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

TL;DR

This study uses extensive multi-band photometry and stellar population models to estimate the age and mass of G1, revealing it as an ancient, massive cluster possibly not a genuine globular cluster, with implications for stellar population analysis.

Contribution

It provides a comprehensive spectral energy distribution of G1 and demonstrates the effectiveness of multi-passband photometry combined with SSP models for age and mass estimation of old stellar populations.

Findings

G1 is among the oldest building blocks of M31, formed within ~1.7 Gyr after the Big Bang.

Mass estimates of G1 are around 10^7 solar masses, making it one of the most massive GCs in the Local Group.

G1 may contain a significant number of UV-bright, hot, extreme horizontal-branch stars.

Abstract

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 $10^7 M_\odot$) 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, $(1.65\pm0.63)\times10^2 L_\odot$ 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.