PACT I: Combining ACT and Planck data for optimal detection of tSZ
  signal
  We present the optimal reconstruction of the thermal Sunyaev-Zel'dovich (tSZ)
effect signal based on the combination of a heterogeneous dataset consisting of
ACT and Planck data, with different numbers of channels, angular resolutions
and noise levels. We combine both datasets using two different approaches, a
Matched Multi-Filter (MMF) technique and an optimised Internal Linear
Combination (ILC). We show that when applying the MMF to the combination of ACT
and Planck data, the size-flux degeneracy is reduced and the signal-to-noise of
clusters detected with their SZ signal improves by up to a factor of three. In
the case of the optimised ILC method, we show that the tSZ map is reconstructed
with a resolution of $\sim 1.5$ arcmin. This is more than a factor two
improvement compared with the Planck resolution, and with a very good control
of noise, i.e. limited only by the intrinsic noise of the individual
experiments. The combination of ACT and Planck data offers a unique opportunity
to improve on the study of the pressure profiles and to study substructure in
clusters through their tSZ.
