When we say a halo of hot gas, what does that mean to me when I fly close to the galaxy and through the "Hot Gas Halo"? Do I get vaporized by the heat or will I notice it at all?
You would not notice it at all. As those motivational gurus who teach you to walk on coals know, the key to not burning your feet is the rate of heat transfer. In the halos of hot gas around galaxies, the gas is so rarified that the heat transfer to a spaceship would be practically nil. Chandra can detect the X-rays from these halos because they are tens of thousands of light years across.
(Photo: Jeff Werner, www.firewalking.com)