Sub-mm clues to elliptical galaxy formation

Abstract

There is growing evidence that, at the S(850) < 1 mJy level, the sub-mm galaxy population (and hence a potentially significant fraction of the sub-mm background) is associated with the star-forming Lyman-break population already detected at optical wavelengths. However, the implied star-formation rates in such objects (typically 3-30 solar masses per year) fall one or two orders of magnitude short of the level of star-forming activity required to produce the most massive elliptical galaxies on a timescale ~ 1 Gyr. If a significant fraction of massive ellipticals did form the bulk of their stars in short-lived massive starbursts at high redshift, then they should presumably be found among the brighter, S(850) ~ 10 mJy sub-mm sources which are undoubtedly not part of the Lyman-break population. A first powerful clue that this is indeed the case comes from our major SCUBA survey of radio galaxies, which indicates that massive dust-enshrouded star-formation in at least this subset of massive ellipticals is largely confined to z > 2.5, with a mean redshift z = 3.5. While radio selection raises concerns about bias, I argue that our current knowledge of the brightest (S(850) ~ 10 mJy) sub-mm sources detected in unbiased SCUBA imaging surveys indicates that they are also largely confined to this same high-z regime. Consequently, while the most recent number counts imply such extreme sources can contribute only 5-10% of the sub-mm background, their comoving number density (in the redshift band 3 < z < 5) is 1-2 x 10^{-5} per cubic megaparsec, sufficient to account for the formation of all ellipticals of comparable mass to radio galaxies (~4L-star) in the present-day universe.