Formation Of Metal-poor Globular Clusters Research Articles

The Formation of Low-metallicity Globular Clusters in Dwarf Galaxy Mergers

We present a hydro-dynamical simulation at sub-parsec and few solar mass resolution of a merger between two gas-rich dwarf galaxies. Our simulation includes a detailed model for the multi-phase interstellar medium (ISM) and is able to follow the entire formation history of spatially resolved star clusters, including feedback from individual massive stars. Shortly after the merger we find a population of $\sim 900$ stellar clusters with masses above $10^{2.5}\; \rm{M_\odot}$ and a cluster mass function (CMF), which is well fitted with a power-law with a slope of $\alpha=-1.70\pm0.08$. We describe here in detail the formation of the three most massive clusters (M$_{*} \gtrsim 10^5$ M$_\odot$), which populate the high-mass end of the CMF. The simulated clusters form rapidly on a timescale of $6$-$8$ Myr in converging flows of dense gas. The embedded merger phase has extremely high star formation rate surface densities of $\Sigma_\mathrm{SFR}>10\; \mathrm{M}_\odot\; \mathrm{yr}^{-1}\; \mathrm{kpc}^{-2}$ and thermal gas pressures in excess of $P_{\rm th}\sim10^7 \; \mathrm{k}_{\rm B}\;(\rm K\;\mathrm{cm}^{-3})^{-1}$. The formation process is terminated by rapid gas expulsion driven by the first generation of supernovae, after which the cluster centers relax and both their structure and kinematics become indistinguishable from observed local globular clusters. The simulation presented here provides a general model for the formation of metal-poor globular clusters in chemically unevolved starbursting environments of low-mass dwarf galaxies, which are common at high redshifts.

Cold Filamentary Accretion and the Formation of Metal-poor Globular Clusters and Halo Stars

Abstract We propose that cold filamentary accretion in massive galaxies at high redshifts can lead to the formation of star-forming clumps in the halos of these galaxies without dark matter substructure. In certain cases, these clumps can be the birthplaces of metal-poor globular clusters (MP GCs). Using cosmological simulations, we show that narrow streams of dense gas feeding massive galaxies from the cosmic web can fragment, producing star-forming clumps. We then derive an analytical model for the properties of streams as a function of halo mass and redshift, and assess when these are gravitationally unstable, when this can lead to collapse and star formation in the halo, and when it may result in the formation of MP GCs. For stream metallicities ≳ 0.01 Z ⊙ , this is likely to occur at z > 4.5 . At z ∼ 6 , the collapsing clouds have masses of ∼ ( 5 – 10 ) × 10 7 M ⊙ , and the average stream pressure is ∼ 10 6 cm − 3 K . The conditions for GC formation are met in the extremely turbulent “eyewall” at ∼ 0.3 R v , where counter-rotating streams can collide, driving very large densities. Our scenario can account for the observed kinematics and spatial distribution of MP GCs, the correlation between their mass and metallicity, and the mass ratio between the GC system and the host halo. For MW-mass halos, we infer that ∼ 30 % of MP GCs could have formed in this way, with the remainder likely accreted in mergers. Our predictions for GC formation along circumgalactic filaments at high redshift are testable with JWST.

FORMATION OF GLOBULAR CLUSTERS IN ATOMIC-COOLING HALOS VIA RAPID GAS CONDENSATION AND FRAGMENTATION DURING THE EPOCH OF REIONIZATION

ABSTRACT We investigate the formation of metal-poor globular clusters (GCs) at the center of two dark matter halos with at using cosmological radiation-hydrodynamics simulations. We find that very compact (≲1 pc) and massive ( ) clusters form rapidly when pristine gas collapses isothermally with the aid of efficient Lyα emission during the transition from molecular-cooling halos to atomic-cooling halos. Because the local free-fall time of dense star-forming gas is very short ( ), a large fraction of the collapsed gas is turned into stars before stellar feedback processes blow out the gas and shut down star formation. Although the early stage of star formation is limited to a small region of the central star-forming disk, we find that the disk quickly fragments due to metal enrichment from supernovae. Sub-clusters formed in the fragmented clouds eventually merge with the main cluster at the center. The simulated clusters closely resemble the local GCs in mass and size but show a metallicity spread that is much wider than found in the local GCs. We discuss a role of pre-enrichment by Pop III and II stars as a potential solution to the latter issue. Although not without shortcomings, it is encouraging that a naive blind (not tuned) cosmological simulation presents a possible channel for the formation of at least some massive GCs.

