Active Galaxies and
Related Objects
What are Active Galactic Nuclei (AGN)?
 Galaxies with a source of non-stellar emission arising in the
nucleus (excessive UV, IR, radio and X-ray light)
Central Black Hole accreting material
from host galaxy (Lynden-Bell 1969)
Quasars – luminous (MB<-23) objects with broad emission lines,
distant, some are strong radio sources
Seyferts – fainter (MB>-23), most identified locally, spiral hosts
Sey 1s – broad emission lines
Sey 2s – narrow emission lines
Starbursts – Extreme star formation in addition to black hole
accretion in the nucleus
Radio Galaxies – display excess radio emission and jets
ULIRGS – ultra-luminous IR galaxies
Quasars – first detected in 1960s as radio sources with star-like optical counterparts
•MB < -23, strong nonthermal continuum, broad permitted (~104 km/s) and
narrow forbidden (~102-3 km/s) emission lines
• Gravitational force required to prevent rapidly moving clouds that produce
these broad emission lines from escaping the nucleus ~ 108 – 109 Msun
It was later realized that most quasars
were not particularly bright in radio
emission - Radio quiet (RQQ)
Radio loud (RLQ) - 5-10% of all quasars
• RL = log(Lradio/Lopt)
• Radio components for RLQs are 1-2
orders of magnitude brighter than radio
galaxies (coming up next....)
Quasar host galaxies
•difficult to detect due to brightness of AGN
•appear to be a mixed bag of galaxy types - from disturbed galaxies
to normal ellipticals and early type spirals.
Quasar Spectra: Lyman Alpha Forest
QSO spectra reveal some
spectral features not
associated with the quasar.
These narrow absorption
lines are caused by
intervening galaxy halos
between us and the QSO.
Redshifts of the material will be equal to or less than the QSO.
Occurs as Lyman alpha absorption since this is the lowest excitation
level for Hydrogen gas found in galaxy halos.
Can be used to measure extent of galaxy halos and map out large
scale structure.
Quasar Spectra: Broad Absorption Lines (BALs)
Occurs when quasar is viewed along the l.o.s. of intervening, fastmoving material, possibly ejected from the quasar
BALs shown in red
SDSS BALQSOs from Trump et al. 2006
Seyfert galaxies were first identified by Carl Seyfert in 1943.
He defined this class based on observational characteristics:
Almost all the luminosity comes from a small (unresolved) region at the
center of the galaxy – the galactic nucleus.
Nuclei have MB > -23 (arbitrary dividing line between quasars/seyferts)
NGC 4151
Seyfert galaxy spectra fall into two classes
•broad & narrow emission line spectra (like quasars – Seyfert type 1)
•narrow emission line only spectra (Seyfert type 2)
Seyfert 1s:
Broad and
narrow
lines
(BLAGNs)
Seyfert 2s:
Only narrow
lines
(NLAGNs)
Structure of AGNs and Unified Theory
Seyfert 2
Observer is looking at
blackhole “edge-on”
through the surrounding
dusty torus - does not see
broad emission lines
produced by gas near BH
Quasar/Seyfert 1
Observer is looking into
the center of the
accretion disk, viewing
motions of gas near
blackhole - sees broad
emission lines
/BL Lac
Structure of AGNs and Unified Theory
Central Black Hole
Rs = 3x1011 m or 2 AU for 108 Msun
(Rs = 2GMBH/c2 ~ 3 x (MBH/M) km)
Accretion disk
UV/visible light from region 1012 - 1013m (Xrays come from a hot corona that surrounds
the disk)
Jets – ionized gas from accretion disk spirals
along magnetic field lines away from the disk
Broad Line Region
• size is few light-days to months
across (from reverberation
mapping)
• gas densities > 109 /cm3
• stratified (higher-ionization lines
from clouds closer to nucleus)
• outer edge defined by dust
sublimation radius (1500K)
Obscuring Torus
• Dusty structure that blocks view
of inner region
• May be composed of different
absorbing components on
different scales (pc-scale to
hundreds of pc)
Narrow Line Region
• few kpc across
• lower gas density ~103/cm3
• part of the host galaxy ISM near
galaxy center
Seyfert 2 galaxies (Narrow-Line AGN)
•can be differentiated from normal emission line galaxies
through the flux ratios of certain emission lines.
•shape of the underlying ionizing source determines how
many photons are available to produce particular
emission lines.
