The X-ray Background from the Warm Gas of the Galactic Halo

Within the framework of the hierarchical scenario of galaxy formation, spiral galaxies like our own Galaxy are still growing at present. This opens a possibility that one might be able to see X-ray galactic halos from gravitationally heated gas with temperatures of 106 K as a result of bremsstrahlung. An interesting issue is whether the X-ray background produced by the warm gas in the halo of our Galaxy is detectable. We present a simple estimate of the strength and spectrum of the X-ray background from the Galactic halo and compare with the recent findings of a spatially variable soft X-ray component seen towards the north Galactic polar cap by Kuntz et al. (2001). It is shown that a good agreement, regardless of cosmological models, can be achieved if the gas fraction is as low as '~ 0.01. This requirement seems to be consistent with the extrapolated result from a number of independent observational and theoretical constraints established for groups and clusters of galaxies. In particular, the ex pected soft X-ray background from the warm gas of the Galactic halo is comparable to, or even exceeds that produced by the warm-hot gas in massive groups, and it may constitute the major source of contamination in the search for missing baryons through the detection of their soft X-ray emission, unless we can work out a way to properly remove the X-ray background (e.g., from anisotropy) from the halo of our Galaxy.

Constraining the cosmic ray propagation halo thickness using Fermi-LAT observations of high-latitude clouds

The diffusive halo is a basic characteristic of cosmic ray(CR)propagation and can advance our understanding of many CR-related phenomena and indirect dark matter.The method used to derive the halo size often has degeneracy problems and is thus affected by large uncertainties.The diffuseγrays from high-latitude clouds might shed light on the halo size independently.Because predictions using the spatially dependent propagation(SDP)model have better agreement with the observed CRs than those of the conventional propagation model,in this work,we investigated halo thickness based on the SDP model using Fermi-LATγ-ray observations of high-and intermediatevelocity clouds.We found that to avoid exceeding the relativeγ-ray emissivity in high-latitude clouds,the halo thickness should be in the range of 3.3-9 kpc.Moreover,the spatial morphology ofγ-rays estimated based on the SDP model for different values of the halo thickness are distinctive,which provides us with a tool to determine the halo size.This newly developed model can be tested and tuned using multi-wavelength observations in future studies.