How and when did galaxies assemble all their stars? Two studies report a census of galaxies across cosmic time and the evolution of star formation rates over the universe's history. Using near-infrared and optical emission data, Kong et al. found that in 80% of distant large galaxies, stars formed at a prodigious rate, much more rapidly than in galaxies of similar mass today. These ancient galaxies appear to have formed all of their stars in a vigorous burst, lasting only a hundred million years.
Caputi et al. observed a similar pattern of exceptionally rapid star formation in old galaxies. By analyzing mid-infrared emission detected with the Spitzer space telescope, they also found evidence for the presence of complex molecules (polyaromatic hydrocarbons) in the interstellar region of these galaxies at early times. Both teams suggest that their findings favor a "cosmic downsizing" phenomenon, with galaxy formation being more active and rapid in the young universe than at present. -- JB
Astrophys. J. 638, 72; 637, 727 (2006).
