The ALPINE-ALMA [CII] survey

Small Lyα-[CII] velocity offsets in main-sequence galaxies at 4.4 < z < 6

¹ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio, 3, 35122 Padova, Italy
e-mail: paolo.cassata@unipd.it
² INAF Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA, 91125, USA
⁴ Observatoire de Genève, Université de Genève 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁵ Aix Marseille Université, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille), 13013 Marseille, France
⁶ The Cosmic Dawn Center (DAWN), University of Copenhagen, Vibenshuset, Lyngbyvej 2, 2100 Copenhagen, Denmark
⁷ Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen, Denmark
⁸ Institut de Recherche en Astrophysique et Planétologie – IRAP, CNRS, Université de Toulouse, UPS-OMP, 14, avenue E. Belin, 31400 Toulouse, France
⁹ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kavli IPMU, WPI, Kashiwa, 277-8583, Japan
¹⁰ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japan
¹¹ Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA, 91125, USA
¹² Department of Physics, University of California, Davis, One Shields Ave., Davis, CA, 95616, USA
¹³ University of Bologna, Department of Physics and Astronomy (DIFA), Via Gobetti 93/2, 40129 Bologna, Italy
¹⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
¹⁵ Centro de Astronomia (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile
¹⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁷ Department of Astronomy, University of Geneva, ch. des Maillettes 51, 1290 Versoix, Switzerland
¹⁸ Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo, 169-8555, Japan
¹⁹ Astronomy Department, University of Massachusetts, Amherst, MA, 01003, USA
²⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
²¹ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
²² Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge, CB3 0HE, UK
²³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
²⁴ Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY, 14853, USA
²⁵ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 16 January 2020
Accepted: 14 June 2020

Abstract

Context. The Lyman-α line in the ultraviolet (UV) and the [CII] line in the far-infrared (FIR) are widely used tools to identify galaxies in the early Universe and to obtain insights into interstellar medium (ISM) properties in high-redshift galaxies. By combining data obtained with ALMA in band 7 at ∼320 GHz as part of the ALMA Large Program to INvestigate [CII] at Early Times (ALPINE) with spectroscopic data from DEIMOS at the Keck Observatory, VIMOS and FORS2 at the Very Large Telescope, we assembled a unique sample of 53 main-sequence star-forming galaxies at 4.4 <  z <  6 in which we detect both the Lyman-α line in the UV and the [CII] line in the FIR.

Aims. The goal of this paper is to constrain the properties of the Lyα emission in these galaxies in relation to other properties of the ISM.

Methods. We used [CII], observed with ALMA, as a tracer of the systemic velocity of the galaxies, and we exploited the available optical spectroscopy to obtain the Lyα-[CII] and ISM-[CII] velocity offsets.

Results. We find that 90% of the selected objects have Lyα-[CII] velocity offsets in the range 0 <  Δv_{Lyα − [CII]} <  400 km s⁻¹, in line with the few measurements available so far in the early Universe, and significantly smaller than those observed at lower redshifts. At the same time, we observe ISM-[CII] offsets in the range −500 <  Δv_ISM−[CII] <  0 km s⁻¹, in line with values at all redshifts, which we interpret as evidence for outflows in these galaxies. We find significant anticorrelations between Δv_Lyα−[CII] and the Lyα rest-frame equivalent width EW₀(Lyα) (or equivalently, the Lyα escape fraction f_esc(Lyα)): galaxies that show smaller Δv_Lyα−[CII] have larger EW₀(Lyα) and f_esc(Lyα).

Conclusions. We interpret these results in the framework of available models for the radiative transfer of Lyα photons. According to the models, the escape of Lyα photons would be favored in galaxies with high outflow velocities, producing large EW₀(Lyα) and small Δv_Lyα-[CII], in agreement with our observations. The uniform shell model would also predict that the Lyα escape in galaxies with slow outflows (0 <  v_out <  300 km s⁻¹) is mainly determined by the neutral hydrogen column density (NHI) along the line of sight, while the alternative model by Steidel et al. (2010, ApJ, 717, 289) would more highly favor a combination of NHI at the systemic velocity and covering fraction as driver of the Lyα escape. We suggest that the increase in Lyα escape that is observed in the literature between z ∼ 2 and z ∼ 6 is not due to a higher incidence of fast outflows at high redshift, but rather to a decrease in average NHI along the line of sight, or alternatively, a decrease in HI covering fraction.