AGN

1. AGN Jason Jeong

2. Intro: AGN AGN stands for active galactic nuclei. It is the core of the galaxy that produces more radiation than the rest of the galaxy.

3. Intro: AGN It is an active galaxy because of one of the following: • Nuclear optical continuum emissionNuclear infra-red emissionBroad optical emission linesNarrow optical emission linesRadio continuum emissionX-ray continuum emissionX-ray line emission

4. Intro: History Astronomers knew early in the twentieth century that some galaxies have emission-line nuclei. However, even the systematic study by Seyfert (1943) was not enough to launch active galactic nuclei (AGN) as a major topic of astronomy. The advances in radio astronomy in the 1950s revealed a new universe of energetic phenomena, and inevitably led to the discovery of quasars. These discoveries demanded the attention of observers and theorists, and AGN have been a subject of intense effort ever since. Only a year after the recognition of the redshifts of 3C 273 and 3C 48 in 1963, the idea of energy production by accretion onto a black hole was advanced. However, acceptance of this idea came slowly, encouraged by the discovery of black hole X-ray sources in our Galaxy and, more recently, supermassive black holes in the center of the Milky Way and other galaxies. Many questions remain as to the formation and fueling of the hole, the geometry of the central regions, the detailed emission mechanisms, the production of jets, and other aspects. The study of AGN will remain a vigorous part of astronomy for the foreseeable future.

5. Intro: types Elliptical Galaxy: Elliptical galaxies are very common in our universe. However, they are faint in the radio spectrum because they are mostly cool stars.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.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.Quasars: Quasars are the most distant and most luminous type of AGN known; and their spectra doesn’t look like normal galaxies at all.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.Seyfert galaxies: are characterized by extremely bright nuclei, and spectra which have very bright emission lines of hydrogen, helium, nitrogen, and oxygen.Optically Violent Variable Quasar (OVV quasar):is a type of highly variable quasar. It is a subtype of blazar that consists of a few rare, bright radio galaxies, whose visible light output can change by 50% in a day. They are similar in appearance to BL Lacs but generally have a stronger broad emission line, and tend to have higher red shift components.

6. Need to know: Spectroscopy Spectroscopy pertains to the dispersion of an object's light into its component colors (i.e. energies). There are three types of spectra: Continuous, Emission, and absorption lines.

7. Types of spectra Continuous is all wavelengths of light produced by hot, dense objects.EX: the sun is hot and dense, it produces the light to us which is the visiblespectrum. Emission line is a short wavelength produced by hot, low-density objects.EX: hydrogen gas, when electricity runs through it, gives off a couple of spectra.I think this could also be like neon lights that are filled with argon and other noblegases (most likely). Absorption line is a wavelength absorbed by cool gas/gases (the light comes from a different source). It absorbs what ever wavelength it would usually emit at high temperatures.EX: any gas

8. Applications of spectra You can use the knowledge of spectra especially the emission lines to identify the element or galaxy depending on its unique spectrograph.

9. Identifying galaxies with spectrographs 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).

10. Identifying galaxies with spectrographs 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)

11. Identifying galaxies with spectrographs 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).

12. Identifying galaxies with spectrographs 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).

13. Identifying galaxies with spectrographs 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).

14. Shifts Redshift is the shift of the light to a longer wavelength due to the object moving away from us the observer.Opposite to this is the blue shift where the object is moving towards the observer giving us a shorter wavelength.

15. Calculating shifts To find the shift you have to identify two strong emission lines. After that you find the ratio between the two lines and use the redshift equation to find the shift.

16. Calculating other information from shifts Velocity, Distance and LuminosityAfter you have found out what the redshift, z, is then you can find out what the velocity, distance and the luminosity of the galaxy that you are looking at.To get Velocity:If an object is pretty close (close enough that the redshift is less than or equal to one) then you can use the equation:v=cz where v is the velocity, c is the speed of light, and z is the redshift.If the redshift is greater than one then use this equation:redshift_greater_than_one_equation.jpg

17. Calculating other information from shifts To get distance:For near galaxies use Hubble’s Law:Hubble's_Law.jpgFor farther galaxies we have to define a model of the universe first. One of the simplest models of the universe we have is the "empty Universe" model and from that we can use:Empty_universe_model_equation.jpgto find the distance of the galaxy we are looking at.AGN is one of the farthest and brightest objects in the universe because we can observe the light from its luminosity (explained below) and the time it takes for the light to reach us is so long (because it's so far away) that it is as if we are looking back in time to the origins of the universe.

18. Calculating other information from shifts To get luminosity:What we see through the telescope is the flux of the galaxy, the energy per second of the light we see.But, what we want is the luminosity or the total amount of light it emits per second. Luminosity is essential because it tells us the amount of energy the galaxy is producing.According to the inverse-square law, flux and luminosity are related by the square of the distance to the object. For example, if two objects have the same luminosity but the first object is twice as far away as the second, the flux of the first object will be one-quarter that of the second. The relationship between flux and luminosity for very distant objects is given by:This equation relates luminosity, L, flux, f, distance, d, and the shift, z.We have to use the flux over the whole wavelength so we have to integrate or find the total flux over the wavelengths (the area under the "curve". We can see a demonstration in the picture below over a part of the wavelength.

19. Ca II break CaII break: The "calcium break" is an overall decrease in flux on the blue side (the left side) of the spectrum beginning somewhere around the calcium absorption doublet ~4000Å. In other words when a "cliff" is made in the spectra on the left side. (only cliff up as shown in picture below).