Abstract
Local starbursts have a higher efficiency of converting gas into stars, as
compared to typical star-forming galaxies at a given stellar mass, possibly
indicative of different modes of star formation. With the peak epoch of galaxy
formation occurring at z > 1, it remains to be established whether such an
efficient mode of star formation is occurring at high-redshift. To address this
issue, we measure the CO molecular gas content of seven high-redshift starburst
galaxies with ALMA and IRAM/PdBI. Our sample is selected from the FMOS-COSMOS
near-infrared spectroscopic survey of star-forming galaxies at z ~ 1.6 with
Subaru. All galaxies have star formation rates (~300-800 Msolar/yr) elevated,
by at least four times, above the star-forming main sequence. We detect CO
emission in all cases at high significance, indicative of plentiful gas
supplies (f_gas ~ 30-50%). Even more compelling, we firmly establish for the
first time that starbursts at high redshift systematically have a lower ratio
of CO to total infrared luminosity as compared to typical 'main-sequence'
star-forming galaxies, although with an offset less than expected based on past
studies of local starbursts. We put forward a hypothesis that there exists a
continuous increase in star formation efficiency with elevation from the main
sequence with galaxy mergers as a possible physical driver. In support of this
scenario, our high-redshift sample is similar in other respects to local
starbursts such as being metal rich and having a higher ionization state of the
ISM.
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