A 1200 μm MAMBO Survey of the ELAISN2 and Lockman Hole East Fields

Using the MPIfR Max Planck Millimeter Bolometer array (MAMBO) on the IRAM 30m Telescope we have mapped the ELAISN2 and Lockman Hole East Fields at 1200μm to a rms noise level of 0.8−1.0mJy per 11′′ beam. The areas surveyed are 326 arcmin in the ELAISN2 field and 212 arcmin in the Lockman Hole, and cover the 260 arcmin previously observed by SCUBA [5]. The 1200μm number counts derived from the survey are shown in Fig. 1a (Greve et al. in prep.). At flux levels ∼ 3.5mJy the power-law slope of the number counts is about α ∼ −1.6, while at the brighter end there is evidence for a turn-over in the number counts, as is illustrated by the fact that the data are well matched by an integrated Schechter function with a knee at 3.5mJy. At a redshift of 2.5, this corresponds to a far-IR luminosity of 10 L⊙ assuming a modified black body law with β = 1.5 and Td = 40K. For comparison we have also plotted the 850μm counts from the HDF-N SCUBA Supermap [1], scaled by a factor of S850μm/S1200μm = 2.5 which is expected for a starburst galaxy at z = 2.5 [2]. Even though this scaling-factor is highly uncertain, the agreement between the 1200μm and scaled 850μm counts in terms of the shape of the number counts is remarkably good. Deep radio observations currently provide the most efficient way of determining the exact positions of (sub)-mm sources, and thus positively identifying them in the optical/NIR [3,6]. Using deep Very Large Array radio maps [4] we have searched for statistically robust radio counterparts within 6 of each of the MAMBO sources in our sample. We find that about two-thirds of the MAMBO sources have counterparts in the radio, which is comparable to the radio-identification fraction found for SCUBA sources [4]. The MAMBO source shown in Fig. 1b is associated with a very strong radio counterpart (S1.4GHz = 189μJy) which lies on top of a compact optical/NIR galaxy. A Keck LRIS-B spectrum of this source reveals that it is a type II QSO at z = 2.6 (Ivison et al. in prep.). This source lies well within the SCUBA map yet is not included in the ≥ 3.0σ SCUBA catalogue [5]. While it is conceivable that a certain fraction of the 1200μm sources might be at extremely high redshifts (z > 8) and 1 The Lockman data are part of the MAMBO 1sq. deg. survey (Bertoldi et al. in prep.)