Active galactic nuclei

 

Active galactic nuclei, powered by accretion onto a supermassive black hole,  are among the most luminous sources in the Universe. We can see these sources over the whole electromagnetic spectrum, from radio all the way to gamma-rays, and even TeV energies.

 

The basic structure of almost all AGN is rather similar and can be seen in the schematic picture (artwork by Lucia Zarantonello). The supermassive black hole, with a mass from one hundred thousand solar masses up to tens of billions of solar masses, resides in the centre. We of course cannot see the black hole, but if it is accreting matter, this matter is emitting light through different processes. In the optical band (light visible to us humans) and the ultraviolet, we see the accretion disk that is feeding the black hole. Also, broad emission lines from the broad-line region consisting of ionised gas orbiting the black hole are visible in this regime. Close to the accretion disk, there are hot electrons that give a kick to the accretion disk photons and produce X-rays. This component is known as the X-ray corona. The centre - black hole, accretion disk, broad-line region, and X-ray corona - is surrounded by a dusty structure that absorbs accretion disk emission and re-emits it in the infrared band. From certain orientations this structure obscures our view of the central engine; this is the basis for orientation-based unification models. The accretion disk emission also ionises the gas in the host galaxy of the active galactic nucleus, usually in a biconical shape due to the dusty component absorbing some of the emission. This gas produces very narrow forbidden lines.

 

In addition, about 10% of active galactic nuclei can host powerful relativistic jets. These jets are launched from the vicinity of the supermassive black hole and consist of plasma collimated by magnetic fields and moving very close to the speed of light.  Only a few active galactic nuclei classes host these jets: blazars - whose jets we observe at small angles - including flat-spectrum radio quasars and BL Lacs; their misaligned radio galaxy counterparts; and a class of unevolved, fast-accreting AGN, called narrow-line Seyfert 1 galaxies. Flat-spectrum radio quasars and BL Lacs are fully evolved active galactic nuclei that differ in accretion modes and environments. On the other hand, the jets in narrow-line Seyfert 1 galaxies were found about 20 years ago, and remain poorly understood. 

Image by benzoix on Freepik