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

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange Nice, France
² School of Physics and Astronomy, Cardiff University Queen’s Buildings The Parade Cardiff CF24 3AA, UK
³ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM 91191 Gif-sur-Yvette, France
⁴ Institut de Radioastronomie Millimétrique (IRAM), Avenida Divina Pastora 7 Local 20 E-18012 Granada, Spain
⁵ Aix Marseille Univ, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille) Marseille, France
⁶ Institut Néel, CNRS, Université Grenoble Alpes Grenoble, France
⁷ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550 67000 Strasbourg, France
⁸ Astronomy Centre, Department of Physics and Astronomy, University of Sussex Brighton BN1 9QH, UK
⁹ Institut de RadioAstronomie Millimétrique (IRAM) Grenoble, France
¹⁰ Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 53 avenue des Martyrs 38000 Grenoble, France
¹¹ Dipartimento di Fisica, Sapienza Università di Roma Piazzale Aldo Moro 5 I-00185 Roma, Italy
¹² Univ. Grenoble Alpes, CNRS, IPAG 38000 Grenoble, France
¹³ Institute for Research in Fundamental Sciences (IPM), School of Astronomy Tehran, Iran
¹⁴ Centro de Astrobiología (CSIC-INTA), Torrejón de Ardoz 28850 Madrid, Spain
¹⁵ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Ioannou Metaxa and Vasileos Pavlou GR-15236 Athens, Greece
¹⁶ Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens Panepistimiopolis GR-15784 Zografos Athens, Greece
¹⁷ High Energy Physics Division, Argonne National Laboratory 9700 South Cass Avenue Lemont IL 60439, USA
¹⁸ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC 75014 Paris, France
¹⁹ Institute of Space Sciences (ICE), CSIC, Campus UAB Carrer de Can Magrans s/n E-08193 Barcelona, Spain
²⁰ ICREA Pg. Lluís Companys 23 Barcelona, Spain
²¹ IRAP, CNRS, Université de Toulouse, CNES, UT3-UPS (Toulouse), France
²² Dipartimento di Fisica, Università di Roma ‘Tor Vergata’ Via della Ricerca Scientifica 1 I-00133 Roma, Italy
²³ School of Physics and Astronomy, University of Leeds Leeds LS2 9JT, UK
²⁴ Institut d’Astrophysique de Paris, CNRS (UMR7095) 98 bis boulevard Arago 75014 Paris, France
²⁵ University of Lyon, UCB Lyon 1, CNRS/IN2P3, IP2I 69622 Villeurbanne, France

^★ Corresponding author.

Received: 14 March 2025
Accepted: 17 June 2025

Abstract

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_−19.2^+15.9±11.9 μJy/beam at 1.2 mm and 38.6_−13.1^+9.6±3.7 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_−24.8^+15.9±6.4 μJy/beam. These values are consistent with the state of the art Simulated Infrared Dusty Extragalactic Sky (SIDES) model.