The ALPINE-ALMA [CII] survey: The population of [CII]-undetected galaxies and their role in the L_[CII]-SFR relation

¹ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
e-mail: michael.romano@studenti.unipd.it
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ National Center for Nuclear Research, ul. Pasteura 7, 02-093 Warsaw, Poland
⁴ Associazione Big Data, Via Piero Gobetti 101, 40129 Bologna, Italy
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁶ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁷ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁸ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
⁹ IPAC, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
¹⁰ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo Kashiwa, Chiba 277-8583, Japan
¹¹ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
¹² The Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
¹³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁴ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile
¹⁵ Cosmic Dawn Center (DAWN), Jagtvej 128, 22000 Copenhagen N, Denmark
¹⁶ Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
¹⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁸ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
¹⁹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
²⁰ Department of Physics and Astronomy, University of California, Davis, One Shields Ave., Davis, CA 95616, USA
²¹ Gemini Observatory, NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, Hawai’i 96720, USA
²² Insituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, 2340000 Valparaíso, Chile
²³ Department of Astronomy, University of Florida, 211 Bryant Space Sciences Center, Gainesville 32611, USA
²⁴ University of Florida Informatics Institute, 432 Newell Drive, CISE Bldg E251, Gainesville 32611, USA
²⁵ Cosmic Dawn Center at the Niels Bohr Institute, University of Copenhagen and DTU-Space, Technical University of Denmark, Denmark
²⁶ University of Bologna – Department of Physics and Astronomy “Augusto Righi” (DIFA), Via Gobetti 93/2, 40129 Bologna, Italy

Received: 20 September 2021
Accepted: 4 February 2022

Abstract

The [CII] 158 μm emission line represents one of the most profitable tools for the investigation of the high-redshift galaxies in the early Universe so far. Being one of the brightest cooling lines in the rest-frame far-infrared regime of star-forming galaxies, it has been successfully exploited as a tracer of the star-formation rate (SFR) in local sources. The picture is more complex at higher redshifts, where its usability in this context is still under investigation. Recent results from the ALMA Large Program to INvestigate [CII] at Early times (ALPINE) survey suggest that there is no (or weak) evolution of the L_[CII]-SFR relation up to z ∼ 6, but their reliability is hampered by the presence of a large population of [CII] nondetected galaxies. In this work, we characterize the population of [CII] nondetections in ALPINE. By stacking their ALMA spectra, we obtained a signal detected at ∼5.1σ, resulting in a [CII] luminosity of log(L_[CII]/L_⊙)∼7.8. When combining this value with those from the [CII] detections, we found a L_[CII]-SFR relation with a slope b = 1.14 ± 0.11, which is in agreement within the uncertainties both with the linear relation found in the local Universe and with the previous findings from ALPINE at z ∼ 5. This suggests that the [CII] line can be considered a good tracer of star formation up to the distant Universe. Finally, we show that the galaxies of our sample that deviate from the observed L_[CII]-SFR relation most could suffer from a less precise redshift estimation, perhaps artificially reducing their [CII] luminosity. In this respect, we claim that there is no evidence in favor of a deficit of [CII] content in high-z galaxies, in contrast with earlier studies.