Globular Cluster Formation Efficiency Research Articles

Clues on the missing sources of reionization from self-consistent modelling of Milky Way and dwarf galaxy globular clusters

Globular clusters are unique tracers of ancient star formation. We determine the formation efficiencies of globular clusters across cosmic time by modelling the formation and dynamical evolution of the globular cluster population of a Milky Way type galaxy in hierarchical cosmology, using the merger tree from the Via Lactea II simulation. All of the models are constrained to reproduce the observed specific frequency and initial mass function of globular clusters in isolated dwarfs. Globular cluster orbits are then computed in a time varying gravitational potential after they are either accreted from a satellite halo or formed in situ, within the Milky Way halo. We find that the Galactocentric distances and metallicity distribution of globular clusters are very sensitive to the formation efficiencies of globular clusters as a function of redshift and halo mass. Our most accurate models reveal two distinct peaks in the globular cluster formation efficiency at z ∼ 2 and 7–12 and prefer a formation efficiency that is mildly increasing with decreasing halo mass, the opposite of what expected for feedback-regulated star formation. This model accurately reproduces the positions, velocities, mass function, metallicity distribution, and age distribution of globular clusters in the Milky Way and predicts that ∼40 per cent formed in situ, within the Milky Way halo, while the other ∼60 per cent were accreted from about 20 satellite dwarf galaxies with vcir > 30 km s−1, and about 29 per cent of all globular clusters formed at redshifts z > 7. These results further strengthen the notion that globular cluster formation was an important mode of star formation in high-redshift galaxies and likely played a significant role in the reionization of the intergalactic medium.

Globular clusters: DNA of early-type galaxies?

This paper explores if the mean properties of Early-Type Galaxies (ETG) can be reconstructed from "genetic" information stored in their GCs (i.e., in their chemical abundances, spatial distributions and ages). This approach implies that the formation of each globular occurs in very massive stellar environments, as suggested by some models that aim at explaining the presence of multi-populations in these systems. The assumption that the relative number of globular clusters to diffuse stellar mass depends exponentially on chemical abundance, [Z/H], and the presence of two dominant GC sub-populations blue and red, allows the mapping of low metallicity halos and of higher metallicity (and more heterogeneous) bulges. In particular, the masses of the low-metallicity halos seem to scale up with dark matter mass through a constant. We also find a dependence of the globular cluster formation efficiency with the mean projected stellar mass density of the galaxies within their effective radii. The analysis is based on a selected sub-sample of galaxies observed within the ACS Virgo Cluster Survey of the {\it Hubble Space Telescope}. These systems were grouped, according to their absolute magnitudes, in order to define composite fiducial galaxies and look for a quantitative connection with their (also composite) globular clusters systems. The results strengthen the idea that globular clusters are good quantitative tracers of both baryonic and dark matter in ETGs.

THE RICH GLOBULAR CLUSTER SYSTEM OF ABELL 1689 AND THE RADIAL DEPENDENCE OF THE GLOBULAR CLUSTER FORMATION EFFICIENCY

We study the rich globular cluster (GC) system in the center of the massive cluster of galaxies Abell 1689 (z=0.18), one of the most powerful gravitational lenses known. With 28 HST/ACS orbits in the F814W bandpass, we reach magnitude I_814=29 with >90% completeness and sample the brightest ~5% of the GC system. Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we estimate a total population of N(GC_total) = 162,850 GCs within a projected radius of 400kpc. As many as half may comprise an intracluster component. Even with the sizable uncertainties, which mainly result from the uncertain GCLF parameters, this is by far the largest GC system studied to date. The specific frequency S_N is high, but not uncommon for central galaxies in massive clusters, rising from S_N~5 near the center to ~12 at large radii. Passive galaxy fading would increase S_N by ~20% at z=0. We construct the radial mass profiles of the GCs, stars, intracluster gas, and lensing-derived total mass, and we compare the mass fractions as a function of radius. The estimated mass in GCs, M(GC_total)=3.9x10^10 Msun, is comparable to ~80% of the total stellar mass of the Milky Way. The shape of the GC mass profile appears intermediate between those of the stellar light and total cluster mass. Despite the extreme nature of this system, the ratios of the GC mass to the baryonic and total masses, and thus the GC formation efficiency, are typical of those in other rich clusters when comparing at the same physical radii. The GC formation efficiency is not constant, but varies with radius, in a manner that appears similar for different clusters; we speculate on the reasons for this similarity in profile.

