The ALMA-CRISTAL Survey

Spatial extent of [C II] line emission in star-forming galaxies at z = 4−6

¹ Department of Astronomy, School of Science, SOKENDAI (The Graduate University for Advanced Studies), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³ Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan
⁴ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
⁵ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Avenida Ejercito Libertador 441, Santiago, Chile
⁶ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281 – S9 B-9000 Ghent, Belgium
⁷ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁸ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Santiago 7820436, Chile
⁹ Las Campanas Observatory, Carnegie Institution of Washington, Raúl Bitrán 1200, La Serena, Chile
¹⁰ Departamento de Astronomía, Universidad de Concepción, Barrio Universitario, Concepción, Chile
¹¹ Department of Physics and Astronomy and George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, 4242 TAMU, College Station, TX 77843-4242, USA
¹² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹³ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22903, USA
¹⁴ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, School of Natural Sciences, The University of Manchester, Manchester M13 9PL, UK
¹⁵ German Aerospace Center (DLR), Institute of Communications and Navigation, Wessling, Germany
¹⁶ International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, M468, 35 Stirling Highway, Crawley, WA 6009, Australia
¹⁷ ARC Center of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
¹⁸ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
¹⁹ Institute of Astrophysics, Foundation for Research and Technology-Hellas (FORTH), Heraklion 70013, Greece
²⁰ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
²¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
²² Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
²⁴ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
²⁵ Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
²⁶ Joint Space-Science Institute, University of Maryland, College Park, Maryland 20742, USA

^⋆ Corresponding author; ryota195ikeda@gmail.com

Received: 6 August 2024
Accepted: 11 December 2024

Abstract

We investigate the spatial extent and structure of the [C II] line emission in a sample of 34 galaxies at z = 4 − 6 from the [C II] Resolved ISM in STar-forming galaxies with ALMA (CRISTAL) Survey. By modeling the distribution of the [C II] line emission in the interferometric visibility data directly, we derive the effective radius of [C II] line emission assuming an exponential profile. These measurements comprise not only isolated galaxies but also interacting systems that were identified thanks to the high spatial resolution of the data. The [C II] line radius ranges from 0.5 to 3.5 kpc with an average value of ⟨R_e, [CII]⟩ = 1.90 kpc. We compare the [C II] sizes with the sizes of rest-frame ultraviolet (UV) and far-infrared (FIR) continua, which were measured from the HST F160W images and ALMA Band-7 continuum images, respectively. We confirm that the [C II] line emission is more spatially extended than the continuum emission, with average size ratios of ⟨R_e, [CII]/R_e, UV⟩ = 2.90 and ⟨R_e, [CII]/R_e, FIR⟩ = 1.54, although about half of the FIR-detected sample shows a comparable spatial extent between the [C II] line and the FIR continuum emission (R_e, [CII] ≈ R_e, FIR). The residual visibility data of the best-fit model do not show statistical evidence of flux excess, indicating that the [C II] line emission in star-forming galaxies can be characterized by an extended exponential disk profile. Overall, our results suggest that the spatial extent of [C II] line emission can primarily be explained by photodissociation regions associated with star formation activity, while the contribution from diffuse neutral medium (atomic gas) and the effects of past merger events may further expand the [C II] line distributions, causing their variations among our sample. Finally, we report the negative correlation between the [C II] surface density (Σ_[CII]) and the Lyα equivalent width (EW_Lyα), and a possible negative correlation between R_e, [CII]/R_e, UV and EW_Lyα, which may be in line with the scenario that atomic gas component largely contributes to the extended [C II] line emission. Future three-dimensional analysis of Lyα and Hα lines will shed light on the association of the extended [C II] line emission with atomic gas and outflows.