The nature of the ISM in galaxies during the star-formation activity peak of the Universe

• Published in: Monthly notices of the Royal Astronomical Society Vol. 444; no. 2; pp. 1301 - 1317
• Main Authors: Popping, G., Pérez-Beaupuits, J. P., Spaans, M., Trager, S. C., Somerville, R. S.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 21-10-2014
• Subjects: Astrophysics; Red shift; Star & galaxy formation; Universe; galaxies: formation; ISM: atoms; ISM: lines and bands; ISM: molecules; galaxies: evolution; galaxies: ISM
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stu1506

We combine a semi-analytic model of galaxy formation, tracking atomic and molecular phases of cold gas, with a three-dimensional radiative-transfer and line tracing code to study the sub-mm emission from atomic and molecular species (CO, HCN, [C i], [C ii], [O i]) in galaxies. We compare the physics that drives the formation of stars at the epoch of peak star formation (SF) in the Universe (z = 2.0) with that in local galaxies. We find that normal star-forming galaxies at high redshift have much higher CO-excitation peaks than their local counterparts and that CO cooling takes place at higher excitation levels. CO line ratios increase with redshift as a function of galaxy star-formation rate, but are well correlated with H2 surface density independent of redshift. We find an increase in the [O i]/[C ii] line ratio in typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to counterparts at z = 0. Our model results suggest that typical star-forming galaxies at high redshift consist of much denser and warmer star-forming clouds than their local counterparts. Galaxies belonging to the tail of the SF activity peak at z = 1.2 are already less dense and cooler than counterparts during the actual peak of SF activity (z = 2.0). We use our results to discuss how future ALMA surveys can best confront our predictions and constrain models of galaxy formation.