ar X iv : a st ro - p h / 02 01 10 4 v 1 8 J an 2 00 2 Properties of mm galaxies : Constraints from K band blank fields 1

We have used the IRAM Plateau de Bure mm interferometer to locate with sub-arcsecond accuracy the dust emission of three of the brightest 1.2 mm sources in the NTT Deep Field (NDF) selected from our 1.2mm MAMBO survey at the IRAM 30m telescope. We combine these results with deep B to K imaging and VLA interferometry. Reliable identifications are an essential step towards an understanding of the high redshift (sub)mm galaxy population, towards testing the common belief that they are scaled up analogs of local dusty ultraluminous galaxies, and in shedding light on the possible connection to spheroid formation. Strikingly, none of the three accurately located mm galaxies MMJ120546-0741.5, MMJ120539-0745.4, and MMJ120517-0743.1 has a K-band counterpart down to the faint limit of K s >21.9. This implies that these three galaxies are either extremely obscured and/or are at very high redshifts (z4). We combine our results with literature data for 11 more (sub)mm galaxies that are identified with similar reliability. In terms of their K band properties, the sample divides into three roughly equal groups: (i) undetected to K∼22, (ii) detected in the near-infrared but not the optical and (iii) detected in the optical with the possibility of optical follow up spectroscopy. We find a correlation in this sample between near-infrared to submm and submm to radio spectral indices, suggesting that the main factor causing the extreme near-infrared faintness of our objects is their high redshift. We show that the near-infrared to radio spectral energy distributions (SEDs) of the sample are inconsistent with SEDs that resemble local far-infrared 1 Based on observations collected at ESO, La Silla, Chile, at the VLA, and on observations carried out with the IRAM PdBI. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The Very Large Array is a facility of the National Radio Astronomy Observatory, which is operated by Associated Universities Inc., under cooperative agreement with the National Science Foundation. – 2 – cool galaxies with moderate luminosities, which were proposed in some models of the submm sky. We briefly discuss the implications of the results for our understanding of galaxy formation.