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Appendix A: Two opposite, extreme cases: Arp 220 and NGC 1068

Arp 220 and NGC 1068 are prototypical sources that have been observed at essentially all wavelengths. With regard to their H₂O submm emission, these galaxies are extreme cases and deserve special consideration.

In the nearby ULIRG Arp 220, discrepancies between the observed SLED (Rangwala et al. 2011, Y13) and the single-component models of Figs. 3a1−c1 are worth noting. The observed high L₆/L_IR ≈ 2.4 × 10^-5 (Fig. 8), together with the high 6/2 ratio of ≈1.4 (Fig. A.1a), suggest T_dust ≳ 65 K and N_H2O ≳ 10¹⁷ cm^-2, consistent with detection of lines 7−8. However, high T_dust and N_H2O are mostly compatible with F₄/F₃ > 1, while the observed ratio is ≈0.7 (Fig. A.1a). As in Mrk 231, a composite model is required to account for the H₂O SLED in this galaxy.

In sources with very optically thick and very warm cores such as Arp 220 (G-A12), the increase in τ₁₀₀ above 1 decreases the submm H₂O fluxes due to the rise of submm extinction (Fig. 9). While higher T_dust generates warmer SEDs, but lowers the L_H2O/L_IR ratios for lines 2−6, the increase in τ₁₀₀ further decreases L_H2O/L_IR. This behavior suggests that the optimal environments for efficient H₂O submm line emission are regions with high far-IR radiation density but moderate extinction, i.e., those that surround the thick core(s) where the bulk of the continuum emission is generated. In contrast, the H₂O absorption at shorter wavelengths is more efficiently produced in the near-side layers of the optically thick cores, primarily if high-lying lines are involved. Absorption and emission lines are thus complementary, providing information on the source structure.

Fig. A.1 a) Proposed composite model for the H2O submm lines in Arp 220 (see G-A12), compared with the observed line fluxes (black squares, from Rangwala et al. 2011). Toward the far-IR optically thick nuclear region (blue symbols), the Eupper< 400 K lines are expected mostly in absorption. The H2O emission is generated around that nuclear region, in the Cextended (G-A12) component (green). Red is total. b) Resulting predicted composite PACS/SPIRE H2O continuum-subtracted spectrum of Arp 220, which is dominated by absorption of the continuum.

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We have taken the models in G-A12 for Arp 220 to predict its submm H₂O emission. In Fig. A.1a, the blue symbols/line indicate the predicted H₂O fluxes towards the optically thick, warm nuclear region (both C_west and C_east , see G-A12), indicating that most submm lines (with the exception of lines 3, 7, and 8) are predicted in absorption. The observed H₂O submm line emission (Rangwala et al. 2011) must therefore arise in the surrounding, optically thinner region, i.e., the C_extended component, where the H₂O abundance in the inner parts (R ≲ 150 pc, where T_dust = 70−90 K) is increased relative to G-A12 (so C_extended has N_H2O = 1.3 × 10¹⁷ cm^-2 in Fig. A.1a). According to our model, the relatively low flux in line 4 is due to line absorption towards the nuclei. The main drawback of the model in Fig. A.1a is that line 7 is underestimated by a factor 2. The submm H₂O emission in Arp 220 traces a transition region between the compact optically thick cores and the extended kpc-scale disk (G-A12). The overall H₂O spectrum is, however, dominated by absorption of the continuum (Fig. A.1b).

Fig. A.2 a) Composite model for the H2O emission in NGC 1068 favored in this work. Blue/red indicate ortho/para lines, and the submm H2O 1−6 lines (Table 1) are indicated. Open squares and triangles show the contribution by a moderate-excitation (ME) and a low-excitation (LE) component, and filled symbols indicate the total emission (see text for details); in both cases the submm H2O lines 2−6 are pumped through far-IR photons emitted by dust. b) Comparison between the observed fluxes of the H2O submm lines (black squares, from Spinoglio et al. 2012) and those predicted with the composite model.

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Just the opposite set of conditions characterizes the nearby Seyfert 2 galaxy NGC 1068, since the nuclear continuum emission is optically thin and collisional excitation is important (S12). All detected H₂O lines, including those in the far-IR (100−200 μm) are seen in emission, and most of them show fluxes (in erg/s/cm²) unrelated to wavelength, upper level energy (up to ≈300 K), or A-Einstein coefficient (S12). In particular, the H₂O 2₂₁−1₁₀ (108 μm) and 2₂₁−2₁₂ (180 μm) lines share the same upper level and show similar fluxes but the A-Einstein coefficient of the 108 μm transition is a factor of 8.4 higher than that of the 180 μm transition. With pure collisional excitation, the only way to account for the observed line ratios is to invoke high densities and H₂O column densities, but also a relatively low T_gas to avoid significantly populating the high-lying levels ( > 300 K). S12 found that T_gas ~ 40 K, and very high N_H2O and n_H2 can provide a reasonable fit to the SLED. However, these conditions are unrelated to the warmer gas conditions in the nuclear region of NGC 1068, as derived from the CO SLED (S12, Hailey-Dunsheath et al. 2012, hereafter H12). In addition, the observed H₂O submm SLED (Fig. A.2) is fairly similar to the SLEDs obtained in optically thin models with significant collisional excitation of the low-lying levels.

