Galactic Halo Globular Clusters Research Articles

Formation of massive globular clusters with dark matter and its implication on dark matter annihilation

ABSTRACT Recent observational studies of γ-ray emission from massive globular clusters (GCs) have revealed possible evidence of dark matter (DM) annihilation within GCs. It is, however, still controversial whether the emission comes from DM or from millisecond pulsars. We here present the new results of numerical simulations, which demonstrate that GCs with DM can originate from nucleated dwarfs orbiting the ancient Milky Way. The simulated stripped nuclei (i.e. GCs) have the central DM densities ranging from 0.1 to several M⊙ pc−3, depending on the orbits and the masses of the host dwarf galaxies. However, GCs born outside the central regions of their hosts can have no/little DM after their hosts are destroyed and the GCs become the Galactic halo GCs. These results suggest that only GCs originating from stellar nuclei of dwarfs can possibly have DM. We further calculate the expected γ-ray emission from these simulated GCs and compare them to observations of ω Cen. Given the large range of DM densities in the simulated GCs, we suggest that the recent possible detection of DM annihilation from GCs should be more carefully interpreted.

Discovery of a New Stellar Subpopulation Residing in the (Inner) Stellar Halo of the Milky Way

Abstract We report the discovery of a unique collection of metal-poor giant stars that exhibit anomalously high levels of 28Si, clearly above typical Galactic levels. Our sample spans a narrow range of metallicities, peaking at −1.07 ± 0.06, and exhibits abundance ratios of [Si, Al/Fe] that are as extreme as those observed in Galactic globular clusters (GCs), and Mg is slightly less overabundant. In almost all the sources we used, the elemental abundances were redetermined from high-resolution spectra, which were reanalyzed assuming LTE. Thus, we compiled the main element families, namely, the light elements (C, N), α-elements (O, Mg, Si), iron-peak element (Fe), s-process elements (Ce, Nd), and the light odd-Z element (Al). We also provide dynamical evidence that most of these stars lie on tight (inner) halo-like and retrograde orbits passing through the bulge. Such kinds of objects have been found in present-day halo GCs, providing the clearest chemical signature of past accretion events in the (inner) stellar halo of the galaxy, possibly formed as the result of dissolved halo GCs. Their chemical composition is, in general, similar to that of typical GC populations, although several differences exist.

Ruprecht 106: A Riddle, Wrapped in a Mystery, inside an Enigma∗

Abstract Galactic globular clusters (GCs) show overwhelming photometric and spectroscopic evidence for the existence of multiple stellar populations. The question of whether or not there exists a GC that represents a true “simple stellar population” remains open. Here we focus on Ruprecht 106 (R106), a halo GC with [Fe/H] = −1.5 and [α/Fe] ≃0. A previous spectroscopic study found no sign of the Na–O anticorrelation among nine of its brightest red giants, which led to the conclusion that R106 is a true simple stellar population GC. Here we present new Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) photometry of R106 that, when combined with archival HST images spanning a 6 year baseline, allows us to create proper-motion cleaned color–magnitude diagrams spanning the ultraviolet (F336W) to the near-infrared (F814W). These data allow us to construct the pseudo-color that is sensitive to the presence of light-element abundance spreads. We find no evidence of a split along the red giant branch (RGB) in the diagram but the width of the RGB ( ) is marginally broader than expected from artificial star tests ( ). The observed spread in is smaller than any other Galactic GC studied to date. Our results raise important questions about the role of formation environment and primordial chemical composition in the formation of multiple stellar populations in GCs.

NGC 5824: a luminous outer halo globular cluster with an intrinsic abundance spread

We present a detailed study of the strengths of the calcium triplet absorption lines in the spectra of a large sample of red giant members of the luminous outer Galactic halo globular cluster NGC 5824. The spectra were obtained with the FORS2 and GMOS-S multi-object spectrographs at the VLT and the Gemini-S telescope, respectively. By comparing the line strengths of the NGC 5824 stars with those for red giants in clusters with well established abundances, we conclude that there is an intrinsic abundance dispersion in NGC 5824 characterized by an inter-quartile range in [Fe/H] of 0.10 dex and a total range of ~0.3 dex. As for omega-Cen and M22, the abundance distribution shows a steep rise on the metal-poor side and a shallower decline on the metal-rich side. There is also some indication that the distribution is not unimodal with perhaps 3 distinct abundance groupings present. NGC 5824 has a further unusual characteristic: the outer surface density profile shows no signs of a tidal cutoff. Instead the profile has a power-law distribution with cluster stars detected to a radius exceeding 400 pc. We postulate that NGC 5824 may be the remnant nuclear star cluster of a now disrupted dwarf galaxy accreted during the formation of the Galaxy's halo. We further speculate that the presence of an intrinsic [Fe/H] spread is the characteristic that distinguishes former nuclear star clusters from other globular clusters.

