Recognizing+different+types+of+AGN+galaxies

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**Galaxies (active and quasars)** - What AGN is all about. Some different kinds and how to differentiate between them.
 * __Elliptical Galaxy__: Elliptical galaxies are very common in our universe. However, they are faint in the radio spectrum because they are mostly cool stars. Since these galaxies are close to us they have very small red shifts. Also their graphs rarely ever have emission lines because of the cool nature of the gases and can be classified by the unique curvature of its graph. Another feature of the elliptical galaxies are that their flux drops about 50% or half its value around wavelengths below the Ca II lines (also known as Ca II break) see graph below for an example of an Elliptical Galaxy. (notes from AGN article edited by JJ).

Picture of: **NGC 3245** a typical elliptical galaxy. The ⊕ signs are the **telluric absorption bands** from the Earth’s atmosphere.
 * __Starburst Galaxy__: 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. Because they are newly formed and super hot, these galaxies give off more blue light than regular galaxies. It is often difficult to differentiate between starburst galaxies and radio galaxies because of their similar narrow emmission lines. 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. They are rare and are weak sources of radio waves. (notes from AGN article edited by JJ)


 * __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. (from AGN article edited by JJ).


 * Above:** From its distinctive O II, O III and N II lines that are much stronger than the Hα and Hβ lines we can identify the Radio Galaxy.
 * __Quasars__: Quasars are the most distant and most luminous type of AGN known; and their spectra doesn’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. 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. (from AGN article edited by JJ).
 * Above:** the average of over 100 quasar spectra averaged together as if there was no redshift.
 * Above:** A typical Quasar spectra with the Mg II and O II clearly visible. You can make out the C III in the blue spectra.


 * __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. (from AGN article edited by JJ).
 * Above:** Clearly a BL Lac object because of its bland waves. No significant emission or absorption lines and no sign of a Ca II break. We can also see the **telluric absorption bands** (⊕).
 * Above:** a common BL Lac object that has a <40% Ca II break. Galactic absorption lines are used to determine the BL Lac objects.

Source: Straight from my pre-AGN page found here