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

Jun Ma*, Richard de Grijs, Zhou Fan, Soo-Chang Rey, Zhen-Yu Wu, Xu Zhou, Jiang-Hua Wu, Zhao-Ji Jiang, Jian-Sheng Chen, Kyungsook Lee, Sangmo Tony Sohn

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

17 Citations (Scopus)


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 U B V R I, and infrared J H K 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 quantitative comparison with a variety of simple stellar population (SSP) models yields a mean age which 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 107 M) 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. Our results also suggest that G1 may contain, on average, (1.650.63) × 102 L far-ultraviolet-bright, hot, extreme horizontal-branch stars, depending on the adopted SSP model. In addition, we demonstrate that extensive multi-passband photometry coupled with SSP analysis enables one to obtain age estimates for old SSPs that have similar accuracies as those from integrated spectroscopy or resolved stellar photometry, provided that some of the free parameters can be constrained independently.

Original languageEnglish
Pages (from-to)641-652
Number of pages12
JournalResearch in Astronomy and Astrophysics
Issue number6
Publication statusPublished - 2009
Externally publishedYes


  • galaxies: individual (M31)
  • galaxies: star clusters
  • galaxies: stellar content


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