Looking for tidal streams around Galactic globular clusters

The picture of building the Milky Way halo from merging protogalactic fragments is considered the local manifestation of the hierarchical galaxy formation process. In this scenario, some observational evidences have suggested that the outer young Galactic halo globular cluster population might be associated (or even the nuclei) to tidal disrupted dwarf spheroidals, now extinct galaxies. If this hypothesis is true, these systems might be surrounded by a distinct and still detectable stellar population. We have carried out a systematic observation of Galactic globulars covering the galactocentric distance range 10< RGC <40kpc in both hemispheres. We have used wide field instruments both in La Palma and in La Silla observatories to obtain deep photometry of wide areas around these globulars to unveil the possible remnants of their progenitor dwarf galaxies.

THE PECULIAR CHEMICAL INVENTORY OF NGC 2419: AN EXTREME OUTER HALO “GLOBULAR CLUSTER”

NGC 2419 is a massive outer halo Galactic globular cluster whose stars have previously been shown to have somewhat peculiar abundance patterns. We have observed seven luminous giants that are members of NGC 2419 with Keck/HIRES at reasonable SNR. One of these giants is very peculiar, with an extremely low [Mg/Fe] and high [K/Fe] but normal abundances of most other elements. The abundance pattern does not match the nucleosynthetic yields of any supernova model. The other six stars show abundance ratios typical of inner halo Galactic globular clusters, represented here by a sample of giants in the nearby globular cluster M30. Although our measurements show that NGC 2419 is unusual in some respects, its bulk properties do not provide compelling evidence for a difference between inner and outer halo globular clusters.

Distant star clusters of the Milky Way in MOND

We determine the mean velocity dispersion of six Galactic outer halo globular clusters, AM 1, Eridanus, Pal 3, Pal 4, Pal 15, and Arp 2 in the weak acceleration regime to test classical vs. modified Newtonian dynamics (MOND). Owing to the non-linearity of MOND's Poisson equation, beyond tidal effects, the internal dynamics of clusters is affected by the external field in which they are immersed. For the studied clusters, particle accelerations are much lower than the critical acceleration a_0 of MOND, but the motion of stars is neither dominated by internal accelerations (a_i >> a_e) nor external accelerations (a_e >> a_i). We use the N-body code N-MODY in our analysis, which is a particle-mesh-based code with a numerical MOND potential solver developed by Ciotti, Londrillo, and Nipoti (2006) to derive the line-of-sight velocity dispersion by adding the external field effect. We show that Newtonian dynamics predicts a low-velocity dispersion for each cluster, while in modified Newtonian dynamics the velocity dispersion is much higher. We calculate the minimum number of measured stars necessary to distinguish between Newtonian gravity and MOND with the Kolmogorov-Smirnov test. We also show that for most clusters it is necessary to measure the velocities of between 30 to 80 stars to distinguish between both cases. Therefore the observational measurement of the line-of-sight velocity dispersion of these clusters will provide a test for MOND.

Searching for Tidal Remnants in the Milky Way: Photometric Survey of Galactic Globular Clusters

It has been suggested that the outer young Galactic halo globular cluster population might be associated (or even the nuclei) to tidal disrupted dwarf spheroidals, now extinct galaxies. If this hypothesis is true, these systems might be still surrounded by a distinct and detectable stellar population similar to that of their progenitor galaxies. We have designed a systematic photometric survey of Galactic globular clusters to unveil the possible remnants of the systems where they were formed and estimate their contribution to the Galatic halo.

Chemical abundances for the outer halo cluster Pal 4 from co-added high-resolution spectroscopy

