Transcript Document

Active Galaxies and
Related Objects
What are Active Galaxies?
Active galaxies have an energy source beyond what can be
attributed to stars. The energy is believed to originate from
accretion onto a supermassive blackhole.
Active galaxies tend to have higher overall luminosities and very
different spectra than “normal” galaxies.
“non-stellar” radiation
Some classes of
active galaxies:
stellar, blackbody radiation
•Quasars
•Seyfert galaxies
(Type I and Type II)
•Radio galaxies
•LINERs
Quasars
• First discovered in the 1960s.
• Detected radio sources with optical counterparts
appearing as unresolved point sources.
• Unfamiliar optical emission lines.
• Maartin Schmidt was the first to recognize that these lines
were normal Hydrogen lines seen at much higher redshifts
than any previously observed galaxies.
D = 660 Mpc (2.2 billion light years) for 3C273
1340 Mpc (4.4 billion light years) for 3C 48
L = 2 x 1013 Lsun for 3C273.
• Within ~2 years, quasars were discovered with:
z > 2 and L  1014 Lsun
• Most distant QSO discovered today - z = 6.42
Quasars
•MB < -23, strong nonthermal continuum, broad permitted (~104
km/s) and narrow forbidden (~102-3 km/s) emission lines
•Radio quiet (RQQ): elliptical or spiral host galaxies
•Radio loud (RLQ): 5-10% of all quasars, elliptical hosts
•Broad Absorption Lines (BAL) Quasars: normal quasars seen at
a particular angle along the l.o.s. of intervening, fast-moving
material.
• High-ionization (HIBAL): Ly, NV, SiIV, CIV
• Low-ionization (LOBAL): AlIII, MgII
If we block out the light of luminous quasar, we can see
evidence of an underlying host galaxy.
Quasar hosts appear to be a mixed bag of galaxy types from disturbed galaxies to normal E’s and early type spirals.
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
short
exposure
long
exposure
10000 times brighter than our galactic nucleus!
Seyfert galaxy spectra fall into two classes: broad emission line
spectra (like quasars) and narrow emission line spectra.
Seyfert 1s:
Broad and
narrow
lines
Seyfert 2s:
Only narrow
lines
NLAGNs can be differentiated from normal emission line
galaxies through the flux ratios of certain emission lines.
The shape of the underlying ionizing source (energy source)
determines how many photons are available to produce
particular emission lines.
Variability in AGNs
• 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
This indicates that the fluctuations are originating from a very tiny object.
Why does rapid variability indicate small physical size of the
emitting object?
Consider an object like the Sun. Any instantaneous flash
would appear “blurred” in time by  t = RSun / c.
RSun
observer
RSun
Time Delay =  t = RSun / c
700,000 km / 300,000 km/s = 2.3 sec
Seyfert continuum luminosity varies significantly in less than a year
(some variation occurs on timescales of days or weeks.
This implies an emitting source less than a few light-weeks across!
Blazars
•Strongly variable, highly polarized nonthermal continua,
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 most of their energy at radio wavelengths
•Emission lines from many ionization states
•Nucleus does not dominate galaxy’s emission
•Host galaxies are 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
FR II radio galaxies: most
emission comes from lobes
Radio “Light”
Centaurus A
Visible Light
The radio lobes span about 10 degrees on the sky!
Lobes consist of material ejected from the nucleus.
Radio image of the FR II radio galaxy Cygnus A.
This galaxy also has HUGE radio lobes.
The lobes occur where the jets plow into intracluster gas.
The thin line through the galaxy is a jet ejected from the nucleus.
FR I radio galaxy: most of the energy comes from a small nucleus
with a halo of weaker emission in a halo around the nucleus.
Visible image of the core-halo (FR I) radio galaxy M87.
This giant elliptical (E1) galaxy is ~100 Kpc across.
It has a “jet” of material coming from the nucleus.
Close-up view of the jet in M87 at radio wavelengths.
galaxy nucleus,
i.e. the radio core
The jet is apparently a series of distinct “blobs”, ejected by the
galaxy nucleus, and moving at up to half the speed of light.
The jet and nucleus are clearly non-stellar.
LINERs and ULIRGs - Starburst or AGN?
What is a starburst?
•May result from a galaxy collision/merger
•Gas streams converge from different directions causing shocks
which compress material and trigger star formation
•Gas which loses enough angular momentum falls into the galaxy
center  bar formation  funnels more gas inward  violent star
formation near center of disk and further out
Nuclear close-up (HST) of NGC
1808 starburst galaxy. Galaxy
has barred-spiral morphology.
