Evidence for environmental effects in the $z=4.3$ protocluster core SPT2349$-$56

Abstract

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 $L_{[\text{CI}](2-1)}^{\prime }/L_{[\text{CI}](1-0)}^{\prime }$ 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$\hat{I}¼$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 [CI](2-1) in the protocluster galaxies for a given 850$\hat{I}¼$m flux density. Assuming local thermal equilibrium, we estimate gas excitation temperatures of $T_{\mathrm{ex}}=59.1_{-6.8}^{+8.1}$K for our protocluster sample and $T_{\mathrm{ex}}=33.9_{-2.2}^{+2.4}$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, and an elevated [CI](2-1) luminosity-to-far-infrared luminosity ratio.