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Local starbursts have a higher efficiency of converting gas into stars, ascompared to typical star-forming galaxies at a given stellar mass, possiblyindicative of different modes of star formation. With the peak epoch of galaxyformation occurring at z > 1, it remains to be established whether such anefficient mode of star formation is occurring at high-redshift. To address thisissue, we measure the molecular gas content of seven high-redshift (z ~ 1.6)starburst galaxies with the Atacama Large (sub-)Millimeter Array andIRAM/Plateau de Bure Interferometer. Our targets are selected from the sampleof Herschel far-infrared detected galaxies having star formation rates(~300-800 Msolar/yr) elevated (>4x) above the star-forming `main sequence', andincluded in the FMOS-COSMOS near-infrared spectroscopic survey of star-forminggalaxies at z ~ 1.6 with Subaru. We detect CO emission in all cases at highlevels of significance, indicative of high gas fractions (~30-50%). Even morecompelling, we firmly establish with a clean and systematic selection thatstarbursts, identified as main-sequence outliers, at high redshift generallyhave a lower ratio of CO to total infrared luminosity as compared to typicalmain-sequence star-forming galaxies, although with a smaller offset thanexpected based on past studies of local starbursts. We put forward a hypothesisthat there exists a continuous increase in star formation efficiency withelevation from the main sequence with galaxy mergers as a possible physicaldriver. Along with a heightened star formation efficiency, our high-redshiftsample is similar in other respects to local starbursts such as being metalrich and having a higher ionization state of the interstellar medium.

A higher efficiency of converting gas to stars push galaxies at z ~ 1.6 well above the star-forming main sequence