Kewley et al (2006) – red line shows extreme starburst, dashed line
separates most AGN from star-forming galaxies
Variability in AGN
• QSOs and Seyfert nuclei have long been recognized as variable
• Optical flux changes occur on timescales of months to years
• Cause of variability? – instabilities in accretion disk, SN or starbursts,
microlensing…..
Quasar light curve ~25 years
Seyfert light curve over ~11 months
Hawkins 2002
Variability occurs at most wavelengths - X-rays through radio
Fluctuations originate from a region no more than a ~few light weeks across
Variability in AGN
• QSOs and Seyfert nuclei have long been recognized as variable
• 90% to 100% of AGN selected through other means (spectroscopic, color
selection, UV excess) are variable when observed over several years (e.g.
Schmidt et al. 2010).
• Optical/IR flux changes occur on timescales of months to years
• Well modeled by DRW with characteristic timescale and max amplitude at
long timescales (MacLeod et al. 2010)
• Variability likely due to accretion disk instabilities (e.g. Li & Cao 2008)
• Variability amplitude is correlated with several AGN parameters (e.g. Lbol, λ, Mbh)
(Trevese+ 1994)
(Wilhite et al. 2008)
 Fainter AGN have a higher amplitude of variability
Blazars
•Strongly variable, highly polarized continuum, weak/absent
emission lines
•Variability faster and higher amplitude than normal quasars and
Seyferts
•BL Lac - high polarization, emission lines have low
equivalent width
•OVVs (Optically Violent Variables) - lower polarization,
emission line EW decreases as continuum brightens
Spectrum
Light Curve
Radio Galaxies
•Emit primarily at radio wavelengths (>108 Lsun)
(MW emits ~2500 Lsun in radio)
•Radio emission is highly polarized synchrotron
radiation
•Radio and optical morphology can be quite different!
•Host galaxies are generally Elliptical/S0
Radio morphology first classified by Fanaroff & Riley (1974)
•FR I: less luminous, 2-sided jets brightest closest to core and
dominate over radio lobes
•FR II: more luminous, edge-brightened radio lobes dominate over
1-sided jet (due to Doppler boosting of approaching jet and
deboosting of receding jet)
Spectroscopic classification of radio galaxies
•NLRGs (Narrow line …): like Seyfert 2s; FR I or II
•BLRGs (Broad line …): like Seyfert 1s; FR II only
FR I - 3C 47
FR II - 3C 449
Radio Galaxies
Radio image of the FR II radio galaxy
Cygnus A.
The lobes occur where the jets plow into intracluster gas.
Jet and Core in M87 at optical/radio wavelengths.
The jet is a series of distinct “blobs”, optically thin and brightest at low
radio frequencies – L ~ ν-α where 0.7 < α < 1.2
ejected by the galaxy nucleus, moving at up to half the speed of light
Core is just ~few pc across, optically thick and brightest at higher
frequencies – α = 0
Superluminal Motion of Jets
radio jet in 3C273
For object moving distance r below:
x = r cos , y = r sin  and t = r/v
Light from P takes x/c less time
to reach us than light emitted
from O.
Time the observer sees for
object to travel from O to P is
tapp = t - x/c
tapp = (r/v) - (r/c) cos 
tapp = (r/v) (1 -  cos )
Apparent velocity across sky
vapp = y/tapp
vapp = (v sin )/(1 -  cos )
For v << c,  is close to 0 and vapp = v sin
For v close to c, vapp can be >> v and even greater than c.
(U)LIRG’s - (Ultra) Luminous IR Galaxies
•First detected in IRAS all-sky survey (infrared – 12 to 100 microns)
•Galaxies that emit most of their light in IR  LIR > 1012 Lsun
(1011 Lsun for LIRGS)
•Few in local universe; most beyond z > 1
•Most luminous are undergoing mergers
•IR light is likely a combination of dust reprocessed AGN emission and
starbursts (Gas streams converge from different directions causing
shocks which compress material and trigger star formation)
•Some AGN may manifest as ULIRGs during different stages of evolution.
Nicmos Near-IR
Image of IRAS
selected ULIRG
Powering Active Galactic Nuclei
(1) A compact central source –
blackhole - produces intense
gravitational field.
MBH = 106 - 109 Msun
(2) Infalling gas forms an
accretion disk around the
black hole.
(3) As the gas spirals inward,
friction heats it to extremely
high temperatures; emission
from the accretion disk at
different radii (T > 104 K)
accounts for optical thru soft
X-ray continuum.
