Evidence for a SMBH

I^11.10 am going to take you through several lines of evidence that QSOs are indeed powered by a SMBH. You may think that the fact that we need an engine the size of the solar system that produces as much energy as 100 galaxies together, would seem enough of a proof on its own. So let me discuss just one alternative: suppose there is a dense cluster of stars in the centre of a galaxy hosting a QSO, and what we are seeing is the combined effect of several tens of SNe explosions. A single SN at its peak luminosity is about as bright as a whole galaxy, so we need about 100 SNe and we're done. Is this a reasonable model for AGN?

The model has withstood for several decades, but it does have some pitfalls. For example, we don't know how such SNe would generate the tremendous radio-lobes that some QSOs have. But then, we don't know how a SMBH does that either. As Luis Ho puts it: `our confidence that SMBHs must power AGN largely rests on the implausibility of alternative explanations'. Most of the arguments in the next sections just suggest the presence of a very massive, dense object (MDO) in the centre of galaxies (not just in galaxies harbouring a QSO - in fact, some of the best evidence is in galaxies which do not have an active QSO), but not really require the MDO to be a black hole, i.e. an object with an event horizon.