Extended Hot Gas Halos Around Starburst Galaxies

Abstract

Reanalysis of Einstein IPC data and new observations from the GINGA LAC indicate the presence of extended X-ray emission (10-50 kpc) around the starburst galaxy M82. Here we model this emission by calculating numerical hydrodynamic simulations of the starburst event to much later times and larger scales than previously considered. For our models, we adopt a supernova rate of 0.1 ${\rm yr}^{-1}$, and an extended low-density static halo that is bound to the galaxy. There are three stages to the evolution of the wind-blown bubble and the propagation of the shock front: the bubble expands in an almost uniform density disk gas, with a deceleration of the shock front ($t \alt $ 3.6 Myr); breakout from the disk and the upward acceleration of the shock front (3.6 Myr $\alt t \alt$ 18 Myr); propagation into the halo, leading to a more spherical system and shock deceleration (18 Myr $\alt t$). For a halo density of $10^{-3} {\rm cm}^{-3}$, the outflow reaches a distance of 40-50 kpc from the center of the starburst galaxy in 50 Myr. We calculate the time evolution of the X-ray luminosity and find that the extended starburst emits $3\times 10^{39}\lcgs$ to $10^{40}\lcgs$ in the GINGA LAC band and $\sim 10^{41}\lcgs$ in the Einstein or ROSAT HRI band. The degree of the ionization equilibrium in the outflow and its effect on the iron K$α$ line emission are discussed.