Context: Chemical element abundances for distant Galactic globular clusters (GCs) hold important clues to the origin of the Milky Way halo and its substructures. Aims: We study the chemical composition of red giant stars in Pal 4 - one of the most remote GCs in the Milky Way - and compare our abundance measurements to those for both low surface brightness dwarf galaxies, and GCs in the inner and the outer halo. Methods: By co-adding high-resolution, low-S/N Keck/HIRES spectra of 19 stars along the red giant branch, we estimate chemical abundance ratios of 20 alpha-, iron peak-, and neutron-capture elements. Our method gives total uncertainties on most element-to-iron ratios of typically 0.2 dex. Results: We measure [Fe/H] = -1.41 +- 0.04 (statistical) +- 0.17 (systematic) and an alpha-enhancement of [alpha/Fe] = +0.38 +- 0.11 dex, which is consistent with the canonical value of +0.4 dex found for Galactic halo field stars and most halo GCs at this metallicity. Although Pal 4 has higher enhancements in the heavier elements with respect to the halo, the majority of the element ratios are, within the measurement errors, consistent with those for local halo field stars. We find, however, evidence for a lower [Mg/Ca] ratio than in other halo clusters. Conclusions: Based on the available evidence, we conclude that the material from which Pal 4 and the Galactic halo formed experienced similar enrichment processes, despite the apparently younger age of this cluster. Within the limitations of our methodology we find no significant indication of an iron spread, as is typical of genuine GCs of the Milky Way. However, abundance ratios for individual stars in Pal 4 and other distant satellites are urgently needed to understand the relationship, if any, between remote GCs and other halo substructures (i.e., luminous and ultra-faint dwarf spheroidal galaxies).

ANCIENT STARS BEYOND THE LOCAL GROUP: RR LYRAE VARIABLES AND BLUE HORIZONTAL BRANCH STARS IN SCULPTOR GROUP DWARF GALAXIES

We have used Hubble Space Telescope ACS images to generate color-magnitude diagrams that reach below the magnitude of the horizontal branch in the Sculptor Group dwarf galaxies ESO294-010 and ESO410-005. In both diagrams blue horizontal branch stars are unambiguously present, a signature of the existence of an ancient stellar population whose age is comparable to that of the Galactic halo globular clusters. The result is reinforced by the discovery of numerous RR Lyrae variables in both galaxies. The occurrence of these stars in the first direct confirmation of the existence of ancient stellar populations beyond the Local Group and indicates that star formation can occur at the earliest epochs even in low density environments.

Globular clusters in NGC147, 185, and 205

AbstractStudying the chemical compositions and color-magnitude diagrams of globular clusters in the nearby low-mass galaxies is critical to compare properties of these long-living objects situated in galaxies of different type and mass, and to establish the role of dwarf galaxies as building blocks of massive early-type and spiral galaxies. We present measurements of ages, metallicities and [α/Fe] ratios for 16 globular clusters (GC) in NGC147, NGC185, and NGC205 and for the central regions of the diffuse galaxy light in NGC185, and NGC205, based on measurements of absorption line indices as defined by the Lick standard system in spectra obtained with the SCORPIO multi-slit spectrograph at the 6-m telescope of the Russian Academy of Sciences. We include in our analysis high-quality HST/WFPC2 photometry of individual stars in the GGs to investigate the influence of their horizontal branch (HB) morphology on the spectroscopic analysis. The HB morphologies for our sample GCs follow the same behavior with metallicity as younger halo Galactic globular clusters. We show that it is unlikely that they bias our spectroscopic age estimates based on Balmer absorption-line indices. Almost all our sample GCs appear to be old (T &gt; 8 Gyr) and metal-poor ([Z/H]&lt;−1.1). We find that most of the GCs in the studied galaxies are weakly or not α-enhanced, in contrast to the population of GCs in nearby early-type galaxies, and to the halo population of GCs in M31 and Milky Way.

Primordial pollution of globular clusters within their host dwarfs embedded in dark matter haloes at high redshifts

Abstract Recent observational studies have revealed star-to-star abundance inhomogeneity among light elements (e.g. C, N, O, Na and Al) of stars on the main sequence in the Galactic globular clusters (GCs). One of the promising interpretations for this result is that the observed abundance inhomogeneity is due to the second generation of stars formed from ejecta of the first generation of evolved stars (e.g. asymptotic giant branch stars) within GCs. However, it remains unclear whether and how this primordial pollution can occur within GCs. We here propose a new scenario in which primordial pollution of GCs is highly likely to occur if GCs are located in the central regions of high-redshift dark matter subhaloes that can host low-mass dwarf galaxies. In this scenario, gas ejected from the first generation of stars of GCs can be effectively trapped in the deep gravitational potential of their host haloes, and consequently can be consumed for the formation of the second generation of stars without losing a significant amount of gas by ram pressure stripping of interstellar and intergalactic medium. During merging of these haloes with the proto-Galaxy, the haloes are completely destroyed owing to the strong tidal field of the Galaxy. The self-polluted GCs located initially in the central regions of the haloes can survive tidal destruction owing to their compactness and finally become the Galactic halo GCs. In this scenario, ejecta of field stars surrounding the central GCs can be also converted into stars within their host dwarfs and finally become the second generation of stars of GCs. We also discuss the origin of the difference in the degree of abundance inhomogeneity between different GCs, such as ω Centauri and NGC 6752, in terms of the difference in physical properties between host haloes from which GCs originate.

