Identifying the Baryons in a Multiphase Intergalactic Medium
  In this white paper, we summarize current observations of the baryon census
at low redshift (Shull, Smith, & Danforth 2012). Measurements of Lya, O-VI, and
broad Lya absorbers, together with more careful corrections for metallicity and
ionization fraction, can now account for approximately 60% of the baryons in
the intergalactic medium (IGM). An additional 5 +/- 3% may reside in the
circumgalactic medium (CGM), 7 +/- 2% in galaxies, and 4 +/- 1.5% in clusters.
This still leaves a substantial fraction, 29 +/- 13%, unaccounted for. We
suggest improvements in measuring the baryons in major components of the IGM
and CGM with future Ultraviolet and X-ray spectrographs. These missions could
find and map the missing baryons, the fuel for the formation and chemical
evolution of galaxies.
