7.2. Dust, the UV and the Submm

As noted above the issue of dust and the extinction of the optically-detected, UV rest-frame, sources has been the subject of considerable discussion over the last few years. In parallel with efforts to characterize the impact of dust on optically-detected high redshift galaxies, a very dramatic set of results has become available over the last year on sources at high redshift that are emitting predominantly in the submm. These detections were made at 850 µm with a new bolometer SCUBA on the 15-m JCMT telescope in Hawaii. It's large 15'' `beam', combined with the generally low UV and optical fluxes from these highly extincted sources, has made it hard to reliably identify the corresponding optical source and derive a redshift. A novel approach at improving the positions, by using radio continuum flux, has improved the source ``identification'' rate, though this may only be useful for the most luminous sources. Nonetheless, enough redshifts have been determined, which, when combined with indirect arguments, suggest that such objects could well be contributing as much (UV-equivalent) flux at high redshift as the optically-detected sources, if not more. Whether this is entirely due to starbursts, or whether AGN activity is also a contributing factor remains to be determined. However, it is likely that they are at least comparable to the optically-detected sources in their contribution to the overall SFR at high redshift, and may actually be the dominant population in the ``Madau plot'' at z > 1!

Such sources will be readily detectable with ALMA, and observable over a wide redshift range because of a shape of their SED. These sources have been detected and measured at 850 µm, which lies on the long wavelength side of the black-body distribution, such that as the redshift increases the flux increases (unlike the usual situation in the optical where the K-corrections are negative), and the increase is such as to largely cancel out the cosmological (1 + z)⁴ surface brightness dimming (see Sanders, this volume, for a good explanatory figure). Much remains to be learnt about these important new objects.