  Galaxy mergers are believed to trigger strong starbursts. This is well
assessed by observations in the local Universe. However the efficiency of this
mechanism has poorly been tested so far for high redshift, actively star
forming, galaxies. We present a suite of pc-resolution hydrodynamical numerical
simulations to compare the star formation process along a merging sequence of
high and low z galaxies, by varying the gas mass fraction between the two
models. We show that, for the same orbit, high-redshift gas-rich mergers are
less efficient than low-redshift ones at producing starbursts: the star
formation rate excess induced by the merger and its duration are both around 10
times lower than in the low gas fraction case. The mechanisms that account for
the star formation triggering at low redshift - the increased compressive
turbulence, gas fragmentation, and central gas inflows - are only mildly, if
not at all, enhanced for high gas fraction galaxy encounters. Furthermore, we
show that the strong stellar feedback from the initially high star formation
rate in high redshift galaxies does not prevent an increase of the star
formation during the merger. Our results are consistent with the observed
increase of the number of major mergers with increasing redshift being faster
than the respective increase in the number of starburst galaxies.
