利用新的Gemini/NIRI K '波段成像和现有的光学V，I测光资料，研究了候选中龄早型星系NGC 3610，NGC 584和NGC 3377中亮球状星团（GC）的年龄和金属丰度分布。发现V-I vs I-K'色-色图打破了光学颜色中存在的年龄-金属丰度简并，因为I-K'主要测量种群的金属丰度，并且与光谱学不同，对已知存在于大质量古老GC中的热水平分支（HB）星的影响相对不敏感。我们推导出GC的光度年龄、Z和质量。一般来说，贫金属（[Z/H]<-0.7dex）GC比富金属GC更老。对于NGC 3610中质量最大的GC（M>6x10^5 M_sol），根据现有的光谱数据，光度年龄要老约2 Gyr，并且这种差异对于贫金属GC更为明显。然而，光度和光谱金属丰度是在良好的协议。我们认为，这表明存在一个热HB在这些大规模的集群，这使得光谱年龄从巴耳末线强度被低估。为了支持这一观点，我们指出，除了47 Tuc外，所有M>6x10^5 M_sol的银河系GC都有热的HB。利用最大质量的GC质量和星系的SFR之间的关系，我们发现星系的峰值SFR是在最古老的（贫金属）GC形成的时期达到的。富金属GC的年龄和[Z/H]分布很宽，表明星系恒星形成历史较长。GC年龄和[Z/H]分布的峰值与宿主星系积分年龄和[Z/H]相关，表明GC确实可以作为星系恒星形成历史的相关代理。（节录）
英文摘要：
We investigate age and metallicity distributions of bright globular clusters (GCs) in the candidate intermediate-age early-type galaxies NGC 3610, NGC 584 and NGC 3377 using a combination of new Gemini/NIRI K'-band imaging and existing optical V,I photometry from HST data. The V-I vs I-K' colour-colour diagram is found to break the age-metallicity degeneracy present in optical colours, as I-K' primarily measures a populations' metallicity and is relatively insensitive, unlike optical spectroscopy, to the effect of hot horizontal branch (HB) stars, known to be present in massive old GCs. We derive GCs' photometric age, Z and masses. In general, metal-poor ([Z/H]<-0.7dex) GCs are older than more metal-rich GCs. For the most massive GCs (M>6x10^5 M_sol) in NGC 3610 with available spectroscopic data, photometric ages are older by ~2 Gyr, and this difference is more pronounced for the metal-poor GCs. However, photometric and spectroscopic metallicities are in good agreement. We suggest that this indicates the presence of a hot HB in these massive clusters, which renders spectroscopic ages from Balmer line strengths to be underestimated. To support this suggestion we show that all Galactic GCs with M>6x10^5 M_sol feature hot HBs, except 47 Tuc. Using the relation between the most massive GC mass and the galaxy's SFR, we find that the galaxies' peak SFR was attained at the epoch of the formation of the oldest (metal-poor) GCs. Age and [Z/H] distributions of the metal-rich GCs are broad, indicating prolonged galaxy star formation histories. The peak value of the age and [Z/H] distributions of the GCs correlates with host galaxy integrated age and [Z/H], showing that GCs can indeed be used as relevant proxies of the star formation histories of galaxies.(Abridged)
Impact of the low solar abundance on the ages of globular clusters
本文介绍了Asplund及其合作者（2004）关于低太阳丰度对基于等时线拟合的最古老星团年龄的影响的研究结果。我们已经建立了新的恒星模型和相应的等时线使用这种新的太阳混合适当的太阳校准。我们发现使用Asplund et al.（2004）的金属丰度会导致银河系中老球状星团的典型年龄大约增加10%。虽然这可能看起来很小，但它对解释银河系球状星团的形成时代有重要影响。Asplund等人（2004）的丰度可能不一定会威胁到目前的和谐宇宙学，但会表明银河系球状星团在再电离之前和主星系体开始形成之前形成。这与目前对星系形成的理解形成对比。
The Age of the Universe with Globular Clusters IV: Multiple Stellar Populations
Berkeley Center for Cosmological Physics and Department of Physics, University of California, Berkeley（加州大学伯克利分校）；ICC, University of Barcelona（巴塞罗那大学）；ICREA（加泰罗尼亚高级研究学院）；Dipartimento di Fisica e Astronomia “Augusto Righi Università di Bologna（博洛尼亚大学）；INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna（意大利国家天体物理研究所波洛尼亚天文与空间科学观测站）；Lawrence Berkeley National Lab（劳伦斯伯克利国家实验室）
