compare the GCs in NGC 3115 and NGC 1399 as derived from the analysis of one square degree u-, g-, and i-band images taken with the VST telescope as part of the VST early-type galaxy survey (VEGAS) and Fornax deep survey (FDS). We selected GC candidates using as reference the morpho-photometric and color properties of confirmed GCs. The surface density maps of GCs in NGC 3115 reveal a morphology similar to the light profile of field stars; the same is true when blue and red GCs are taken separately. The GC maps for NGC 1399 are richer in structure and confirm the existence of an intra-cluster GC component. We confirm the presence of a spatial offset in the NGC 1399 GC centroid and find that the centroid of the GCs for NGC 3115 coincides well with the galaxy center. Both GC systems show unambiguous color bimodality in (g − i) and (u − i); the color–color relations of the two GC systems are slightly different with NGC 3115 appearing more linear than NGC 1399. The azimuthal average of the radial density profiles in both galaxies reveals a larger spatial extent for the total GCs population with respect to the galaxy surface brightness profile. For both galaxies, the red GCs have radial density profiles compatible with the galaxy light profile, while the radial profiles for blue GCs are shallower. As for the specific frequency of GCs, SN, we find it is a factor of two higher in NGC 1399 than for NGC 3115; this is mainly the result of extra blue GCs. By inspecting the radial behavior of the specific frequency, SN(<r), for the total, blue, and red GCs, we find notable similarities between the trends for red GCs in the two targets. In spite of extremely different host environments, the red GCs in both cases appear closely linked to the light distribution of field stars. Blue GCs extend to larger galactocentric scales than red GCs, marking a significant difference between the two galaxies: the blue/red GCs and field stellar components of NGC 3115 appear well thermalized with each other and the blue GCs in NGC 1399 appear to fade into an unrelaxed intra-cluster GC population.

DETECTION OF A DISTINCT METAL-POOR STELLAR HALO IN THE EARLY-TYPE GALAXY NGC 3115

We present the resolved stellar populations in the inner and outer halo of the nearby lenticular galaxy NGC~3115. Using deep HST observations, we analyze stars two magnitudes fainter than the tip of the red giant branch (TRGB). We study three fields along the minor axis of this galaxy, 19, 37 and 54 kpc from its center -- corresponding to 7, 14, 21 effective radii (r_{e}). Even at these large galactocentric distances, all of the fields are dominated by a relatively enriched population, with the main peak in the metallicity distribution decreasing with radius from [Z/H] ~ -0.5 to -0.65. The fraction of metal-poor stars ([Z/H] < -0.95) increases from 17%, at 16-37 kpc, to 28%, at ~54 kpc. We observe a distinct low metallicity population (peaked at [Z/H] ~ -1.3 and with total mass 2*10^{10}M_{\odot} ~ 14% of the galaxy's stellar mass) and argue that this represents the detection of an underlying low metallicity stellar halo. Such halos are generally predicted by galaxy formation theories and have been observed in several late type galaxies including the Milky Way and M31. The metallicity and spatial distribution of the stellar halo of NGC~3115 are consistent with the galaxy's globular cluster system, which has a similar low metallicity population that becomes dominant at these large radii. This finding supports the use of globular clusters as bright chemo-dynamical tracers of galaxy halos. These data also allow us to make a precise measurement of the magnitude of the TRGB, from which we derive a distance modulus of NGC~3115 of 30.05\pm0.05\pm0.10_{sys} (10.2\pm0.2\pm0.5_{sys} Mpc).

