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abstract
H$\alpha$ images of M33 and other spiral galaxies have revealed a vast mosaic of ionized gas shells and filaments surrounding and connecting HII regions. Energy introduced into the interstellar medium (ISM) by massive stars is expected to produce bubbles in areas where the ISM is relatively homogeneous, leaving gas shells as their legacy. According to models, the diameter of a shell depends on its age, the number of stars contributing energy, and the density of the surrounding ISM. Stars interior to the gas shells can then continue to ionize them once they are formed. A study of the formation and ionization of three gas shells in M33 is presented here. All three shells or filaments surround OB associations. We present photometry of these OB associations measured from BVI CCD images and compare the stellar populations to ionization requirements measured from H$\alpha$ images and to models for the formation and evolution of shells. Our analysis shows that two of the regions contain more than enough stars to both form and ionize the shell while the third does not. This, and other peculiarities of the shells, imply that gas shell formation is more complicated than present models assume.
