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

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. 1 MSCs as Young Counterparts of Old MRGCs Are merger of gas–rich spirals and multiphase collapse the only contributors to the formation of old MRGC populations? I argue that MSCs (compact populous and super–star clusters with M≥ 104M⊙) in the LMC and other irregulars are counterparts of the old MRGCs and that another reason (quantitative and qualitative changes of the dust?) leads to (favors) their formation. Peak metallicities of MRGCs in early–type galaxies with stellar masses differing by nearly 2.5 order of magnitude are estimated by [1] to fall between −0.7 ≤[Fe/H]≤ −0.2. The MRGC populations are assumed to be coeval, and their color trend is fully attributed to their metallicity trend. However, this is not supported by data on timing of spheroids’ formation: the more massive spheroid, the shorter timescale of its formation ([2], [3], [4]). Real scatter of the MRGC peak metallicities around mean, [Fe/H]∼ −0.5, may be at least twice as lower, by accepting conservative estimate of possible systematic age difference of ∼5 Gyr between MRGCs in spheroids of the range of mass. According to [5], a mean metallicity of the populous star clusters formed in the LMC 1–3 Gyr ago is close to [Fe/H ]= -0.5, irrespective of their age and location in the galaxy. However, metallicity of the field stars near these clusters exhibits obvious dependence on age (see Fig. 1, where squares and asterisks are data from [6] and [7], respectively). Moreover, the MDF for the