The ALMA-CRISTAL Survey

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 Ly α equivalent width (EW Ly α ), and a possible negative correlation between R e , CII / R e , UV and EW Ly α , 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 Ly α and H α lines will shed light on the association of the extended C II line emission with atomic gas and outflows.