Colloquium Series: Direct Evidence for Radiatively Accelerating Quasar Outflows

Abstract: Quasar outflows play a fundamental role in galaxy evolution, serving as the primary mechanism for quenching star formation and unbinding inter- stellar gas from host galaxies. This large-scale feedback is expected to be driven by fast winds launched near the central supermassive black hole (SMBH) . While outflow speeds exceeding 10, 000 km/s have been routinely observed for 50 years, the physical mechanism driving them remains a mystery, largely because their acceleration phase has eluded observations. Using HST observations, in 2017 we detected the largest acceleration for a quasar outflow, observed to date. We then used a well-known radiation-driven wind model to predict the future acceleration of the outflow. In January 2026 we obtained a third HST observation epoch that showed a further velocity increase of ∆v = 1350 km/s, which is within 6% of the predicted value for this epoch. Thus, providing the first conclusive evidence for the nature of the acceleration mechanism of quasar outflows: Line-driven radiation acceleration.