We have explored an alternative composite solution for the H₂O emission in NGC 1068 with lower densities and H₂O columns and higher T_gas , based on the far-IR pumping of the lines by an external anisotropic radiation field. In this framework, we can account for the weakness of the 108 μm line by the absorption of continuum photons, and indeed we would have to explain why this line is not observed to be even weaker than it is or in absorption. The higher lying far-IR 3₂₂−3₁₃ emission line at 156.2 μm is in this scenario pumped through absorption of continuum photons in the 3₂₂−2₁₁ line at 90 μm.

For the first component, we closely follow H12 in modeling the moderate-excitation (ME) component as an ensemble of clumps, which are described by T_dust = 55 K, τ₁₀₀ = 0.18, n_H2 = 10⁶ cm^-3, T_gas = 150 K, and N_H2O = 6.5 × 10¹⁶ cm^-2, and V_turb = 15 kms^-1 (giving K_vir ~ 10, see H12). With a mass of 7.5 × 10⁶M_⊙, this component is unable to account for the H₂O submm lines 2−6, but generates a significant fraction of the observed emission in line 1 and some far-IR lines (Fig. A.2a and panel b).

We then added another, low-excitation (LE) component, which is identified with the gas generating the low-J CO lines (Krips et al. 2011, S12) and is thus assigned a density of n_H2 = 2 × 10⁴ cm^-3. For simplicity, we also assume T_dust = 55 K, τ₁₀₀ = 0.18, and N_H2O = 6.5 × 10¹⁶ cm^-2 as for the ME, but adopt the higher V_turb of 60 kms^-1 (giving K_vir ~ 7). For the LE component, and besides the internal far-IR field described by its T_dust and τ₁₀₀, we also follow H12 in including an external field (associated with the emission from the whole region), which is described as a graybody with T_BG = 55 K and . The resulting mean specific intensity at 100 μm of the external field, , matches the value estimated by H12 within a factor of 2 (their Eq. (1)). A crucial aspect of the present approach is that this external field is assumed to be anisotropic, that is, it does not impinge into the LE clumps on the back side (in the direction of the observer). As a result, the external field contributes to the H₂O excitation without generating absorption in the pumping far-IR lines (though some absorption is nevertheless produced by the internal field). As shown in Fig. A.2a, the LE component is expected to dominate the emission of the submm lines 2−6, as well as the emission of the majority of the far-IR lines. The required mass of the LE component is 3.5 × 10⁷M_⊙, consistent with the mass inferred from the CO lines for the CND (S12), and the IR luminosity is 2.6 × 10¹⁰L_⊙.

A key assumption of the present model is that the external radiation field does not produce absorption in the far-IR lines, as otherwise (that is, in a perfectly isotropic radiation field) the strengths of the far-IR lines would weaken, and in particular, the H₂O 2₂₁−1₁₀ line at 108 μm line would be predicted to be observed in absorption. The proposed anisotropy could be associated with the heating by the central AGN, and it seems possible as long as the source is optically thin in the far-IR. Radiative transfer in 3D would be required to check this feature. On the other hand, the external field, while having an important effect on the far-IR lines, has a secondary effect on the submm lines, which are primarily pumped by the internal (isotropic) radiation field (that is, by the dust that is mixed with H₂O). With the caveat of the assumed intrinsic radiation anisotropy in mind, we preliminary favor this model over the pure collisional one in predicting the H₂O submm fluxes and conclude that radiative pumping most likely plays an important role in exciting the H₂O in the CND of NGC 1068.

From the models for these two very different sources and the case of Mrk 231 studied previously (G-A10), we conclude that the excitation of the submm H₂O lines other than the 1₁₁−0₀₀ one is dominated by radiative pumping, though the relatively low-lying 2₀₂−1₁₁ line may still have a significant “collisional” contribution in some very warm/dense nuclear regions, and the radiative pumping may be enhanced with collisional excitation of the low-lying 1₁₁ and 2₁₂ levels. These individual cases also show that composite models to account for the full H₂O far-IR/submm spectrum in a given source may be a rather general requirement.

© ESO, 2014