The ALMA-CRISTAL survey: Extended [CII] emission in an interacting galaxy system at z ∼ 5.5

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago 8370191, Chile
² Instituto de Astrofísica, Facultad de Física, Pontifícia Universidad Católica de Chile, Santiago 7820436, Chile
³ Las Campanas Observatory, Carnegie Institution of Washington, Casilla 601, La Serena, Chile
⁴ Max-Planck-Institut für Extraterrestiche Physik (MPE), Giessen-bachstr., 85748, Garching, Germany
⁵ Institute of Astrophysics, Foundation for Research and Technology-Hellas (FORTH), Heraklion, GR-70013, Greece
⁶ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
⁷ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁸ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22903, USA
⁹ Departamento de Astronomía, Facultad Ciencias Físicas y Matemáticas, Universidad de Concepción, Av. Esteban Iturra s/n Barrio Universitario, Casilla 160, Concepción, Chile
¹⁰ INAF, Istituto di Radioastronomia – Italian node of the ALMA Regional Centre (It-ARC), Via Gobetti 101, 40129 Bologna, Italy
¹¹ Dipartimento di Chimica, Università degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, 00185 Roma, Italy
¹² Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, School of Natural Sciences, The University of Manchester, Manchester, M13 9PL, UK
¹³ European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748, Garching, Germany
¹⁴ International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Hwy, Crawley 26WA 6009, Australia
¹⁵ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
¹⁶ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281-S9, B-9000 Ghent, Belgium
¹⁷ Scuola Normale Superiore, Piazza dei Cavalieri 7, 50126 Pisa, Italy
¹⁸ Centre for Astrophysics and Supercomputing, Swinburne Univ. of Technology, PO Box 218, Hawthorn, VIC, 3122, Australia
¹⁹ Department of Astronomical Science, SOKENDAI (The Graduate University for Advanced Studies), Mitaka, Tokyo 181-8588, Japan
²⁰ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²¹ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
²² Dept. Fisica Teorica y del Cosmos, Universidad de Granada, Granada, Spain
²³ Instituto Universitario Carlos I de Física Teórica y Computacional, Universidad de Granada, E-18071 Granada, Spain
²⁴ Department of Physics and Astronomy and George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, USA
²⁵ Max-Planck Institute for Astrophysics, Karl Schwarzschildstrasse 1, 85748, Garching, Germany
²⁶ Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan
²⁷ Ioffe Institute, 26 Politekhnicheskaya, St. Petersburg, 194021, Russia
²⁸ Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Avenue, Cambridge, CB3 0HE, UK
²⁹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK

^⋆ Corresponding author: ana.posses@mail.udp.cl

Received: 4 March 2024
Accepted: 9 May 2025

Abstract

The ALMA [C II] Resolved Ism in STar-forming gALaxies (CRISTAL) survey is a Cycle 8 ALMA Large Program that studies the cold- gas component of high-redshift galaxies. Its subarcsecond-resolution observations are key to distinguishing physical mechanisms that shaped galaxies during cosmic dawn. In this paper, we explore the morphology and kinematics of the cold gas, star-forming, and stellar components in the star-forming main-sequence galaxy CRISTAL-05/HZ3, at z = 5.54. Our analysis includes ALMA observations at a spatial resolution of 0.3″ (∼2 kpc) of the [C II] line. While CRISTAL-05 was previously classified as a single source, our observations reveal that the system is a close interacting pair that is surrounded by an extended component of carbon-enriched gas. This is imprinted in the disturbed elongated [C II] morphology and in the separation of the two components in the position-velocity diagram (∼100  km s⁻¹ ). The central region is composed of two components, named C05-NW and C05-SE, and the former is the dominant component. A significant fraction of [C II] arises beyond the close pair up to 10 kpc, while the regions forming new massive stars and the stellar component seem compact (r_[C II]∼4 × r_UV), as traced by rest-frame UV and optical imaging obtained with the Hubble Space Telescope and the James Webb Space Telescope. Our kinematic model, constructed using the DYSMALpy software, yields a minor contribution of dark matter of C05-NW within a radius of ∼2 × R_eff. Finally, we explore the resolved [C II] /far-IR ratios as a proxy for shock-heating produced by this merger. We argue that the extended [C II] emission is mainly caused by the merger of the galaxies, which could not be discerned with lower-resolution observations. Our work emphasizes the need for high-resolution observations to fully characterize the dynamic stages of infant galaxies and the physical mechanisms that drive the metal enrichment of the circumgalactic medium.