Active Galactic Nuclei

Some galaxies have in their centres immensely powerful sources of radiation, detected from the longest (radio) to the shortest wavelengths (X-rays). We think these are powered by accretion of matter onto super massive black holes (SMBH), which have masses ranging from $10^6$ to $10^9\hbox{$M_\odot$}$. In fact, we think that most or even all reasonably massive galaxies have a SMBH lurking in their centres, but only a small fraction of these are `active'. The evidence that the MW has its own SMBH is particularly impressive.

Where do these SMBHs come from? Are they really black holes (i.e. objects with an event horizon) or are they just very massive, extremely dense objects (MDO)? Why are some associated with strong radio-sources (radio-loud AGN), but others are not? How do radio-loud AGN power the immense radio-lobes we observe? What is the effect of the AGN on the galaxy itself? Most of these questions remain without a clear answer, even today.

I will start by discussing some of the observations of AGN. Surely, this must be one of the most exciting topics in Astronomy, or even physics. Then I'll take you through the evidence that these really are black holes, but leave it up to you to decide whether you believe it or not. My own view is that the evidence for super massive and very dense objects is very convincing - but I have not seen evidence for an event horizon (yet).