Probable member stars of the gravitational theory-testing globular clusters AM 1, Pal 3 and Pal 14

Some of the Galactic outer halo globular clusters are excellent tools to probe gravitational theories in the regime of weak accelerations. The measurement of the line-of-sight velocity dispersion among stars in these clusters will differentiate between the validity of Newtonian dynamics (low velocity dispersion) and the possiblity of modified Newtonian dynamics (MOND) or dark matter dominated globular clusters (high velocity dispersion). In this paper, the properties of probable member stars of the three best-case gravitational theory-testing clusters AM 1, Pal 3 and Pal 14 are presented. The member selection is based on VLT photometry in Johnson $\it BV$. The positions of the stars were determined with an accuracy of the order of ≤ $0\farcs2$, allowing their direct use for follow-up spectroscopy. The distance, reddening, age and metallicities of the clusters were estimated from isochrone fitting. Furthermore, improved structural parameters, like central coordinates, ellipticity, half-light radius, King model core and tidal radius, are presented.

New light on the initial mass function of the Galactic halo globular clusters

We present new constraints on the initial mass spectrum of the Galactic Old Halo globular clusters. This has remained poorly known so far, as both an initial power-law mass spectrum and an initial lognormal mass function could have evolved into the presently observed globular cluster mass distribution, making the initial contribution of now lost low-mass objects ill-determined. Our approach consists of comparing the evolution with time of both the radial mass density profile and the number density profile of the globular cluster system. Using the analytical expression established by Vesperini & Heggie for the temporal evolution of the mass of a globular cluster on a circular orbit in a stable Galactic potential, we evolve the mass and number density profiles of many putative globular cluster systems, each starting with a different initial cluster mass spectrum and initial cluster space density. We then compare the modelled profiles with those of the Old Halo cluster system in order to investigate which system(s) provide(s) the best consistency with the data. Specifically, we build on the following points: (i) the presently observed mass density profile and number density profile of the Old Halo cluster system show the same shape; (ii) assuming that globular clusters were initially distributed the same way in mass at all galactocentric distances, the mass and number density profiles had also the same shape initially; and (iii) according to our simulations, the mass density profile remains well preserved, irrespective of the initial cluster mass spectrum, while the temporal evolution of its number counterpart depends sensitively on initial conditions. We show that to obtain a mass density profile and a number density profile that are identical in shape, both initially and after a Hubble time of evolution, the globular cluster system must have been depleted in low-mass objects from the beginning. We deduce that the initial distribution in mass of the globular clusters was either a lognormal mass function, similar to that today, or a power-law mass spectrum with a slope ≃−2 and truncated at large mass, say, around 105 M⊙. In contrast, a power-law mass spectrum with a similar slope but extending down to low cluster mass (i.e. a few thousand solar masses) seems to be ruled out.

On the origin of the radial mass density profile of the Galactic halo globular cluster system

We investigate what may be the origin of the presently observed spatial distribution of the mass of the Galactic Old Halo globular cluster system. We propose its radial mass density profile to be a relic of the distribution of the cold baryonic material in the protogalaxy. Assuming that this one arises from the profile of the whole protogalaxy minus the contribution of the dark matter (and a small contribution of the hot gas by which the protoglobular clouds were bound), we show that the mass distributions around the Galactic centre of this cold gas and of the Old Halo agree satisfactorily. In order to demonstrate our hypothesis even more conclusively, we simulate the evolution with time, up to an age of 15 Gyr, of a putative globular cluster system whose initial mass distribution in the Galactic halo follows the profile of the cold protogalactic gas. We show that beyond a galactocentric distance of order 2-3 kpc, the initial shape of such a mass density profile is preserved despite the complete destruction of some globular clusters and the partial evaporation of some others. This result is almost independent of the choice of the initial mass function for the globular clusters, which is still ill determined. The shape of these evolved cluster system mass density profiles also agrees with the presently observed profile of the Old Halo globular cluster system, thus strengthening our hypothesis. Our result might suggest that the flattening shown by the Old Halo mass density profile at short distances from the Galactic centre is, at least partly, of primordial origin.

