Modeling the H2O submillimeter emission in extragalactic sources

الأصل · EN

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 (Eupper>400 K) are formed in very warm (Tdust> 90 K) regions and require high H2O columns (Nₕ₂ₒ> 3x10¹⁷ cm⁻²), while lower lying lines can be efficiently excited with Tdust 45-75 K and Nₕ₂ₒ (0.5-2)x10¹⁷ cm⁻²; (ii) significant collisional excitation of the lowest lying (Eupper<200 K) levels, which enhances the overall Lₕ₂ₒ-LIR ratios, is identified in sources where the ground-state para-H2O 1₁₁-0₀₀ line is detected in emission; (iii) the H2O-to-infrared (8-1000 um) luminosity ratio is expected to decrease with increasing Tdust for all lines with Eupper< 300 K, as has recently been reported in a sample of LIRGs, but increases with Tdust for the highest lying H2O lines (Eupper>400 K); (iv) we find theoretical upper limits for Lₕ₂ₒ/LIR 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 Lₕ₂ₒ-LIR correlation previously reported in observational studies indicates depletion or exhaustion time scales, tdep=Sigmagas/SigmaSFR, of < 12 Myr for star-forming sources where lines up to Eupper=300 K are detected, in agreement with the values previously found for (U)LIRGs from HCN millimeter emission...