New insights into the star formation histories of candidate intermediate-age early-type galaxies from K-band imaging of globular clusters
New insights into the star formation histories of candidate intermediate-age early-type galaxies from K-band imaging of globular clusters
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DOI：
10.1111/j.1365-2966.2011.20115.x
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发表时间：
2011-11
影响因子：
4.8
通讯作者：
I. Georgiev;P. Goudfrooij;Thomas Bonn;H Germany;Stsci;Baltimore.;Usa;P. U. C. D. Chile;Santiago;Chile.
中科院分区：
文献类型：
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作者：
I. Georgiev;P. Goudfrooij;Thomas Bonn;H Germany;Stsci;Baltimore.;Usa;P. U. C. D. Chile;Santiago;Chile.
我们利用新的双子座近红外成像仪和光谱仪K�波段成像和现有的哈勃太空望远镜光学VI光度测量相结合的方法，研究了候选中龄早型星系NGC3610、584和3377中明亮球状星团(GC)的年龄和金属丰度分布。发现V−I-I−K色彩图打破了光学颜色和光谱中存在的年龄-金属丰度简并性，因为I−K主要测量布居的金属丰度。此外，它对已知存在于大质量老GC中的热水平分支(HB)星的影响相对不敏感。通过简单恒星布居模型轨道间的内插，我们得到了光度学星团的年龄、金属度和质量。总体而言，我们发现具有(Z/H)−0.7dex的贫金属GC比富金属GC更古老。对于有光谱数据的NGC3610中质量最大的GC(M≥6×105M�)，其光度年龄比∼2 Gyr老，并且这种差异对贫金属的GC更加明显。然而，光度金属活性和光谱金属活性是一致的。我们认为，这表明在这些大质量星系团中存在热的HB，这使得从巴尔默谱线强度计算的光谱年龄被低估。为了支持这一观点，我们发现，除47个TUC外，所有M≥为6×105M的银河系GC都具有热HBs。利用最大质量GC的光度与星系在给定年龄的恒星形成率(SFR)之间的最新观测关系，我们发现星系的峰值SFR是在最古老的(贫金属元素)GC形成时期获得的。富含金属的GC的年龄和(Z/H)分布广泛，表明银河系恒星形成历史较长。样本星系NGC3610、584和3377中富金属GC的峰值年龄分别为3.7、5.9和8.9 Gyr。GCs的年龄和金属丰度分布的峰值分别与宿主星系的光度加权年龄和金属丰度相关，表明GCs确实可以作为星系恒星形成历史的相关指标。
We investigate the age and metallicity distributions of bright globular clusters (GCs) in the candidate intermediate-age early-type galaxies NGC 3610, 584 and 3377 using a combination of new Gemini's Near InfraRed Imager and Spectrometer K � -band imaging and existing optical VI photometry from Hubble Space Telescope data. The V − I versus I − Kcolour- colour diagram is found to break the age-metallicity degeneracy present in optical colours and spectroscopy, as I − Kprimarily measures a population's metallicity. In addition, it is relatively insensitive to the effect of hot horizontal branch (HB) stars that are known to be present in massive old GCs. By interpolation between simple stellar population model tracks, we derive photometric cluster ages, metallicities and masses. In general, we find that 'metal- poor' GCs with (Z/H) −0.7 dex are older than more metal rich GCs. For the most massive GCs (M ≥ 6×10 5 M� ) in NGC 3610 with available spectroscopic data, the photometric ages are older by ∼2 Gyr, and this difference is more pronounced for the metal-poor GCs. However, the 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 ≥ 6 × 10 5 Mfeature hot HBs, except 47 Tuc. Using a recent observational relation between the luminosity of the most massive GC and the galaxies' star formation rate (SFR) at a given age, we find that the galaxies' peak SFR was attained at the epoch of the formation of the oldest (metal-poor) GCs. The age and (Z/H) distributions of the metal-rich GCs are broad, indicating prolonged galaxy star formation histories. The peak ages of the metal-rich GCs in the sample galaxies NGC 3610, 584 and 3377 are 3.7, 5.9 and 8.9 Gyr, respectively. The peak value of the age and metallicity distributions of the GCs is correlated with the host galaxies' luminosity-weighted age and metallicity, respectively, indicating that the GCs can indeed be used as relevant proxies of the star formation histories of galaxies.