Tracing Molecular Gas Mass in z ≃ 6 Galaxies with [C ii]
David Vizgan, T. R. Greve, Karen P. Olsen, Anita Zanella, Desika Narayanan, Romeel Davé, G. Magdis, Gergö Popping, Francesco Valentino, K. E. Heintz
Abstract
Abstract We investigate the fine-structure [C ii ] line at 158 μ m as a molecular gas tracer by analyzing the relationship between molecular gas mass ( M mol ) and [C ii ] line luminosity ( L [C II ] ) in 11,125 z ≃ 6 star-forming, main-sequence galaxies from the simba simulations, with line emission modeled by the Simulator of Galaxy Millimeter/Submillimeter Emission. Though most (∼50%–100%) of the gas mass in our simulations is ionized, the bulk (>50%) of the [C ii ] emission comes from the molecular phase. We find a sublinear (slope 0.78 ± 0.01) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathsize="small" mathvariant="normal">II</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> </mml:msub> <mml:mo>–</mml:mo> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mol</mml:mi> </mml:mrow> </mml:msub> </mml:math> relation, in contrast with the linear relation derived from observational samples of more massive, metal-rich galaxies at z ≲ 6. We derive a median [C ii ]-to- M mol conversion factor of α [C II ] ≃ 18 M ⊙ / L ⊙ . This is lower than the average value of ≃30 M ⊙ / L ⊙ derived from observations, which we attribute to lower gas-phase metallicities in our simulations. Thus, a lower, luminosity-dependent conversion factor must be applied when inferring molecular gas masses from [C ii ] observations of low-mass galaxies. For our simulations, [C ii ] is a better tracer of the molecular gas than CO J = 1–0, especially at the lowest metallicities, where much of the gas is CO-dark . We find that L [C II ] is more tightly correlated with M mol than with star formation rate (SFR), and both the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathsize="small" mathvariant="normal">II</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> </mml:msub> <mml:mo>–</mml:mo> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mol</mml:mi> </mml:mrow> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathsize="small" mathvariant="normal">II</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> </mml:msub> <mml:mo>–</mml:mo> <mml:mi>log</mml:mi> <mml:mspace width="0.25em"/> <mml:mi>SFR</mml:mi> </mml:math> relations arise from the Kennicutt–Schmidt relation. Our findings suggest that L [C II ] is a promising tracer of the molecular gas at the earliest cosmic epochs.