Illustration of the need for several OH components in Mrk 231, as inferred from the OH119, OH84, and OH65 doublets. In panels d)–i), red curves indicate the absorption and emission by all considered components. a)–c) An outflow-free, high-excitation component (QC) generates absorption in the high-lying OH lines (blue curves), but cannot account for the blue wings in the three doublets or the redshifted emission in the OH119 transition. e), f) The HVC (light-blue curves) and LVC (dashed light-blue curves) reproduce the blue wings in the OH85 and OH65 doublets, but fail to account for both the full blueshifted absorption and redshifted emission in the ground-state OH119 doublet. g) A low-excitation component (LEC, green curves) is therefore required to match the ground-state OH119 (and also OH79) blueshifted absorption and redshifted emission (panel g)). The composite fit to all lines is shown in Fig. 10.
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