Triggering the formation of the supergiant H II region NGC 604 in M33
  Formation mechanism of a supergiant H II region NGC 604 is discussed in terms
of collision of H I clouds in M33. An analysis of the archival H I data
obtained with the Very Large Array (VLA) reveals complex velocity distributions
around NGC 604. The H I clouds are composed of two velocity components
separated by ~ 20 km s^-1 for an extent of ~ 700 pc, beyond the size of the the
H II region. Although the H I clouds are not easily separated in velocity with
some mixed component represented by merged line profiles, the atomic gas mass
amounts to 6 x 10^6 M_Sol and 9 x 10^6 M_Sol for each component. These
characteristics of H I gas and the distributions of dense molecular gas in the
overlapping regions of the two velocity components suggest that the formation
of giant molecular clouds and the following massive cluster formation have been
induced by the collision of H I clouds with different velocities. Referring to
the existence of gas bridging feature connecting M33 with M31 reported by
large-scale HI surveys, the disturbed atomic gas possibly represent the result
of past tidal interaction between the two galaxies, which is analogous to the
formation of the R136 cluster in the LMC.
