The Orion OB Association as a Generator for the Hot Circumgalactic Medium

Feedback from star formation may play a key role in energizing the hot, diffuse, X-ray emitting circumgalactic medium (CGM). The nearby (~400 pc) Orion star forming region has built the Orion-Eridanus Superbubble (OES), including the hot, diffuse, X-ray emitting gas that fills it. We analyzed the hot (10⁶-10⁷ K) gas on the interior of the OES using HaloSat, a CubeSat X-ray observatory. We covered the majority of the OES with eleven observation fields, each with a 10 degree diameter and found that the gas is well described by two thermal plasma model components. There are regions of enhanced emission measure (EM) that coincide with the Eridanus X-ray Enhancement and the Orion OB association. The temperatures of the two components are statistically consistent with their weighted averages: a warm temperature kT = 0.17(2) keV and a hot temperature kT = 0.79(12) keV. We interpret our two-temperature model as describing a single body of gas with a temperature distribution. The gas is over-pressured in comparison with typical interstellar medium pressures, and the hot gas pressures indicate that the gas is likely not in hydrostatic equilibrium. The radiative cooling timescale of the hot gas, about 30 Myr, is long in comparison with the supernova rate of 1-1.5 supernovae per million years in the Orion OB association. The temperatures and EMs of the gas agree with properties of the bulk CGM elsewhere in the Milky Way. These factors together suggest that the hot CGM is energized by star formation activity.