Globular cluster formation in the Virgo cluster

Metal poor globular clusters (MPGCs) are a unique probe of the early universe, in particular the reionization era. Systems of globular clusters in galaxy clusters are particularly interesting as it is in the progenitors of galaxy clusters that the earliest reionizing sources first formed. Although the exact physical origin of globular clusters is still debated, it is generally admitted that globular clusters form in early, rare dark matter peaks (Moore et al. 2006; Boley et al. 2009). We provide a fully numerical analysis of the Virgo cluster globular cluster system by identifying the present day globular cluster system with exactly such early, rare dark matter peaks. A popular hypothesis is that that the observed truncation of blue metal poor globular cluster formation is due to reionization (Spitler et al. 2012; Boley et al. 2009; Brodie & Strader 2006); adopting this view, constraining the formation epoch of MPGCs provides a complementary constraint on the epoch of reionization. By analyzing both the line of sight velocity dispersion and the surface density distribution of the present day distribution we are able to constrain the redshift and mass of the dark matter peaks. We find and quantify a dependence on the chosen line of sight of these quantities, whose strength varies with redshift, and coupled with star formation efficiency arguments find a best fitting formation mass and redshift of $\simeq 5 \times 10^8 \rm{M}_\odot$ and $z\simeq 9$. We predict $\simeq 300$ intracluster MPGCs in the Virgo cluster. Our results confirm the techniques pioneered by Moore et al. (2006) when applied to the the Virgo cluster and extend and refine the analytic results of Spitler et al. (2012) numerically.

Metal-Poor Globular Clusters and Galaxy Formation

We demonstrate a significant (>5 σ) correlation between the mean color of metal-poor globular cluster (GC) systems and parent galaxy luminosity. A Bayesian Markov Chain Monte Carlo method is introduced to find the mean color and is easily generalizable to quantify multimodality in other astronomical data sets. We derive a GC color-galaxy luminosity relation of the form Z ∝ L0.15±0.03. When combined with evidence against a single primordial GC metallicity-galaxy luminosity relation for protogalactic fragments, the existence of such a correlation is evidence against both accretion and major merger scenarios as an explanation of the entire metal-poor GC systems of luminous galaxies. However, our relation arises naturally in an in situ picture of GC formation and is consistent with the truncation of metal-poor GC formation by reionization. A further implication is that the ages of metal-poor GCs in dwarf galaxies constrain the main epoch of galaxy formation in hierarchical models. If the ages of old metal-poor GCs in Local Group dwarfs (11 Gyr) are typical of those in dwarfs elsewhere, then the bulk of galaxy assembly (at least in clusters and groups) must have occurred at z 2.5, contrary to the predictions of some structure formation models.

Globular Cluster Systems. II. On the Formation of Old Globular Clusters and Their Sites of Formation

We studied the metal-poor globular cluster populations of a large variety of galaxies and compared their mean metallicity with the properties of the host galaxies. For this purpose, we constructed a comprehensive database of old metal-poor globular cluster populations, hosted by 47 galaxies, spanning about 10 mag in absolute brightness. The mean metallicities of the systems are found to be very similar and lie in the -1.65 < [Fe/H] ≤ -1.20 range (74% of the population). Using only globular cluster systems with more than six objects detected, we find that 85% of the population are within -1.65 < [Fe/H] ≤ -1.20. The relation between the mean metallicity of the metal-poor globular cluster systems and the absolute V magnitude of their host galaxies presents a very low slope that includes zero. An analysis of the correlation of the mean metallicity of the populations with other galaxy properties (such as velocity dispersion, metallicity, and environment density) also leads to the conclusion that no strong correlation exists. The lack of correlation with galaxy properties suggests a formation of all metal-poor globular clusters in very similar gas fragments. A weak correlation (to be confirmed) might exist between the mean metallicity of the metal-poor clusters and the host galaxy metallicity. This would imply that at least some fragments in which metal-poor globular clusters formed were already embedded in the larger dark matter halo of the final galaxy (as opposed to being independent satellites that were accreted later). Our result suggests a homogeneous formation of metal-poor globular clusters in all galaxies in typical fragments of masses around 109–1010 M⊙ with very similar metallicities, compatible with hierarchical formation scenarios for galaxies. We further compare the mean metallicities of the metal-poor globular cluster populations with the typical metallicities of high-redshift objects. If we add the constraint that globular clusters need a high column density of gas to form, damped Lyα systems are the most likely sites among the known high-redshift objects for the formation of metal-poor globular cluster populations.