The impact of galaxy harassment on the globular cluster systems of early-type cluster dwarf galaxies

The dynamics of globular cluster systems (GCSs) around galaxies are often used to assess the total enclosed mass, and even to constrain the dark matter distribution. The globular cluster system of a galaxy is typically assumed to be in dynamical equilibrium within the potential of the host galaxy. However cluster galaxies are subjected to a rapidly evolving and, at times, violently destructive tidal field. We investigate the impact of the harassment on the dynamics of GCs surrounding early type cluster dwarfs, using numerical simulations. We find that the dynamical behaviour of the GCS is strongly influenced by the fraction of bound dark matter f_{DM} remaining in the galaxy. Only when f_{DM} falls to ~15%, do stars and GCs begin to be stripped. Still the observed GC velocity dispersion can be used to measure the true enclosed mass to within a factor of 2, even when f_{DM} falls as low as ~3%. This is possible partly because unbound GCs quickly separate from the galaxy body. However even the distribution of {\it{bound}} GCs may spatially expand by a factor of 2-3. Once f_{DM} falls into the <3% regime, the galaxy is close to complete disruption, and GCS dynamics can no longer be used to reliably estimate the enclosed mass. In this regime, the remaining bound GCS may spatially expand by a factor of 4 to 8. It may be possible to test if a galaxy is in this regime by measuring the dynamics of the stellar disk. We demonstrate that if a stellar disk is rotationally supported, it is likely that a galaxy has sufficient dark matter, that the dynamics of the GCS can be used to reliably estimate the enclosed mass.

GLOBULAR CLUSTER SYSTEMS OF SPIRAL AND S0 GALAXIES: RESULTS FROM WIYN IMAGING OF NGC 1023, NGC 1055, NGC 7332, AND NGC 7339

We present results from a study of the globular cluster (GC) systems of four spiral and S0 galaxies imaged as part of an ongoing wide-field survey of the GC systems of giant galaxies. The target galaxies -- the SB0 galaxy NGC1023, the SBb galaxy NGC1055, and an isolated pair comprised of the Sbc galaxy NGC7339 and the S0 galaxy NGC7332 -- were observed in BVR filters with the WIYN 3.5-m telescope and Minimosaic camera. For two of the galaxies, we combined the WIYN imaging with previously-published data from the Hubble Space Telescope and the Keck Observatory to help characterize the GC distribution in the central few kiloparsecs. We determine the radial distribution (surface density of GCs versus projected radius) of each galaxy's GC system and use it to calculate the total number of GCs (N_GC). We find N_GC = 490+/-30, 210+/-40, 175+/-15, and 75+/-10 for NGC1023, NGC1055, NGC7332, and NGC7339, respectively. We also calculate the GC specific frequency (N_GC normalized by host galaxy luminosity or mass) and find values typical of those of the other spiral and E/S0 galaxies in the survey. The two lenticular galaxies have sufficient numbers of GC candidates for us to perform statistical tests for bimodality in the GC color distributions. We find evidence at a high confidence level (>95%) for two populations in the B-R distribution of the GC system of NGC1023. We find weaker evidence for bimodality (>81% confidence) in the GC color distribution of NGC7332. Finally, we identify eight GC candidates that may be associated with the Magellanic dwarf galaxy NGC1023A, a satellite of NGC1023.

The age structure of globular cluster systems in early-type galaxies and its dependence on host galaxy properties

Aims. We present the results of an optical – near-infrared photometric survey of globular cluster systems in early-type galaxies. Our project aims to study correlations between the age structure of the globular cluster systems and environmental as well as structural parameters of their host galaxies. Methods. To derive the age structure in globular cluster systems we use combined optical and near-infrared photometry. This combination facilitates the detection of young globular cluster sub-populations if their age differs significantly from the old (∼13 Gyr) population, i.e. if their age is <7 Gyr. We investigate globular cluster systems in galaxies located in the central regions of galaxy clusters (NGC 1399, M 87, NGC 4478), galaxies which are members of (sub-) groups (NGC 4365, NGC 5846) and isolated galaxies (NGC 3115 and NGC 7192). Further, we look for correlations between globular cluster system properties and structural parameters of the galaxies (velocity dispersion σ, color ((V − K), (B − V)), and luminosity MV ). For a quantitative analysis we define the “Methuselah” parameter, Mth – based on the fraction of young and old metal-rich globular clusters weighted by age, characterizing the age structure of a globular cluster system. Results. We find that, within our galaxy sample, group galaxies appear to host an intermediate-age globular cluster population, whereas the globular cluster systems in cluster galaxies and in rather isolated galaxies are dominated by an old globular cluster population. A weak correlation of Mth with the Hβ index of the integrated light of the host galaxies is the only trend found with respect to internal galaxy parameters.

