The molecular gas outflow from the quasar includes hydroxyl (OH) (top)

The molecular gas outflow from the quasar includes hydroxyl (OH) (top). Due to the motion of the molecular gas toward the observer, the OH peak in the absorption spectrum (bottom, dashed blue line) appears at a shorter wavelength (solid blue line), a phenomenon known as the Doppler effect.
Illustration: ALMA (ESO/NAOJ/NRAO) modified from Dragan Salak, et al. The Astrophysical Journal. February 1, 2024