2021A&A...652A..66P

We present a multiline survey of the interstellar medium (ISM) in two z>6 quasar host galaxies, PJ231-20 (z=6.59) and PJ308-21 (z=6.23), and their two companion galaxies. Observations were carried out using the Atacama Large (sub-)Millimeter Array (ALMA). We targeted 11 transitions including atomic fine-structure lines (FSLs) and molecular lines: [NII]_205µm, [CI]_369µm, CO (J_up=7, 10, 15, 16), H₂O 3₁₂-2₂₁, 3₂₁-3₁₂, 3₀₃-2₁₂, and the OH_163µm doublet. The underlying far-infrared (FIR) continuum samples the Rayleigh-Jeans tail of the respective dust emission. By combining this information with our earlier ALMA [CII]_158µm observations, we explored the effects of star formation and black hole feedback on the ISM of the galaxies using the CLOUDY radiative transfer models. We estimated dust masses, spectral indexes, IR luminosities, and star-formation rates from the FIR continuum. The analysis of the FSLs indicates that the [CII]_158µm and [CI]_369µm emission arises predominantly from the neutral medium in photodissociation regions (PDRs). We find that line deficits agree with those of local luminous IR galaxies. The CO spectral line energy distributions (SLEDs) reveal significant high-J CO excitation in both quasar hosts. Our CO SLED modeling of the quasar PJ231-20 shows that PDRs dominate the molecular mass and CO luminosities for J_up≤7, while the J_up≥10 CO emission is likely driven by X-ray dissociation regions produced by the active galactic nucleus (AGN) at the very center of the quasar host. The J_up>10 lines are undetected in the other galaxies in our study. The H₂O 3₂₁-3₁₂ line detection in the same quasar places this object on the L_H2_O-L_TIR relation found for low-z sources, thus suggesting that this water vapor transition is predominantly excited by IR pumping. Models of the H₂O SLED and of the H₂O-to-OH_163µm ratio point to PDR contributions with high volume and column density (n_H∼0.8x10⁵cm^–3, N_H=10²⁴cm^–2) in an intense radiation field. Our analysis suggests a less highly excited medium in the companion galaxies. However, the current data do not allow us to definitively rule out an AGN in these sources, as suggested by previous studies of the same objects. This work demonstrates the power of multiline studies of FIR diagnostics in order to dissect the physical conditions in the first massive galaxies emerging from cosmic dawn.