扩展基线：半人马座A和草帽星系中球状星团系统的Spitzer中红外测光
Extending the baseline: Spitzer Mid-Infrared Photometry of Globular Cluster Systems in the Centaurus A and Sombrero Galaxies
摘要：
介绍了NGC 5128（“半人马座A”）和NGC 4594（“宽边帽”）星系的球状星团（GC）系统的Spitzer IRAC中红外测光。现有的光学光度和光谱与这一新的数据相结合，在一个全面的光学中红外颜色目录的260 GC。经验的颜色，金属丰度的关系推导出所有的光学中红外颜色组合。这些颜色被证明是非常有效的量来测试光度预测的简单恒星星族（SSP）模型。一般来说，四个SSP模型显示彼此之间的差异较大，在蓝波长的数据，特别是在高金属丰度。当试图使用颜色-颜色模型预测来限制恒星星族的年龄时，这种差异变得非常重要。此外，从颜色-颜色图和Beasley等人（2008）的光谱年龄的91个NGC 5128 GC确定的年龄-子结构是不一致的，这表明任何由颜色-颜色分析暗示的明显GC系统年龄-子结构必须独立验证。与蓝色波段不同，某些光学到中红外的颜色对来自热水平分支恒星的通量不敏感，因此提供了一个很好的金属丰度代理。NGC 5128 GC系统在光学R波段至中红外颜色分布中显示出强烈的双峰性，因此证明它在金属丰度方面是双峰的。在这个新的颜色空间中，在NGC 5128贫金属GC数据中发现了一种颜色星等趋势，即“蓝色倾斜”。NGC 5128年轻的GC对这一趋势没有贡献。[删节]
英文摘要：
Spitzer IRAC mid-infrared photometry is presented for the globular cluster (GC) systems of the NGC 5128 ("Centaurus A") and NGC 4594 ("Sombrero") galaxies. Existing optical photometric and spectroscopic are combined with this new data in a comprehensive optical to mid-IR colour catalogue of 260 GCs. Empirical colour-metallicity relationships are derived for all optical to mid-IR colour combinations. These colours prove to be very effective quantities to test the photometric predictions of simple stellar population (SSP) models. In general, four SSP models show larger discrepancies between each other and the data at bluer wavelengths, especially at high metallicities. Such differences become very important when attempting to use colour-colour model predictions to constrain the ages of stellar populations. Furthermore, the age-substructure determined from colour-colour diagrams and 91 NGC 5128 GCs with spectroscopic ages from Beasley et al. (2008) are inconsistent, suggesting any apparent GC system age-substructure implied by a colour-colour analysis must be verified independently. Unlike blue wavebands, certain optical to mid-IR colours are insensitive to the flux from hot horizontal branch stars and thus provide an excellent metallicity proxy. The NGC 5128 GC system shows strong bimodality in the optical R-band to mid-IR colour distributions, hence proving it is bimodal in metallicity. In this new colour space, a colour-magnitude trend, a "blue tilt", is found in the NGC 5128 metal-poor GC data. The NGC 5128 young GCs do not contribute to this trend. [abridged]
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