Evidence for Environmental Effects in the z=4.3 Protocluster Core SPT2349-56

We present Atacama Large Millimeter/submillimeter Array observations of the [C I] 492 and 806 GHz 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 850 mu m 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 [C i](2-1) in the protocluster galaxies for a given 850 mu m flux density. Assuming local thermal equilibrium, we estimate gas excitation temperatures of Tex=59.1-6.8+8.1 K for our protocluster sample and Tex=33.9-2.2+2.4 K 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, as well as an elevated [C i](2-1) luminosity-to-far-infrared-luminosity ratio.