Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220

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Table 2

Inferred column densities, OH⁺ abundances, and OH⁺/H₂O⁺ and OH⁺/H₃O⁺ column density ratios.

Source	NGC 4418		Arp 220
Component	C_core-C_warm	C_extended	C_west	C_extended

T_dust (K)	110−150	Variable^c	90−110	variable^c
N(OH⁺)^a (10¹⁶ cm^-2)	0.6−1	~0.3	0.5−1.1	2−4
N(H₂O⁺)^a (10¹⁶ cm^-2)	0.3−0.4		0.25−0.9	~0.5^d
N(H₃O⁺)^a (10¹⁶ cm^-2)	0.5−0.8		0.9−2.7^e
χ(OH⁺)^b (10^-8)	1.5−2.5	~2	1.1−2.8	~4
OH⁺/H₂O⁺	1.5−2.5		1.0−2.0	5−10
OH⁺/H₃O⁺	~1		~1^e

Notes.

^(a)

Columns toward the nuclei are calculated within a screen approach.

^(b)

Estimated OH⁺ abundance relative to H nuclei, using a normalization column density of N(H) = 4 × 10²³ cm^-2 for the nuclear components (Paper I).

^(c)

T_dust varies with radial position as it is calculated from the balance between heating and cooling.

^(d)

The p-H₂O⁺ ground-state lines at 183−184 μm in Arp 220 require an additional absorbing halo component (C_halo) with N(H₂O⁺) ~ (2.5−3.0) × 10¹⁵ cm^-2.

^(e)

In Arp 220, the total H₃O⁺ column density is twice the nuclear OH⁺ column, but since the H₃O⁺ lines are redshifted relative to the OH⁺ lines, we roughly estimate an OH⁺/H₃O⁺ ratio of 1 in the nuclear region where the OH⁺ lines are formed. In the C_extended component, the OH⁺/H₃O⁺ ratio is uncertain.

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