2016ApJ...832..209U

We present Herschel observations of the far-infrared (FIR) fine-structure (FS) lines [C ii]158 µm, [O i]63 µm, [O iii]52 µm, and [Si ii]35 µm in the z = 2.56 Cloverleaf quasar, and combine them with published data in an analysis of the dense interstellar medium (ISM) in this system. Observed [C ii]158 µm, [O i]63 µm, and FIR continuum flux ratios are reproduced with photodissociation region (PDR) models characterized by moderate far-ultraviolet (FUV) radiation fields with G₀ = 0.3-1 x 10³ and atomic gas densities n_H = 3-5 x 10³ cm^–3, depending on contributions to [C ii]158 µm from ionized gas. We assess the contribution to the [C ii]158 µm flux from an active galactic nucleus (AGN) narrow line region (NLR) using ground-based measurements of the [N ii]122 µm transition, finding that the NLR can contribute at most 20%-30% of the observed [C ii]158 µm flux. The PDR density and far-UV radiation fields inferred from the atomic lines are not consistent with the CO emission, indicating that the molecular gas excitation is not solely provided via UV heating from local star formation (SF), but requires an additional heating source. X-ray heating from the AGN is explored, and we find that X-ray-dominated region (XDR) models, in combination with PDR models, can match the CO cooling without overproducing the observed FS line emission. While this XDR/PDR solution is favored given the evidence for both X-rays and SF in the Cloverleaf, we also investigate alternatives for the warm molecular gas, finding that either mechanical heating via low-velocity shocks or an enhanced cosmic-ray ionization rate may also contribute. Finally, we include upper limits on two other measurements attempted in the Herschel program: [C ii]158 µm in FSC 10214 and [O i]63 µm in APM 08279+5255.