Virial Shock Heating
Virial shocks appear when gas falls in toward a galaxy, and the shocked gas stays hot so the pressure in the post-shock gas can support the shock and keep it away from the galactic disk. 
 The alternative is if the post-shock gas cools and loses pressure support, and the (non-virial) shock collapses right down to the galactic disk.  The significance is that if gas falls directly onto the high-density disk, it has different effects than if it meets a low-density shock well away from the galaxy.  There's a lot of information at https://academic.oup.com/mnras/article/345/1/349/984798