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Modeling the H2O submillimeter emission in extragalactic sources

E. Gonzalez-Alfonso ; J. Fischer ; Susanne Aalto (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; Niklas Falstad (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik)
Astronomy and Astrophysics (0004-6361). Vol. 567 (2014), p. Art. no. A91.
[Artikel, refereegranskad vetenskaplig]

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.

Nyckelord: line: formation, galaxies: ISM, infrared: galaxies, submillimeter: galaxies



Denna post skapades 2014-10-10. Senast ändrad 2015-02-05.
CPL Pubid: 204085

 

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