Historically, GHz radio emission has been used extensively to characterize the star-formation activity in galaxies. In this work, we look for empirical relations amongst the radio luminosity, the infrared luminosity, and the CO-based molecular gas mass. We assemble a sample of 278 nearby galaxies with measurements of radio continuum and total infrared emission, and the $^{12}$CO (J = 1-0) emission line. We find a correlation between the radio continuum and the CO emission line (with a scatter of 0.36 dex), in a large sample of different kind of galaxies. Making use of this correlation, we explore the evolution of the molecular gas mass function and the cosmological molecular gas mass density in six redshift bins up to $z = 1.5$. These results agree with previous semi-analytic predictions and direct measurements: the cosmic molecular gas density increases up to $z=1.5$. In addition, we find a single plane across five orders of magnitude for the explored luminosities, with a scatter of 0.27 dex. These correlations are sufficiently robust to be used for samples where no CO measurements exist.

中文翻译：

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从 L1.4 GHz 和 L'CO 之间的经验关系看分子气体质量密度的宇宙演化

从历史上看，GHz 无线电发射已被广泛用于表征星系中的恒星形成活动。在这项工作中，我们寻找射电光度、红外光度和基于 CO 的分子气体质量之间的经验关系。我们收集了 278 个附近星系的样本，并测量了无线电连续谱和总红外发射，以及 $^{12}$CO (J = 1-0) 发射线。在不同种类星系的大样本中，我们发现射电连续谱和 CO 发射线（散射为 0.36 dex）之间存在相关性。利用这种相关性，我们探索了分子气体质量函数和宇宙学分子气体质量密度在 6 个红移箱中直至 $z = 1.5$ 的演变。这些结果与之前的半解析预测和直接测量一致：宇宙分子气体密度增加到$z=1.5$。此外，我们发现所探索的光度跨越五个数量级的单个平面，散布为 0.27 dex。这些相关性足以用于不存在 CO 测量的样品。