Active Galaxies and Related Objects

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Active Galaxies and Related Objects
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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
• Quasars
• Seyfert galaxies (Type I and Type
  II)
• Radio galaxies
• LINERs

stellar, blackbody radiation
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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 gt 2 and L ? 1014 Lsun
• Most distant QSO discovered today - z 6.42

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Quasars

• MB lt -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

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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
Es and early type spirals.
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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 gt -23 (arbitrary dividing
line between quasars/seyferts)
10000 times brighter than our galactic nucleus!
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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
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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.
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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.
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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.
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!
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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

Light Curve
Spectrum
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Radio Galaxies

• Emit most of their energy at radio wavelengths
• Emission lines from many ionization states
• Nucleus does not dominate galaxys 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

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FR I - 3C 47
FR II - 3C 449
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FR II radio galaxies most emission comes from
lobes
The radio lobes span about 10 degrees on the sky!
Lobes consist of material ejected from the
nucleus.
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Radio image of the FR II radio galaxy Cygnus A.
The lobes occur where the jets plow into
intracluster gas.
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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.
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Close-up view of the jet in M87 at radio
wavelengths.
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.
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• 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.
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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?

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ULIRGs - Ultra Luminous IR Galaxies

• First detected in IRAS all-sky survey
• Galaxies that emit most of their light in IR -
  LIR gt 1012 Lsun
• Few in local universe most beyond z gt 1
• Nearly all are undergoing mergers - forming Es
• 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
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What Powers Active Galactic Nuclei??

• A compact central source provides a very intense
  gravitational field. For active galaxies, the
  black hole has MBH 106 - 109 Msun
• 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 (Tgt104 K)
  accounts for optical thru soft X-ray continuum.
• (4) Some of the gas is driven out into jets,
  focused by magnetic fields.

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Broad Emission line region photoionized by
continuum emission size is few light-days to
months densities gt 109 /cm3 stratified
(higher-ionization lines from smaller
radii) Narrow Emission line region also
photoionized size is 10 to 1000 pc densities
103 - 106 /cm3 complex morphology
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Seyfert 1
Obscuring Torus of dust is believed to form
around perimeter of accretion disk
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Unified Theory of Active Galaxies
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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
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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 Rs2GM/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.
How efficient is the energy production?
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Direct evidence of the blackhole/accretion disk
hypothesis HST image of the core of the lobe
radio galaxy NGC 4261
galaxy nucleus
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Velocities derived from Doppler shifted lines on
either side of nucleus require 3 billion solar
masses. ? a blackhole!
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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.

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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)
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• Quasars were more common in the past - during
  the epoch of galaxy formation
• Whats 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.

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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.