Ionized and atomic interstellar medium in the z = 6.003 quasar SDSS
  J2310+1855
  Observing the interstellar medium (ISM) in $z \gtrsim 6$ quasars host
galaxies is essential for understanding the co-evolution between the
supermassive black holes and their hosts. To probe the gas physical conditions
and search for imprints of Active Galactic Nuclei (AGN) on the ISM, we report
ALMA observations of the $\rm [N\ II]_{122 \mu m}$ and $\rm [O\ I]_{146 \mu m}$
lines and the underlying continuum from the $z=6.003$ quasar SDSS
J231038.88+185519.7. Together with previous $\rm [C\ II]_{158 \mu m}$ and $\rm
[O\ III]_{88 \mu m}$ 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 $\rm [C\ II]_{158 \mu m}$/$\rm [O\ I]_{146 \mu m}$ 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 $\rm [O\ III]_{88 \mu m}$/$\rm [O\ I]_{146 \mu m}$ ratio is
lower than that of other local and high-$z$ systems, indicating a smaller
ionized gas fraction in this quasar. The $\rm [O\ III]_{88 \mu m}$/$\rm [N\
II]_{122 \mu m}$ ratio is comparable to that of local systems, and suggests a
metallicity of $Z/Z_{\odot}$=1.5$-$2.1. Based on the $\rm [N\ II]_{122 \mu m}$
detection, we estimate that $17\%$ of the $\rm [C\ II]_{158 \mu m}$ emission is
associated with ionized gas. The $\rm [N\ II]_{122 \mu m}$ line shows a "flux
deficit" comparable to local systems. The $\rm [O\ I]_{146 \mu m}$ line, with a
$\rm [O\ I]_{146 \mu m}$/FIR ratio $\ge 2\times$ than expected from the local
relation, indicates no $\rm [O\ I]_{\rm 146 \mu m}$ deficit. The low $\rm [C\
II]_{158 \mu m}$/$\rm [O\ I]_{146 \mu m}$ ratio, together with the high $\rm
[O\ I]_{146 \mu m}$/FIR ratio in J2310+1855, reveals that the warm and dense
gas is likely a result of AGN heating to the ISM.