Secondary Globular Cluster populations

This study is motivated by two facts: 1. The formation of populous star cluster systems is widely observed to accompany violent star formation episodes in gas-rich galaxies as e.g. those triggered by strong interactions or merging. 2. The Globular Cluster (GC) systems of most but not all early-type galaxies show bimodal optical color distributions with fairly universal blue peaks and somewhat variable red peak colors, yet their Luminosity Functions (LFs) look like simple Gaussians with apparently universal turn-over magnitudes that are used for distance measurements and the determination of . Based on a new set of evolutionary synthesis models for Simple (= single burst) Stellar Populations (SSPs) of various metallicities using the latest Padova isochrones I study the color and luminosity evolution of GC populations over the wavelength range from U through K, providing an extensive grid of models for comparison with observations. I assume the intrinsic widths of the color distributions and LFs to be constant in time at the values observed today for the Milky Way or M 31 halo GC populations. Taking the color distributions and LFs of the Milky Way or M 31 halo GC population as a reference for old metal-poor GC populations in general, I study for which combinations of age and metallicity a secondary GC population formed in some violent star formation event in the history of its parent galaxy may or may not be detected in the observed GC color distributions. I also investigate the effect of these secondary GCs on the LFs of the total GC system. Significant differences are found among the diagnostic efficiencies in various wavelength regions. In particular, we predict the NIR to be able to reveal the presence of GC subpopulations with different age – metallicity combinations that may perfectly hide within one inconspicuous optical color peak. If the entire manifold of possible age – metallicity combinations is admitted for a secondary GC population, we find several cases where the resulting LF of the whole GC system is significantly affected and its turn-over could not serve as a reliable distance indicator. If, on the other hand, we assume some age – metallicity relation for GC populations, the second peak of the LFs vanishes and models indicate single-peak GC LFs even in GC systems with bimodal color distributions. A broad but sufficient age – metallicity relation is, for example, obtained if the secondary GC populations form in mergers of various spiral galaxy types from the ISM pre-enriched over the redshift range from to . As a first illustrative example we apply our models to V- and I-band data presented by Larsen et al. ([CITE]) for blue and red peak GCs in three early-type galaxies. We point out the importance of having multi-band information to independently constrain ages and metallicities of different GC subpopulations and again stress the diagnostic potential of K-band data in addition to optical observations. The models presented here will be used for the interpretation of multi-wavelength data on GC systems in galaxies of various types, luminosities and environments as well as on young star cluster systems in interacting galaxies and mergers. By independently constraining ages and metallicities of individual clusters we expect to shed light on both cluster and galaxy formation scenarios.

The Globular Cluster System of the Spiral Galaxy NGC 7814

We present the results of a wide-field photometric study of the globular cluster (GC) system of the edge-on Sab spiral NGC 7814. This is the first spiral to be fully analyzed from our survey of the GC systems of a large sample of galaxies beyond the Local Group. NGC 7814 is of particular interest because a previous study estimated that it has 500–1000 GCs, giving it the largest specific frequency (SN) known for a spiral. Understanding this galaxy's GC system is important in terms of our understanding of the GC populations of spirals in general and has implications for the formation of massive galaxies. We observed the galaxy in BVR filters with the WIYN 3.5 m telescope and used image classification and three-color photometry to select GC candidates. We also analyzed archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images of NGC 7814, both to help quantify the contamination level of the WIYN GC candidate list and to detect GCs in the inner part of the galaxy halo. Combining HST data with high-quality ground-based images allows us to trace the entire radial extent of this galaxy's GC system and determine the total number of GCs directly through observation. We find that rather than being an especially high-SN spiral, NGC 7814 has 200 GCs and SN ~ 1, making it comparable to the two most well-studied spiral galaxies, the Milky Way and M31. We explore the implications of these results for models of the formation of galaxies and their GC systems. The initial results from our survey suggest that the GC systems of typical elliptical galaxies can be accounted for by the merger of two or more spirals, but that for highly luminous elliptical galaxies, additional physical processes may be needed.

