  We explore the evolution of the ~107 degree hot gas in normal galaxies out to
redshift = 0.5 (lookback time = 5 Gyr), using X-ray luminosity functions (XLF)
built from a sample of 575 normal galaxies with z < 0.6 detected in five high
galactic latitude Chandra wide-field surveys. After estimating the emission due
to the hot gas component (reducing the sample to ~400 galaxies), we compared
the XLF in three redshift bins (z = 0.1, 0.3, and 0.5), finding increases in
the number of galaxies per unit co-moving volume from z = 0.1 to 0.3 and then
from z = 0.3 to 0.5. These XLF changes suggest a significant (~5s) X-ray
luminosity evolution of the hot gas, with LX,GAS decreasing by a factor of 6-10
in the last 5 Gyr (from z = 0.5 to 0.1). The relative abundance of LX,GAS~1041
erg s-1 galaxies at higher z, suggests that high z, moderate LX,GAS galaxies
may be the optimal target to solve the missing baryon problem. In early-type
galaxies, this observational trend is qualitatively consistent with (but larger
than) the expected time-dependent mass-loss rate in cooling flow models without
AGN feedback. In late-type galaxies, the observational trend is also
qualitatively consistent with (but larger than) the effect of the z-dependent
SFR.
