  Cloud-cloud collisions (CCCs) are expected to compress gas and trigger star
formation. However, it is not well understood how the collisions and the
induced star formation affect galactic-scale properties. By developing an
on-the-fly algorithm to identify CCCs at each timestep in a galaxy simulation
and a model that relates CCC-triggered star formation to collision speeds, we
perform simulations of isolated galaxies to study the evolution of galaxies and
giant molecular clouds (GMCs) with prescriptions of self-consistent CCC-driven
star formation and stellar feedback. We find that the simulation with the
CCC-triggered star formation produces slightly higher star formation rates and
a steeper Kennicutt-Schmidt relation than that with a more standard star
formation recipe, although collision speeds and frequencies are insensitive to
the star formation models. In the simulation with the CCC model, about 70 per
cent of the stars are born via CCCs, and colliding GMCs with masses of $\approx
10^{5.5}\,M_{\odot}$ are the main drivers of CCC-driven star formation. In the
simulation with the standard star formation recipe, about 50 per cent of stars
are born in colliding GMCs even without the CCC-triggered star formation model.
These results suggest that CCCs may be one of the most important star formation
processes in galaxy evolution. Furthermore, we find that a post-processing
analysis of CCCs, as used in previous studies in galaxy simulations, may lead
to slightly greater collision speeds and significantly lower collision
frequencies than the on-the-fly analysis.
