The NIKA2 cosmological legacy survey at 2 mm: Catalogs, colors, redshift distributions, and implications for deep surveys

¹ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
² Aix Marseille Univ, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille), Marseille, France
³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁴ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK
⁵ Université Paris Cité, Université Paris-Saclay, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette, France
⁶ Institut de Radioastronomie Millimétrique (IRAM), Avenida Divina Pastora 7, Local 20, E-18012 Granada, Spain
⁷ Institut Néel, CNRS, Université Grenoble Alpes, Grenoble, France
⁸ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, United Kingdom
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 Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
¹⁶ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Ioannou Metaxa and Vasileos Pavlou, GR-15236 Athens, Greece
¹⁷ Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis, GR-15784 Zografos, Athens, Greece
¹⁸ High Energy Physics Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
¹⁹ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC, 75014 Paris, France
²⁰ Institute of Space Sciences (ICE), CSIC, Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
²¹ ICREA, Pg. Lluís Companys 23 Barcelona, Spain
²² IRAP, CNRS, Université de Toulouse, CNES, UT3-UPS, (Toulouse), France
²³ Dipartimento di Fisica, Università di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1, I-00133 Roma, Italy
²⁴ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
²⁵ Institut d’Astrophysique de Paris, CNRS (UMR7095), 98 bis boulevard Arago, 75014 Paris, France
²⁶ University of Lyon, UCB Lyon 1, CNRS/IN2P3, IP2I, 69622 Villeurbanne, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 June 2025
Accepted: 28 January 2026

Abstract

Aims. Millimeter (mm) galaxy surveys are particularly effective in detecting dusty star-forming galaxies at high redshift. While such observations are typically conducted at ∼1 mm, various authors have suggested over the last 20 years that 2 mm observations may be better suited for selecting sources at even higher redshifts. In this work, we use the unprecedented 2 mm data from the NIKA2 Cosmological Legacy Survey (N2CLS), together with the simulated infrared dusty extragalactic sky (SIDES), to study and interpret the statistical properties of galaxies selected at this wavelength.

Methods. We used the N2CLS robust sample (95% purity) at 2 mm (∼18 arcsec resolution), which contains 25 sources in the deep GOODS-N field (159 arcmin², 0.047 mJy/beam RMS) and 90 sources in the wide COSMOS field (1130 arcmin², 0.09 mJy/beam RMS). The sources were matched with the N2CLS 1.2 mm sources, ancillary 850 μm sources, and redshift catalogs to study the colors and redshift distributions. We also produced end-to-end simulations based on SIDES and the observed N2CLS detector timelines to interpret the data.

Results. We found a mean S_2 mm/S_1.2 mm color of 0.222 ± 0.008 with a standard deviation of 0.070 ± 0.010, mainly caused by resolution and source extraction effects according to the SIDES simulation. We measured a mean redshift of 3.6 ± 0.3 in GOODS-N, which is marginally higher than the expectations from SIDES (2.9 ± 0.2) because of an overdensity at z ∼ 5.2, and 3.2 ± 0.2 in COSMOS, which is identical to that obtained in SIDES. We also show that the observed S_2 mm/S_1.2 mm colors exhibit a weak dependence with redshift but a large dispersion, which limits its efficiency with respect to selecting high-z sources. We compared the measured fluxes of 2 mm sources detected by both the N2CLS and the Ex-MORA surveys. The results only agree if we take into account the impact of the bandwidth, source blending, and source size. Finally, we studied the eight 2 mm sources that had not been detected at 1.2 mm and found that two of them are radiogalaxies and one is a z ∼ 2 galaxy, while the remaining six (all in COSMOS) are compatible with the expected number of spurious detections. Consequently, the N2CLS survey shows no evidence of any exotic dusty galaxy population with either very cold dust or at very high redshift, which would only be detectable at 2 mm. Using SIDES, we show that 2 mm samples have a higher mean redshift compared to 1.2 mm because they miss dusty galaxies around cosmic noon (z ∼ 2). Finally, we discuss the efficiency of single-dish and interferometric blind surveys to build samples of high-z dusty galaxies.