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

TL;DR

This study investigates the efficiency of gas-to-star conversion in high-redshift starburst galaxies around z ~ 1.6, revealing they are more efficient than typical galaxies and share properties with local starbursts, driven possibly by galaxy mergers.

Contribution

It provides the first systematic measurement of molecular gas content in high-redshift starbursts, showing increased star formation efficiency compared to main-sequence galaxies.

Findings

High gas fractions (~30-50%) in z ~ 1.6 starbursts.

Starbursts have lower CO to infrared luminosity ratios than main-sequence galaxies.

Star formation efficiency increases with elevation from the main sequence.

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.