  The effect of compressibility in hydrodynamic vortex merging has been
discussed. In the past, in incompressible limit it has been observed that the
merging of a collection of intense point-like vortices arranged uniformly
outside a circular vortex, can lead to quasistationary vortex patch and
transient hole pattern inside the patch via nonlinear merger process. These
patterns are akin to \textquoteleft vortex crystals\textquoteright.
Compressibility can introduce a natural acoustic scale to the problem. We find
that the natural mode is independent of the number of point-like vortices and
the amplitude scales linearly with compressibility. Further it has been
identified that after merging, the system exhibits oscillation at a natural
frequency together with its harmonics and beats with its own harmonics. The
power of the frequency is found to scale as $M^{-2}$, where $M$ is the Mach
number. Also the vortex crystals formed out of the merging process are found to
melt faster as compressibility is increased.
