Modeling the H2O submillimeter emission in extragalactic sources
Journal article, 2014
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 > 400 K) are formed in very warm (T-dust greater than or similar to 90 K) regions and require high H2O columns (N-H2O greater than or similar to 3 x 10(17) cm(-2)). while lower lying lines can be efficiently excited with T-dust similar to 45-75 K and N-H2O similar to (0.5-2) x 10(17) cm(-2); (ii) significant collisional excitation of the lowest lying (E-upper < 200 K) levels, which enhances the overall L-H2O-L-IR ratios, is identified in sources where the ground state para-H2O 1(11)-0(00) line is detected in emission, (iii) the H2O-to-infrared (8-1000 mu m) luminosity ratio is expected to decrease with increasing Td st for all lines with E-upper less than or similar to 300 K, as has recently been reported in a sample of L1RGs. but increases with T-dust for the highest lying H2O lines (E-upper > 400 K); (iv) we find theoretical upper limits for L-H2O/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-H2O-L-IR correlation previously reported in observational studies indicates depletion or exhaustion time scales, t(dep) = Sigma(gas)/Sigma(SFR) of less than or similar to 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 H2O line emission other than the para-H2O 1(11)-0(00) transition is pumped primarily by far -infrared radiation, though some cohisional pumping may contribute to the low-lying para-H2O 2(02)-1(11) line and that coffisional pumping of the para-1(11) m and ortho-2(12) levels enhances the radiative pumping of the hither lying levels.