Unveiling the warm dense ISM phase in z>6 QSOs

We modeled the H_2_O lines using the MOLPOP-CEP radiative transfer code, finding that water vapor lines in our quasar host galaxies are primarily excited in the warm, dense (with a gas kinetic temperature and density of T_kin_=50K, n_H2_~10^4.5^-10^5^cm^-3^) molecular medium with a water vapor column density of N_H2O_~2x10^17^-3x10^18^cm^-3^. High-J H_2_O lines are mainly radiatively pumped by the intense optically-thin far-IR radiation field associated with a warm dust component at temperatures of T_dust_~80-190K that account for <5-10% of the total dust mass.

In the case of J2310+1855, our analysis points to a relatively high value of the continuum optical depth at 100{mu}m ({tau}_100_~1). Our results are in agreement with expectations based on the H_2_O spectral line energy distribution of local and high-z ultra-luminous IR galaxies and active galactic nuclei (AGN). The analysis of the Boltzmann diagrams highlights the interplay between collisions and IR pumping in populating the high H_2_O energy levels and it allows us to directly compare the excitation conditions in the targeted quasar host galaxies.

In addition, the observations enable us to sample the high-luminosity part of the H_2_O-total-IR (TIR) luminosity relations (L_H2O_-L_TIR_). Overall, our results point to supralinear trends that suggest H_2_O-TIR relations are likely driven by IR pumping, rather than the mere co-spatiality between the FIR continuum- and line-emitting regions. The observed L_H2O/L_TIR_ ratios in our z>6 quasars do not show any strong deviations with respect to those measured in star-forming galaxies and AGN at lower redshifts.

This supports the notion that H_2_O can be likely used to trace the star formation activity buried deep within the dense molecular clouds.