AN EXPANDING NEUTRAL HYDROGEN SUPERSHELL EVACUATED BY MULTIPLE SUPERNOVAE IN M101

Several neutral hydrogen (H I) cavities have been detected in the Milky Way and other nearby star-forming galaxies. It has been suggested that at least a fraction of them may be expanding supershells driven by the combined mechanical feedback from multiple supernovae (SNe) occurring in an OB association. Yet most extragalactic Hi holes have neither a demonstrated expansion velocity nor an identified OB association inside them. In this work, we report on the discovery of an unbroken expanding H I supershell in the nearby spiral galaxy M101, with a UV-emitting young stellar association inside it. We measure its size (500 pc) and expansion velocity (20 km s(-1)) by identifying both its approaching and receding components in the position-velocity space, using 21 cm emission spectroscopy. This provides us with an ideal system to test the theory of supershells driven by the mechanical feedback from multiple SNe. The UV emission of the cluster inside the supershell is compared with simulated spectral energy distribution of synthetic clusters of the appropriate age (similar to 15 Myr). The observed UV flux is found to be consistent with an association of the appropriate mass (similar to 10(5) M-circle dot) and age required by the energy budget of the supershell. Properties of this supershell and another previously reported in the same galaxy are used to infer its neutral hydrogen scale height and mean neutral hydrogen density in the disk. The presence of another UV-emitting stellar association in overdense swept-up gas is discussed in the context of propagating star formation.