THE FAINT COUNTERPARTS OF MAMBO MILLIMETER SOURCES NEAR THE NEW TECHNOLOGY TELESCOPE DEEP FIELD

摘要

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 opticalear-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 opticalear-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 opticalear-infrared photometric redshifts for the objects with counterparts is ～2.1. This suggests that those radio-identified millimeter sources without opticalear-infrared counterparts tend to lie at higher redshifts than those with opticalear-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 (approx> 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 zapprox> 2.5. This fraction is probably larger when including sources without radio counterparts.