We present ALMA observations of the [CI] 492 and 806GHz fine-structure lines in 25 dusty star-forming galaxies (DSFGs) at z=4.3 in the core of the SPT2349−56 protocluster. The protocluster galaxies exhibit a median L[CI](2−1)′/L[CI](1−0)′ ratio of 0.94 with an interquartile range of 0.81-1.24. These ratios are markedly different to those observed in DSFGs in the field (across a comparable redshift and 850μm flux density range), where the median is 0.55 with an interquartile range of 0.50-0.76, and we show that this difference is driven by an excess of [CI](2-1) in the protocluster galaxies for a given 850μm flux density. Assuming local thermal equilibrium, we estimate gas excitation temperatures of Tex=59.1−6.8+8.1K for our protocluster sample and Tex=33.9−2.2+2.4K for the field sample. Our main interpretation of this result is that the protocluster galaxies have had their cold gas driven to their cores via close-by interactions within the dense environment, leading to an overall increase in the average gas density and excitation temperature, and an elevated [CI](2-1) luminosity-to-far-infrared luminosity ratio.
@article{arxiv.2412.03790,
title = {Evidence for environmental effects in the $z\,{=}\,4.3$ protocluster core SPT2349$-$56},
author = {Chayce Hughes and Ryley Hill and Scott Chapman and Manuel Aravena and Melanie Archipley and Veronica J. Dike and Anthony Gonzalez and Thomas R. Greve and Gayathri Gururajan and Chris Hayward and Kedar Phadke and Cassie Reuter and Justin Spilker and Nikolaus Sulzenauer and Joaquin D. Vieira and David Vizgan and George Wang and Axel Weiss and Dazhi Zhou},
journal= {arXiv preprint arXiv:2412.03790},
year = {2025}
}