Direct Evidence for Radiatively Accelerating Quasar Outflows

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 driv-

ing 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. From

−19, 550 km/s in 2011 to −21, 040 km/s in 2017 (∆v = −1490 km/s).

We then used a well-known radiation-driven wind model (7)

to predict the future acceleration of the outflow. In January 2026 we

obtained a third HST observation epoch that showed a further velocity

increase to −22, 390 km/s. This ∆v = −1350 km/s is within

5% 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.