Metallicity distributions of globular cluster systems in galaxies

AbstractWe collected a sample of 100 galaxies for which different observers have determined colour indices of globular cluster candidates. The sample includes representatives of galaxies of various morphological types and different luminosities. Colour indices (in most cases (V–I), but also (B–I) and (C–T1)) were transformed into metallicities [Fe/H] according to a relation by Kissler‐Patig (1998). These data were analysed with the KMM software in order to estimate similarity of the distribution with uni‐ or bimodal Gaussian distribution. We found that 45 of 100 systems have bimodal metallicity distributions. Mean metallicity of the metal‐poor component for these galaxies is 〈[Fe/H]〉 = –1.40 ± 0.02, of the metalrich component 〈[Fe/H]〉 = –0.69 ± 0.03. Dispersions of the distributions are 0.15 and 0.18, respectively. Distribution of unimodal metallicities is rather wide. These data will be analysed in a subsequent paper in order to find correlations with parameters of galaxies and galactic environment.

Constraints on the merger models of elliptical galaxies from their globular cluster systems

The discovery of protoglobular cluster candidates in many present-day mergers allows us to understand better the possible effects of a merger event on the globular cluster system of a galaxy, and to foresee the properties of the end-product. By comparing these expectations with the properties of globular cluster systems of elliptical galaxies at the present time we can constrain merger models. The observational data indicate that (i) every gaseous merger induces the formation of new star clusters, and (ii) the number of new clusters formed in such a merger increases with the gas content of the progenitor galaxies. Low-luminosity (about MV > −21), discy ellipticals are generally thought to be the result of a gaseous merger. As such, new globular clusters are expected to form but have not been detected to date. We investigate various reasons for the non-detection of subpopulations in low-luminosity ellipticals, i.e. absence of an old population, absence of a new population, destruction of one of the populations and, finally, an age–metallicity conspiracy that allows old and new globular clusters to appear indistinguishable at the present epoch. All of these possibilities lead us to a similar conclusion, namely that low-luminosity ellipticals did not form recently (z < 1) in a gas-rich merger, and might not have formed in a major merger of stellar systems at all. High-luminosity ellipticals do reveal globular cluster subpopulations. However, it is difficult to account for the two populations in terms of mergers alone and, in particular, we can rule out scenarios in which the second subpopulation is the product of a recent, gas-poor merger.

The Globular Cluster Luminosity Function as a Distance Indicator: Dynamical Effects

The dynamical evolution of the globular cluster systems in galaxies is predicted, based on the standard dynamical theory normalized to the example of the Milky Way. The major processes varying with the galactocentric distance are the tidal shocks and dynamical friction. Our simple model explains, on a quantitative basis, the observed differences of the inner and outer populations of globular clusters. We can thus calculate corrections for dynamical evolution for the luminosity function of globular clusters with the assumption that the initial luminosity function is identical in all galaxies (and we can test this assumption as well, in certain cases). Then we can compute the expected distribution of absolute magnitudes and compare it with the observed distribution of apparent magnitudes to estimate the distance moduli for M31 and M87. Using this new method we find dm(M31)=24.05 +- 0.23, dm(M87)=30.83 +- 0.17, as compared to current best estimates using other methods of dm(M31)=24.30 +- 0.20, dm(M87)=31.0 +- 0.1. As a check on the method we compute, and compare with observations, the differences between the inner and outer globular clusters in all three galaxies. This new method, coupled with HST observations, promises to provide an independent method of estimating distances to galaxies with recession velocities < 10,000 km/s, or D < 100 Mpc.

