Local ISM 3D distribution and soft X-ray background. Inferences on nearby hot gas and the North Polar Spur

Three-dimensional (3D) interstellar medium (ISM) maps can be used to locate not only interstellar (IS) clouds, but also IS bubbles between the clouds that are blown by stellar winds and supernovae, and that are filled by hot gas. To demonstrate this and to derive a clearer picture of the local ISM, we compare our recent 3D maps of the IS dust distribution to the ROSAT diffuse X-ray background maps after removing heliospheric emission. In the Galactic plane, there is a good correspondence between the locations and extents of the mapped nearby cavities and the soft (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 10⁶ K hot gas filling the cavities, we modeled the 0.25 keV surface brightness in a simple way along the Galactic plane as seen from the Sun, taking the absorption by the mapped clouds into account. The data-model comparison favors the existence of hot gas in the solar neighborhood, the so-called Local Bubble (LB). The inferred average mean pressure in the local cavities is found to be on the order of 10 000 cm^-3 K, in agreement with previous studies, providing a validation test for the method. On the other hand, the model overestimates the emission from the huge cavities located in the third quadrant. Using CaII absorption data, we show that the dust-to-CaII ratio is very low in this region, implying there is a large quantity of lower temperature (non-X-ray emitting) ionized gas and, as a consequence, a reduction in the volume filled by hot gas, explaining at least part of the discrepancy. In the meridian plane, the main two brightness enhancements coincide well with the LB's most elongated parts and chimneys connecting the LB to the halo, but no particular nearby cavity is found towards the enhancement in the direction of the bright North Polar Spur (NPS) at high latitude. We searched in the 3D maps for the source regions of the higher energy (0.75 keV) enhancements in the fourth and first quadrants. Tunnels and cavities are found to coincide with the main bright areas; however, no tunnel or cavity is found to match the low-latitude b ≳ 8°, brightest part of the NPS. In addition, the comparison between the 3D maps and published spectral data do not favor the nearby cavities being located within ~200 pc as potential source regions for the NPS. Those examples illustrate the potential use of more detailed 3D distributions of the nearby ISM for interpreting the diffuse soft X-ray background.