The Sunyaev-Zel'dovich (SZ) effect is a direct probe of thermal energy
content of the Universe, induced in the cosmic microwave background (CMB) sky
through scattering of CMB photons off hot electrons in the intracluster medium
(ICM). We report a 9-sigma detection of the SZ signal in the CMB maps of
Wilkinson Microwave Anisotropy Probe (WMAP) 3yr data, through study of a sample
of 193 massive galaxy clusters with observed X-ray temperatures greater than 3
keV. For the first time, we make a model-independent measurement of the
pressure profile in the outskirts of the ICM, and show that it closely follows
the profiles obtained by X-ray observations and numerical simulations. We find
that our measurements of the SZ effect would account for only half of the
thermal energy of the cluster, if all the cluster baryons were in the hot ICM
phase. Our measurements indicate that a significant fraction (35 +/- 8 %) of
baryonic mass is missing from the hot ICM, and thus must have cooled to form
galaxies, intracluster stars, or an unknown cold phase of the ICM. There does
not seem to be enough mass in the form of stars or cold gas in the cluster
galaxies or intracluster space, signaling the need for a yet-unknown baryonic
component (at 3-sigma level), or otherwise new astrophysical processes in the
ICM.Comment: 9 pages, 4 figures, references added, a mismatch between X-ray and SZ
properties of simulated clusters is corrected, marginally increasing the
significance of missing baryon fraction, Accepted for publication in MNRA
