Finding starburst galaxies

Only a handful of new stars form every year in the Milky Way, a typical quiescent galaxy. But the cosmic history of star formation is dominated by galaxies in their brief starburst phases, when prolific star formation makes them glow in the far-IR with thermal radiation around 100 μm from cocoons of dust warmed by very bright young stars. Earth's atmosphere blocks far-IR radiation at wavelengths below 800 μm, and no space telescope has yet had adequate resolution above 24 μm to resolve into discrete sources the diffuse cosmic far-IR background (FIRB) discovered 15 years ago by the COBE orbiter. Now the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST) has produced maps of patches of sky that reveal more than 500 starburst galaxies luminous at submillimeter wavelengths from 200 to 600 μm, where the FIRB is brightest. The BLAST collaboration, led by Mark Devlin (University of Pennsylvania), concludes that the entire FIRB can be accounted for by individual galaxies. The group finds that at those submillimeter wavelengths, the FIRB is dominated by distant, high-redshift starburst galaxies from long ago. Augmented by redshift measurements of the starburst galaxies at other wavelengths, the BLAST observations make it possible to document the falling cosmic rate of star formation since its heyday at redshifts around 3,  just 2 billion years after the Big Bang (see the figure). (M. J. Devlin et al., Nature 458, 737, 2009; E. Pascale et al., http://arxiv.org/abs/0904.1206v1.) — Bertram Schwarzschild