Free Access
Volume 567, July 2014
Article Number A91
Number of page(s) 13
Section Extragalactic astronomy
Published online 17 July 2014

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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. 3a1c1 are worth noting. The observed high L6/LIR ≈ 2.4 × 10-5 (Fig. 8), together with the high 6/2 ratio of 1.4 (Fig. A.1a), suggest Tdust ≳ 65 K and NH2O ≳ 1017 cm-2, consistent with detection of lines 7−8. However, high Tdust and NH2O 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 Tdust generates warmer SEDs, but lowers the LH2O/LIR ratios for lines 2−6, the increase in τ100 further decreases LH2O/LIR. 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.

thumbnail 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 Cwest and Ceast , 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 Cextended component, where the H2O abundance in the inner parts (R ≲ 150 pc, where Tdust = 70−90 K) is increased relative to G-A12 (so Cextended has NH2O = 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).

thumbnail 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 Tgas to avoid significantly populating the high-lying levels ( > 300 K). S12 found that Tgas ~ 40 K, and very high NH2O and nH2 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 Tgas , 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 Tdust = 55 K, τ100 = 0.18, nH2 = 106 cm-3, Tgas = 150 K, and NH2O = 6.5 × 1016 cm-2, and Vturb = 15 kms-1 (giving Kvir ~ 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 nH2 = 2 × 104 cm-3. For simplicity, we also assume Tdust = 55 K, τ100 = 0.18, and NH2O = 6.5 × 1016 cm-2 as for the ME, but adopt the higher Vturb of 60 kms-1 (giving Kvir ~ 7). For the LE component, and besides the internal far-IR field described by its Tdust 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 TBG = 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

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