Circumgalactic medium and galactic baryon budget

4 problems

• How do cold gas clouds survive inside hot galactic halos?

  Partially resolved

  Halos contain cool clouds at about ten thousand degrees moving through gas a hundred times hotter. Such clouds should be shredded and evaporated quickly, yet they are common.

• Mass, metal and energy flow rates through galactic halo gas

  Open

  Gas flows into galaxies to make stars and is pushed out by supernovae and black holes. How much gas, heavy elements and energy cross the halo per year has not been measured directly.

• How far do hot X-ray halos extend around Milky Way-mass galaxies?

  Partially resolved

  Galaxies like ours should be wrapped in gas at a million degrees, which glows faintly in X-rays. Its density profile and how far out it reaches are only beginning to be measured.

• Where are the missing baryons of Milky Way-mass galaxies?

  Partially resolved

  A galaxy like ours should contain about one sixth of its total mass as ordinary matter, but stars and disk gas account for only a fraction of that. The rest may be in hot halo gas or may have been blown out of the halo.