The ALPINE-ALMA [CII] survey

Circumgalactic medium pollution and gas mixing by tidal stripping in a merging system at z ∼ 4.57

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
e-mail: michele.ginolfi@unige.ch
² Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge, CB3 0HE, UK
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁴ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁵ IPAC, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA, 91125, USA
⁶ Department of Physics, University of California, Davis, One Shields Ave., Davis, CA, 95616, USA
⁷ Cosmic Dawn Center (DAWN), Lyngby, Denmark
⁸ Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen, Denmark
⁹ Scuola Normale Superiore, Piazza dei Cavalieri 7, Pisa, 56126, Italy
¹⁰ University of Padova, Department of Physics and Astronomy, Vicolo Osservatorio 3, 35122 Padova, Italy
¹¹ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japan
¹² Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, (Kavli IPMU, WPI), Kashiwa, 277-8583, Japan
¹³ Caltech Optical Observatories, Cahill Center for Astronomy and Astrophysics, 1200 East California Boulevard, Pasadena, CA, 91125, USA
¹⁴ Istituto Nazionale di Astrofisica – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁵ University of Hawai’i, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI, 96822, USA
¹⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, 21218, USA
¹⁷ University of Bologna, Department of Physics and Astronomy (DIFA), Via Gobetti 93/2, 40129 Bologna, Italy

Received: 28 April 2020
Accepted: 20 July 2020

Abstract

We present ALMA observations of a merging system at z ∼ 4.57, observed as a part of the ALMA Large Program to INvestigate [CII] at Early times (ALPINE) survey. Combining ALMA [CII]158 μm and far-infrared continuum data with multi-wavelength ancillary data, we find that the system is composed of two massive (M_⋆ ≳ 10¹⁰ M_⊙) star-forming galaxies experiencing a major merger (stellar mass ratio r_mass ≳ 0.9) at close spatial (∼13 kpc; projected) and velocity (Δv <  300 km s⁻¹) separations, and two additional faint narrow [CII]-emitting satellites. The overall system belongs to a larger scale protocluster environment and is coincident to one of its overdensity peaks. Additionally, ALMA reveals the presence of [CII] emission arising from a circumgalactic gas structure, extending up to a diameter-scale of ∼30 kpc. Our morpho-spectral decomposition analysis shows that about 50% of the total flux resides between the individual galaxy components, in a metal-enriched gaseous envelope characterised by a disturbed morphology and complex kinematics. Similarly to observations of shock-excited [CII] emitted from tidal tails in local groups, our results can be interpreted as a possible signature of interstellar gas stripped by strong gravitational interactions, with a possible contribution from material ejected by galactic outflows and emission triggered by star formation in small faint satellites. Our findings suggest that mergers could be an efficient mechanism of gas mixing in the circumgalactic medium around high-z galaxies, and thus play a key role in the galaxy baryon cycle at early epochs.