A HIGHER EFFICIENCY OF CONVERTING GAS TO STARS PUSHES GALAXIES AT <i>z</i> ∼ 1.6 WELL ABOVE THE STAR-FORMING MAIN SEQUENCE

Abstract

Local starbursts have a higher efficiency of converting gas into stars, as\ncompared to typical star-forming galaxies at a given stellar mass, possibly\nindicative of different modes of star formation. With the peak epoch of galaxy\nformation occurring at z &gt; 1, it remains to be established whether such an\nefficient mode of star formation is occurring at high-redshift. To address this\nissue, we measure the molecular gas content of seven high-redshift (z ~ 1.6)\nstarburst galaxies with the Atacama Large (sub-)Millimeter Array and\nIRAM/Plateau de Bure Interferometer. Our targets are selected from the sample\nof Herschel far-infrared detected galaxies having star formation rates\n(~300-800 Msolar/yr) elevated (&gt;4x) above the star-forming `main sequence', and\nincluded in the FMOS-COSMOS near-infrared spectroscopic survey of star-forming\ngalaxies at z ~ 1.6 with Subaru. We detect CO emission in all cases at high\nlevels of significance, indicative of high gas fractions (~30-50%). Even more\ncompelling, we firmly establish with a clean and systematic selection that\nstarbursts, identified as main-sequence outliers, at high redshift generally\nhave a lower ratio of CO to total infrared luminosity as compared to typical\nmain-sequence star-forming galaxies, although with a smaller offset than\nexpected based on past studies of local starbursts. We put forward a hypothesis\nthat there exists a continuous increase in star formation efficiency with\nelevation from the main sequence with galaxy mergers as a possible physical\ndriver. Along with a heightened star formation efficiency, our high-redshift\nsample is similar in other respects to local starbursts such as being metal\nrich and having a higher ionization state of the interstellar medium.\n