  This letter presents a numerical study of the evolution of an emission line
cloud of initial density 10 cm$^{-3}$, temperature $10^4$ K, and size 200 pc,
being overtaken by a strong shock wave. Whereas previous simple models proposed
that such a cloud would either be completely destroyed, or simply shrink in
size, our results show a different and more complex behaviour: due to rapid
cooling, the cloud breaks up into many small and dense fragments, which can
survive for a long time. We show that such rapid cooling behaviour is in fact
expected for a wide range of cloud and shock properties. This process applies
to the evolution of emission line clouds being overtaken by the cocoon of a
radio jet. The resulting small clouds would be Jeans unstable, and form stars.
Our results thus give theoretical credibility to the process of jet induced
star formation, one of the explanations for the alignment of the optical/UV and
radio axis observed in high redshift radio galaxies.
