Extra-planar X-ray emission from disc-wide outflows in spiral galaxies

Abstract

We study the effects of mass and energy injection due to OB associations spread across the rotating disc of a Milky Way-type galaxy, with the help of 3D hydrodynamic simulations. We compare the resulting X-ray emission with that produced from the injection of mass and energy from a central region. We find that the predicted X-ray image shows a filamentary structure that arises even in the absence of disc gas inhomogeneity. This structure stems from warm clumps made of disc material being lifted by the injected gas. We show that as much as half of the total X-ray emission comes from regions surrounding warm clumps that are made of a mix of disk and injected gas. This scenario has the potential to explain the origin of the observed extra-planar X-ray emission around star forming galaxies and can be used to understand the observed sublinear relation between the $L_X$ and SFR. We quantify the mass contained in these `bow-shock' regions. We also show that the top-most region of the outer shock above the central area emits harder X-rays than the rest. Further, we find that the mass distribution in different temperature ranges is bimodal, peaking at $10^4\hbox{-}10^5$ K (in warm clumps) and $10^6\hbox{-}10^7$ K (X-ray emitting gas). The mass loading factor is found to decrease with increasing SFR, consistent with previous theoretical estimates and simulations.Comment: 18 pages, 18 figures- Accepted in MNRAS. Comments and suggestions welcome