26 November 2026: David Alexander

Speaker: David Alexander (Durham University)

Date: Thursday 26 November 2026

Time: 14:00

Location: Physics 3.34

I'm a quasar get me out of here!

A quasar is the observed manifestation of a rapidly growing super-massive black hole. The majority of quasars discovered to date are blue in colour due to us having a direct view of the mass accretion onto the black hole. However, a minority are found to be optically red due to the presence of obscuring dust along the line of sight. The standard model postulates that these red quasars are more inclined towards our line of sight than blue quasars such that the accreting black holes are partially hidden from us. On the basis of this model red quasars would have essentially the same basic properties (on average) as blue quasars, when the effects of obscuration and orientation are taken into account.

I will report on a series of systematic and controlled multi-wavelength experiments where we identify fundamental differences between red and blue quasars, particularly in the radio waveband. I will show that these results suggest a connection between opacity and the production of radio emission, almost certainly due to shocks from winds and/or jets as the quasar tries to break free of a dusty shroud. These results appear in conflict with the standard model and I argue that they point towards red quasars representing a distinct phase in the evolution of quasars.

I will finish with our latest research which extends these results to even fully obscured quasars, where we also identify differences in the host galaxy and large scale properties. I will discuss plans to extend these results with our upcoming 4MOST survey to provide a comprehensive census of optically obscured infrared-selected quasars.