Probing the missing baryons with the Sunyaev-Zel'dovich effect from
  filaments
  Observations of galaxies and galaxy clusters in the local universe can
account for only $\sim\,10\%$ of the total baryon content. Cosmological
simulations predict that the `missing baryons' are spread throughout
filamentary structures in the cosmic web, forming a low-density gas with
temperatures of $10^5-10^7\,\!$K. We search for this warm-hot intergalactic
medium (WHIM) by stacking the Planck Compton $y$-parameter map of the thermal
Sunyaev-Zel'dovich (tSZ) effect for 1,002,334 pairs of CMASS galaxies from the
Sloan Digital Sky Survey. We model the contribution from the galaxy halo pairs
assuming spherical symmetry, finding a residual tSZ signal at the
$2.9\mbox{$\sigma$}$ level from a stacked filament of length $10.5\,h^{-1}\,\rm
Mpc$ with a Compton parameter magnitude $y=(0.6\pm0.2)\times10^{-8}$. We
consider possible sources of contamination and conclude that bound gas in
haloes may contribute only up to $20\%$ of the measured filamentary signal. To
estimate the filament gas properties we measure the gravitational lensing
signal for the same sample of galaxy pairs; in combination with the tSZ signal,
this yields an inferred gas density of $\rho_{\rm b}=(5.5\pm
2.9)\times\bar{\rho_{\rm b}}$ with a temperature $T=(2.7\pm 1.7) \times
10^6\,$K. This result is consistent with the predicted WHIM properties, and
overall the filamentary gas can account for $ 11\pm 7\%$ of the total baryon
content of the Universe. We also see evidence that the gas filament extends
beyond the galaxy pair. Averaging over this longer baseline boosts the
significance of the tSZ signal and increases the associated baryon content to
$28\pm 12\%$ of the global value.
