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Starbursts, normal galaxies and the molecular gas history of the Universe

by Mark Sargent (University of Sussex, UK)

Apr 01, 2015 from 02:00 PM to 03:30 PM

Where Seminars' room, floor -1, via Ranzani 1

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The interpretation of the CO-line emission from the roughly 150 'normal' star-forming galaxies detected to date at z>0 suggests that there is a high degree of homogeneity between low- and high-redshift main-sequence galaxies in terms of their molecular gas properties. I will show that the variations of star-formation efficiency and gas fraction about the characteristic value of a "typical" main sequence galaxy do not change over the range z < 3. For starbursts - i.e. objects displaying specific star-formation rates and star formation efficiencies (SFE) that can exceed those of main-sequence galaxies by an order of magnitude - the change in SFE scales supralinearly with the SFR enhancement, as expected for merging events, implying a continuous distribution of galaxies in the Schmidt-Kennicutt plane that is not strictly bimodal, but nevertheless more clearly separated into a locus for starbursts and normal galaxies than their relative distribution in the space of SFR and stellar mass. I will discuss how we can exploit these scaling relations to infer the molecular gas properties (e.g. H2 mass functions and CO-luminosity functions) of the large samples of distant star-forming galaxies that await detection in upcoming blind sub-mm surveys with ALMA and the JVLA. I will also apply our formalism for distinguishing between starbursts and normal galaxies to constrain the fraction of the total molecular gas content of the Universe that fuels burst-like and secular star-formation activity, respectively. Finally, I will report on our recent search for molecular gas in a quiescent z~1.4 elliptical galaxy, for which we found that the gas fraction is at least tenfold lower than those measured for star-forming galaxies at this redshift. This allowed us to carry out a first comparison of the gas content of high-z and low-z early-type galaxies and to gain insight into the importance of different quenching mechanisms for producing passive galaxies at the peak epoch of galaxy assembly.