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 with
theoretical stellar population synthesis models. Based on far- and
near-ultraviolet GALEX photometry, broad-band UBVRI, and infrared JHK_s 2MASS
data, we construct the most extensive spectral energy distribution of G1 to
date, spanning the wavelength range from 1538 to 20,000 A. A quantitative
comparison with a variety of simple stellar population (SSP) models yields a
mean age that is consistent with G1 being among the oldest building blocks of
M31 and having formed within ~1.7 Gyr after the Big Bang. Irrespective of the
SSP model or stellar initial mass function adopted, the resulting mass
estimates (of order 107Mββ) indicate that G1 is one of the most massive
GCs in the Local Group. However, we speculate that the cluster's exceptionally
high mass suggests that it may not be a genuine GC. We also derive that G1 may
contain, on average, (1.65Β±0.63)Γ102Lββ far-ultraviolet-bright,
hot, extreme horizontal-branch stars, depending on the SSP model adopted. On a
generic level, we demonstrate that extensive multi-passband photometry coupled
with SSP analysis enables one to obtain age estimates for old SSPs to a similar
accuracy as from integrated spectroscopy or resolved stellar photometry,
provided that some of the free parameters can be constrained independently.Comment: Accepted for Publication in RAA, 12 pages, 1 figure, 2 table