Galaxy Evolution Explorer Ultraviolet Photometry of Globular Clusters in M31

We present ultraviolet photometry for globular clusters (GCs) in M31 from 15 square deg of imaging using the Galaxy Evolution Explorer (GALEX). We detect 200 and 94 GCs with certainty in the near-ultraviolet (NUV; 1750 - 2750 Angstroms) and far-ultraviolet (FUV; 1350 - 1750 Angstroms) bandpasses, respectively. Our rate of detection is about 50% in the NUV and 23% in the FUV, to an approximate limiting V magnitude of 19. Out of six clusters with [Fe/H]>-1 seen in the NUV, none is detected in the FUV bandpass. Furthermore, we find no candidate metal-rich clusters with significant FUV flux, because of the contribution of blue horizontal-branch (HB) stars, such as NGC 6388 and NGC 6441, which are metal-rich Galactic GCs with hot HB stars. We show that our GALEX photometry follows the general color trends established in previous UV studies of GCs in M31 and the Galaxy. Comparing our data with Galactic GCs in the UV and with population synthesis models, we suggest that the age range of M31 and Galactic halo GCs are similar.

Self-enrichment of Galactic halo globular clusters: stimulated star formation and consequences for the halo metallicity distribution

We explore the self-enrichment hypothesis for globular cluster formation with respect to the star formation aspect. Following this scenario, the massive stars of a first stellar generation chemically enrich the globular progenitor cloud up to Galactic halo metallicities and sweep it into an expanding spherical shell of gas. This paper investigates the ability of this swept protoglobular cloud to become gravitationally unstable and, therefore, to seed the formation of second-generation stars which may form a globular cluster later on. We use a simple model based on a linear perturbation theory for transverse motions in a shell of gas to demonstrate that the pressures by which the progenitor clouds are bound and the supernova numbers required to achieve Galactic halo metallicities support the successful development of the shell transverse collapse. Interestingly, the two parameters controlling the metallicity achieved through self-enrichment, namely the number of supernovae and the external pressure, also rule the surface density of the shell and thus its ability to undergo a transverse collapse. Such a supernova-induced origin for the globular cluster stars therefore opens the way to the understanding of the halo metallicity distributions. This model is also able to explain the lower limit of the halo globular cluster metallicity.

Building Up the Globular Cluster System of the Milky Way: The Contribution of the Sagittarius Galaxy

We demonstrate that there is a clear statistical correlation between the (X, Y, Z, Vr) phase-space distribution of the outer halo Galactic globular clusters (having 10 kpc ≤ RGC ≤ 40 kpc) and the orbital path of the Sagittarius dwarf spheroidal galaxy (Sgr dSph), as derived by Ibata & Lewis. At least four of the sample of 35 globular clusters in this distance range were formerly members of the Sagittarius galaxy (at the 95% confidence level) and are now distributed along the Sagittarius Stream, a giant tidal structure that surrounds the Milky Way. This is the first instance that a statistically significant structure associated with the Sgr dSph has been detected in the globular cluster population of the Galactic halo. Together with the four well-known globular clusters that are located near the center of this tidally disrupting dwarf galaxy, these clusters constitute 20% of the population of outer halo (RGC ≥ 10 kpc) clusters. The Sgr dSph was therefore not only an important contributor to the halo field-star population, but it also had a significant role in building up the globular cluster system of the Milky Way.

The self-enrichment of galactic halo globular clusters

We discuss the existence of a mass-metallicity relation among galactic halo globular clusters. The lack of any luminosity-metallicity correlation in globular cluster systems has been used as an argument against self-enrichment models of cluster formation. We show that such a relation is statistically present among the galactic Old Halo globulars. This observational correlation implies that the least massive old clusters are the most metal-rich. This is in contradiction with the idea that, if globular clusters were self-enriched systems, the most metal-rich clusters would also be the most massive ones. We further show that this anti-correlation is as predicted by self-enrichment models.

Is the Variable X-ray Source in M82 due to Gravitational Lensing?

We explore the possibility of attributing the recent discovery of the variable hard X-ray source CXO M82 J095550.2+694047 in M82 to the gravitational magnification by an intervening stellar object along the line of sight acting as a microlens. The duration of the event (> 84 days) allows us to set robust constraints on the mass and location of the microlensing object when combined with the dynamical properties of the Galactic halo, M82 and typical globular clusters. Except for the extremely low probability, the microlensing magnification by MACHO in either the Galactic halo or M82 halo is able to explain the X-ray variability of CXO M82 J095550.2+694047. It is hoped that the lensing hypothesis can be tested soon by measurement of the light curve.