标题： H₂质量密度的宇宙演化和分子气体的时代 显示英文标题

标题： Cosmic evolution of the H2 mass density and the epoch of molecular gas

摘要： 我们提出了对 $\rho_{\rm H_2}$ （宇宙中分子氢的宇宙质量密度）演化的新观测限制，时间范围追溯到 $z\approx2.5$。 我们采用统计方法，通过红移测量近红外选择星系的平均观测 $850\mu{\rm m}$ 通量密度。 所考虑的红移范围对应于一个区间，在该区间内 $850\mu{\rm m}$ 带探测了静止框架下的热尘埃辐射的瑞利-金斯尾部，因此可以将其用作星际介质（ISM）质量的估计值。 我们的样本包括英国红外深空巡天-广域巡天（UKIDSS-UDS）领域的 ${\approx}150,000$ 个星系，这些星系具有近红外星等 $K_{\rm AB}\leq25$ 魔法，并且具有从深度12波段测光数据推导出的对应概率分布函数的光谱红移。 利用比以往工作大约大两个量级的样本，我们显著减少了从$\rho_{\rm H_2}$到$z\approx2.5$的统计不确定性。 我们的测量结果与最近来自空白场分子气体调查的直接估算结果大致一致，发现分子气体的时期与恒星形成峰值时期的重合性在$z\approx2$时具有$\rho_{\rm H_2}\approx2\times10^7\,{\rm M_\odot}\,{\rm Mpc^{-3}}$。 我们演示了$\rho_{\rm H_2}$可以通过反转恒星形成率密度来广泛建模，同时采用固定或弱演化的恒星形成效率。 这个“恒定效率”模型显示出与我们统计推导的$\rho_{\rm H_2}$类似的演化趋势，这表明驱动$z\approx2$恒星形成历史达到峰值的主要因素是星系中分子气体供应量的增加，而不是单个星系内恒星形成效率的显著变化。 显示英文摘要

摘要： We present new empirical constraints on the evolution of $\rho_{\rm H_2}$, the cosmological mass density of molecular hydrogen, back to $z\approx2.5$. We employ a statistical approach measuring the average observed $850\mu{\rm m}$ flux density of near-infrared selected galaxies as a function of redshift. The redshift range considered corresponds to a span where the $850\mu{\rm m}$ band probes the Rayleigh-Jeans tail of thermal dust emission in the rest-frame, and can therefore be used as an estimate of the mass of the interstellar medium (ISM). Our sample comprises of ${\approx}150,000$ galaxies in the UKIDSS-UDS field with near-infrared magnitudes $K_{\rm AB}\leq25$ mag and photometric redshifts with corresponding probability distribution functions derived from deep 12-band photometry. With a sample approximately 2 orders of magnitude larger than in previous works we significantly reduce statistical uncertainties on $\rho_{\rm H_2}$ to $z\approx2.5$. Our measurements are in broad agreement with recent direct estimates from blank field molecular gas surveys, finding that the epoch of molecular gas coincides with the peak epoch of star formation with $\rho_{\rm H_2}\approx2\times10^7\,{\rm M_\odot}\,{\rm Mpc^{-3}}$ at $z\approx2$. We demonstrate that $\rho_{\rm H_2}$ can be broadly modelled by inverting the star-formation rate density with a fixed or weakly evolving star-formation efficiency. This 'constant efficiency' model shows a similar evolution to our statistically derived $\rho_{\rm H_2}$, indicating that the dominant factor driving the peak star formation history at $z\approx2$ is a larger supply of molecular gas in galaxies rather than a significant evolution of the star-formation rate efficiency within individual galaxies.