High-redshift surveys

The 70 and 160 micron SIRTF surveys are likely to probe far deeper into the high-redshift IR-background population than what is currently possible from the ground. However, these data alone will not permit a characterization of those sources which escape optical identification and spectroscopy, like most of those sources detected in the recent SCUBA and MAMBO deep field surveys. BLAST would provide deep photometry to 500 micron, but is severely limited by confusion at the long wavelengths, which are critical to constrain the source redshifts. Ground-based large format bolometer cameras will therefore be a crucial supplement for SIRTF and BLAST surveys, in order to determine the nature of the high-redshift starburst and AGN populations. The 117-element MAMBO camera at the IRAM 30 m telescope, LABOCA on APEX, and BOLOCAM on the CSO are the important instruments in this respect. All three will be available when the SIRTF and BLAST data arrive. SIRTF, BLAST, LABOCA, SCUBA-2, and, eventually, Herschel are expected to provide a sufficiently large number of sources to study the luminosity distribution and clustering of IR-luminous sources at high redshifts. With the vast survey data to be accumulated within the coming three to five years, it is likely that the interest in additional wide field ground-based surveys will diminish. More targeted studies, especially deep, multi-wavelength photometry of selected classes of objects, will be of increasing importance. Currently there already exist many interesting, marginally detectable targets for on-off observations, such as high-redshift QSOs, Lyman-break galaxies, gamma ray burst afterglows, comets, and Kuiper belt objects. Pointed follow-up observations at multiple wavelengths of sources found in deep mapping surveys are also of high interest to determine the source SEDs, and thereby estimate their redshifts and dust temperatures. For photometric pointed observations, the most sensitive, compact bolometer arrays must therefor be used at all accessible wavelengths. In the long term ALMA and the EVLA will do this job with unprecedented sensitivity and resolution. Although at longer wavelenghts a higher fraction of high-z sources are detectable in blank field surveys, the difference is not large at wavelengths longer than 0.8 mm. At redshifts up to 3, the 0.87 mm window would yield the best sensitivity since the flux ratio 0.87mm/1.2mm is typically around 3, and the 30 m beam efficiency at 0.87 mm is 30% compared to 50% at 1.2 mm. At Pico Veleta adequate weather conditions to observe at 0.87 mm are rare, but if observations are scheduled flexibly, a non-negligible amount of time would be available with sub-mm conditions. For deep field surveys however, the available time would not be sufficient, and the 1.2 mm window is the better choice. At 2 mm the high-redshift thermal sources are at least 4 times fainter, too faint to permit efficient blank field searches for thermal sources in the early universe.