# Gas halo of our Milky Way Galaxy

This question relates to a diffuse hot gas halo of our Milky Galaxy. I've read that there is a hot diffuse halo of gas surrounding our Galaxy (NED, Caltech)

I was wondering why such a halo can exist? Why doesn't it collapse to a disk shape? Is it because the gas itself is still hot and so remains largely unaffected by the Galaxy potential?

The scale height of gas in a disk (if it were in equilibrium) is roughly $kT/mg$, where $T$ is the temperature, $g$ is the gravitational field, $m$ the mean mass of agas particle, and $k$ the Boltzmann constant.
If we assume most of the mass is in a thin disk, then Gauss's law for gravitation tells us that that $g = 2\pi G \sigma$, where $\sigma$ is the mass per unit area in the disk. According to Rix & Bovy, $\sigma \simeq 70 M_{\odot}$ pc$^{-2}$ at the location of the Sun (http://arxiv.org/abs/1309.0809).
If we assume hydrogen gas, then the effective particle mass is that of a proton, and this means the gas scale height is $$H = 4300 \left(\frac{T}{10^6\ K}\right)\ pc$$
• @MichaelJRoberts Yes, hydrostatic equilibrium is a very crude approximation. There are energy inputs - shockwaves from supernovae for example. Also, the gas is not very dense and the cooling times will be long and cooling is less efficient once temperature exceed a million degrees. If I've done my sums right, for gas densities of $10^{-4}$ per cc and $T=2\times10^{6}$ K, the gas takes billions of years to cool, even without energy input. – ProfRob Nov 26 '15 at 12:09