2014A&A...567A..91G

Recent observational studies have shown that H₂O emission at (rest) submillimeter wavelengths is ubiquitous in infrared galaxies, both in the local and in the early Universe, suggestive of far-infrared pumping of H₂O by dust in warm regions. In this work, models are presented that show that (i) the highest-lying H₂O lines (E_upper>400K) are formed in very warm (T_dust>90K) regions and require high H₂O columns (N_H2_O>3x10¹⁷cm^–2), while lower lying lines can be efficiently excited with T_dust∼45-75K and N_H2_O~(0.5-2)x10¹⁷cm^–2; (ii) significant collisional excitation of the lowest lying (E_upper<200K) levels, which enhances the overall L_H2_O-L_IR ratios, is identified in sources where the ground-state para-H₂O 111000 line is detected in emission; (iii) the H₂O-to-infrared (8-1000µm) luminosity ratio is expected to decrease with increasing T_dust for all lines with E_upper≲300K, as has recently been reported in a sample of LIRGs, but increases with T_dust for the highest lying H₂O lines (E_upper>400K); (iv) we find theoretical upper limits for L_H2_O/L_IR in warm environments, owing to H₂O line saturation; (v) individual models are presented for two very different prototypical galaxies, the Seyfert 2 galaxy NGC 1068 and the nearest ultraluminous infrared galaxy Arp 220, showing that the excited submillimeter H₂O emission is dominated by far-infrared pumping in both cases; (vi) the L_H2_O-L_IR correlation previously reported in observational studies indicates depletion or exhaustion time scales, t_dep=Σ_gas/Σ_SFR, of ≲12Myr for star-forming sources where lines up to E_upper=300K are detected, in agreement with the values previously found for (U)LIRGs from HCN millimeter emission. We conclude that the submillimeter H₂O line emission other than the para-H₂O 111000 transition is pumped primarily by far-infrared radiation, though some collisional pumping may contribute to the low-lying para-H₂O 202111 line, and that collisional pumping of the para-1₁₁ and ortho-2₁₂ levels enhances the radiative pumping of the higher lying levels.