I'm interested to hear about the ways we can study AGN! I would also be interested in seeing a diagram of one explained. That's a question some of the grad students here have gotten on their qual exam ("Diagram an AGN.") and I still feel a little blurry about the details.

What does it mean that AGN influence the color of their host galaxies?

Now that the horror that is Phys 12, which is by the way the sophomore physics course for astrophysics and physics students is over for the week, I am free to answer.

To properly understand the parts of an AGN we must consider the two types. The currently accepted "unified" structure of an AGN looks this (personal communication with Dr. Andy Goulding, origin unknown):

I'd assume you know what a black hole is, but let's talk about the accretion disk. According to NASA, an accretion disk is a flat sheet of gas and dust that surrounds a black hole. The presence of an accretion disk indicates that there is material being incorporated into the black hole and that the black hole is accumulating mass.

It's important to note that jets do not occur in all of AGN, but when they do occur they are characterised by a beam of particles ejected in opposite directions. These are observed in the radio wavelength.

The Broad Line Region (BLR) of an AGN is the region of gas clouds immediately surrounding the black hole. It is characterised by relatively broad emission lines, hence its name. The Narrow Line Region (NLR) of an AGN is an outer layer of gas clouds in the area of the jets that produce strong "forbidden lines" which are not present in denser gases.

This is simply an obscuring torus around the accretion disk. Current research indicates that it is probably made up of gas and dust, but the distribution of gas and dust in the torus are not yet known (Antonucci, 1993).

Now let's put it all together. The two types refer to the inclination of the AGN with respect to us (the observer).

A type 1 AGN refers to an AGN in which the BLR is visible. To visualise this situation, a type 1 looks a bit like a donut with an accretion disk at the centre. i.e., a type 1 AGN is face-on as opposed to edge-on. Viewing a type 1 AGN will show a clear view of the BLR as well as the accretion disk and black hole.

A true type 2 AGN is a bit less straightforward. We call an AGN type 2 when the BLR/accretion disk/black hole area is obscured by the dusty torus. This leaves us only the NLR visible for observation. There are some AGN that are neither entirely edge-on or face-on, since the inclination of galaxies does not change in intervals of 90°. However, a true type 2 AGN is completely edge-on.

I have to admit, when I wrote that AGNs affect galaxy colour, I did so because my SURF mentor told me to rather than because I truly understood. I still can't claim to fully understand. However, I've spent some time reading through the relevant paper and found an interesting diagram to share (Hickox et al., 2009):

This diagram shows (a) the colour of host galaxies plotted against absolute magnitude with yellow circles, green stars, and red squares showing radio, X-ray, and infrared AGN, respectively and (b) the colour distribution in each of these wavelengths. Notice how in both the X-ray and infrared spectra, the distributions for AGN are shifted to the left in what is called the "green valley". To summarise, AGN do not influence the colour of their host galaxies beyond what is normally seen, but they may influence the proportion of a particular colour in the X-ray and infrared spectra.