2004ApJ...606..664D

We discuss identifications for 18 sources from our Max-Planck-Millimeter-Bolometer (MAMBO) 1.2 mm survey of the region surrounding the NTT Deep Field. We have obtained accurate positions from Very Large Array 1.4 GHz interferometry, and in a few cases IRAM millimeter interferometry, and have also made deep BVRIzJK imaging at ESO. We find thirteen 1.2 mm sources associated with optical/near-infrared objects in the magnitude range K=19.0-22.5, while five are blank fields at K>22. We argue from a comparison of optical/near-infrared photometric redshifts and radio/millimeter redshift estimates that two of the 13 optical/near-infrared objects are likely foreground objects distinct from the dust sources, one of them possibly lensing the millimeter source. The median redshift of the radio-identified millimeter sources is ∼2.6 from the radio/millimeter estimator, and the median optical/near-infrared photometric redshifts for the objects with counterparts is ∼2.1. This suggests that those radio-identified millimeter sources without optical/near-infrared counterparts tend to lie at higher redshifts than those with optical/near-infrared counterparts. Compared to published identifications of objects from 850 µm surveys of similar depth, the median K and I magnitudes of our counterparts are roughly 2 mag fainter, and the dispersion of I-K colors is less. Real differences in the median redshifts, residual misidentifications with bright objects, cosmic variance, and small-number statistics are likely to contribute to this significant difference, which also affects redshift measurement strategies. Some of the counterparts are red in J-K (≳20%), but the contribution of such millimeter objects to the recently studied population of near-infrared-selected (J_s-K_s>2.3) high-redshift galaxies is only of order a few percent. The recovery rate of MAMBO sources by preselection of optically faint radio sources is relatively low (∼25%), in contrast to some claims of a higher rate for Submillimeter Common-User Bolometric Array (SCUBA) sources (∼70%). In addition to this difference, the MAMBO sources also appear significantly fainter (∼1.5 mag in the I band) than radio-preselected SCUBA sources. We discuss the basic properties of the near-infrared/(sub)millimeter/radio spectral energy distributions of our galaxies and of interferometrically identified submillimeter sources from the literature. From a comparison with submillimeter objects with CO-confirmed spectroscopic redshifts, we argue that roughly two-thirds of the (sub)millimeter galaxies are at z≳2.5. This fraction is probably larger when including sources without radio counterparts.