The Fine Line Between Normal and Starburst Galaxies

Nicholas Lee, Kartik Sheth, Kimberly S. Scott, Sune Toft, Georgios Magdis, Ivana Damjanov, H. Jabran Zahid, Caitlin M. Casey, Isabella Cortzen, Carlos Gomez Guijarro, Alexander Karim, Sarah K. Leslie, Eva Schinnerer

Published 2017-10-07Version 1

Recent literature suggests that there are two modes through which galaxies grow their stellar mass - a normal mode characterized by quasi-steady star formation, and a highly efficient starburst mode possibly triggered by stochastic events such as galaxy mergers. While these differences are established for extreme cases, the population of galaxies in-between these two regimes is poorly studied and it is not clear where the transition between these two modes of star formation occurs. We utilize ALMA observations of the CO J=3-2 line luminosity in a sample of 20 infrared luminous galaxies that lie in the intermediate range between normal and starburst galaxies at z ~ 0.25-0.6 in the COSMOS field to examine the gas content and star formation efficiency of these galaxies. We compare these quantities to the galaxies' deviation from the well-studied "main sequence" correlation between star formation rate and stellar mass (MS) and find that at log($SFR/SFR_{MS}$) < 0.6, a galaxy's distance to the main sequence is mostly driven by increased gas content, and not a more efficient star formation process.