Local ISM 3D distribution and soft X-ray background: Inferences on
  nearby hot gas and the North Polar Spur
  3D maps of the ISM can be used to locate not only IS clouds, but also IS
bubbles between the clouds that are blown by stellar winds and supernovae. We
compare our 3D maps of the IS dust to the ROSAT diffuse X-ray background maps.
In the Plane, there is a good correspondence between the locations and extents
of the mapped nearby cavities and the 0.25 keV background emission
distribution, showing that most of these nearby cavities contribute to this
soft X-ray emission. Assuming a constant dust to gas ratio and homogeneous 1MK
hot gas filling the cavities, we modeled in a simple way the 0.25 keV surface
brightness along the Galactic plane as seen from the Sun, taking into account
the absorption by the mapped clouds. The data-model comparison favors the
existence of hot gas in the Local Bubble (LB). The average mean pressure in the
local cavities is found to be on the order of about 10,000 cm-3K, in agreement
with previous studies. The model overestimates the emission from the huge
cavities in the 3rd quadrant. Using CaII absorption data, we show that the dust
to CaII ratio is very small in this region, implying the presence of a large
quantity of lower temperature (non-X-ray emitting) ionized gas, explaining at
least part of the discrepancy. In the meridian plane, the two main brightness
enhancements coincide well with the chimneys connecting the LB to the halo. No
nearby cavity is found towards the bright North Polar Spur (NPS) at high
latitude. We searched in the maps for the source regions of the 0.75 keV
enhancements in the 4th and 1st quadrants. Tunnels and cavities are found to
coincide with the main bright areas, however no tunnel nor cavity is found to
match the low-latitude, brightest part of the NPS. In addition, the comparison
between the maps and published spectra do not favor the nearby cavities located
within about 200pc as potential source regions for the NPS.
