  Sub-structures in the hot gas of galaxy clusters are related to their
formation history and to the astrophysical processes at play in the
intracluster medium (ICM). The thermal Sunyaev-Zel'dovich (tSZ) effect is
directly sensitive to the line-of-sight integrated ICM pressure, and is thus
particularly adapted to study ICM sub-structures. We apply
structure-enhancement filtering algorithms to high resolution tSZ observations
of distant clusters, in order to search for pressure discontinuities,
compressions, as well as secondary peaks in the ICM. The same filters are
applied to synthetic tSZ images extracted from RHAPSODY-G hydrodynamic
simulations, in order to better interpret the extracted features. We also study
the noise propagation trough the filters and quantify the impact of systematic
effects, point source residuals being identified as the dominant potential
contaminant. In 3 of our 6 NIKA clusters we identify features at high S/N that
show clear evidence for merger events. In MACSJ0717 (z=0.55), three strong
pressure gradients are observed on the E, SE and W sectors, and two main peaks
in the pressure distribution are identified. We observe a lack of tSZ compact
structure in the cool-core cluster PSZ1G045.85 (z=0.61), and a tSZ gradient
ridge dominates in the SE. In the highest z cluster, CLJ1227 (z=0.89), we
detect a ~45" (360 kpc) long ridge pressure gradient associated with a
secondary pressure peak in the W region. Our results show that current tSZ
facilities have now reached the angular resolution and sensitivity to allow an
exploration of the details of pressure sub-structures in clusters, even at high
z. This opens the possibility to quantify the impact of the dynamical state on
the relation between the tSZ signal and the mass of clusters, which is
important when using tSZ clusters to test cosmological models. This work also
marks the first NIKA cluster sample data release.
