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

Abstract

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 ${L}_{[\,\rm{C}\,{\rm\small{I}}](2-1)}^{{\prime} }/{L}_{[\,\rm{C}\,{\rm\small{I}}](1-0)}^{{\prime} }$ 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 μ 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 μ m flux density. Assuming local thermal equilibrium, we estimate gas excitation temperatures of ${T}_{{\rm{ex}}}=59.{1}_{-6.8}^{+8.1}\,$ K for our protocluster sample and ${T}_{{\rm{ex}}}=33.{9}_{-2.2}^{+2.4}\,$ 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.

Keywords

• Galaxy environments
• Protoclusters
• High-redshift galaxies