An Introduction to Radio Astronomy and Radio Galaxies

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An Introduction to Radio Astronomy and Radio
Galaxies

• Bruce Partridge, Haverford College
• bpartrid_at_haverford.edu

• High redshift sources 3C48 3C273 and Cyg A first
  found in radio.
• Why, when Lrad lt Lopt?
• At stellar temperatures, Lrad lt 10-6 Lopt.
  Cyg A
• However
  optical
• Non-thermal emission boosts Lrad
  radio
• And radio receivers are
• very sensitive.

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Properties of Radio Telescopes

• Sensitivity of radio instruments
• by radio I mean ? 100-105 MHz
• flux density S in W/m2 Hz 1 Jy 10-26 W/m2 per
  Hz
• 3C sources gt 1 Jy
• present observations reach S 10 ?Jy 10-31
  W/m2 Hz
• Resolution of radio instruments ? ?/d
• - for single antenna worse than optical (1 not
  1)
• - but for arrays of size D, ? ? /D
• can achieve 10-3 arcsec resolution
• but arrays are inefficient collectors of energy
  ? N(d/D)2 , for array of N antennae.

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The VLA, with D 3 km
4

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• Luminosities of Radio Galaxies
• Lrad lt Lopt with huge range
• most powerful radio sources have Lrad 108-9 Lo
• consider Lrad 108 Lo, source freq. range 1010
  Hz
• and receiver properties as at VLA
• then source can be seen to 4000 Mpc in 1
  min!
• Morphology of Radio Galaxies
• ordinary, not radio, galaxies
• starburst systems
• classical radio galaxies (morphology very
  different from optical)

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An Ordinary Galaxy M81 in Optical and Radio

• Optical Radio note concentration in
  spiral arms core.

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A Classical Radio Galaxy Cen A in optical and
(false color) radio
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One of the radio sources in clusters studied by
Y.T. Lin RBP

• Classical radio galaxies (morphology very
  different from optical)
• core, jet, lobe structure
• FRI core and jet prominent (younger?)
• FRII lobes (and shock fronts) prominent (older?)
  yes in this case

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FR Classes

• FR I
• FR II

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• Emission Mechanisms
• not blackbody (wrong morphology and expected S ?
  ?-2 tiny)
• hence evoke nonthermal mechanisms
• 1.) Synchrotron
• 2.) Thermal Bremsstrahlung (free-free)
• 3.) Re-emission from warm dust
• All present (to some degree) in most sources

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• 1.) Synchrotron
• spiraling of relativistic electrons around (weak)
  magnetic fields
• expected B 1 ?G, for which the cyclotron
  frequency 2? ? eB/mec
• 18 Hz check this at home
• to get radio frequencies require relativistic
  electron energies actual
• frequency ? ?2 calculate energy of electrons
  needed to produce
• 10 GHz radio waves
• spectrum of assembly of e set by e energy
  spectrum
• if N(E) ? E-p, then S ? ? -(p-1)/2 not
  hard to derive
• typical values of p 2-3, for which S ? ?
  -(0.5 -1.0)
• if B uniform, synchrotron radiation is highly
  polarized
• ? (p 1)/(p 7/3) 69-75, ?r B field
• In real sources, B is NOT uniform, and Faraday
  rotation depolarizes both reduce ?

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• Loss of energy (by synchrotron rad.) fastest for
  highest energy results in ?p -1
• Or (gradual) change in synchrotron spectrum by
  -1/2
• Time scale (2.5 x 1013)/B2 ? yrs, with B in
  microgauss
• for 100 GeV e- and B 1, t 108 yrs

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• 2.) Thermal Bremsstrahlung or Free-Free
  Emission
• non-relativistic e in a plasma
• move past positive ions
• produced in HII regions (T 104 K)
• for T 104 K, optical depth ? ?-2.1
• A useful formula
• when ? ltlt 1, easy to show S ? ?-0.1
• when ? gt 1, we have opaque cloud at T 104 so S
  ? ?2

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• 2.) Thermal Bremsstrahlung or Free-Free
  Emission
• non-relativistic e in a plasma
• move past positive ions
• produced in HII regions (T 104 K)
• for T 104 K, optical depth ? ?-2.1
• A useful formula
• where ne2 dl is emission measure in cm-6 pc
• when ? ltlt 1, easy to show S ? ?-0.1
• when ? gt 1, we have opaque cloud at T 104 so S
  ? ?2

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A Thermal Bremsstrahlung Spectrum Orion Nebula
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• 3.) Re-emission by Warm Dust
• dust absorbs uv and optical photons (for some
  sources
• ? gtgt 1)
• dust re-emits at its characteristic T
• T 15-25 K in our Galaxy
• T 40-50 K in starburst systems
• so peak of emission 100?
• long-wavelength tail extends to high radio
  frequencies
• (especially for high-z sources)
• radio spectrum not Rayleigh-Jeans, but S ? ?3-4
  because emission efficiency declines sharply with
  increasing ?

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Unified Spectrum (Arp 220)

• Note small role of Bremsstrahlung
• (dust reemission strong because Arp 220 is a
  starforming galaxy)

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Unified Model of AGN

• Beaming
• Morphology
• core as origin
• (often) symmetrical, collimated jets
• lobes
• Unified model to explain morphology
• (see Urry and Padovani, 1998)
• Black Hole at core
• fed by accretion disk
• spin axis axis of symmetry
• relativistic mater escapes along axis ?r
  accretion disk
• magnetic fields (?) collimate jets
• lobes are shock fronts

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• At radio wavelengths,
• see core jets
• In optical, depending
• on alignment, see
• narrow line or broad
• line region

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An Example 3C31
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Other Features of Unified Model

• Orientation
• line of sight ?r jet axisclassical radio
  galaxies
• line of sight inclined to jet axisasymmetrical
  sources
• line of sight ?? jet axisblazars
• Optical effects
• torus absorbs
• ? see broad line region only if line of sight
  parallel axis
• degree of obscuration distinguishes Type 1 and
  Type 2 QSO

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Asymmetrical Jets

• An optical jet --
• synchrotron emission
• from M87
• and a radio
• jet

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Evidence that Jets ARE One-Sided
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AGN and the Central Black Hole

• All galaxies above a certain mass?? contain a
  central Black Hole
• and MBH ? mass of bulge component (Gebhardt et
  al., 2000)
• Why are only 10 of galaxies active?
• BH present, but not fed dM/dt too low
• as exercise, calculate dM/dt needed to power
  107 Lo radio galaxy

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Raising dM/dt the Role of Collisions

• Collisions help randomize motions, increasing
  radial component, thus increasing dM/dt to Black
  Hole
• explains larger fraction of AGN in early Universe
• explains large number of radio-active galaxies in
  clusters
• Collisions trigger star formation as well

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A Collision NGC 6240

• 
  check NED for radio properties

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Starburst systems in radio

• Active star formation ? SN
• synchrotron emission (broadly distributed)
• Can feed AGN as well
• (emission from core
• small jets)
• May be heavily
• obscured in optical

M82 in radio
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