The specific frequency and the globular cluster formation efficiency in Milgromian dynamics

Previous studies of globular cluster (GC) systems show that there appears to be a universal specific GC formation efficiency $\eta$ which relates the total mass of GCs to the virial mass of host dark matter halos, $M_{vir}$ (Georgiev et al 2010, Spitler & Forbes2009). In this paper, the specific frequency, $S_N$, and specific GC formation efficiency, $\eta$, are derived as functions of $M_{vir}$ in Milgromian dynamics, i.e., in modified Newtonian dynamics (MOND). In Milgromian dynamics, for the galaxies with GCs, the mass of the GC system, $M_{GC}$, is a two-component function of $M_{vir}$ instead of a simple linear relation. An observer in a Milgromian universe, who interprets this universe as being Newtonian/Einsteinian, will incorrectly infer a universal constant fraction between the mass of the GC system and a (false) dark matter halo of the baryonic galaxy. In contrast to a universal constant of $\eta$, in a Milgromian universe, for galaxies with $M_{vir} <= 10^{12}\msun$, $\eta$ decreases with the increase of $M_{vir}$, while for massive galaxies with $M_{vir}>10^{12}\msun$, $\eta$ increases with the increase of $M_{vir}$.

A CATALOG OF GLOBULAR CLUSTER SYSTEMS: WHAT DETERMINES THE SIZE OF A GALAXY'S GLOBULAR CLUSTER POPULATION?

We present a catalog of 422 galaxies with published measurements of their globular cluster (GC) populations. Of these, 248 are E galaxies, 93 are S0 galaxies, and 81 are spirals or irregulars. Among various correlations of the total number of GCs with other global galaxy properties, we find that N_GC correlates well though nonlinearly with the dynamical mass of the galaxy bulge M_dyn = 4 \sigma_e^2 R_e /G, where \sigma_e is the central velocity dispersion and R_e the effective radius of the galaxy light profile. We also present updated versions of the GC specific frequency S_N and specific mass S_M versus host galaxy luminosity and baryonic mass. These graphs exhibit the previously known U-shape: highest S_N or S_M values occur for either dwarfs or supergiants, but in the midrange of galaxy size (10^9 - 10^10 L_Sun) the GC numbers fall along a well defined baseline value of S_N ~ 1 or S_M ~ 0.1, similar among all galaxy types. Along with other recent discussions, we suggest that this trend may represent the effects of feedback, which systematically inhibited early star formation at either very low or very high galaxy mass, but which had its minimum effect for intermediate masses. Our results strongly reinforce recent proposals that GC formation efficiency appears to be most nearly proportional to the galaxy halo mass M_halo. The mean "absolute" efficiency ratio for GC formation that we derive from the catalog data is M_GCS/M_halo = 6 \times 10^-5. We suggest that the galaxy-to-galaxy scatter around this mean value may arise in part because of differences in the relative timing of GC formation versus field-star formation. Finally, we find that an excellent empirical predictor of total GC population for galaxies of all luminosities is N_GC \sim (R_e \sigma_e)^1.3$, a result consistent with Fundamental Plane scaling relations.

Formation Efficiencies of Old Globular Clusters – form Dwarf to Giant Galaxies

For the full galaxy mass range, we find that previously observed trends of globular cluster (GC) system scaling parameters (number, luminosity or mass of all GCs in a galaxy normalized to the host galaxy luminosity or mass, e.g. SL) as a function of galaxy mass, holds irrespective of galaxy type or environment. The SL value of early-type galaxies is, on average, twice that of late-types. We derive theoretical predictions which describe remarkably well the observed GC system scaling parameter distributions given an assumed GC formation efficiency (η), i.e. the ratio of total mass in GCs to galaxy halo mass. It has a mean value of η ≃ 5.5 × 10−5, and increasing scatter toward low galaxy mass. The excess η-values of some massive galaxies compared to expectations from the mean model prediction, may be attributed to an efficient GC formation, inefficient production of field stars, accretion of low-mass high-η galaxies or likely a mixture of all these effects.

