Massive Star Cluster Populations in Irregulars as Probable Younger Counterparts of Old Metal-Rich Globular Cluster Populations in Spheroids

Abstract

Peak metallicities of metal-rich(MR) populations of GCs belonging to spheroids of different mass fall within the somewhat conservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible age effects are taken into account,this metallicity range might become smaller. Irregulars, like the LMC, with longer timescales of their formation and lower star formation (SF) efficiency do not contain the old MRGCs with [Fe/H]>-1.0,but they are observed to form populations of young/ intermediate-age massive star clusters (MSCs) with masses exceeding 10^4 Msol. Their formation is widely believed to be accidental process fully depending on external factors. From analysis of data available on the populations and their hosts, including populous star clusters in the LMC, we find that their most probable mean metallicities fall within -0.7<=[Fe/H]<=-0.3, as the peak metallicities of MRGCs do, irrespective of sings of interaction. Moreover, both the disk giant metallicity distribution function (MDF) in the LMC and the MDFs for old giants in the halos of massive spheroids exhibit significant increasing toward [Fe/H]~-0.5. That is in agreement with a correlation found between SF activity in galaxies and their metallicity. The formation of both the old MRGCs in spheroids and MSC populations in irregulars probably occurs approximately at the same stage of the host galaxies' chemical evolution and is related to the essentially increased SF activity in the hosts around the same metallicity that is achieved very soon in massive spheroids, later in lower-mass spheroids, and much more later in irregulars. (Abridged)