Galactic Astronomy

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Galactic Astronomy

• Absolute distance estimators
• - Baad-Wesselink method
• - Sunyaev-Zeldovich effect

Dong-hyun Lee 2007/07/18
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Nearby star simple geometric tech.

• Trigonometric parallax (d 20pc)
• Moving cluster method
• Secular parallax (d - 500pc)
• Statistical parallax
• ? major drawback
• Suffer from a variety of large sys. errors
• Compare results from others to check for
  consistancy

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Baade-Wesselink method

• Where L intrinsic lum. R known stars
  rad.
• bolometric abs. mag.
• Cepheids RR Lyrae variable stars
• Lines in the spectrum show Doppler shift that
  vary cyclically with the same period
• Integrating the line-of-sight vel. of atmosphere
  over time

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Baade-Wesselink method

• Simple case
• Classical weakness
• Ass. overall exp. Vs. real non-uniform exp.
• Ass. sph. sym. Vs. real not sph. sym.
• Ass. stellar color-T_eff rel.
• Vs. obs. not a universal 1-to-1 rel. b/w color
  T_eff

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Sunyaev-Zeldovich effect

• CMBR highly-redshifted th. em.
• very uniform from all direction
• BR through rich cluster of galaxies
• ? excess of photons at higher freq. deficit at
  lower freq.
• SZ effect
• Size of distortion depends only on probability
• Where n_e electron density in clusters plasma
• sigma_T Thomson scattering cross-section

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Sunyaev-Zeldovich effect

• Distance-dep measure of electron density by X-ray
  obs.
• in hot cluster plasma, emit X-ray via
  Bremsstrahlung
• Bremsstrahlung emissivity of gas at freq. nu
• Where T_x temp of plasma
• Obs. X-ray flux dep. only on element density
  distance
• Model isothermal uniform density sph. sym

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Sunyaev-Zeldovich effect

• Angular extent of X-ray emission
• Reality n_e T_x will vary with position
• sph. sym ??