Molecular gas reservoirs during the winding-down of star formation
Intervenant : Jonathan Freundlich
Star formation in the Universe decreased by an order of magnitude in the last ten billion years. To understand this winding-down of star formation, it is crucial to probe the molecular gas reservoirs from which stars are formed. The PHIBSS2 legacy program at the IRAM NOEMA interferometer surveys the molecular gas properties of galaxies at different redshifts on and around the main sequence, where most of star formation occurs. It contributes to show that the cosmic evolution of the star formation rate is mainly driven by that of the molecular gas fraction, albeit with a small inflexion of the star formation efficiency. At z=0.5-0.8, we observe the CO(2-1) line for 61 targets, from which we determine molecular gas masses, gas fractions and depletion times. The Kennicutt-Schmidt relation between molecular gas and star formation rate surface densities is strikingly linear, which argues in favor of uniform star formation processes across cosmic time. We further obtain disk sizes and bulge-to-total luminosity ratios, which enable us to characterize morphological changes. Our results suggest an ongoing supply of molecular gas to compensate for star formation and a contribution of disk stars to the growing bulge.
Picture: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team.