旧恒星星族合成：Mayall II = G1的新年龄和质量估计
Old stellar population synthesis: New age and mass estimates for Mayall II = G1
摘要：
Mayall II = G1是M31中最明亮的球状星团（GC）之一。在这里，我们确定它的年龄和质量的比较与理论恒星星族合成模型的测光。基于远紫外和近紫外GALEX测光、宽波段UBVRI和红外JHK_s 2MASS数据，我们构建了迄今为止G1最广泛的光谱能量分布，其波长范围从1538到20，000 A。与各种简单的恒星星族（SSP）模型的定量比较得出的平均年龄与G1是M31最古老的组成部分之一，并在大爆炸后约1.7 Gyr内形成。无论采用何种SSP模型或恒星初始质量函数，得到的质量估计（$10^7 M_\odot$阶）表明G1是本星系群中质量最大的星系团之一。然而，我们推测该星系团异常高的质量表明它可能不是真正的GC。我们还得出，G1可能包含，平均而言，$(1.65\pm0.63)\times10^2 L_\odot$远紫外亮，热，极端的水平分支恒星，这取决于所采用的SSP模型。在一个通用的水平上，我们表明，广泛的多通带光度加上SSP分析，使之能够获得年龄估计老SSP从集成光谱或解决恒星光度的类似精度，只要一些自由参数可以独立约束。
英文摘要：
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.
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