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Galactic Nuclei

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... due to super-massive black holes at the center ... only light-days across Nuclear activity propagates all around within days Size ~ 0.01 pc ~ 10 light-days ... – PowerPoint PPT presentation

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Title: 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
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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
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17
Quasar 3C273 and Relativistic Jet
18
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19
Radio lobes at endpoints of jets
20
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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
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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
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