Galactic Astronomy chap. 9.3.2 3

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Galactic Astronomy- chap. 9.3.2 3

• - Pulsars the galactic magnetic field
• - Diffuse Halpha radiation

Dong-hyun Lee 2007/11/20
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Pulsars the Galactic B field

• Measurements of D_M along l.o.s to pulsar, where
• Determine mean value of free-electron density in
  the disk
• Distance to pulsar is estimated from the
  structure of 21cm absorption in its spectrum
• In a few cases, model method

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Pulsars the Galactic B field

• Galactic B field tendency of the
  Davis-Greenstein effect (Polarization of
  starlight by dust alignment of dust grains with
  B field spinning dust grains to align their
  spin axes with B) to align the
  electric vector of radiation from obscured stars
  parallel to B fig.3.19
• When both the D_M, R_M of a pulsar are known,
  
• one can determine mean value along l.o.s of
  component of B that is parallel to l.o.s
• Galactic B field is highly organized on kpc
  scales enable us to determine magnitude of field

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Pulsars the Galactic B field

• Fig 3.19 on small scales B field wiggles
• B field is a superposition of an ordered
  large-scale component and a random small-scale
  component
• Measurements of D_M, R_M for pulsars are
  sensitive only to the ordered filed
• Magnitude of disordered component of B is harder
  to estimate
• Most reliable estimates obtained from the
  typical intensity of Galactic synchrotron
  emission ( synchrotron emission is polarized
  perpendicular to direction of local B field)
  fig 8.45
• In a few cases there is convincing evidence that
  B is larger in molecular clouds than in
  lower-density interstellar space

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Pulsars the Galactic B field

• The strongest interstellar B field known occurs
  in the filaments of synchrotron emission fig
  9.30
• Filaments located about 30pc from Galactic center
  extend perpendicular to the Galactic plane for
  a few pcs
• Intense field exert forces that are 10000 times
  larger than those exerted by B at a typical point
  in ISM

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Diffuse Halpha radiation

• Fabry-Perot spectrometers detect Halpha radiation
  at all Galactic latitudes
• Intensity of this rad. Varies irregularly over
  the sky
• Avg intensity within a window is roughly
  proportional to cscb where b latitude of
  windows center
• Scaling with cscb suggests that the radiation
  originates in a layer centered on the plane fig
  3.20
• Production of observed Halpha emission within
  this layer requires a large power input to the
  gas of the layer minimum power requirement is
  41034W for the disk out to a Galactocentric
  distance of 12kpc
• Compare with supernovae
• Diffuse Halpha emission is associated with a
  major item in the energy budget of the ISM

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Diffuse Halpha radiation

• Observed Halpha emission is not associated with
  detectable emission lines of neutral O N
• Diffuse Halpha emission comes from gas nearly
  fully ionized
• Free electron density is a reliable guide to
  total mass density
• Column density of free electrons along l.o.s can
  be determined from pulsars D_M
• If we assume particle-density n of the ionized
  gas varies with height z above the plane
• Then we can estimate scale-height z_0 of gas from
  the value of n(0) that is obtained by observing
  nearby pulsars
• From irregularity of observed Halpha flux on the
  sky it follows that the ionized gas from which it
  comes is inhomogeneously distributed