Globular cluster systems in nearby dwarf galaxies - III. Formation efficiencies of old globular clusters★

We investigate the globular cluster (GC) system scaling parameters as a function of galaxy mass, i.e. specific frequency (S_N), specific luminosity (S_L), specific mass (S_M), and specific number (^T) of GCs. We sample the entire range in galaxy luminosity (Mv = -11 to -23 mag = 10^6 - 10^11 L_sol), environment, and morphology. Irrespective of galaxy type, we confirm the increase of the S_N-value above and below a galaxy magnitude of Mv = -20 mag. Over the full mass range, the S_L-value of early-type galaxies is, on average, twice that of late-types. To investigate the observed trends we derive theoretical predictions of GC system scaling parameters as a function of host galaxy mass based on the models of Dekel & Birnboim (2006) in which star-formation processes are regulated by stellar and supernova feedback below a stellar mass of 3x10^10 M_sol, and by virial shocks above it. We find that the analytical model describes remarkably well the shape of the GC system scaling parameter distributions with a universal specific GC formation efficiency, eta, which relates the total mass in GCs to the total galaxy halo mass. Early-type and late-type galaxies show a similar mean value of eta = 5.5e-5, with an increasing scatter towards lower galaxy masses. This can be due to the enhanced stochastic nature of the star and star-cluster formation processes for such systems. Some massive galaxies have excess eta values compared to what is expected from the mean model prediction for galaxies more luminous than Mv = -20 mag (Lv=10^10L_sol). This may be attributed to a very efficient early GC formation, less efficient production of field stars or accretion of predominantly low-mass/luminosity high-eta galaxies, or a mixture of all these effects. (Abridged)

The ACS Virgo Cluster Survey. XV. The Formation Efficiencies of Globular Clusters in Early‐Type Galaxies: The Effects of Mass and Environment

The fraction of stellar mass contained in globular clusters (GCs), also measured by number as the specific frequency, is a fundamental quantity that reflects both a galaxy's early star formation and its entire merging history. We present specific frequencies, luminosities, and mass fractions for the globular cluster systems of 100 early-type galaxies in the ACS Virgo Cluster Survey, the largest homogeneous catalog of its kind. We find the following: (1) GC mass fractions can be high in both giants and dwarfs but are universally low in galaxies with intermediate luminosities. (2) The behavior of specific frequency across galaxy mass is dominated by the blue GCs. (3) GC fractions of low-mass galaxies exhibit a dependence on environment. Nearly all dwarf galaxies with high GC fractions are within 1 Mpc of the cD galaxy M87, presenting the first strong evidence that GC formation in dwarfs is biased toward dense environments. (4) GC formation in central dwarfs is biased because their stars form earliest and most intensely. Comparisons to the Millennium Simulation show that central dwarfs have older stellar populations and form more stars at higher star formation rates (SFRs) and SFR surface densities. The SFR surface density in simulated dwarfs peaks before the total SFR, naturally producing GC populations that are older and more metal-poor than the field stars. (5) Dwarfs within ~40 kpc of the giant ellipticals M87 and M49 are red and have few or no GCs, suggesting that they have been tidally stripped and have contributed their GCs to the halos of their giant neighbors. The central dwarfs with high GC mass fractions are thus likely to be the survivors most similar to the protogalaxies that assembled the rich M87 globular cluster system.

The connection between globular cluster systems and their host galaxy and environment: a case study of the isolated elliptical NGC 821★

In an effort to probe the globular cluster (GC) system of an isolated elliptical galaxy, a comprehensive analysis of the NGC 821 GC system was performed. New imaging from the WIYN Mini-Mosaic imager, supplemented with Hubble Space Telescope (HST) WFPC2 images reveals a GC system similar to those found in counterpart ellipticals located in high-density environments. To put these results into the context of galaxy formation, a robustly determined census of GC systems is presented and analysed for galaxies spanning a wide range of masses (> M*), morphologies and environments. Results from this meta-study: (1) confirm previous findings that the number of GCs normalized by host galaxy stellar mass increases with host stellar mass. Spiral galaxies in the sample show smaller relative GC numbers than those of massive ellipticals, suggesting the GC systems of massive ellipticals were not formed from major spiral–spiral mergers; (2) indicate that GC system numbers per unit galaxy baryon mass increases with host baryon mass and that GC formation efficiency may not be universal as previously thought; (3) suggest previously reported trends with environment may be incorrect due to sample bias or the use of galaxy stellar masses to normalize GC numbers. Thus claims for environmentally dependent GC formation efficiencies should be revisited; (4) in combination with weak-lensing halo mass estimates, suggest that GCs formed in direct proportion to the halo mass; (5) are consistent with theoretical predictions whereby the local epoch of reionization did not vary significantly with environment or host galaxy type.

