ALMA Lensing Cluster Survey: A spectral stacking analysis of [C II] in lensed z ∼ 6 galaxies

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: jean.jolly@chalmers.se
² Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁴ CRAL, Observatoire de Lyon, Université Lyon 1, 9 Avenue Ch. André, 69561 Saint Genis Laval Cedex, France
⁵ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁶ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁷ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
⁸ Research Center for the Early Universe, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁹ Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, Nanjing, PR China
¹⁰ School of Astronomy and Space Science, University of Science and Technology of China, Hefei, Anhui, PR China
¹¹ Instituto de Astrofisica, Facultad de Fisica, Pontificia Universidad Catolica de Chile Av. Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile
¹² Millennium Institute of Astrophysics (MAS), Nuncio Monse nor Santero Sanz 100, Providencia, Santiago, Chile
¹³ Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson 85721, USA
¹⁴ SKA Organisation, Lower Withington, Macclesfield, Cheshire SK11 9DL, UK
¹⁵ Departamento de Física Teórica y del Cosmos, Campus de Fuentenueva, Universidad de Granada, E18071 Granada, Spain
¹⁶ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
¹⁷ DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹⁸ CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
¹⁹ Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan
²⁰ Physics Department, Ben-Gurion University of the Negev, PO Box 653, Be’er-Sheva 84105, Israel

Received: 25 March 2021
Accepted: 7 June 2021

Abstract

Context. The properties of galaxies at redshift z > 6 hold the key to our understanding of the early stages of galaxy evolution and can potentially identify the sources of the ultraviolet radiation that give rise to the epoch of reionisation. The far-infrared cooling line of [C II] at 158 μm is known to be bright and correlate with the star formation rate (SFR) of low-redshift galaxies, and hence is also suggested to be an important tracer of star formation and interstellar medium properties for very high-redshift galaxies.

Aims. With the aim to study the interstellar medium properties of gravitationally lensed galaxies at z > 6, we search for [C II] and thermal dust emission in a sample of 52 z ∼ 6 galaxies observed by the ALMA Lensing Cluster Survey.

Methods. We perform our analysis using LINESTACKER, stacking both [C II] and continuum emission. The target sample is selected from multiple catalogues, and the sample galaxies have spectroscopic redshift or low-uncertainty photometric redshifts (σ_z < 0.02) in nine galaxy clusters. Source properties of the target galaxies are either extracted from the literature or computed using spectral energy distribution fitting. Both weighted-average and median stacking are used, on both the full sample and three sub-samples.

Results. Our analyses find no detection of either [C II] or continuum. An upper limit on L_[CII] is derived, implying that [C II] remains marginally consistent for low-SFR z > 6 galaxies but likely is under-luminous compared to the local L_[CII]-SFR relationship. We discuss potential biases and possible physical effects that may be the cause of the non-detection. Further, the upper limit on the dust continuum implies that less than half of the star formation is obscured.