Metal-rich Globular Clusters: An Unaccounted Factor Responsible for Their Formation?

A presently unaccounted but quite probable “chemical factor” may be responsible for the formation of old metal-rich globular clusters (MRGCs) in spheroids, as well as of their conterparts, young (intermediate–age) massive star clusters (MSCs) in irregulars. Their formation presumably occurs ~ 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, ~Z⊙/3 ([Fe/H]~-0.5). It is achieved very soon in massive spheroids, later in lower-mass spheroids, and (much) more later in irregulars.