Galactic Nuclei

1
Galactic Nuclei
2
Active Galactic Nucleus

• The centers of galaxies are found to be
• 1. a dense stellar cluster, with composite
  stellar-type absorption and emission spectra
• 2. extremely bright and compact nucleus,
  sometimes brighter than the entire galaxy, with
  non-stellar spectra
• The second type are the AGN, up to 10 of all
  galaxies, possibly an active phase of all
  galaxies
• AGN have strong, broad and rapidly variable
  emission lines from hot gas the nuclei are a
  few light-days across !

3
Active Galactic Nuclei and Quasars

• Centers of galaxies are extremely bright and
  active due to super-massive black holes at the
  center
• Emit light strongly in ALL wavelength bands,
  from gamma rays and X-rays to Radio
• The spectra do NOT look like that of stars
  evidence of unusual non-stellar activity
• Quasars Quasi-Stellar Radio Sources, appear
  star-like owing to large distances and
  red-shifted spectra
• Hence QSOs were formed early in the Universe,
  and are otherwise also active galactic nuclei
  (AGN)

4
Quasar
5
Redshifted H Balmer Lines
6
Redshift vs. speed (distance)
7
Number of Quasars vs. Age
8
AGN and Quasars

• There is considerable evidence of evolution
  Quasars are early (high-z) versions of AGN,
  before evolving into normal galaxies
• AGN and quasars are thought to harbor
  supermassive black holes that are the engines
  which power the highly energetic activity leading
  to extreme luminosities
• Quasars are the most luminous objects in the
  Universe (note that supernovae can be equally
  luminous but are transient, and remain that
  bright only for a few months at most)

9
Quasars and host galaxies at same redshift
10
AGN Seyfert Galaxies

• Spiral galaxies with bright compact nuclei
  showing broad emission lines like quasars are
  called Seyfert galaxies (after Carl Seyfert)
• Broad emission lines are due to Doppler
  broadening of light from gas clouds moving at
  high velocities, about 10,000 Km/sec
• Radiation from AGN and Quasars indicates a
  plasma source with temperatures 100,000 to
  millions of degrees (stars have temperatures much
  less than 100,000 K)
• How does the Black Hole power AGN and quasars ?

11
(No Transcript)
12
Blackbody Stellar vs. Non-thermal AGN Spectra

• Stellar spectra are that of a BB with peak
  emission around in one wavelength region
• AGN and Quasar spectra are non-thermal (non-BB),
  with radiation flux decreasing monotonically with
  frequency (energy), but remains significant at
  all wavelengths

13
Solar Radiation vs. Wavelength (Black Dots)
Spectral Fit to Blackbody at 5700 K (Solid Line)
14
AGN Spectra Constant Radiation at All Wavelengths
15
Active Radio Galaxies

• Radio loud powerful radio sources
• - Low Power Radio galaxies
• - High Power Quasars
• Radio Quiet weak radio sources
• - Low Power Seyfert galaxies
• - High Power quasi-stellar radio
  sources
• Radio sources have associated jets of
  relativistic particles synchrotron radiation
  and radio lobes

16
(No Transcript)
17
Quasar 3C273 and Relativistic Jet
18
(No Transcript)
19
Radio lobes at endpoints of jets
20
(No Transcript)
21
Supermassive Black Hole ParadigmStructure of
AGN and Quasars

• Geometry of AGN/Quasars Black Hole surrounded
  by an accretion disc, embedded in a torus
  (doughnut shaped), with jet streams perpendicular
  to the disc
• High velocity clouds moving around, with Doppler
  broadened emission lines
• Orientation of disc determines our view
• - face on looking at the nucleus OR
• - edge-on view of the obscuring torus
• Most extreme Quasars BLAZARS (BL Lac
  objects), viewing the jets end-on, no emission or
  absorption lines, but surrounding elliptical
  galaxies seen as fuzz

22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
AGN Variability or Reverberation

• The luminosity of AGN often varies on a
  time-scale of days ? the emitting nucleus must be
  very compact size, only light-days across
• Nuclear activity propagates all around within
  days
• Size 0.01 pc 10 light-days 1000 AU size
  of the solar system (give or take a factor of 2)
• AGN/Quasars emit light equal to an entire galaxy
  from a region not much bigger than a stellar
  system !

28
Evolutionary Sequence of Galaxies

• Quasars are nuclei of active galaxies in the
  distant past ultra-luminous, high-z objects
• AGN are the link between quasars and normal
  galaxies, activity continues to present day as
  they evolve
• Quasar/AGN activity fueled but a super-massive
  black-hole (SMBH) engine

29
Geometry and Physical Properties

• Observed image and spectral properties depend on
  orientation
• Obscuring torus hides the active nucleus and
  black hole activity if seen edge-on, e.g.
  narrow-line spectra similar to H II regions or
  nebulae with low-velocity clouds
• Face-on view reveals highly non-thermal
  (non-stellar) spectra with broad emission lines