Q

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Quasars Cosmology
Quasars and Cosmology
ASTR-1510-01 ASTR-1510-02
Department of Physics, Applied Physics
Astronomy, RPI
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Week 6

• Active Galaxies
• Jones Lambourne
• Chapter 3
• October 5, 2009
• 300 PM
• Darrin 330

Department of Physics, Applied Physics
Astronomy, RPI
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Outline

• Astronomical Spectroscopy
• Spectra of Galaxies
• Observations of Active Galaxies
• Models of Active Galaxies

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Spectral Energy Distribution (SED)

• Light has a continuous range of colors.
• Each color corresponds to one wavelength, l.
• SED plot energy received vs l over broad
• range of l without fine detail.
• Above SED of a solid object at T5000 K.

Fig http//www.egglescliffe.org.uk/physics/astron
omy/blackbody/bbody.html
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SED of the Sun
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Spectrum

• spectrum like SED but with very fine detail.
• Some spectra (above) have spectral lines.
• Can be emission lines or absorption lines.
• Spectrum above has absorption (dark) lines only.
• Each line produced by one kind of atom.

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Spectrum of a Star (the Sun)

• Visible wavelengths 0.3 mm to 0.9 mm.
• Shown above tiny fraction of visible region.
• See several narrow absorption lines.
• They reveal presence of Fe, La, Cr, Sc, Nd, Y.
• Normal stars have absorption lines only.

Spectrum E.C. Olson, Mt. Wilson Observatory
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Spectrum of a Nebula

• Nebulae like this are clouds of very hot gas.
• The atoms produce emission lines.
• There is little or no energy between the lines.

Spectrum http//homepage.oma.be/gsteene/poster.h
tml
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Image of a Nebula
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Doppler Shift
atoms

• Wavelength changes by Dl if source is moving.
• Moving away shift toward red (Dlgt0,
  redshift).
• Moving toward shift toward blue (Dllt0,
  blueshift).
• If speed v of source is much less than c

Dl/l0 v/c l0 rest wavelength
Fig http//zebu.uoregon.edu/soper/Light/doppler.
html
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Doppler Broadening
atoms
Dl
Energy
Dl/l0 Dv/c
wavelength
l0

• A galaxy contains billions of stars.
• Each star produces narrow lines.
• Some are redshifted and some blueshifted.
• The combined effect is a broader line.

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Spectra of Galaxies
several percent of the speed of light!
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Key Concepts

• Normal stars have absorption spectra.
• Nebulae have emission spectra.
• Ordinary galaxies have absorption and some
  (spirals) have weak emission (from nebulae). The
  lines are always narrow (Dv 100 km/s) lines.
• Active galaxies have much stronger emission
  lines. All have narrow lines. Some also have
  broad (Dv 10,000 km/s) lines.

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Active Galaxies Seyfert Galaxies

• Usually (but not always) spiral galaxies.
• Strong emission from point-like nucleus.
• Strong emission from x-rays to infrared.
• Light varies over days to months.
• Come in two types
• Type I narrow lines broad-lines.
• Type II narrow lines only.
• Discovered in 1940s by Carl Seyfert.

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Arrows indicate nuclei.
Image http//www.astr.ua.edu/keel/agn/seyferts.g
if
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Spectrum of a Type 2 Seyfert
Spectrum http//praxis.pha.jhu.edu/papers/papers/
afdscirev_b/node9.html
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Active Galaxies Quasars (QSOs)

• Very bright pointlike optical emission.
• Varies over days to months.
• Broad and narrow emission lines.
• Spectra have redshifts z 0.1-6!!!
• Some (1) also emit radio waves.
• Enormous energy output L gt 1010 Lsun.
• Energy from region smaller than solar
• system!!! (to be explained later)
• HST showed they are nuclei of galaxies.
• Some have jets.

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Typical Quasar Ground-Based Telescope
Image http//www.astr.ua.edu/keel/agn/pks1117r.
gif
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HST Images of Quasar Host Galaxies
Image J.N. Bahcall, M. Disney, NASA
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Quasar 3C175 and its Jet
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Active Galaxies Radio Galaxies

• Usually (but not always) elliptical galaxies.
• Radio jets lobes extend from point nucleus.
• Lobes can extend 100 kpc from nucleus.
• Have one or two jets.
• Nucleus narrow lines or broadnarrow lines.

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Radio Galaxy Centaurus A
radio image
optical image
Images http//bdaugherty.tripod.com/gcseAstrono
my/active.html
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Optical and Radio Image of NGC 315
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Image http//www.astr.ua.edu/keel/agn/cygnusa.ht
ml
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Active Galaxies Blazars

• Powerful point-like optical radio source.
• Rapid variability over less than one day.
• Have a jet pointed directly at Earth (really!).
• Two types
• BL Lacertae (BL Lac) objects.
• Optically Violent Variables.

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The Unified Model

• Active galaxy galaxy black hole (BH) in
  nucleus.
• Matter falls into BH releasing huge energy.
• Broad lines emission from fast gas close to BH.
• Narrow lines emission from slower gas farther
  out.
• Dusty torus (doughnut) around BH obscures some
  views.
• Different types explained by different
  viewpoints and
• how much matter is falling into BH.

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Implications of Variability
dr
r
d

• Lamp turns on suddenly.
• Shade (radius r) illuminated at time t0.
• Signal from front arrives at td/c
• Signal from side arrives at td/cr/c

Unless the is a conspiracy, objects of size r
cannot vary faster than times of about r/c. AGN
vary on times of hours hence smaller than light
hours!!!
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Do we really know there is a black hole in the
center?

• Images of the centers of AGN show a small size.
• (2) The smallest time scales of luminosity
  variation indicate the central source is half the
  diameter of the solar system (much smaller than
  we can resolve in the images).
• (3) Rotation curves of stuff around the centers
  indicate a mass greater than that required for
  the central source to be a black hole.

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Where are Quasars today?

• Remember that the quasar phenomenon depends on a
  black hole that is consuming material from its
  surroundings. It is believed that almost all big
  galaxies today have black holes in the centers of
  them (the Milky Way included), but that they are
  no longer consuming large amounts of material.
  So, it is likely that active galaxies are the
  younger versions of todays normal galaxies.

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Era when QSOs were most common
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Black Hole at Center of the Milky Way
Laser points toward Galactic Center
Stars orbiting central black hole
Images Y. Beletsky Max Planck Institute for
Extraterrestrial Physics
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Summary

• Active galaxies have strong emission lines.
• There are 4 types
• Seyfert galaxies, quasars, radio galaxies, and
  blazars.
• All have pointlike nuclei.
• All have narrow emission lines.
• Some also have broad emission lines.
• All vary rapidly in time.
• All at very large distances, up to billions of
  light yr.
• Unified model explains many (but not all) we see.