Active galaxy is a broad term than refers to many loosely connected objects:
quasars/QSOs
Seyfert galaxies
BL Lac objects / Blazars / OVV quasars
Radio galaxies
LINERs
AGN
The term Active Galactic Nuclei (AGN) is used because most of these objects have been located at the centre of a host galaxy.
First observations of AGNs showed them as point-like, hence "quasi-stellar object", for example.
Subsequent observations with superior techniques found a host galaxy in almost every case.
The AGN is typically far brighter than the diffuse galactic emission that surrounds it.
The red shifts of AGNs are large, indicating they are moving away from us at great speeds, but also that they are very distant (see Hubble's law later in the course).
All spectra in the last slide have been corrected for redshift of the emitting object.
An angstrom (Å) is a tenth of a nanometre (nm). So, for example, 5000 Å = 500 nm
Narrow spectral lines
These can easily be identified with atoms found here on Earth.
For example, the hydrogen alpha line (Hα) can be seen at 6563 Å in most spectra; Hβ is at 4861 Å. The prominent line near 5000 Å is the OIII oxygen line.
Broad spectral lines
If emitting atoms are moving with a range of speeds, the Doppler shift causes a broad line.
A very high temperature can produce broadened lines.
Rapidly rotating or orbiting objects can also produce broadened lines.
Generally the temperatures implied would be too high and are inconsistent with observations at short wavelengths (X-rays and gamma rays), so rapid orbiting is believed to be responsible in most cases.
Radio-quiet AGN
Seyferts
Radio quiet quasars or QSOs
LINERs - Low Ionization Nuclear Emission-line Regions (not very active, barely qualifies as an AGN)
significant emission in all spectral regions except the far infrared
significant time variability: short timescales plus speed of light place limits on the physical size of the extremely luminous source.
lack of spectral line features, but OVV quasars generally have some broad line features, less-so for blazars and BL Lac objects.
Radio Galaxies
strong radio emission
generally found in elliptical galaxies
Can be divided into two classes:
Low excitation: lack strong narrow or broad emission lines
High excitation: strong narrow emission lines, similar to Seyfert type 2s.
Unification
It is now understood that the variety of AGN objects observed is partly due to different views of the same type of object.
For example, jets are highly directional so similar objects will appear very different if the jet is aligned along of the light of sight or if it is perpendicular.
Seyfert type 1s are believed to be where we can see the AGN directly, whereas in type 2 the AGN is wholly or partly obscured.
It seems clear that unification does not lead us back to one type of object, e.g. radio loud AGNs cannot be easily unified with radio quiet ones.
A common conclusion is that a supermassive blackhole is present, though this is also true for non-AGN galaxies, such as the Milky Way. Therefore, something else is needed to trigger AGN-like activity.
Implications for galactic evolution
Because AGNs are seen at high red shifts it seems that they were more common in the past (though care is needed to account for bias when studying high luminosity objects over last distances).
This suggests that AGNs may be involved in the formation of galaxies, though it is also possible that AGN are "awakened" by interactions between galaxies.
The fact that AGNs require a supermassive blackhole to be present, and that AGNs are seen in the young Universe (less than a billion years old), means that either the supermassive black holes formed first, or they formed with galaxies.
