NIKA2 Cosmological Legacy Survey

Survey description and galaxy number counts

¹ Aix-Marseille Univ., CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille), Marseille, France
e-mail: longji.bing@lam.fr
² Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
⁴ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff, CF24 3AA, UK
⁵ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris-Diderot, Sorbonne Paris-Cité, 91191 Gif-sur-Yvette, France
⁶ Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 53, avenue des Martyrs, 38000 Grenoble, France
⁷ Max-Planck-Institut fur Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁸ Institut Néel, CNRS, Université Grenoble Alpes, France
⁹ Institut de RadioAstronomie Millimétrique (IRAM), Grenoble, France
¹⁰ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, Versoix, Switzerland
¹¹ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
¹² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹³ Centro de Astrobiología (CSIC-INTA), Torrejón de Ardoz, 28850 Madrid, Spain
¹⁴ High Energy Physics Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
¹⁵ Instituto de Radioastronomía Milimétrica (IRAM), Granada, Spain
¹⁶ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC, 75014 Paris, France
¹⁷ IFPU-Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
¹⁸ INAF-Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
¹⁹ School of Earth and Space Exploration and Department of Physics, Arizona State University, Tempe, AZ 85287, USA
²⁰ Laboratoire de Physique de l’École Normale Supérieure, ENS, PSL Research University, CNRS, Sorbonne Université, Université de Paris, 75005 Paris, France
²¹ INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
²² Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA
²³ Institut d’Astrophysique de Paris, Sorbonne Université, CNRS (UMR7095), Paris, France
²⁴ University of Lyon, UCB Lyon 1, CNRS/IN2P3, IP2I, 69622 Villeurbanne, France

Received: 3 April 2023
Accepted: 11 May 2023

Abstract

Context. Finding and characterizing the heavily obscured galaxies with extreme star formation up to very high redshift is key for constraining the formation of the most massive galaxies in the early Universe. It has been shown that these obscured galaxies are major contributors to the accumulation of stellar mass to z ~ 4. At higher redshift, and despite recent progress, the contribution of dust-obscured galaxies remains poorly known.

Aims. Deep surveys in the millimeter domain are necessary in order to probe the dust-obscured galaxies at high redshift. We conducted a large observing program at 1.2 and 2 mm with the NIKA2 camera installed on the IRAM 30m telescope. This NIKA2 Cosmological Legacy Survey (N2CLS) covers two emblematic fields: GOODS-N and COSMOS. We introduce the N2CLS survey and present new 1.2 and 2 mm number counts measurements based on the tiered N2CLS observations (from October 2017 to May 2021) covering 1169 arcmin².

Methods. After a careful data reduction and source extraction, we develop an end-to-end simulation that combines an input sky model with the instrument noise and data reduction pipeline artifacts. This simulation is used to compute the sample purity, flux boosting, pipeline transfer function, completeness, and effective area of the survey (taking into account the non-homogeneous sky coverage). For the input sky model, we used the 117 square degree SIDES simulations, which include galaxy clustering. Our formalism allows us to correct the source number counts to obtain galaxy number counts, the difference between the two being due to resolution effects caused by the blending of several galaxies inside the large beam of single-dish instruments.

Results. The N2CLS-May2021 survey is already the deepest and largest ever made at 1.2 and 2 mm. It reaches an average 1σ- noise level of 0.17 and 0.048 mJy on GOODS-N over 159 arcmin², and 0.46 and 0.14 mJy on COSMOS over 1010 arcmin², at 1.2 and 2 mm, respectively. For a purity threshold of 80%, we detect 120 and 67 sources in GOODS-N and 195 and 76 sources in COSMOS at 1.2 and 2 mm, respectively. At 1.2 mm, the number counts measurement probes consistently 1.5 orders of magnitude in flux density, covering the full flux density range from previous single-dish surveys and going a factor of 2 deeper into the sub-mJy regime. Our measurement connects the bright single-dish to the deep interferometric number counts. At 2 mm, our measurement matches the depth of the deepest interferometric number counts and extends a factor of 2 above the brightest constraints. After correcting for resolution effects, our results reconcile the single-dish and interferometric number counts, which can be further accurately compared with model predictions.

Conclusions. While the observation in GOODS-N have already reached the target depth, we expect the final N2CLS survey to be 1.5 times deeper for COSMOS. Thanks to its volume-complete flux selection, the final N2CLS sample will be an ideal reference for conducting a full characterization of dust-obscured galaxies at high redshift.