用多色测光研究z ~ 2星系的恒星形成率和质量
Star formation rates and masses of z ~ 2 galaxies from multicolour photometry
摘要：
拟合合成光谱能量分布（SED）的星系的多波段测光，以获得他们的恒星形成率（SFR），恒星质量，年龄等，需要作出先验假设的恒星形成历史（SFH）。一种被广泛采用的SFH参数化方法，即所谓的τ模型，其中SFR为e^（-t/τ），当应用于z ~ 2处的恒星形成星系时，会导致不切实际的低年龄，这是其他SFH在拟合中留下年龄作为自由参数时所共有的问题。这是因为这些星系的SED在所有波长上都由它们最年轻的恒星星族主导，它们的亮度超过了较老的星系。因此，这些星系的SED几乎没有提供恒星形成开始的信息。为了解决这个问题，我们探索了各种SFH，例如常数SFR和倒τ模型-SFR为e^（+t/τ）-以及各种年龄的先验，包括假设所有星系的恒星形成都始于高红移。我们发现，倒τ模型与后者的假设，使SFR和projections在很好的协议，仅使用紫外部分的SED得出的值。这些模型也被证明可以准确地恢复由半解析模型构造的模拟星系在z ~ 2处的SFR和质量。所有其他探索的SFH模板不满足这两个测试。特别是，在拟合过程中不受年龄约束的直接τ模型高估了SFR，低估了恒星质量，并会加剧SFR和恒星质量的体积密度的宇宙演化之间的明显不匹配。我们的结论是，高红移恒星形成星系的指数增加的SFR与高形成红移是最好的SFH的其他形式，迄今为止在文献中采用。
英文摘要：
Fitting synthetic spectral energy distributions (SED) to the multi-band photometry of galaxies to derive their star formation rates (SFR), stellar masses, ages, etc. requires making a priori assumptions about their star formation histories (SFH). A widely adopted parameterization of the SFH, the so-called tau-models where SFR goes as e^{-t/tau) is shown to lead to unrealistically low ages when applied to star forming galaxies at z ~ 2, a problem shared by other SFHs when the age is left as a free parameter in the fitting. This happens because the SED of such galaxies, at all wavelengths, is dominated by their youngest stellar populations, which outshine the older ones. Thus, the SED of such galaxies conveys little information on the beginning of star formation. To cope with this problem, we explore a variety of SFHs, such as constant SFR and inverted-tau models - with SFR as e^{+t/tau) - along with various priors on age, including assuming that star formation started at high redshift in all the galaxies. We find that inverted-tau models with such latter assumption give SFRs and extinctions in excellent agreement with the values derived using only the UV part of the SED. These models are also shown to accurately recover the SFRs and masses of mock galaxies at z ~ 2 constructed from semi-analytic models. All other explored SFH templates do not fulfil these two test. In particular, direct-tau models with unconstrained age in the fitting procedure overstimate SFRs and underestimate stellar mass, and would exacerbate an apparent mismatch between the cosmic evolution of the volume densities of SFR and stellar mass. We conclude that for high-redshift star forming galaxies an exponentially increasing SFR with a high formation redshift is preferable to other forms of the SFH so far adopted in the literature.
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