The ALMA Spectroscopic Survey in the HUDF: CO luminosity functions and the molecular gas content of galaxies through cosmic history

21 Mar 2019  ·  Decarli Roberto, Walter Fabian, Gónzalez-López Jorge, Aravena Manuel, Boogaard Leindert, Carilli Chris, Cox Pierre, Daddi Emanuele, Popping Gergö, Riechers Dominik, Uzgil Bade, Weiss Axel, Assef Roberto J., Bacon Roland, Bauer Franz Erik, Bertoldi Frank, Bouwens Rychard, Contini Thierry, Cortes Paulo C., da Cunha Elisabete, Díaz-Santos Tanio, Elbaz David, Inami Hanae, Hodge Jacqueline, Ivison Rob, Fèvre Olivier Le, Magnelli Benjamin, Novak Mladen, Oesch Pascal, Rix Hans-Walter, Sargent Mark T., Smail Ian R., Swinbank A. Mark, Somerville Rachel S., van der Werf Paul, Wagg Jeff, Wisotzki Lutz ·

We use the results from the ALMA large program ASPECS, the spectroscopic survey in the Hubble Ultra Deep Field (HUDF), to constrain CO luminosity functions of galaxies and the resulting redshift evolution of $\rho$(H$_2$). The broad frequency range covered enables us to identify CO emission lines of different rotational transitions in the HUDF at $z>1$... We find strong evidence that the CO luminosity function evolves with redshift, with the knee of the CO luminosity function decreasing in luminosity by an order of magnitude from $\sim$2 to the local universe. Based on Schechter fits, we estimate that our observations recover the majority (up to $\sim$90%, depending on the assumptions on the faint end) of the total cosmic CO luminosity at $z$=1.0-3.1. After correcting for CO excitation, and adopting a Galactic CO-to-H$_2$ conversion factor, we constrain the evolution of the cosmic molecular gas density $\rho$(H$_2$): this cosmic gas density peaks at $z\sim1.5$ and drops by factor of $6.5_{-1.4}^{+1.8}$ to the value measured locally. The observed evolution in $\rho$(H$_2$) therefore closely matches the evolution of the cosmic star formation rate density $\rho_{\rm SFR}$. We verify the robustness of our result with respect to assumptions on source inclusion and/or CO excitation. As the cosmic star formation history can be expressed as the product of the star formation efficiency and the cosmic density of molecular gas, the similar evolution of $\rho$(H$_2$) and $\rho_{\rm SFR}$ leaves only little room for a significant evolution of the average star formation efficiency in galaxies since $z\sim 3$ (85% of cosmic history). read more

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Astrophysics of Galaxies