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Active Galactic Nuclei
(F. Miniati HIT J12.2)
• Seyfert Galaxies
• Quasars
i) Radio Galaxies
ii) QSOs
iii)Blazars
Active Galactic Nuclei
AGNs are classified in many different ways but the two
“main” classes:
• Seyfert Galaxies:
• Quasars:
LAGN ∼ LGalaxy
LAGN ≥ 100 x LGalaxy
Probably related but historically discovered very
differently. Quasars are rare objects, found at large
distances and so bright that they hide the light from the
galaxy. This lead to the separate classification.
Quasars include radiogalaxies and blazars which have
supermassive BH in their centers.
NGC 1275
NGC 5548
courtesy of A.V. Filippenko
courtesy of K.T. Korista
First optical spectrum of an AGN was obtained at Lick Observatory by
E.A. Fath in 1908 as part of his dissertation work. He noted the
presence of strong emission lines in the nebula NGC 1068.
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But it took Carl Seyfert (1943) to identify the new class of objects
(NGC 1068, NGC 1275 1, NGC 3516, NGC 4051, NGC 4151, NGC 7469):
i)
The lines are broad (up to 8500 km/s, full width at zero
intensity)
ii) The hydrogen lines sometimes are broader than the other lines.
Seyfert Galaxies
Type 1 and 2
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Type 1 Seyfert galaxies have both narrow and
broadened optical spectral emission lines. The broad
lines imply gas velocities of 1000 - 5000 km/s very
close to the nucleus.
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Type 2 Seyfert galaxies have narrow emission lines
only (but still wider than emission lines in normal
galaxies) implying gas velocities of ~ 500-1000 km/s.
These narrow lines are due to low density gas clouds
at larger distances (than the broad line clouds) from
the nucleus.
Seyfert: Active phase of a
Spiral Galaxy (?)
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The Seyfert are
found in spiral
galaxies, about
2-5% of all spiral
galaxies.
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They have bright
central nuclei,
P~1036 - 1039 erg/s
NGC 4258. Credit: Nature.
Seyfert Lifetimes
(Woltjer 1959)
Seyfert galaxies 2% of spiral galaxies
=> τSyefert ≥ 2×108 yrs
i) If 2% of spirals are always Seyferts
=> τSyefert = τHubble (1010 yrs)
ii) If all spirals pass through a Seyfert phase
=> τSyefert = 2% τHubble = 2×108 yrs
Seyfert Nuclear Mass
(Woltjer 1959)
The nucleus is unresolved => R < 100pc
The emission lines characteristic of a lowdensity gas, => R > 1pc
If the nucleus is gravitationally bound, virial
2
2
argument gives:
u r
u
⎞ ⎛ r ⎞
9⎛
M≈
G Newton
= 10 ⎜ 3
M
⎟
⎜
⎟
⎝ 10 km / s ⎠ ⎝ 10 pc ⎠
Thus the mass of the nucleus can be inferred to
be in the range: M ≈ 10 9 ±1 M 
Quasars
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Quasars are the most luminous AGN, and among the most luminous
objects in the sky at every wavelength.
•
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Unlike Seyfert, quasars are typically found in elliptic galaxies
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The angular resolution of radio observations was good enough to
identify the strongest radio sources with individual optical objects,
often galaxies, but sometimes stellar-appearing sources
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However, the photographic spectra were very confusing, as they
showed strong very broad emission lines at unidentified wavelengths.
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Photometry of these objects revealed that they are anomalously blue
relative to normal stars.
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The nature of these ``radio stars'' was very uncertain. They became
known simply as quasi-stellar-radio-sources.
Originally discovered as a result of the first radio surveys of the sky
in the late 1950s
Quasars 3C 273, breakthrough
(Schmidt 1963)
Emission lines are hydrogen Balmer-series and Mg II 2798 at z =
0.158. This was about 10 x zSeyfert and among the largest measured at
the time
d = cz / H 0 = 470 h0−1Mpc
Disturbing enormous luminosity implied; 3C 273 was and remains the
brightest known quasar (B = 13.1 mag). Using the formula for the
distance modulus
m − M = 5 log(cz / H 0 / Mpc) + 25
the absolute magnitude of 3C 273 is MB = -25.3 + 5log h0, which is
about 100 times as luminous as normal bright spirals like the Milky
Way or M31.
