Active Galactic NucleiThomas Schlenker

University of Washington Department of Physics

PHYS 496

-What is an AGN?

-What kinds of AGN’s are out there?

-Unified theory for a single kind of AGN.

What is an AGN?

• At the center of each galaxy is a region of densely packed matter called the nucleus.

• In Active Galaxies, these nuclei are actually more luminous than all other objects within that galaxy through out all (or at least most) of the spectrum, from radio to gamma rays.– i.e. AGN’s are among the most luminous objects

in the universe.

An AGN is a nucleus (of a galaxy) that has a higher luminosity than the rest of the galaxy.

What kinds of AGN’s are out there?• Several classes of AGN’s are strong radio emitters (Radio Loud)

others are not (Radio Quiet) so it serves as a good way to break the classes into two groups.

– Radio Quiet AGN’s:• Seyfert Galaxies

– Type 1– Type 2

• Quasars – Radio Quiet

– Radio Loud AGN’s:• Quasars

– Radio Loud • Blazars

– BL Lacertaes (BL Lacs)– Optically Violently Variable Quasars (OVV’s)

• Radio Galaxies– Narrow Line– Broad Line

Seyfert Galaxies

• Characteristics of both Type 1 and Type 2 Seyfert Galaxies:– Seyferts are strong sources of low energy gamma rays, with

luminosity peaking around 100 KeV then dropping off for higher energy photons.

– Their host galaxies are often spiral or irregularly shaped galaxies.

– Seyferts are actually fairly common. (There is thought to be a Seyfert AGN at the center of about 1% of spiral galaxies.)

– Seyfert Galaxies are broken into two sub classes, Type 1 and Type 2, based on the emission lines appearing in their spectra.

Seyfert Galaxies – Continued• Type 1:

– These have both narrow line AND broad line peaks in their spectrum.

Emission Lines (Type 1):

Top graph shows full peaks, bottom zooms in along y axis to better show the difference between peak widths.

X-axis: Wavelength (Angstroms)

from the AGN’s frame of reference.

Y-axis: Intensity of flux (no units)

Seyfert Galaxies – Continued• Type 2:

– These have only narrow emission lines visible in their spectrum.

Emission Lines (Type 2):

Only narrow peaks are present.

Wavelength in Angstroms

4000 5000 6000

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Quasars• Radio Quiet Quasars:

– Strong emissions in both the optical and X-ray spectrums.

– Within the optical spectrum, both broad and narrow emission lines are present, similar to a Type 1 Seyfert Galaxy.

– Host is usually an elliptical galaxy. But less commonly, it might be a spiral.

• Radio Loud Quasars:– All the same characteristics of a Radio Quiet Quasar

with the addition of having strong radio emissions.

Blazars

• Characteristics of both classes of Blazars:– Blazars are strong sources of high energy emissions

(energies greater than 100 MeV). However they are luminous over the entire range of the spectrum, from radio up through gamma emission.

– The host galaxies of Blazars are often Giant Elliptical galaxies.

– There are two subclasses of Blazars, BL Lac objects and Optically Violently Variable quasars (OVV’s). Again, classified by their emission spectrum.

Blazars – Continued• BL Lacs:

– Emission spectrum of BL Lacs are relatively flat and featureless when compared to other AGN’s (such as a quasar bellow).

– Highly polarized in the optical spectrum.– Luminosity varies rapidly (on the order of a day).

• OVV’s:– Spectrum has features unlike BL Lacs.– Luminosity varies rapidly as with BL Lacs (on the

order of one day) but by a very large amount (around 10% of it’s overall luminosity)

Blazars – ContinuedComparison of BL Lac and quasar

Emission Lines:BL Lac emissions shown on top, emisson of a quasar shown bellow. Note: The BL Lac has no emission lines.

Wavelength (Angstroms)

Radio Galaxies

• Except for being highly luminous along the radio wavelengths, Radio Galaxies do not have many common characteristics. Their properties may be different from object to object. However, they can be classified into two smaller sub groups. (Again, based on the widths of their emission lines.)

• Broad Line Radio Galaxies:– Emission line widths similar to those in a Seyfert Type 1

• Narrow Line Radio Galaxies:– Emission line widths similar to those in a Seyfert Type 2

Ties Between Classes of AGN’sAs more was learned about each class of AGN scientists began to link classes together, theorizing that two objects of different classes might be similar to one another, only appearing dissimilar due to the observers orientation.

Seyfert AGN’s were theorized to be similar objects. If the observer (Earth) had and un-obscured view of the active nucleus it then it appeared as a Type 1 (with both narrow and broad lines still visible) however if the observers view is obscured by an accretion disk the broad line emissions and low energy X-rays would be blocked and it would appear as a Type 2.

Unification of AGN’s:

• Current model of an AGN:– Within it’s nucleus many galaxies are thought to have

an SMBH. A Super Massive Black Hole of mass >> 100,000 Solar Masses.

– In AGN’s this SMBH is surrounded by an accretion disk (a flat disk of matter orbiting the black hole) which is then surrounded by a Torus, a cloud of matter orbiting the black hole like an inner tube.

– As matter from the accretion disk orbits closer and closer to the SMBH some is accelerated to near c and propelled out away from the SMBH (just before entering the event horizon) perpendicular to the orbital plane of the accretion disk and Torus.

Basic sketch of an AGN.

Different classifications.The same object!

• Blazars– BL Lacs– OVV

• Quasars– Radio Loud– Radio Quiet

• Radio– Narrow Line– Broad Line

• Seyfert– Type 1– Type 2

Sources:• http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html• http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html• http://en.wikipedia.org/wiki/Active_galaxy• http://rocinante.colorado.edu/~pja/astr3830/lecture22.pdf• http://en.wikipedia.org/wiki/Quasar• http://www-xray.ast.cam.ac.uk/xray_introduction/AGN.gif