Direct detection of the kinetic Sunyaev-Zel'dovich effect in galaxy
  clusters
  We report the direct detection of the kinetic Sunyaev-Zel'dovich (kSZ) effect
in galaxy clusters with a 3.5 sigma significance level. The measurement was
performed by stacking the Planck map at 217 GHz at the positions of galaxy
clusters from the Wen-Han-Liu (WHL) catalog. To avoid the cancelation of
positive and negative kSZ signals, we used the large-scale distribution of the
Sloan Digital Sky Survey (SDSS) galaxies to estimate the peculiar velocities of
the galaxy clusters along the line of sight and incorporated the sign in the
velocity-weighted stacking of the kSZ signals. Using this technique, we were
able to measure the kSZ signal around galaxy clusters beyond 3R500. Assuming a
standard beta-model, we also found that the gas fraction within R500 is
fgas,500 = 0.12 +- 0.04 for the clusters with the mass of M500 ~ 1e14 Msun/h.
We compared this result to predictions from the Magneticum cosmological
hydrodynamic simulations as well as other kSZ and X-ray measurements, most of
which show a lower gas fraction than the universal baryon fraction for the same
mass of clusters. Our value is statistically consistent with results from the
measurements and simulations and also with the universal value within our
measurement uncertainty.
