The Missing Baryons in the Milky Way and Local Group

Abstract

The Milky Way and all other galaxies are missing most of their baryons in that the ratio of the known baryonic mass to the gravitating mass (within the virial radius), is several times less than the cosmic ratio determined from WMAP. This implies that either the baryons never fell into galaxies or that powerful galactic winds removed most of the baryons. It is possible to discriminate between these two pathways if we can discover the missing baryons and measure its properties: spatial distribution; temperature; and metallicity. The missing baryons from galaxies are expected to have temperatures of 1-3E6 K, so X-ray observations are required. In this white paper, we show how X-ray observations, obtained with a facility such as IXO, can be used to identify and study these elusive baryons.