Hubble Space Telescope Imaging of the Globular Cluster System Around NGC 5846

view Abstract Citations (51) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope Imaging of the Globular Cluster System Around NGC 5846 Forbes, Duncan A. ; Brodie, Jean P. ; Huchra, John Abstract Bimodal globular cluster metallicity distributions have now been seen in a handful of large ellipticals. Here we report the discovery of a bimodal distribution in the dominant group elliptical NGC 5846, using the Hubble Space Telescope's Wide Field and Planetary Camera 2 (WFPC2). The two peaks are located at V-I=0.96 and 1.17, which roughly correspond to metallicities of [Fe/H]=-1.2 and -0.2, respectively. The luminosity functions of the blue and red subpopulations appear to be the same, indicating that luminosity does not correlate with metallicity within an individual galaxy's globular cluster system. Our WFPC2 data cover three pointings allowing us to examine the spatial distribution of globular clusters out to 30 kpc (or 2.5 galaxy effective radii). We find a power law surface density with a very fiat slope, and a tendency for globular clusters to align close to the galaxy minor axis. An extrapolation of the surface density profile, out to 50 kpc, gives a specific frequency SN=4.3±1.1 Thus NGC 5846 has a much lower specific frequency than other dominant ellipticals in clusters but is similar to those in groups. The central galaxy regions reveal some filamentary dust features, presumably from a past merger or accretion of a gas-rich galaxy. This dust reaches to the very nucleus and so provides an obvious source of fuel for the radio core. We have searched for proto-globular clusters that may have resulted from the merger/accretion and find none. Finally, we briefly discuss the implications of our results for globular cluster formation mechanisms. Publication: The Astronomical Journal Pub Date: March 1997 DOI: 10.1086/118308 arXiv: arXiv:astro-ph/9612172 Bibcode: 1997AJ....113..887F Keywords: Astrophysics E-Print: 22 pages, Latex. To be published in the Astronomical Journal. Full paper available at http://www.ucolick.org/~forbes/home.html full text sources arXiv | ADS | data products SIMBAD (5) NED (1) MAST (1) ESA (1)

Abundances and Kinematics of the Globular Cluster Systems of the Galaxy and of the Sagittarius Dwarf

view Abstract Citations (252) References (104) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Abundances and Kinematics of the Globular Cluster Systems of the Galaxy and of the Sagittarius Dwarf Da Costa, G. S. ; Armandroff, T. E. Abstract Spectra have been obtained at the Ca II triplet of a number of member giants in seven galactic globular clusters for which current metallicity estimates are either lacking or poorly determined. Similar spectra have also been obtained of giants in eight clusters whose abundances are well established. These latter data have been combined with other existing CaII triplet observations to define a (Ca II triplet line strength, abundance) relation that encompasses the full range of metallicities exhibited by the Galaxy's globular cluster system. This calibration is then used to determine abundances for the program clusters. Specifically, we find [Fe/Il] -2.00+/-0.08, -0.67+/-0.07 -128+/-0.12, -0.36+/-0.09, -1.70+/-0.11, -1.55+/-0.10, and -1.99+/-0.08 for Pal 15, NGC 6366, NGC 6626 (M28), Terzan 7, Asp 2, NGC 6715 (M54), and Terzan 8, respectively. Further, although our sample in M54 is small (five stars), our results are consistent with the color-magnitude diagram based findings of Sarajedini & Layden [AJ, 109,1086 (1995)], who have suggested that M54 may possess an internal abundance range characterized by σ([Fe/H])~0.15 dex. Radial velocities for the program clusters have also been derived from our spectra. With these new abundance and velocity data, and using other recent results from the literature, we have repeated the analysis of Zinn [ASP ConE. Ser. Vol. 48, p. 38 (1993)]. We confirm his finding of kinematic differences between two subsamples of the halo globular clusters. In particular, we find for the "younger halo" clusters, which have redder horizontal branch morphologies and/or younger ages at fixed abundance, a velocity dispersion σ_los_ = 163+/-25 km/s and a (retrograde) rotation of V_rot_ = -46+/-81 km/s. For the "old halo" clusters, which have bluer horizontal branch morphologies, the corresponding numbers are σ_los_= 106+/-11 km/s and V_rot_ 50+/-27 km/s. We note also that four of the clusters studied here (M54, Terzan 7, Arp 2, and Terzan 8) locations in the sky, radial velocities, and distances that associate them with the recently discovered Sagittarius dwarf spheroidal galaxy. We then compare the properties of this presumed Sagittarius cluster system with the cluster system of the Fornax dwarf spheroidal galaxy, noting in particular that for both systems the mean cluster abundance is considerably less than the mean abundance of the field star population. We note also that the Sagittarius cluster system, if integrated into the galactic halo, will contribute clusters to both the younger halo and old halo subsamples. Publication: The Astronomical Journal Pub Date: June 1995 DOI: 10.1086/117469 Bibcode: 1995AJ....109.2533D Keywords: GLOBULAR CLUSTERS: GENERAL; GALAXY: KINEMATICS AND DYNAMICS; GALAXY: HALO full text sources ADS | data products SIMBAD (55) NED (1)

