Download PH607lec12-10agn2

4. Quasars & Blazars (AGN – Part 2)
Some radio sources were not associated with galaxies, but with
stellar-like sources on optical images
In 1963, Maarten Schmidt made the breakthrough that the
spectrum of 3C273 had nebular emission lines at the then unheard
of redshift of z=0.158
Soon afterward, 3C48 was found to be at a redshift of z=0.367
(thus began the race for the highest redshift quasar!)
z = 0.2 and z = 1.0 shifts…….
Quasars are very luminous – L ~ 1013-1015L
These objects are AGN whose nuclear light dominates so
that we can’t distinguish the host galaxy
These “3C” objects are known as quasi-stellar radio
sources (quasars); there are also quasi-stellar objects
(QSO’s) identified via the spectra that are radio quiet
Radio quiet QSO’s outnumber quasars by 10-30, the
term quasar is often used for both
Quasars are variable over small time scales – hours to days!
Current record for the highest redshift quasar is z=6.4 found
in the Sloan Digital Sky Survey:
PKS 1117-248, optical :
Some quasars display extended radio structures:
The entire SED of radio loud (top) and
radio quiet quasars (lower panel):
Quasar host galaxies:
Many are interacting – do interactions trigger AGN
activity? Promote fueling gas into centres?
Quasar Evolution
Note: not all galaxies with SMBHs (Super-Massive
Black Holes) are AGN (even the Milky Way)
Quasars were much (~1000 times) more
numerous at z~3 than today:
Is an AGN a requirement for galaxy formation? Does every galaxy
have a SMBH? (Remember -- lots have been discovered in nearby
ellipticals and bulges!)
More interactions in the past:
Quasars were also much more luminous in the past
There is probably some combination of luminosity and
density evolution. Here are the Luminosity Functions for
quasars at different redshifts (the numbers actually start to
drop off above z=2). (This is the co-moving space density)
Co-moving volume = 1/(1+z)3
2df quasar luminosity function:
5. Blazars: BL Lacs
Catalogued as variable stars (e.g., BL Lacertae)
Highly variable, highly polarized
Variability occurs on the time scale of days
Virtually featureless continuous spectra
Also: optically violent variables (OVV) quasars
Collectively OVV’s and BL Lac’s are known as blazers
6. Interpretation of AGN: accretion
1. Size. Variability on short time scales – hours - implies a small
region:
 R = ct
 ~ 7-10 AU !! (for timescale variability on the order of
hours)
2. Mass.
Eddington luminosity: the largest luminosity that can pass
through a layer of gas in hydrostatic equilibrium, supposing
spherical symmetry.
Using the mass-luminosity relation, it can be used to set limits on
the mass. For an AGN with an observed (bolometric) luminosity L,
can estimate the minimum mass of the black hole involved.
Suppose the gas around the black hole is:
• Spherically symmetric
• Fully ionized hydrogen
At distance r, flux is:

F  L / 4r2
This is flux of energy. Since momentum of a photon of
energy E is E / c, momentum flux due to radiation is:

Prad  L/ 4r2c
This is the pressure that would be exerted on a totally
absorbing surface at distance r from the source.
Force exerted on the gas depends upon the opacity (i.e. the
fraction of the radiation absorbed per unit mass of gas).
Minimum force is given by the absorption due just to free
electrons. This is given by the Thomson cross-section:

T 83)e2mec2)26.65 1025 cm2
Outward radiation force on a single electron is:

Frad LT / (4r2c)
Inward force due to gravity of a central point mass M is:

Fgrav GM(mp me )r2 = GMmpr2
Note: include proton mass in Fgrav since electrons and protons
coupled due to electrostatic forces.
Setting Frad = Fgrav, and solving for L:

