Triggering the formation of the supergiant H II region NGC 604 in M 33

Abstract

Formation mechanism of a supergiant H II region NGC 604 is discussed in terms of collision of H I clouds in M 33. 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 × 10⁶ M_{⊙} and 9 × 10⁶ M_{⊙} 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 a gas bridging feature connecting M 33 with M 31 reported by large-scale H I surveys, the disturbed atomic gas possibly represents the result of past tidal interaction between the two galaxies, which is analogous to the formation of the R 136 cluster in the LMC.