Globular Cluster Systems in Galaxies: Main Trends and Future Directions

This was a really exciting conference! The main problem in summarizing it is that so many new and important results were presented, making it impossible to mention each one individually. In reviewing this conference I shall first discuss new results, then problem areas revealed by papers and during discussions and finally desiderata and trends for future work.

The appearance, number, and history of highly compact binary systems in globular clusters

view Abstract Citations (57) References (71) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The appearance, number, and history of highly compact binary systems in globular clusters Krolik, J. H. Abstract The author analyzes the various evolutionary paths possible for highly compact binaries in globular clusters, where they come under the simultaneous influence of gravitational radiation and gravitational encounters with field stars. The observational appearance of those stages in which mass transfer occurs is discussed, with particular attention given to a new class of objects characterized by massive, nonequilibrium accretion disks created by catastrophic gravitational encounters. The author then constructs an analytic model to predict the number of highly compact binaries in each stage of evolution existing at present in the Galaxy's globular cluster system. Both steady transfer binaries, and systems in which a collapsed stellar remnant acquires a massive, nonequilibrium accretion disk, are shown to be present in significant numbers. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162223 Bibcode: 1984ApJ...282..452K Keywords: Binary Stars; Globular Clusters; Gravitational Collapse; Stellar Evolution; Stellar Mass Accretion; Dwarf Novae; Neutron Stars; Star Distribution; X Ray Sources; Astrophysics full text sources ADS |

Main-sequence binaries, contact binaries, and blue stragglers in globular clusters

The existence of binary stars in globular clusters has a long history of controversy1–3. At present the only evidence for binaries in globular clusters comes from binaries in extreme states: cataclysmic variables4,5 and X-ray sources (see refs 6 and 7 for reviews, and ref. 8 for new observations of very weak X-ray sources). The existence of the latter is customarily ascribed to tidal capture of red dwarf companions by neutron stars (or possibly solar-mass black holes)9. In fact, tidal capture should be at least as effective in creating close binaries comprising two main-sequence stars9,10. Subsequent evolution of such binaries is very likely to lead to mass exchange between the two, and possibly merger. In conventional calculations, gravitational radiation drives the binary orbital evolution11,12; recently, it has been shown that in the denser globular clusters, where tidal capture is most effective, gravitational encounters with passing field stars also have an important role, both by slow, cumulative compression of the binary orbit, and by occasional catastrophic physical collisions13. On the basis of these mechanisms, we predict that globular clusters should contain substantial numbers of: close binaries; low-mass contact binaries, perhaps similar to the somewhat more massive WUMa stars; and ‘blue stragglers’, stars lying on the extrapolated main sequence beyond the turn-off. These objects should be most common in the densest regions of the Galaxy's globular cluster system.