THE LESSER ROLE OF STARBURSTS IN STAR FORMATION AT <i>z</i> = 2

Abstract

Two main modes of star formation are know to control the growth of galaxies:\na relatively steady one in disk-like galaxies, defining a tight star formation\nrate (SFR)-stellar mass sequence, and a starburst mode in outliers to such a\nsequence which is generally interpreted as driven by merging. Such starburst\ngalaxies are rare but have much higher SFRs, and it is of interest to establish\nthe relative importance of these two modes. PACS/Herschel observations over the\nwhole COSMOS and GOODS-South fields, in conjunction with previous\noptical/near-IR data, have allowed us to accurately quantify for the first time\nthe relative contribution of the two modes to the global SFR density in the\nredshift interval 1.5&lt;z&lt;2.5, i.e., at the cosmic peak of the star formation\nactivity. The logarithmic distributions of galaxy SFRs at fixed stellar mass\nare well described by Gaussians, with starburst galaxies representing only a\nrelatively minor deviation that becomes apparent for SFRs more than 4 times\nhigher than on the main sequence. Such starburst galaxies represent only 2% of\nmass-selected star forming galaxies and account for only 10% of the cosmic SFR\ndensity at z~2. Only when limited to SFR&gt;1000M(sun)/yr, off-sequence sources\nsignificantly contribute to the SFR density (46+/-20%). We conclude that\nmerger-driven starbursts play a relatively minor role for the formation of\nstars in galaxies, whereas they may represent a critical phase towards the\nquenching of star formation and morphological transformation in galaxies.\n