Probing the submillimetre number counts at f(850um) < 2mJy

TL;DR

This study presents the deepest submillimetre survey of gravitationally lensed sources, revealing numerous faint sources below previous confusion limits and providing detailed number counts that resolve almost all of the 850um background emission.

Contribution

It is the largest and deepest survey to date, detecting faint sources below 2 mJy and accurately determining number counts and background contributions at 850um.

Findings

Detected 59 sources, including 10 below 2 mJy.

Number counts at 0.5 mJy are approximately 2x10^4 deg^-2.

Resolved nearly all of the 850um background emission.

Abstract

We have conducted a submillimetre mapping survey of faint, gravitationally lensed sources, where we have targeted twelve galaxy clusters and additionally the NTT Deep Field. The total area surveyed is 71.5 arcmin^2 in the image plane; correcting for gravitational lensing, the total area surveyed is 40 arcmin^2 in the source plane for a typical source redshift z=2.5. In the deepest maps, an image plane depth of 1sigma r.m.s. ~0.8 mJy is reached. This survey is the largest survey to date to reach such depths. In total 59 sources were detected, including three multiply-imaged sources. The gravitational lensing makes it possible to detect sources with flux density below the blank field confusion limit. The lensing corrected fluxes ranges from 0.11 mJy to 19 mJy. After correcting for multiplicity there are 10 sources with fluxes <2 mJy of which 7 have sub-mJy fluxes, doubling the number of such sources known. Number counts are determined below the confusion limit. At 1 mJy the integrated number count is ~10^4 deg^-2, and at 0.5 mJy it is ~2x10^4 deg^-2. Based on the number counts, at a source plan flux limit of 0.1 mJy, essentially all of the 850um background emission has been resolved. The dominant contribution (> 50 per cent) to the integrated background arises from sources with fluxes S(850) between 0.4 and 2.5 mJy, while the bright sources S(850) > 6mJy contribute only 10 per cent.