Planck intermediate results. XL. The Sunyaev-Zeldovich signal from the
  Virgo cluster
  The Virgo cluster is the largest Sunyaev-Zeldovich (SZ) source in the sky,
both in terms of angular size and total integrated flux. Planck's wide angular
scale and frequency coverage, together with its high sensitivity, allow a
detailed study of this large object through the SZ effect. Virgo is well
resolved by Planck, showing an elongated structure, which correlates well with
the morphology observed from X-rays, but extends beyond the observed X-ray
signal. We find a good agreement between the SZ signal (or Compton paranmeter,
y_c) observed by Planck and the expected signal inferred from X-ray
observations and simple analytical models. Due to its proximity to us, the gas
beyond the virial radius can be studied with unprecedented sensitivity by
integrating the SZ signal over tens of square degrees. We study the signal in
the outskirts of Virgo and compare it with analytical models and a constrained
simulation of the environment of Virgo. Planck data suggest that significant
amounts of low-density plasma surround Virgo out to twice the virial radius. We
find the SZ signal in the outskirts of Virgo to be consistent with a simple
model that extrapolates the inferred pressure at lower radii while assuming
that the temperature stays in the keV range beyond the virial radius. The
observed signal is also consistent with simulations and points to a shallow
pressure profile in the outskirts of the cluster. This reservoir of gas at
large radii can be linked with the hottest phase of the elusive warm/hot
intergalactic medium. Taking the lack of symmetry of Virgo into account, we
find that a prolate model is favoured by the combination of SZ and X-ray data,
in agreement with predictions.
