  Young massive clusters (YMCs) are dense aggregates of young stars and are
often speculated as potential precursors to globular clusters. However, the
formation mechanism of massive and compact gas clumps that precede YMCs remains
unknown. In this paper, we study the formation of such massive clumps via fast
HI gas collisions (~100 km/s) as suggested by recent observations and their
subsequent evolution into YMCs by using three-dimensional magnetohydrodynamics
simulations involving self-gravity and detailed thermal/chemical processes. In
particular, the impact of ionization feedback from stellar radiation is
included in an approximate fashion where the temperature within the HII regions
is elevated to 10,000 K, while supernova feedback is not included. We examine
whether the resulting massive clumps can survive this ionization feedback and
evolve into YMCs. Our simulations reveal the emergence of gas clumps that do
not only possess substantial mass (~10^5 M_sun) but also sufficient compactness
(~5 pc). Notably, these clumps exhibit significantly higher escape velocities
compared to the sound speed of the HII region, indicating effective
gravitational retention of gas against feedback-induced evaporation.
Consequently, these conditions foster efficient star formation within the
massive gas clumps, ultimately leading to their evolution into YMCs. We also
perform simulations involving lower-velocity gas collisions, approximately 15
km/s, typical shock velocities induced by galactic superbubbles.
