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 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 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 = 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 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.

Herschelspectroscopic observations of the compact obscured nucleus in Zw 049.057

“…During the fitting, the frequencies for these components were held fixed at the positions of maximum emission and absorption in the feature. The analysis performed by González-Alfonso et al (2014a) further indicates that the absorptions are most efficiently produced close to the continuum source while the low-excitation The IRAS data points at 25, 60, and 100 μm as well as the upper limit at 12 μm come from Sanders et al (2003), the (sub)millimeter points at 850 and 1250 μm are from Dunne et al (2000) and Carico et al (1992) of these wavelength ranges provides us with information on the source structure.…”

“…Observations of highly excited H 2 O over the broad spectral range provided by the combination of PACS and SPIRE give us an unique opportunity to constrain the parameters of the underlying continuum source with unprecedented accuracy. Note that the lines in the PACS range are all detected primarily in absorption while the lines detected with SPIRE are mainly observed in emission, indicating that the rotational levels are pumped at far-IR wavelengths and then relaxed by emission in the submillimeter , 2014a. Notes.…”

“…In doing so, we use the spherically symmetric radiative transfer code described in González-Alfonso & Cernicharo (1997, 1999, including line overlaps between between the Λ-components of the OH doublets in calculations for OH. Dust emission is simulated by using a mixture of silicate and amorphous carbon grains with optical constants from Draine (1985) and Preibisch et al (1993), the adopted mass absorption coefficient as a function of wavelength is described in González-Alfonso et al (2014a). Rates for collisional excitation of H 2 O are taken from Dubernet et al (2009) and Daniel et al (2011) while the rates for OH were taken from Offer et al (1994).…”

Molecular line emission in NGC 1068 imaged with ALMA

“…Hence, our somewhat degenerate results are consistent with multiple gas phases spatially and temporally co-existing. Finally, we emphasize that this analysis did not take into consideration infrared pumping, which in fact may be responsible for the excitation of several of the observed transitions, as has been recently shown to be the case for high-lying H 2 O lines (González-Alfonso et al 2014). It is clear from our preliminary analysis that a higher number of molecular transitions, including vibrationally excited A28, page 16 of 21 transitions, at ALMA resolution is necessary to determine the chemical and physical characteristics of each sub-region within the CND.…”

HERUS: A CO ATLAS FROM SPIRE SPECTROSCOPY OF LOCAL ULIRGs

“…For Mrk 273, the CO(10−9) line and possibly the CO(8 −7) line appear partially resolved and the sinc-Gauss profile provides a better fit. Note, however, that this line may suffer contamination from an H 2 O line (H 2 O 3_12-2_21 on the blue side of the CO line), which could broaden the 2 CO(10−9) line (González- Alfonso et al , 2014. In both cases a broad N II line at 205 μm (1461 GHz) is present and is better fit by the sincGauss profile.…”