2005ApJ...619L.135J

We examine the cosmic star formation rate (SFR) and its dependence on galaxy stellar mass over the redshift range 0.8<z<2 using data from the Gemini Deep Deep Survey (GDDS). The SFR in the most massive galaxies (M_*>10^10.8 M_☉) was 6 times higher at z=2 than it is today. It drops steeply from z=2, reaching the present-day value at z∼1. In contrast, the SFR density of intermediate-mass galaxies (10^10.2 M_☉≤M_*<10^10.8 M_☉) declines more slowly and may peak or plateau at z∼1.5. We use the characteristic growth time t_SFR≡ρ_M*/ρ_SFRto provide evidence of an associated transition in massive galaxies from a burst to a quiescent star formation mode at z∼2. Intermediate-mass systems transit from burst to quiescent mode at z∼1, while the lowest mass objects undergo bursts throughout our redshift range. Our results show unambiguously that the formation era for galaxies was extended and proceeded from high- to low-mass systems. The most massive galaxies formed most of their stars in the first ∼3 Gyr of cosmic history. Intermediate-mass objects continued to form their dominant stellar mass for an additional ∼2 Gyr, while the lowest mass systems have been forming over the whole cosmic epoch spanned by the GDDS. This view of galaxy formation clearly supports ``downsizing'' in the SFR where the most massive galaxies form first and galaxy formation proceeds from larger to smaller mass scales.