In light of Jackie's question from last week, let's talk about what exactly an AGN is. I could quote Wikipedia and say "An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum," but what's the fun in that? That sentence tells us everything and nothing about AGN. What causes AGN? And why do we even care about this mysterious AGN? Those are the real questions.

First of all, to clarify further from the Wikipedia definition, an AGN isn't just a region at the centre, it is the core of a galaxy which emits far greater radiation than the rest of the galaxy or galaxies that are not considered "active" (do not have AGN). These cores, or nuclei, occur due to accreting (growing) black holes at the centres of galaxies (Cappelluti et al., 2010). Now we have "what" and "how", but how about "why"? AGN are important because of the effect they have on galactic properties. They influence things such as stellar mass, star formation rate, and colour in their host galaxies. As a result, it is important to study AGN in order to better understand this influence (Coil et al., 2009; Hickox et al., 2009).

Next Time: In what ways can we study AGN?