A Higher Efficiency of Converting Gas to Stars Pushes Galaxies at z ∼ 1.6 Well Above the Star-forming Main Sequence
Berta, S.; Lutz, D.; Daddi, E.; Rodighiero, G.; Béthermin, M.; Rujopakarn, W.; Silverman, J. D.; Sanders, D.; Magdis, G.; Sargent, M.; Koekemoer, A.; Zamorani, G.; Kartaltepe, J.; Feruglio, C.; Renzini, A.; Mancini, C.; Arimoto, N.; Kashino, D.; Liu, D.; Onodera, M.; Nagao, T.
Japan, France, Italy, Thailand, United Kingdom, United States, Germany, Greece, Switzerland
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 molecular gas content of seven high-redshift (z ∼ 1.6) starburst galaxies with the Atacama Large Millimeter/submillimeter Array and IRAM/Plateau de Bure Interferometer. Our targets are selected from the sample of Herschel far-infrared-detected galaxies having star formation rates (∼300-800 M⊙ yr-1) elevated (≳4×) above the star-forming main sequence (MS) and included in the FMOS-COSMOS near-infrared spectroscopic survey of star-forming galaxies at z ∼ 1.6 with Subaru. We detect CO emission in all cases at high levels of significance, indicative of high gas fractions (∼30%-50%). Even more compelling, we firmly establish with a clean and systematic selection that starbursts, identified as MS outliers, at high redshift generally have a lower ratio of CO to total infrared luminosity as compared to typical MS star-forming galaxies, although with a smaller offset 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 MS with galaxy mergers as a possible physical driver. Along with a heightened star formation efficiency, 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 interstellar medium.