Modeling the H₂O submillimeter emission in extragalactic sources

Abstract

Recent observational studies have shown that H 2 O 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 H 2 O by dust in warm regions. In this work, models are presented that show that (i) the highest-lying H 2 O lines (E upper > 400 K) are formed in very warm (T dust 90 K) regions and require high H 2 O columns (N H 2 O 3 10 17 cm -2 ), while lower lying lines can be efficiently excited with T dust 45-75 K and N H 2 O (0.5-2) 10 17 cm -2 ; (ii) significant collisional excitation of the lowest lying (E upper < 200 K) levels, which enhances the overall L H 2 O -L IR ratios, is identified in sources where the ground-state para-H 2 O 1 11 -0 00 line is detected in emission; (iii) the H 2 O-toinfrared (8-1000 m) luminosity ratio is expected to decrease with increasing T dust for all lines with E upper 300 K, as has recently been reported in a sample of LIRGs, but increases with T dust for the highest lying H 2 O lines (E upper > 400 K); (iv) we find theoretical upper limits for L H 2 O /L IR in warm environments, owing to H 2 O 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 H 2 O emission is dominated by far-infrared pumping in both cases; (vi) the L H 2 O -L IR correlation previously reported in observational studies indicates depletion or exhaustion time scales, t dep = gas / SFR , of 12 Myr for star-forming sources where lines up to E upper = 300 K are detected, in agreement with the values previously found for (U)LIRGs from HCN millimeter emission. We conclude that the submillimeter H 2 O line emission other than the para-H 2 O 1 11 -0 00 transition is pumped primarily by far-infrared radiation, though some collisional pumping may contribute to the low-lying para-H 2 O 2 02 -1 11 line, and that collisional pumping of the para-1 11 and ortho-2 12 levels enhances the radiative pumping of the higher lying levels.