Toward an understanding of the rapid decline of the cosmic star formation rate

We present a first analysis of deep 24 mu m observations with the Spitzer Space Telescope of a sample of nearly 1500 galaxies in a thin redshift slice, 0: 65 <= z <= 0: 75. We combine the infrared data with redshifts, rest- frame luminosities, and colors from COMBO- 17 and with morphologies from Hubble Space Telescope images collected by the Galaxy Evolution from Morphology and SEDs ( GEMS) and Great Observatories OriginsDeep Survey ( GOODS) projects. To characterize the decline in star formation rate ( SFR) since z similar to 0: 7, we estimate the total thermal IR luminosities, SFRs, and stellar masses for the galaxies in this sample. At z similar to 0: 7, nearly 40% of intermediate- and high- mass galaxies ( with stellarmasses >= 2; 10(10) M-circle dot) are undergoing a period of intense star formation above their past- averaged SFR. In contrast, less than 1% of equally massive galaxies in the local universe have similarly intense star formation activity. Morphologically undisturbed galaxies dominate the total infrared luminosity density and SFR density: at z similar to 0: 7, more than half of the intensely star- forming galaxies have spiral morphologies, whereas less than similar to 30% are strongly interacting. Thus, a decline in major merger rate is not the underlying cause of the rapid decline in cosmic SFR since z similar to 0: 7. Physical properties that do not strongly affect galaxy morphology - for example, gas consumption and weak interactions with small satellite galaxies - appear to be responsible.