Hot Gas Halo Detected Around Galaxy NGC 4631

Is our Milky Way Galaxy surrounded by a halo of hot gas? A step toward solving this long-standing mystery was taken recently with Chandra X-ray observations of nearby galaxy NGC 4631 . In the above composite picture , newly resolved diffuse X-ray emission is shown in blue, superposed on an HST image showing massive stars in red. Since NGC 4631 is similar to the Milky Way , this observation indicates that our own Galaxy is indeed surrounded by a halo of hot X-ray emitting gas, although we are too close to clearly differentiate it from more nearby extended X-ray sources. The clusters of massive stars probably heat the halo gas. Exactly how this gas gets ejected into a halo is a topic of continuing research .