Lecture 16, AGN Evolution

87 Pages·2007·13.02 MB·English

AGN
• The term ``Active Galactic Nucleus’’ (AGN) refers to the
existence of energetic phenomena in the nuclei of galaxies
that cannot be attributed directly to stars
• AGN are classified based (in large part) on their luminosity
in different wavebands (especially the optical and radio)
Seyfert Galaxies
• In 1943, Carl Seyfert identified a class of spiral galaxies
w/unusually bright central regions (stellar appearing cores),
whose spectra are dominated by high-excitation nuclear
emission lines
Seyfert galaxies make up about 1% of spirals, so
there are numerous nearby examples; however, at
the time, they were viewed as oddities
Seyfert Galaxy NGC 7742
Discovery of Quasars
• In late 1950s, w/development of radio telescopes, some
celestial objects were found to emit profusely
• Catalogs compiled, largely at Cambridge (e.g., 3rd such
catalog known as “3C,” & objects in it given numerical
designation)
• The positions of some of these radio sources were
found to be coincident with objects that looked like
stars on normal photographs
The nature of these bluish-looking “radio stars” (a
typical star in the sky is red) was very uncertain
(they became known as quasi-stellar radio sources)
No known mechanism by which stars could produce so
much radio radiation. Moreover, the spectra showed
very strong broad emission lines at unidentified
wavelengths
• Maarten Schmidt made a breakthrough in 1963
– realized emission lines seen in the spectrum of
3C 273 were the hydrogen Balmer-series
emission lines
– but at the uncommonly large redshift z=0.158
• More disturbing than this vast distance was the
enormous luminosity implied
– 3C 273 was and remains the brightest known
quasar (B=13.1 mag)
– It is about 100 times as luminous as normal
bright spirals like the Milky Way or M31
• Probable importance of quasars was recognized
immediately, providing strong motivation to find more
• Subsequent searches for blue stars revealed a class of
“radio-quiet” quasars
– Optical spectra similar to those of quasars, yet radio
emission weak or absent
– Now called QSOs (“quasi-stellar objects”), but
common to use terms interchangeably
– About 10x more numerous than radio-loud quasars
AGN SEDs cannot be described as blackbodies
Reflects amount of energy
emitted in each equally
spaced interval on the
logarithmic frequency axis
Quasars are among the most luminous objects at every
wavelength at which they have been observed
Working model for AGN phenomenon is a `central engine’ that
consists of a hot accretion disk surrounding a supermassive
black hole. Energy is generated by gravitational infall of
material, which is heated to high temperatures in the disk
Radiation Force
• The energy produced propagates outwards & can
interact with infalling matter by absorption or scattering
• Through this interaction, the momentum of the
radiation is transferred to the matter, i.e., the infalling
matter experiences an outward-directed radiation force
• For matter to fall onto the SMBH at all, this radiation
force needs to be smaller than the gravitational force
Source of Radiation
•
Assume
-Spherically symmetric emission
-Completely ionized hydrogen gas
• Outward energy flux at distance r from center is S=L/4πr2
• The momentum carried by a photon (energy E=hν) is E/c, so the
outward momentum flux, or pressure, is S/c= L/4πr2 c
• The outward radiation force on a single electron is then
obtained by multiplying by the cross-section for interaction with
a photon (Thomson scattering), f = σ L/4πr2c
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