Modeling the H2O submillimeter emission in extragalactic sources

摘要

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 ? 3 × 1017 cm-2), while lower lying lines can be efficiently excited with Tdust ~ 45?75 K and NH2O ~ (0.5?2) × 1017 cm-2; (ii) significant collisional excitation of the lowest lying (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 = Σgas/ ΣSFR, 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. We conclude that the submillimeter H2O line emission other than the para-H2O 111?000 transition is pumped primarily by far-infrared radiation, though some collisional pumping may contribute to the low-lying para-H2O 202?111 line, and that collisional pumping of the para-111 and ortho-212 levels enhances the radiative pumping of the higher lying levels.