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

摘要

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.