AGN+Types

AGN has many different types of galaxies. These include elliptical, quasars, radio, starburst, and BL Lac objects. They have many unique characteristics about their spectroscopy that we can use to identify them.

__Elliptical Galaxy__

Elliptical galaxies are very common in our universe. However they are faint in the radio spectrum, so only nearby ones can be observed by us. Since these galaxies are close to use they have very small red shifts. Also their graphs rarely ever have emission lines and can be classified by the unique curvature of its graph.

__Starburst Galaxy__

Most galaxies go through a continual cycle of star birth and death. However, some galaxies are currently forming stars at a furious rate, going through a stellar “baby boom.” These galaxies are known as starburst galaxies. Often rapid star formation is induced in a galaxy by gravitational interaction or collision with another galaxy. Newly-formed massive stars in the starburst galaxy heat up gas in the interstellar medium and create strong, narrow emission lines which are seen in addition to the galaxy’s spectrum. Like “radio galaxies” (described below), starburst galaxies usually have several narrow emission lines. For both of these reasons it is often difficult to differentiate between starburst galaxies and radio galaxies. One difference is that the Hβ and [O III] emission lines in starburst galaxies are usually about the same strength. The same is true for the Hα and [N II] emission lines; however since these two lines are so close to each other they are usually “blended” together, as is the case in the example below. The [O II] and [S II] emission lines are also common is starbursts, but not always present.

__Radio Galaxy__

The term radio galaxy was coined to describe objects that look like normal galaxies in optical images, but were found to emit enormous amounts of radio waves. Their optical spectra reveal the presence of strong, narrow lines and a CaII break strength that is <40%. For these reasons radio galaxies are easily confused with starburst galaxies. The primary difference is the strength of the emission lines: in radio galaxies the forbidden lines [O II], [O III] and [N II] are typically much stronger than Hα and Hβ (as in the example below, where [O III] is much stronger than Hβ). Unlike quasars, radio galaxies tend not to have broad emission lines.

__Quasars__

Quasars are the most distant and most luminous type of AGN known; and their spectra don’t look like normal galaxies at all. Instead of having an optical spectrum which looks like a galaxy (e.g., with many absorption lines and a CaII break), quasars have a very smooth continuum spectrum with strong and broad emission lines. The continuum you see is not due to starlight but synchrotron radiation from the AGN. Synchrotron radiation is produced by electrons in the AGN’s jet which are moving near the speed of light. The quasar’s emission lines are produced by clouds of gas within the galaxy which are heated by the AGN. Quasars are so luminous they usually outshine their host galaxy, often by as much as 1000 times or more. Imagine: something about the size of our Solar System can outshine over 100 billion stars by a factor of 1000!

__BL Lac Objects__

Most AGN have strong emission lines, but a special class of AGN are notorious for having only very weak emission lines, if any at all. They are known as “BL Lacertae objects,” or BL Lacs for short. Because they lack strong emission lines, it is often difficult to determine redshifts for these objects. BL Lacs are most easily differentiated from radio galaxies and quasars by their emission lines: quasars and radio galaxies have strong lines, BL Lacs do not. Like radio galaxies, BL Lacs often show a Ca II break in their spectrum whereas quasars rarely do. The above spectrum is clearly a BL Lac because it has no emission or absorption lines, nor does it have a CaII break. The only spectral l