The Halo Stars in NGC 5128. III. An Inner Halo Field and the Metallicity Distribution

We present new Hubble Space Telescope WFPC2 (V, I) photometry for field stars in NGC 5128 at a projected distance of 8 kpc from the galaxy center, which probes a mixture of its inner and outer bulge. The color-magnitude diagram shows an old red giant branch that is even broader in color than our two previously studied outer fields (at 21 and 31 kpc), with significant numbers of stars extending to solar metallicity and higher. The peak frequency of the metallicity distribution function (MDF) is at [m/H] -0.4, with even fewer metal-poor stars than in the outer fields. If we use the 21 and 31 kpc fields to define template halo MDFs and subtract these from the 8 kpc field, the residual bulge population has a mean [m/H] -0.2, similar to the bulges of other large spiral and elliptical galaxies. We find that the main features of the MDF can be reproduced by a simple chemical evolution model in which early star formation goes on simultaneously with an initial stage of rapid infall of very metal-poor gas, after which the infall dies away exponentially. Finally, by comparison with the MDFs for the NGC 5128 globular clusters, we find that in all the fields we have studied there is a clear decrease of specific frequency SN (number of clusters per unit light) with increasing metallicity. At the lowest-metallicity range ([Fe/H] -1 it has dropped to 1.5. This trend may indicate that globular cluster formation efficiency is a strong function of the metallicity of the protocluster gas. However, we suggest an alternate possibility, which is that globular clusters form preferentially sooner than field stars. If most of the cluster formation within a host giant molecular cloud takes place sooner than most of the distributed field-star formation and if the earliest most metal-poor star-forming clouds are prematurely disrupted by their own first bursts of star formation, then they would leave relatively few field stars with a high-SN population. The high specific frequency at low metallicity may therefore be related to the comparably large SN values found in the most metal-poor dwarf elliptical galaxies.

Near-Infrared Imaging of the Central Regions of Metal-Poor Inner Spheroid Globular Clusters

JHK images obtained with the Canada-France-Hawaii Telescope (CFHT) Adaptive Optics Bonnette (AOB) are used to investigate the near-infrared photometric properties of red giant branch (RGB) and horizontal branch (HB) stars in eight metal-poor globular clusters with R_GC < 2 kpc. The slope of the RGB on the (K, J-K) CMDs confirms the metal-poor nature of these clusters, four of which appear to have metallicities comparable to M92. The luminosity functions of RGB stars in inner spheroid and outer halo clusters have similar slopes, although there is a tendency for core-collapsed clusters to have slightly flatter luminosity functions than non-collapsed clusters. The distribution of red HB stars on the (K, J-K) CMDs of inner spheroid clusters with [Fe/H] ~ -1.5 is very different from that of clusters with [Fe/H] ~ -2.2, suggesting that metallicity is the main parameter defining HB content among these objects. The RGB-bump is detected in four of the inner spheroid clusters, and this feature is used to compute distances to these objects. Finally, the specific frequency of globular clusters in the inner Galaxy is discussed in the context of the early evolution of the bulge. If the globular cluster formation efficiency for the inner Galaxy is similar to that measured in other spheroidal systems, then the main body of the bulge could have formed from gas that was chemically enriched in situ.

The Efficiency of Globular Cluster Formation

(Abridged): The total populations of globular cluster systems (GCSs) are discussed in terms of their connection to the efficiency of globular cluster formation---the mass fraction of star-forming gas that was able to form bound stellar clusters rather than isolated stars or unbound associations---in galaxy halos. Observed variations in GCS specific frequencies (S_N=N_gc/L_gal), both as a function of galactocentric radius in individual systems and globally between entire galaxies, are reviewed in this light. It is argued that trends in S_N do not reflect any real variation in the underlying efficiency of cluster formation; rather, they result from ignoring the hot gas in many large ellipticals. This claim is checked and confirmed in each of M87, M49, and NGC 1399, for which existing data are combined to show that the volume density profile of globular clusters, rho_cl, is directly proportional to the sum of (rho_gas+rho_stars) at large radii. The constant of proportionality is the same in each case: epsilon=0.0026 +/- 0.0005 in the mean. This is identified with the globular cluster formation efficiency. The implication that epsilon might have had a universal value is supported by data on the GCSs of 97 early-type galaxies, on the GCS of the Milky Way, and on the ongoing formation of open clusters. These results have specific implications for some issues in GCS and galaxy formation, and they should serve as a strong constraint on more general theories of star and cluster formation.