Three Little Papers about the Milky Way Halo

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Three Little Papers about the Milky Way Halo
John Dickey 13 March 2002 Astro Journal Club
Reynolds et al. astro-ph 0201392 Howk et al.
astro-ph 0202239 Kappes et al. astro-ph 0203104
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A Spitzer halo ...
Hot gas has a high scale height, so it will not
be confined to the disk as the stars and the
cool gas are. This is because of a balance
between the shape of the gravitational potential
and the rms velocities of the particles.
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kz
The kz curve describes the gravitational
acceleration toward the plane (in z), like g on
earth.
z
z
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kz
Cool gas is confined close to midplane.
z
z
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kz
Hot gas can rise to many kpc in z.
Cool gas is confined close to midplane.
z
z
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A conference review of the WHAM survey. This
paper presents several of the important results
very clearly and succinctly. The overall
result is that the WIM has quite different
properties from HII region gas.
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WHAM images centered on l 120o.
http//www.astro.wisc.edu/wham/
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Normandeaus chimney
Line ratio maps in a small area in the second
quadrant.
see next figure
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Lower halo z 1 kpc (Reynolds layer)
Spectra toward l130o, b -7.5o
Note varying OIII to SII ratio.
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and The WIM extends further into the Milky
Way halo than the HI does ! The Reynolds Layer
is thicker than the Lockman Layer by a factor of
1.5 to 2.
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A study of the HIM (hot ionized medium) as traced
by OVI absorption. This gas is much hotter than
the WIM, and it reaches MUCH higher into the halo.
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Spectra toward two hot stars in the LMC.
OVI traces the hot phase of the ISM (T3x105 K).
Fe II absorption traces the WNM (also seen in HI).
Galactic Plane gas
LMC gas
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OVI spectra toward hot stars in the LMC.
The velocity range between the MW disk and the
LMC seems to be filled with OVI gas !
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Spectra toward two hot stars in the SMC.
OVI traces the hot phase of the ISM (T3x105 K).
Fe II absorption traces the WNM (also seen in HI).
SMC gas
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OVI spectra toward hot stars in the SMC.
The velocity range between the MW disk and the
SMC seems to be filled with OVI gas !
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Fluctuations in the Galactic OVI absorption over
various angular separations.
LMC
SMC
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Maps of the integrated Galactic OVI absorption
toward stars in the LMC and SMC.
LMC
SMC
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The OVI absorbing gas is hot. Some is in a
thick disk distribution, some more is in the
halo. How far does the halo go ?
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Howk et al. are primarily interested in the
structure of the Galactic disk component of the
OVI. But I think the halo component is more
striking in their data !
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This paper is a study of the diffuse, soft X-ray
background. They use the ROSAT survey data plus
LDSS 21-cm data to model the sources of the
X-rays. They find that about 75 of the soft
X-ray flux comes from the Milky Way halo. The
emitting gas must have temperature between 1 and
2 times 106 K. The halo gas is a smoothly
distributed halo plasma which surrounds the Milky
Way.
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Data
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model
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data
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model
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M81 group
model
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red C band, blue M band
Three component model data - prediction
distribution
Kuntz and Snowden model (with two halo
components) data - prediction distribution
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Conclusion of Kappes, Pradas, and Kerp
The soft x-ray data can be fit with
three components in emission plus absorption by
the neutral ISM traced by the HI.

• The three components are
• the local bubble
• the Milky Way halo (T1.6 x 106)
• spherical (?) around MW center
• the extragalactic background

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even more even Hotter I M
HIM
WIM
CNM
z
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even more even Hotter I M
HIM
WIM
CNM
z
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Conclusion There is a lot of hot gas in the
halo. It looks as if it goes all the way to the
Magellanic Clouds. If it is in a hydrostatic
atmosphere, the density and pressure are
significant.