Modeling the H₂O submillimeter emission in extragalactic sources^⋆

¹ Universidad de AlcaláDepartamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
e-mail: eduardo.gonzalez@uah.es
² Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave SW, Washington, DC 20375, USA
³ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden

Received: 11 April 2014
Accepted: 22 May 2014

Abstract

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 ≳ 90 K) regions and require high H₂O columns (N_H2O ≳ 3 × 10¹⁷ cm^-2), while lower lying lines can be efficiently excited with T_dust ~ 45−75 K and N_H2O ~ (0.5−2) × 10¹⁷ 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 1₁₁−0₀₀ line is detected in emission; (iii) the H₂O-to-infrared (8−1000 μm) 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 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 = Σ_gas/ Σ_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 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.