(for those who got the reference in the title, it was from the CBBC series “Watt on Earth”)
Two different models involved in the evolution of galaxies have been put together to form an idea of why quasars do what they do. The first model is the idea of what a quasar is – it is a supermassive black hole at the centre of a galaxy, that is overfeeding on gas and dust and releasing jets of relativistic particles and other forms of radiation that are so intense, we can see them in telescopes on Earth at the limits of the observable universe (they initially appeared a little like stars, hence the name “quasi-stellar”, but later observations have shown the galaxies that are host to quasars).
The second model thrown in the mix is that early galaxies went through a period of birthing whereby the dense gas in the early universe collapsed onto the gravitationally attractive galaxies. Put the two models together and during the collapse of these gas clouds, the massive amounts of matter falling into the galaxy sees quite a substantial amount heading straight for the jaws of the black hole.
Previous ideas of where the matter that fed the black holes held it came from mergers or through the funnelling mechanism available in barred spiral galaxies. The trouble with these is quasars don’t happen in the local universe, whereas mergers are common, furthermore, observations of galaxies hosting quasars show they aren’t all barred spirals. This new theory has the advantage of explaining that at a certain point in time, the gas clouds in the background of the expanding universe dropped below a critical density and shut down the quasars, which is why they’re not around now. Furthermore, as there are still wisps of denser dust about, the occasional stutters from the supermassive black holes, known as Active Galactic Nuclei when putting out some radiation, still around can also be explained.
More information on the theory can be found here and further information on a study of active galactic nuclei, which came out of the Galaxy Zoo project, can be found here in a podcast interview between Chris Lintott, the Sky at Night presenter and Kevin Schawinski, who along with Chris and the team published the new paper.