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

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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 [ C I ] ( 2 − 1 ) ′ / L [ C I ] ( 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 μm 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 μm flux density. Assuming local thermal equilibrium, we estimate gas excitation temperatures of T ex = 59 . 1 − 6.8 + 8.1 K for our protocluster sample and T ex = 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.