Planck Intermediate Results. XI: The gas content of dark matter halos:
  the Sunyaev-Zeldovich-stellar mass relation for locally brightest galaxies
  We present the scaling relation between Sunyaev-Zeldovich (SZ) signal and
stellar mass for almost 260,000 locally brightest galaxies (LBGs) selected from
the Sloan Digital Sky Survey (SDSS). These are predominantly the central
galaxies of their dark matter halos. We calibrate the stellar-to-halo mass
conversion using realistic mock catalogues based on the Millennium Simulation.
Applying a multi-frequency matched filter to the Planck data for each LBG, and
averaging the results in bins of stellar mass, we measure the mean SZ signal
down to $M_\ast\sim 2\times 10^{11} \Msolar$, with a clear indication of signal
at even lower stellar mass. We derive the scaling relation between SZ signal
and halo mass by assigning halo properties from our mock catalogues to the real
LBGs and simulating the Planck observation process. This relation shows no
evidence for deviation from a power law over a halo mass range extending from
rich clusters down to $M_{500}\sim 2\times 10^{13} \Msolar$, and there is a
clear indication of signal down to $M_{500}\sim 4\times 10^{12} \Msolar$.
Planck's SZ detections in such low-mass halos imply that about a quarter of all
baryons have now been seen in the form of hot halo gas, and that this gas must
be less concentrated than the dark matter in such halos in order to remain
consistent with X-ray observations. At the high-mass end, the measured SZ
signal is 20% lower than found from observations of X-ray clusters, a
difference consistent with Malmquist bias effects in the X-ray sample.
