We present a CO and atomic fine-structure line luminosity function analysis using the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). ASPECS consists of two spatially-overlapping mosaics that cover the entire ALMA 3mm and 1.2mm bands. We combine the results of a line candidate search of the 1.2mm data cube with those previously obtained from the 3mm cube. Our analysis shows that $\sim$80% of the line flux observed at 3mm arises from CO(2-1) or CO(3-2) emitters at $z$=1-3 (`cosmic noon'). At 1.2mm, more than half of the line flux arises from intermediate-J CO transitions ($J_{\rm up}$=3-6); $\sim12$% from neutral carbon lines; and $< 1$% from singly-ionized carbon, [CII]. This implies that future [CII] intensity mapping surveys in the epoch of reionization will need to account for a highly significant CO foreground. The CO luminosity functions probed at 1.2mm show a decrease in the number density at a given line luminosity (in units of $L'$) at increasing $J_{\rm up}$ and redshift. Comparisons between the CO luminosity functions for different CO transitions at a fixed redshift reveal sub-thermal conditions on average in galaxies up to $z\sim 4$. In addition, the comparison of the CO luminosity functions for the same transition at different redshifts reveals that the evolution is not driven by excitation. The cosmic density of molecular gas in galaxies, $\rho_{\rm H2}$, shows a redshift evolution with an increase from high redshift up to $z\sim1.5$ followed by a factor $\sim 6$ drop down to the present day. This is in qualitative agreement with the evolution of the cosmic star-formation rate density, suggesting that the molecular gas depletion time is approximately constant with redshift, after averaging over the star-forming galaxy population.

中文翻译：

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哈勃超深场中的 ALMA 光谱巡天：线光度函数的多波段约束和分子气体的宇宙密度

我们使用哈勃超深场 (ASPECS) 中的 ALMA 光谱调查提供了 CO 和原子精细结构线光度函数分析。ASPECS 由两个空间重叠的马赛克组成，覆盖了整个 ALMA 3 毫米和 1.2 毫米波段。我们将 1.2mm 数据立方体的线候选搜索结果与先前从 3mm 立方体获得的结果相结合。我们的分析表明，在 3mm 处观察到的线通量的 $\sim$80% 来自于 $z$=1-3（“宇宙正午”）的 CO(2-1) 或 CO(3-2) 发射器。在 1.2mm 处，超过一半的线通量来自中间 J CO 跃迁（$J_{\rm up}$=3-6）；$\sim12$% 来自中性碳系；和 $<1$% 来自单电离碳，[CII]。这意味着在再电离时代未来的 [CII] 强度绘图调查将需要考虑非常重要的 CO 前景。在 1.2mm 处探测的 CO 光度函数显示，在增加 $J_{\rm up}$ 和红移时，给定线光度（以 $L'$ 为单位）的数密度降低。固定红移下不同 CO 跃迁的 CO 光度函数之间的比较揭示了星系中平均低于 $z\sim 4$ 的亚热条件。此外，不同红移下相同跃迁的 CO 光度函数的比较表明，演化不是由激发驱动的。星系中分子气体的宇宙密度 $\rho_{\rm H2}$ 显示出红移演化，从高红移增加到 $z\sim1.5$，随后因子 $\sim 6$ 下降到今天。这与宇宙恒星形成率密度的演化定性一致，