17116

Less than one-third of the baryons in a galaxy are in the stars and gaseous disk comma with the rest being in a gaseous halo region that may extend to or beyond R_200. According to models comma this is because feedback from stars heats the halo gas comma shaping galaxy evolution and regulating star formation today. Most of the extended gaseous mass is near the virial temperature comma virgul10elev6.3 K for L* galaxies comma and this gas is measured by the O VII and O VIII lines in the X-rays. The other essential halo gas property is the net cooling rate comma measured from the O VI line comma which carries the cooling as it passes through the virgul10elev5.5 K range. These lines define the most important aspects of halo gas models -- a cooling flow with feedback. The X-ray lines in external galaxies will be measured with future X-ray observatories comma such as Athena or Arcus comma while only HST-COS can measure the O VI line. This proposed legacy program will nearly triple the redshift space for which all three lines will be observed along the same sightlines comma producing about 30 absorption systems. This will provide enough statistical power to determine halo properties comma including the feedback efficiency comma metallicity comma and departures from steady state.