Evidence for Environmental Effects in the <i>z</i> = 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mspace width="0.1em"/> <mml:mtext>C</mml:mtext> <mml:mspace width="0.1em"/> <mml:mi mathsize="small" mathvariant="normal">I</mml:mi> <mml:mo stretchy="false">]</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>2</mml:mn> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mi>′</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>/</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mspace width="0.1em"/> <mml:mtext>C</mml:mtext> <mml:mspace width="0.1em"/> <mml:mi mathsize="small" mathvariant="normal">I</mml:mi> <mml:mo stretchy="false">]</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mi>′</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">ex</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>59</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>8.1</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> </mml:math> K for our protocluster sample and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">ex</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>33</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2.2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2.4</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> </mml:math> 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.