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Mayall II = G1 is one of the most luminous globular clusters (GCs) in M31.Here, we determine its age and mass by comparing multicolor photometry withtheoretical stellar population synthesis models. Based on far- andnear-ultraviolet GALEX photometry, broad-band UBVRI, and infrared JHK_s 2MASSdata, we construct the most extensive spectral energy distribution of G1 todate, spanning the wavelength range from 1538 to 20,000 A. A quantitativecomparison with a variety of simple stellar population (SSP) models yields amean age that is consistent with G1 being among the oldest building blocks ofM31 and having formed within ~1.7 Gyr after the Big Bang. Irrespective of theSSP model or stellar initial mass function adopted, the resulting massestimates (of order $10^7 M_\odot$) indicate that G1 is one of the most massiveGCs in the Local Group. However, we speculate that the cluster's exceptionallyhigh mass suggests that it may not be a genuine GC. We also derive that G1 maycontain, on average, $(1.65\pm0.63)\times10^2 L_\odot$ far-ultraviolet-bright,hot, extreme horizontal-branch stars, depending on the SSP model adopted. On ageneric level, we demonstrate that extensive multi-passband photometry coupledwith SSP analysis enables one to obtain age estimates for old SSPs to a similaraccuracy as from integrated spectroscopy or resolved stellar photometry,provided that some of the free parameters can be constrained independently.

Old stellar population synthesis: New age and mass estimates for Mayall II = G1