TY - JOUR

T1 - Direct Measurement of the [C i] Luminosity to Molecular Gas Mass Conversion Factor in High-redshift Star-forming Galaxies

AU - Heintz, Kasper E.

AU - Watson, Darach

PY - 2020/1/20

Y1 - 2020/1/20

N2 - The amount of cold, molecular gas in high-redshift galaxies is typically inferred from proxies of molecular hydrogen (H-2), such as carbon monoxide (CO) or neutral atomic carbon ([C i]) and molecular gas mass conversion factors. The use of these proxies, however, relies on modeling and observations that have not been directly measured outside the local universe. Here, we use recent samples of high-redshift gamma-ray burst (GRB) and quasar molecular gas absorbers to determine this conversion factor from the column density of H-2, which gives us the mass per unit column, and the [C i](J = 1) column density, which provides the luminosity per unit column. This technique allows us to make direct measurements of the relative abundances in high-redshift absorption-selected galaxies. Our sample spans redshifts of z = 1.9-3.4 and covers two orders of magnitude in gas-phase metallicity. We find that the [C i]-to-M-mol conversion factor is metallicity dependent, with alpha([C i]) scaling linearly with the metallicity: with a scatter of sigma alpha([CI]) = 0.2 dex. Using a sample of emission-selected galaxies at z similar to 0-5, with both [C i] and CO line detections, we apply the alpha([C i]) conversion to derive independent estimates of the molecular gas mass and the CO-to-M-mol, alpha(CO), conversion factor. We find a remarkable agreement between the molecular gas masses inferred from the absorption-derived alpha([C i]) compared to typical alpha(CO)-based estimates, which we confirm here to be metallicity-dependent as well, with an inferred slope that is consistent with alpha(CI) and previous estimates from the literature. These results thus support the use of the absorption-derived alpha([C i]) conversion factor for emission-selected star-forming galaxies and demonstrate that both methods probe the same universal properties of molecular gas in the local and high-redshift universe.

AB - The amount of cold, molecular gas in high-redshift galaxies is typically inferred from proxies of molecular hydrogen (H-2), such as carbon monoxide (CO) or neutral atomic carbon ([C i]) and molecular gas mass conversion factors. The use of these proxies, however, relies on modeling and observations that have not been directly measured outside the local universe. Here, we use recent samples of high-redshift gamma-ray burst (GRB) and quasar molecular gas absorbers to determine this conversion factor from the column density of H-2, which gives us the mass per unit column, and the [C i](J = 1) column density, which provides the luminosity per unit column. This technique allows us to make direct measurements of the relative abundances in high-redshift absorption-selected galaxies. Our sample spans redshifts of z = 1.9-3.4 and covers two orders of magnitude in gas-phase metallicity. We find that the [C i]-to-M-mol conversion factor is metallicity dependent, with alpha([C i]) scaling linearly with the metallicity: with a scatter of sigma alpha([CI]) = 0.2 dex. Using a sample of emission-selected galaxies at z similar to 0-5, with both [C i] and CO line detections, we apply the alpha([C i]) conversion to derive independent estimates of the molecular gas mass and the CO-to-M-mol, alpha(CO), conversion factor. We find a remarkable agreement between the molecular gas masses inferred from the absorption-derived alpha([C i]) compared to typical alpha(CO)-based estimates, which we confirm here to be metallicity-dependent as well, with an inferred slope that is consistent with alpha(CI) and previous estimates from the literature. These results thus support the use of the absorption-derived alpha([C i]) conversion factor for emission-selected star-forming galaxies and demonstrate that both methods probe the same universal properties of molecular gas in the local and high-redshift universe.

KW - LYMAN-ALPHA SYSTEMS

KW - DAMPED LYMAN

KW - METALLICITY RELATION

KW - PHYSICAL CONDITIONS

KW - ATOMIC CARBON

KW - EVOLUTION

KW - EMISSION

KW - HYDROGEN

KW - LINES

KW - H-2

U2 - 10.3847/2041-8213/ab6733

DO - 10.3847/2041-8213/ab6733

M3 - Letter

VL - 889

JO - The Astrophysical Journal Letters

JF - The Astrophysical Journal Letters

SN - 2041-8205

IS - 1

M1 - L7

ER -