2020ApJ...900..131L

Observing the interstellar medium (ISM) in the z >= 6 quasar host galaxies is essential for understanding the coevolution between supermassive black holes and their hosts. To probe the gas physical conditions and search for imprints of active galactic nuclei (AGNs) on the ISM, we report ALMA observations of the [N II]_122 µm_ and [O I]_146 µm_ lines and the underlying continuum from the z = 6.003 quasar SDSS J231038.88+185519.7. Together with previous [C II]_158 µm_ and [O III]_88 µm_ observations, we use the ratios of these fine-structure lines to probe the ISM properties. Similar to other high-z systems, this object exhibits a [C II]_158 µm_/[O I]_146 µm_ ratio comparable to the lowest values found in local (ultra)luminous infrared galaxies, suggesting a "warmer" and "denser" gas component compared to typical local systems. The [O III]_88 µm_/[O I]_146 µm_ ratio is lower than that of other local and high-z systems, indicating a smaller ionized gas fraction in this quasar. The [O III]_88 µm_/[N II]_122 µm_ ratio is comparable to that of local systems and suggests a metallicity of Z/Z_☉ = 1.5-2.1. Based on the [N II]_122 µm_ detection, we estimate that 17% of the [C II]_158 µm_ emission is associated with ionized gas. The [N II]_122 µm_ line shows a "flux deficit" comparable to local systems. The [O I]_146 µm_ line, with an [O I]_146 µm_/FIR ratio >2x than expected from the local relation, indicates no [O I]_146 µm_ deficit. The low [C II]_158 µm_/[O I]_146 µm_ ratio, together with the high [O I]_146 µm_/FIR ratio in J2310+1855, reveals that the warm and dense gas is likely a result of AGN heating of the ISM.