Evidence for environmental effects in the $z\,{=}\,4.3$ protocluster core SPT2349$-$56

TL;DR

This study uses ALMA observations to compare the [CI] line ratios in dusty star-forming galaxies within a $z=4.3$ protocluster to those in the field, revealing environmental effects on gas excitation and density.

Contribution

It provides the first detailed comparison of [CI] line ratios in protocluster versus field galaxies at high redshift, highlighting environmental influences on gas properties.

Findings

Protocluster galaxies show higher [CI](2-1) emission than field galaxies.

Gas excitation temperatures are significantly higher in protocluster galaxies.

Environmental effects likely drive gas inflows and increase gas density in dense regions.

Abstract

We present ALMA observations of the [CI] 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 $L^\prime_{[\text{CI}](2-1)}/L^\prime_{[\text{CI}](1-0)}$ 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 [CI](2-1) in the protocluster galaxies for a given 850$\,\mu$m flux density. Assuming local thermal equilibrium, we estimate gas excitation temperatures of $T_{\rm ex}\,{=}\,59.1^{+8.1}_{-6.8}\,$K for our protocluster sample and $T_{\rm ex}\,{=}\,33.9^{+2.4}_{-2.2}\,$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.