The Dawn of Galaxies: Deep MAMBO Imaging Surveys

We discuss results from sensitive, wide-field imaging of the millimeter extragalactic background using the Max-Planck Millimeter Bolometer array (MAMBO) at the IRAM 30 m telescope. The detection of a far-infrared and sub-millimeter wavelength background by COBE, and the first sensitive, high-resolution SCUBA images of the sub-mm background have caused significant revisions of our picture of the star-formation history of the universe. SCUBA at 850 μm discovered a population of what appear to be star forming galaxies at high redshifts, most of which are invisible at optical and near-IR wavelengths (Smail et al. 1997; Hughes et al. 1998; Barger et al. 1999; Eales et al. 1999; Bertoldi et al. 2000). The infrared luminosities of these objects are comparable to the bolometric luminosities of QSOs, but their optical faintness shows that, unlike for QSOs, nearly all of the bolometric luminosity arises from thermal emission of dust grains. The emitting dust is probably heated in massive, optically obscured star forming regions, with star formation rates ∼ 103M⊙yr . The objects discovered with SCUBA and MAMBO can account for the integrated (sub)mm background, and thereby about 25% of the total infrared background radiation. The relation between metal production and the extragalactic background (Eales et al. 1999) would then imply that at least a quarter of all stars were formed during the extreme starbursts we now see as thermal background sources. The total number of (sub)mm background sources discovered with SCUBA at 850 μm (350 GHz), and with the 37-channel MAMBO array at 1.2 mm (250 GHz) now exceeds 100, and great efforts have been made trying to identify the class of known objects to which these (sub)mm sources belong. However, most of them defy a clear optical identification, and thereby an accurate redshift determination. The positional accuracies of the SCUBA and MAMBO sources are of order 5, which for each source allows for a number of faint optical and near-IR sources as possible counterparts, objects which are usually too faint for spectroscopic studies. To date, only two MAMBO sources have clear near-IR identifications, and three SCUBA sources have clear optical or near-IR identifications with a spectroscopic redshift (Lilly et al. 1999; Ivison et al. 2000). More promising are recent attempts to identify the (sub)mm background sources with radio sources in deep VLA images. We found that the majority of several dozen brightest and highest-S/N MAMBO sources in our 100 arcmin map of the Abell 2125 region have 20 cm radio counterparts within 5 of the