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arXiv 1009.2040astro-ph.CO
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0.2 < z < 0.6时星系演化与恒星形成效率

Galaxy Evolution and Star Formation Efficiency at 0.2 < z < 0.6

F. Combes, S. Garcia-Burillo, J. Braine, E. Schinnerer, F. Walter, L. Colina

备注：
11 pages, 13 figures, accepted in A&A
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摘要：

本文利用IRAM 30 m望远镜对中等红移（z \sim 0.2-0.6）的30个星系进行了CO谱线测量，目的是跟踪星系演化，特别是由远红外光度与分子气体质量之比（LFIR/M（H2））定义的恒星形成效率（SFE）。源被选择为超亮红外星系（ULIRG），LFIR大于2.8 × 10^{12} Lsol，经历恒星爆发;采用低ULIRG CO到H2转换因子，其气体消耗时间尺度低于10^8年。到目前为止，只有很少的CO观测存在于这个红移范围内，跨越了宇宙年龄的近25%。在此期间，已经观察到恒星形成率的相当大的演变。在我们的30个样本中，有18个星系被探测到（其中16个是新探测到的），相当于60%的探测率。探测到的18个星系的平均CO光度为L 'CO = 2 × 10^{10} K km/s pc ^2，对应的平均H2质量为1.6 × 10^{10} Msol。远红外光度与CO光度相关性很好，与低和高红移ULIRGs的相关性一致。虽然CO光度和H2质量之间的转换因子是不确定的，我们发现，一个单一的星系可用的气体的最大量是迅速增加的红移的函数。使用相同的转换因子，SFE的z \sim 0.2-0.6 ULIRGs被发现是显着更高的，由一个因素3，比本地ULIRGs，并与高红移的。我们将这种演变与预期的宇宙H2丰度和宇宙恒星形成历史进行比较。

英文摘要：

We present the results of a CO line survey of 30 galaxies at moderate redshift (z \sim 0.2-0.6), with the IRAM 30m telescope, with the goal to follow galaxy evolution and in particular the star formation efficiency (SFE) as defined by the ratio between far-infrared luminosity and molecular gas mass (LFIR/M(H2)). The sources are selected to be ultra-luminous infrared galaxies (ULIRGs), with LFIR larger than 2.8 10^{12} Lsol, experiencing starbursts; adopting a low ULIRG CO-to-H2 conversion factor, their gas consumption time-scale is lower than 10^8 yr. To date only very few CO observations exist in this redshift range that spans nearly 25% of the universe's age. Considerable evolution of the star formation rate is already observed during this period. 18 galaxies out of our sample of 30 are detected (of which 16 are new detections), corresponding to a detection rate of 60%. The average CO luminosity for the 18 galaxies detected is L'CO = 2 10^{10} K km/s pc^2, corresponding to an average H2 mass of 1.6 10^{10} Msol. The FIR luminosity correlates well with the CO luminosity, in agreement with the correlation found for low and high redshift ULIRGs. Although the conversion factor between CO luminosity and H2 mass is uncertain, we find that the maximum amount of gas available for a single galaxy is quickly increasing as a function of redshift. Using the same conversion factor, the SFEs for z\sim 0.2-0.6 ULIRGs are found to be significantly higher, by a factor 3, than for local ULIRGs, and are comparable to high redshift ones. We compare this evolution to the expected cosmic H2 abundance and the cosmic star formation history.

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