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arXiv · 1505.04977

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

J. D. Silverman·E. Daddi·G. Rodighiero·W. Rujopakarn·M. Sargent·A. Renzini·D. Liu·C. Feruglio·D. Kashino·D. Sanders·J. Kartaltepe·T. Nagao·N. Arimoto·S. Berta·M. Bethermin·A. Koekemoer·D. Lutz·G. Magdis·C. Mancini·M. Onodera·G. Zamorani
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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 (sub-)Millimeter 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 Msolar/yr) elevated (>4x) above the star-forming `main sequence', 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 main-sequence outliers, at high redshift generally have a lower ratio of CO to total infrared luminosity as compared to typical main-sequence 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 main sequence 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.

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J. D. Silverman, E. Daddi, G. Rodighiero, W. Rujopakarn, M. Sargent, A. Renzini, D. Liu, C. Feruglio, D. Kashino, D. Sanders, J. Kartaltepe, T. Nagao, N. Arimoto, S. Berta, M. Bethermin, A. Koekemoer, D. Lutz, G. Magdis, C. Mancini, M. Onodera, G. Zamorani. 2015-10-22. A higher efficiency of converting gas to stars push galaxies at z ~ 1.6 well above the star-forming main sequence. https://doi.org/10.1088/2041-8205%2F812%2F2%2Fl23

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