  We present optical integral field unit (IFU) observations of two gas pillars
surrounding the Galactic young massive star cluster NGC 3603. The high S/N and
spectral resolution of these data have allowed us to accurately quantify the
H-alpha, [NII] and [SII] emission line shapes, and we find a mixture of broad
(FWHM~70-100 km/s) and narrow (<50 km/s) components. The broad components are
found close to the edges of both pillars, suggesting that they originate in
turbulent mixing layers (TMLs) driven by the effect of the star cluster wind.
Both pillars exhibit surprisingly high ionized gas densities of >10000 cm-3. In
one pillar we found that these high densities are only found in the narrow
component, implying it must originate from deeper within the pillar than the
broad component. From this, together with our kinematical data, we conclude
that the narrow component traces a photoevaporation flow, and that the TML
forms at the interface with the hot wind. On the pillar surfaces we find a
consistent offset in radial velocity between the narrow (brighter) components
of H-alpha and [NII] of ~5-8 km/s, for which we were unable to find a
satisfactory explanation. We urge the theoretical community to simulate
mechanical and radiative cloud interactions in more detail to address the many
unanswered questions raised by this study.