LINERs
•Low-Ionization Nuclear Emission Region
•Narrow low-excitation emission lines
•Weak nonthermal continuum
•Spiral host galaxies
•Observed emission could be due to AGN or shocks/winds
from a starburst
•Some appear as unresolved compact sources in the UV
•Some have radio sources: AGN or supernovae remnant?
ULIRG’s - Ultra Luminous IR Galaxies
•First detected in IRAS all-sky survey
•Galaxies that emit most of their light in IR - LIR > 1012 Lsun
•Few in local universe; most beyond z > 1
•Nearly all are undergoing mergers - forming E’s
•IR light is likely a combination of dust reprocessed AGN emission
and starbursts.
•Some AGN may manifest as ULIRGs during different stages of
evolution.
Nicmos Near-IR Image
of IRAS selected
ULIRG
What Powers Active Galactic Nuclei??
(1) A compact central source
provides a very intense
gravitational field. For active
galaxies, the black hole has
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.
Broad Emission line
region photoionized by
2 size
continuum emission;
is ~few light-days to
months; densities > 109
/cm3; stratified (higherionization lines from
smaller radii)
Obscuring Torus of
dust is believed to
form around
perimeter of
accretion disk
NarrowSeyfert
Emission
1 line
region also
photoionized; size is ~10
to 1000 pc; densities
~103 - 106 /cm3; complex
morphology
Unified Theory of Active Galaxies
2
Observer is looking at
blackhole “edge-on” through
the surrounding dusty torus
- does not see broad
emission lines produced by
gas near BH
Seyfert 1
Observer is looking into
the center of the
accretion disk, viewing
motions of gas near
blackhole - sees broad
emission lines
How efficient
is the energy
production?
Before disappearing into the event horizon
of a blackhole, some fraction of the infalling
mass is converted into energy. 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 (which sets minimum variability
timescale). Smallest stable orbit is at 3Rs.
Max efficiency occurs when all potential
energy released during fall from infinity to
3Rs is extracted. GR gives efficiency = 6%
to 40% depending on BH rotation.
Example: By consuming 1 – 10 solar
masses per year, black hole accretion disk
can radiate ~100 – 1000 LMilkyWay.
Direct evidence of the blackhole/accretion disk hypothesis: HST image
of the core of the lobe radio galaxy NGC 4261
radio
lobes
galaxy
nucleus
Velocities derived from Doppler shifted lines on either side of nucleus
require ~3 billion solar masses.  a blackhole!
Hosts and Environments
•Most quasars, NLRGs/BLRGs, blazars are E/S0 hosts (some early
type spirals for radio quiet quasars)
•Seyferts/LINERs are typically spirals
•The maximum luminosity of the AGN correlates with the bulge mass
(Ferrerese et al. 2000) - larger bulge/ greater mass BH
•Bars appear to be no more common in Seyferts than normal galaxies
(Mulchaey & Regan 1997).
•Conflicting evidence regarding whether or not Seyferts are found in
more interacting systems than normal galaxies (Dahari et al. 1984;
DeRobertis & Yee 1988). May be that minor mergers are more
important than major mergers for instigating AGN.
•Generally, luminous AGN tend to be in denser than average
environments and low-luminosity AGN in normal/slightly dense
environments.
It is now believed that most if not all galaxies contain
supermassive blackholes in their nuclei. Whether or not these
galaxies appear as Active Galaxies depends on whether or not fuel is
available in the vicinity of BH.
The Milky Way is believed to harbor a supermassive
blackhole in the nucleus!
Sagittarius A:
bright radio source at the
center of the Galaxy
Sagittarius (Sgr) A*:
object at the very center of
the Galaxy a million times
more luminous than the
Sun (IR, radio, X-ray, and
gamma ray source)
• Quasars were more
common in the past during the epoch of galaxy
formation
• What’s the connection?
• Black Holes form in the
centers of young Galaxies.
• Black Holes “shine” as
Active Galaxies (Quasars)
until the fuel (infalling gas)
is used up.
• Most Quasars are now
gone, but the Black Holes
remain.
Active Galaxies as part of Galaxy Evolution
•As small galaxies
merge to form larger
ones, blackholes
may form at the
nucleus.
•With plenty of fuel
available early on,
the galaxy light is
dominated by
emission of the
blackhole (Quasar).
•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.