Missing baryons traced by the galaxy luminosity density in the
  large-scale WHIM filaments
  We propose a new approach to the missing baryons problem. Building on the
common assumption that the missing baryons are in the form of the Warm Hot
Intergalactic Medium (WHIM), we further assumed here that the galaxy luminosity
density can be used as a tracer of the WHIM. The latter assumption is supported
by our finding of a significant correlation between the WHIM density and the
galaxy luminosity density in the hydrodynamical simulations of Cui et al.
(2012). We further found that the fraction of the gas mass in the WHIM phase is
substantially (by a factor of $\sim$1.6) higher within the large scale galactic
filaments, i.e. $\sim$70\%, compared to the average in the full simulation
volume of $\sim$0.1\,Gpc$^3$. The relation between the WHIM overdensity and the
galaxy luminosity overdensity within the galactic filaments is consistent with
linear: $\delta_{\rm whim}\,=\,0.7\,\pm\,0.1\,\times\,\delta_\mathrm{LD}^{0.9
\pm 0.2}$. We applied our procedure to the line of sight to the blazar
H2356-309 and found evidence for the WHIM in correspondence of the Sculptor
Wall (z $\sim$0.03 and $\log{N_H}$ = $19.9^{+0.1}_{-0.3}$) and Pisces-Cetus
superclusters (z $\sim$0.06 and $\log{N_H}$ = $19.7^{+0.2}_{-0.3}$), in
agreement with the redshifts and column densities of the X-ray absorbers
identified and studied by Fang et al. (2010) and Zappacosta et al. (2010). This
agreement indicates that the galaxy luminosity density and galactic filaments
are reliable signposts for the WHIM and that our method is robust in estimating
the WHIM density. The signal that we detected cannot originate from the halos
of the nearby galaxies since they cannot account for the large WHIM column
densities that our method and X-ray analysis consistently find in the Sculptor
Wall and Pisces-Cetus superclusters.
