Triggering the formation of the supergiant H II region NGC 604 in M33

TL;DR

This paper investigates how the collision of H I clouds in M33 likely triggered the formation of the supergiant H II region NGC 604, revealing complex gas dynamics and possible tidal interactions influencing star formation.

Contribution

It provides new evidence linking H I cloud collisions and tidal interactions to the formation of supergiant H II regions in M33.

Findings

H I clouds around NGC 604 have two velocity components separated by ~20 km/s.

The atomic gas mass of these clouds is approximately 6-9 million solar masses.

The gas dynamics suggest cloud collisions induced molecular cloud and star formation.

Abstract

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.