Where does the Disk turn into the Halo

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Where does the Disk turn into the Halo?
John Dickey -- University of Tasmania with data
from Simon Strasser (Univ. of Minnesota Ph.D.
2006) and the SGPS, CGPS, VGPS, and LAB surveys
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The disk turns into the halo at some z (altitude)
above and below midplane, but that is not the
question I am asking. I want to know where and
how the disk turns into the halo at large radius,
i.e. where is the outer edge of the disk, beyond
which there is only halo?
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The atomic hydrogen measured by 21-cm emission
and absorption is an excellent tracer of the disk
gas. In emission, all atoms are equally
represented, but in absorption only the cool
gas, with excitation temperature below a few
hundred K, is visible.
Comparing 21-cm emission and absorption shows the
mixture of WNM and CNM phases, and thus traces
how the disk surface density of these two
constituents varies with radius.
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With the mosaic Galactic Plane Surveys we measure
21-cm emission spectra in millions of independent
directions. We also measure about a thousand
absorption spectra (554299131) toward bright
continuum background sources. We construct pairs
of emission and absorption spectra corresponding
to (almost) the same lines of sight. Analysing
these together shows the column density and
excitation temperature or spin temperature of
the interstellar hydrogen as functions of
velocity.
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Galactic Plane interferometer mosaic plus single
dish surveys
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radial scale length 3.1 kpc (for illustration)
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The warp in the first quadrant shows up in the
VGPS and some of the CGPS data.
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21-cm emission longitude- velocity diagram at
latitude 0o (GBTParkes) scale 5K
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LAB survey Kalberla Dedes (2008)
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What is it like in the clouds that cause the
absorption lines at the outer edge of the disk?
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20 kpc
25 kpc
Strassers collection of high Rg absorption
lines, superposed on the HI emission l-v diagrams.
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14 kpc
12 kpc
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20 kpc
12 kpc
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• Typical conditions in cloud complexes at Rg 20
  kpc
• column density 3 to 10 x 1020 cm-2
• kinetic temperature (Tcool) 25 to 50 K
• cloud masses 105 to 106 Msun
• sizes 150 to 400 pc
• velocity dispersion (linewidths) 3 to 5 km s-1
• marginally gravitational instability in cold
  phase
• free-fall time 108 yr

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Strassers longitude-velocity diagrams of 21-cm
absorption
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• Conclusions
• The value of 250K for the mean spin
  temperature is quite robust, which means that the
  mixture of warm and cool HI phases does not
  change much with Galactic radius out to 20 kpc.
• The cool HI (T50K) in the outer Galaxy is in
  large complexes, including lots of warm gas.
• The confinement of these clouds is difficult
  to understand. The pressure must be much lower,
  both on average and in the extremes, than it is
  in the solar neighborhood.

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the stellar disk sets the gravitational field
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