Planck intermediate results. XXXVII. Evidence of unbound gas from the
  kinetic Sunyaev-Zeldovich effect
  By looking at the kinetic Sunyaev-Zeldovich effect (kSZ) in Planck nominal
mission data, we present a significant detection of baryons participating in
large-scale bulk flows around central galaxies (CGs) at redshift $z\approx
0.1$. We estimate the pairwise momentum of the kSZ temperature fluctuations at
the positions of the CGC (Central Galaxy Catalogue) samples extracted from
Sloan Digital Sky Survey (DR7) data. For the foreground-cleaned maps, we find
$1.8$-$2.5\sigma$ detections of the kSZ signal, which are consistent with the
kSZ evidence found in individual Planck raw frequency maps, although lower than
found in the WMAP-9yr W band ($3.3\sigma$). We further reconstruct the peculiar
velocity field from the CG density field, and compute for the first time the
cross-correlation function between kSZ temperature fluctuations and estimates
of CG radial peculiar velocities. This correlation function yields a
$3.0$-$3.7$$\sigma$ detection of the peculiar motion of extended gas on Mpc
scales, in flows correlated up to distances of 80-100 $h^{-1}$ Mpc. Both the
pairwise momentum estimates and kSZ temperature-velocity field correlation find
evidence for kSZ signatures out to apertures of 8 arcmin and beyond,
corresponding to a physical radius of $> 1$ Mpc, more than twice the mean
virial radius of halos. This is consistent with the predictions from hydro
simulations that most of the baryons are outside the virialized halos. We fit a
simple model, in which the temperature-velocity cross-correlation is
proportional to the signal seen in a semi-analytic model built upon N-body
simulations, and interpret the proportionality constant as an "effective"
optical depth to Thomson scattering. We find $\tau_T=(1.4\pm0.5)\times
10^{-4}$; the simplest interpretation of this measurement is that much of the
gas is in a diffuse phase, which contributes little signal to X-ray or thermal
SZ observations.
