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arXiv 1105.2294astro-ph.CO
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星系原子和分子气体含量的宇宙演化

Cosmic evolution of the atomic and molecular gas content of galaxies

Claudia del P. Lagos, Carlton M. Baugh, Cedric G. Lacey, Andrew J. Benson, Han-Seek Kim, Chris Power

摘要:

我们利用LCDM框架下的星系形成模型GALFORM,通过将星际介质分裂为氢原子和氢分子,研究了星系冷气含量的演化.我们使用两种不同的方法计算每个星系中氢分子与原子的质量比,H2/HI; Blitz和Rosolowsky基于压力的经验关系和Krumholz,McKenzie和Tumlinson的理论模型,并应用它们来计算星系的恒星形成率。我们发现基于Blitz & Rosolowsky定律的模型预测了HI质量函数、CO(1-0)光度函数、H2/HI比与恒星和冷气体质量之间的相关性以及红外-CO光度关系,与本地和高红移观测结果符合得很好。HI质量函数随红移的变化很弱,高质量星系的数密度随红移的增加而减小。在H2质量函数的情况下,大质量星系的数密度从z=0到z=2强烈增加,然后到z=4弱演化。我们还发现,星系的H2/HI比是强烈依赖于恒星和冷气体的质量,也对红移。H2/HI与恒星和冷气体质量之间的相关性的斜率几乎没有变化,但归一化从z=0-8增加了两个数量级。H2质量函数和H2/HI比的强烈演化主要是由于星系大小的演化,其次是气体分数的演化。预测的HI宇宙密度演化与从DLAs推断的观测演化一致,并以低/中质量晕为主。我们发现,以前的理论研究在很大程度上高估了红移的全球H2/HI比的演变,由于有限的分辨率。我们预测在z~3.5处rho_H2/rho_H1 ~1.2的最大值。

英文摘要:

We study the evolution of the cold gas content of galaxies by splitting the interstellar medium into its atomic and molecular hydrogen components, using the galaxy formation model GALFORM in the LCDM framework. We calculate the molecular-to-atomic hydrogen mass ratio, H2/HI, in each galaxy using two different approaches; the pressure-based empirical relation of Blitz & Rosolowsky and the theoretical model of Krumholz, McKeee & Tumlinson, and apply them to consistently calculate the star formation rates of galaxies. We find that the model based on the Blitz & Rosolowsky law predicts an HI mass function, CO(1-0) luminosity function, correlations between the H2/HI ratio and stellar and cold gas mass, and infrared-CO luminosity relation in good agreement with local and high redshift observations. The HI mass function evolves weakly with redshift, with the number density of high mass galaxies decreasing with increasing redshift. In the case of the H2 mass function, the number density of massive galaxies increases strongly from z=0 to z=2, followed by weak evolution up to z=4. We also find that the H2/HI ratio of galaxies is strongly dependent on stellar and cold gas mass, and also on redshift. The slopes of the correlations between H2/HI and stellar and cold gas mass hardly evolve, but the normalisation increases by up to two orders of magnitude from z=0-8. The strong evolution in the H2 mass function and the H2/HI ratio is primarily due to the evolution in the sizes of galaxies and secondarily, in the gas fractions. The predicted cosmic density evolution of HI agrees with the observed evolution inferred from DLAs, and is dominated by low/intermediate mass halos. We find that previous theoretical studies have largely overestimated the redshift evolution of the global H2/HI ratio due to limited resolution. We predict a maximum of rho_H2/rho_HI~1.2 at z~3.5.

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