(4) Some of the gas is driven out
into jets focused by magnetic
fields.
How efficient is
AGN energy
production?
Some fraction of the infalling mass is converted into
energy (the rest goes into building BH mass). Matter
is heated to high temps by dissipation in accretion
disk and radiates away its gravitational potential
energy.
BH radius is Rs=2GM/c2 = 0.25 M8 light hours. What
energy is available (via gravitational potential
energy) for a mass m falling from far away to the Rs?
Emax ≅ GMm/Rs
Substituting equation for Rs
Emax ≅ (1/2)mc2 (~up to half the rest energy)
Energy generation rate (Luminosity) depends on rate of mass infall
Lmax = (1/2)(dm/dt)c2
Infalling matter must spiral in for most energy to escape…
The efficiency of this conversion is typically ~10% (η=0.1) for AGN
L = η(dm/dt)c2
Consuming 1 – 10 solar masses per year, black hole accretion disk
can radiate ~100 – 1000 LMilkyWay.
So...can you just shovel in mass at higher and higher rates to create a
superluminous AGN?
The Eddington Limit – the maximum L for a given black hole
mass (any higher and radiation pressure will blow away
surrounding, accreting gas)
Force caused by outward flow of photons balances gravitational
force of infalling matter (assuming pure ionized H gas)
σeL / (4πr2c) =
GMbhmp / r2
LE = 4πcGMbhmp/σe = 3.3x1012L (Mbh/108M)
If L is greater than LE, ionized gas will be accelerated outward and
accretion will cease. This leads to a maximum accretion rate for
black holes:
.
ME = LE / ηc2 = 2M/yr (Mbh/108M) (0.1/η)
.
. .
The Eddington ratio is then m = M / ME or L / LE
The Milky Way’s Blackhole
All galaxies may contain SMBHs at their centers, with their appearance as
AGN dependent on the availability of fuel in the vicinity of BH. The Milky
Way galaxy is an example of a “starved” SMBH – low Eddington ratio
Radio image (80 pc
across) shows feature
SgrA and radio filaments
Radio image (10 pc
across) shows feature
known as SgrA* thought to be position
of SMBH
IR image showing
stars near the central
parsec - can be used
to estimate BH mass
•Measure proper motions of stars in GC
•Adaptive optics at Keck improved groundbased resolution to 0.5” in IR (stellar positions
measured to 0.002”)
•90 stars identified and proper motions (largest
at 1400 km/s!) centered about SgrA* to within
0.1”
•Velocities consistent with Keplarian motion (all
mass at center)
•M = 2.6 +/- 0.2 x 106 Msun
The Milky Way’s Blackhole – X-ray Emission
•Chandra X-ray image of Sgr A*
showing nucleus and several
thousand other X-ray sources.
•During 2-week observation
period, several X-ray flares
occurred.
•Rapidity of flares indicates they
originate near the Schwarzchild
radius of the BH.
•Even during the flares, X-ray
emission from the nucleus is
relatively weak. Suggests that Sgr
A* is a starved black hole, possibly
because explosive events in the
past have cleared much of the gas
from around it.
Blackhole Mass - Bulge Mass Relation
•clear relationship between mass of spheriodal component of host galaxy and
blackhole mass
•indicates connection between galaxy formation (star formation) and
growth/evolution of central blackhole
(Gebhardt et al. 2000; Ferrarese & Merritt 2000)
Active Galaxies as part of Galaxy Evolution
•As small galaxies merge
to form larger ones,
blackholes may form at the
nucleus (ULIRG stage?)
•With plenty of fuel
available early on
(accretion triggered by
merger?), the galaxy light
is dominated by emission
from accretion disk
(Quasar). Feedback from
AGN radiation may
slow/stop star formation in
the host galaxy, setting
spheriodal component size
•Additional mergers and depletion of fuel may result in powerful radio galaxies and Seyfert
galaxies.
•Further fuel depletion results in a normal galaxy with a dormant blackhole at the nucleus.
Active Galaxies as part of Galaxy Evolution
AGN phase triggered as a result of galaxy mergers
(Hopkins et al 2008)
Active Galaxies as part of Galaxy Evolution
Hopkins et al. (2006)
0.6
0.4
Red Sequence
U-B
0.2
-0.0
-0.2
-0.4
Blue Cloud
-0.6
8
9
10
11
Log Stellar Mass
12