Ledd  4GMm p c /  T
Ledd  1.3  1038

M
M
ergs 1  3.3  10 4
Lsun
M sun
M sun
This luminosity is known as the Eddington limit. It is the
maximum luminosity of a source of mass M, which is
powered by spherical accretion of gas.
Can invert the argument. If a source with observed luminosity
L is radiating at the Eddington limit, the mass would be a minimum
mass - source could actually be radiating at much less than the
Eddington limit.
Need a large mass in a small space
Therefore, probably powered by a black hole!!
Check for consistency: Schwarzschild radius is the radius
where the “escape velocity” equals the speed of light around
a black hole of mass M:
 RS = 2GM/c2
It relates mass and size!
Accretion: Continuum emission powered by gas falling onto
central black hole,
At a rate of one solar mass per year, losing potential energy and
radiating
Since gas has angular momentum it forms an accretion disk:
an accretion disk around a supermassive black hole
Gas clouds in surrounding galaxy are ionized by this central
engine.
Clouds closest to the central engine are dense and have
high velocities (otherwise they would fall in!). So emission
lines from this region are broad (~2000 km/s) – the broad
line region (BLR)
Clouds farther away from the central engine have a lower
density and lower velocities. Line emission is narrow (5001000 km/s) – the narrow line region (NLR)
Dust torus surrounds the central engine and BLR in the
same plane as the accretion disk. The orientation of the dust
torus relative to the line of sight affects the appearance of the
AGN.
How fast must gas be supplied to the black hole to produce
typical AGN luminosities (1044 - 1046 erg s-1)?
L  M˙ c2
Accretion rate: units g s-1 or Solar masses per year
A mass m of gas at infinity has zero potential energy.
Energy available if the gas spirals in to radius r is:

E GMBHm / r

L GMBH (dm/dt) / r
Really an upper limit - not all the potential energy will be
radiated as the gas falls in.
Standard estimate is 
to sustain a quasar is:
w needed
1046 erg s-1
c21026 g s-1 
Mass supply needed is fairly modest…
-1
Summary
Seyfert 1: Broad/narrow emission, weak radio, strong Xray,spirals, variable
Seyfert 2: narrow emission, weak X-ray, spirals, not variable
Quasars QSR: broad/narrow, strong radio, FRII, polarisation,
variable
Quasars QSO: broad/narrow
polarisation, variable
lines,
weak
radio,
weak
Blazars BL Lacs: strong polarisation, absent/weak lines,
strong radio, highly variable, mainly ellipticals
OVV quasars: powerful BL Lacs but with emission lines
Radio Galaxies BLRG: Broad/narrow lines, strong radio
emission, FR II, weak polarisation, elliptical variable, variable
Radio Galaxies NLRG: narrow lines only, strong radio, FR I/II,
no polarisation, elliptical, not variable
In Seyfert 1s we are observing the active nucleus directly. In
Seyfert 2s we observe it through an obscuring structure which
prevents us getting a direct view of the optical continuum:
At higher luminosities, quasars take the place of Seyfert 1s, but the
corresponding 'quasar 2s' are elusive at present. If they do not
have the scattering component of Seyfert 2s they would be hard to
detect except through their luminous narrow-line and hard X-ray
emission.
High-luminosity radio-loud quasars can be unified with narrow-line
radio galaxies in a manner directly analoguous to the Seyfert 1/2
unification.
X-ray evidence, where available, supports the unified picture:
powerful radio galaxies show evidence of obscuration from a torus,
while quasars do not.
The population of radio galaxies is completely dominated by lowluminosity, low-excitation objects. There is no evidence for a torus
in these objects at all. Most likely, they form a separate class in
which only jet-related emission is important. At small angles to the
line of sight, they will appear as BL Lac objects
7. Intervening gas:
Lyman-alpha forests: the Ly-alpha (1215A at rest) peak shifted to
3730A :
Ly-alpha wavelength of the neutral H in intervening galaxies lies
between 1215A and the longer wavelength.
Narrow absorption lines: low column, intergalactic clouds at various
redshifts.
Damped Ly absorbers – intervening high density (N(HI) > 2 x 1020 cm2) gas along the line of sight to a distant (and bright!) quasar
 Probably due to the outer disks of galaxies
 High density implies the absorption line is optically thick
(“damped”)
 We can trace the evolution of HI in damped Ly absorbers
 We can also measure the metal abundances of DLAs,
Lyman break galaxies, the Ly forest (intergalactic
medium) and quasars.
Next, a Lyman break galaxy (utilises 912A also, below which HI can be
ionized from the ground state:
These breaks can be utilized to derive ‘photometric’ redshift using
UBVRI photometry.
THE END