Science, Maths & Technology

### Become an OU student

Introduction to active galaxies

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

# 4.3 The luminosity of AGNs

It is instructive to express the luminosity of an AGN in terms of the luminosity of a galaxy like our own. The figure may then be converted into solar luminosities, if we adopt the figure of 2 × 1010L for the luminosity of our Galaxy.

Consider a Seyfert galaxy first. At optical wavelengths the point-like AGN is about as bright as the remainder of the galaxy, which radiates mainly at optical wavelengths. But the AGN also emits brightly in the ultraviolet and the infrared, radiating at least three times its optical luminosity. So one concludes that for a typical Seyfert, the AGN has at least four times the luminosity of the rest of the galaxy.

We have seen that a characteristic of a quasar is that its luminous output is dominated by emission from its AGN. However quasar host galaxies are not less luminous than normal galaxies, so the AGNs of quasars must be far brighter than normal galaxies and must also be considerably more luminous than the AGNs of Seyfert galaxies.

In the case of a radio galaxy, the AGN may not emit as much energy in the optical as Seyfert and quasar AGNs, but an analysis of the mechanism by which the lobes shine shows that the power input into the lobes must exceed the luminosity of a normal galaxy by a large factor, and the AGN at the centre is the only plausible candidate for the source of all this energy.

A similar conclusion for AGN luminosity follows for blazars, which appear to be even more luminous than quasars. We examine why in Section 4.7.

## Question 8

Calculate the luminosity of an AGN that is at a distance of 200 Mpc, and appears as bright in the optical as a galaxy like our own at a distance of 100 Mpc. Assume that one-fifth of the energy from the AGN is at optical wavelengths.