NIKA2 Cosmological Legacy Survey. First measurement of the confusion noise at the IRAM 30 m telescope

The NIKA2 Cosmological Legacy Survey (N2CLS) is a large programme using the NIKA2 dual-band camera on the IRAM 30 m telescope. Its goal is to improve our understanding of the physics of distant Dusty Star Forming Galaxies (DSFGs) by carrying out deep surveys of two fields, GOODS-North and COSMOS. This work is focussed on GOODS-North, which was observed for 78.2 hours, simultaneously at 1.2 and 2 mm, with a field of view of ∼240 arcmin2. With such a deep integration, we were able to measure, for the first time, the confusion noise limits at the 30 m telescope using the best sampled ∼62 arcmin2 and masking sources with a flux greater than 0.54 or 0.17 mJy at 1.2 or 2 mm, respectively. We found a confusion noise of $ 139.1^{+ 15.9}_{- 19.2}\pm11.9 $ μJy/beam at 1.2 mm and $ 38.6^{+ 9.6}_{- 13.1} \pm3.7 $ μJy/beam at 2 mm (the first uncertainty is statistical, the second is the cosmic variance). In this region, this corresponds to half the instrumental noise. To derive these estimates, we devised a novel estimator, referred to as the cross variance, which also enabled us to estimate the correlated confusion noise between the two bands. Thus, we obtained a result of $ 49.6^{+ 15.9}_{- 24.8}\pm 6.4 $ μJy/beam. These values are consistent with the state of the art Simulated Infrared Dusty Extragalactic Sky (SIDES) model.