Active Galactic Nuclei

Some galaxies harbour immensely powerful sources of radiation in their centres, detected from the longest (radio) to the shortest wavelengths (X-rays): these are called Active Galactic Nuclei (or AGN for short). We think they are powered by the accretion of matter onto super massive black holes (SMBH), the same process that powers some galactic X-ray binaries. However, the SMBH have masses ranging from $10^6$ to $10^9\hbox{$M_\odot$}$, and so, unlike galactic black holes, cannot be stellar remnants. 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' at any given time. 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. 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).