Quasar 3C 273 with a jet
(Chandra X-ray Observatory)
Quasars spectral properties
Fλ = C λ α
Alloin et al. (1995) + Elvis et al. (1994)
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Very broad spectral energy distribution (SED): radio, infrared, optical, X-ray,
gamma-ray
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Unlike spectra of stars or galaxies, quasars spectra cannot be described in terms
of blackbody emission at a single temperature, or as a composite over a small
range in temperature.
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Non-thermal processes, primarily synchrotron radiation, were thus invoked early
on to explain quasar spectra.
Quasars’ variability
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•
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Quasars are variable in every waveband, in the
continuum and in the broad emission lines.
Optical continuum variability of quasars was
established even before the emission-line redshifts were
understood (e.g, Matthews and Sandage 1963), and
variability was one of the first properties of quasars to
be explored in detail (e.g., Smith and Hoffleit 1963).
Many quasars were found to be variable at the 0.3-0.5
mag level over time scales of a few months. A few
sources were found to vary significantly on time scales
as short as a few days.
Quasars’ Variability
courtesy of Bill Keel
Quasars’ Size
Short time variability and coherence arguments imply:
LQuasar ≤ cτ var  1 light day = 2.5 × 1015 cm  200 AU
This was immediately perceived as a major problem,
since a nucleus comparable in size to the Solar System
is emitting hundreds of times as much energy as an
entire galaxy !!
Quasars’ importance...
• Might involve black holes (Zel’dovich &
Novikov 1963, Salpeter 1963)
• They might play a role in galaxy formation
(Burbidge, Burbidge &Sandage 1963)
• Hight luminosity, potential for cosmological
probes at the largest distances
Radio Galaxies: Cygnus A
relativistic electrons
Synchrotron radiation
N(E)dE = N 0 E − s dE
⇒ Fν ∝ N 0 B
1+ s
2
Cygnus A radio galaxy. Image R.A. Perley, J.J. Cowan, J.W. Dreher, and NRAO/AUI/VLA
Extended
Compact
ν
−
s −1
2
Radio Properties of
Quasars
Compact
Extended
Angular size
Unresolved,
R≲10-2pc
Resolved, R≲Mpc
Emission
Synchrotron
Synchrotron
Optical Depth
Thick
Thin
Spectral Index
Flat, α≲0.5
Steep, α≳0.7
Radio Properties of Quasars
Faranoff-Riley I
Not edge brightened,
subsonic outflows
Faranoff-Riley II
Edge brightened,
supersonic outflows
Bridle et al. (1984)
Laing & Bridle (1987)
• Bridle & Perley (1984) give L (1.4 GHz) = 1032 erg s-1 Hz-1
as the transition specific luminosity between the two types.
• Quasars are FR II sources.
Centaurus A
(HST + VLA)
Centaurus A radio galaxy. Image:
NRAO/AUI/VLA and NASA/ESA
Hubble Space Telescope
QSOs
Because of their particularly blue spectra, quasars can
be isolated as U-excess objects (Ryle & Sandage,1964)
Hewitt & Burbidge (1993)
QSOs
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Ryle and Sandage (1964) found a large population of
optically selected, quasi-stellar-objects (QSO) that
could be quasars
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However, at long wavelengths (radio) they were
typically 100 times fainter than radio selected quasars
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QSOs: subclass of radio quiet AGN, 10-20 times more
numerous than radio loud quasars
Rr − o =
Fradio (6cm)
o
Foptical (4400 A)
Radio Loud
Radio Quiet
Rr − o = 10 − 1000
0.1 < Rr − o < 1
Blazars
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Blazars are a class of AGN that are radio sources and
consist of both Optically Violent Variables (OVVs)
and BL Lac objects (sources showing a featureless
optical spectrum) and Flat Spectrum Radio Quasars
(sources showing strong and broad emission lines)
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Their spectra are flat and, among AGNs, blazars emit
over the widest range of frequencies, and have been
detected from radio to gamma-ray.
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They are highly variable AGNs, visible and gamma-ray
variability from minutes to days.