Institution(s): 1. Harvard University, 2. Tata Institute of Fundamental Research
A subset of neutral hydrogen (H I) cavities detected in our Milkyway galaxy and other nearby star forming galaxies may represent expanding supershells driven by the combined mechanical feedback from multiple supernovae occurring in OB associations. However, most extragalactic H I cavities do not have a demonstrated expansion velocity nor an identified OB association inside them. In the nearby, face-on spiral galaxy M101, an unbroken expanding H I supershell was found with a UV-emitting young stellar association inside it. From The HI Nearby Galaxy Survey (THINGS) datacubes (undertaken at the NRAO Very Large Array, VLA) to study the H I emission from nearby galaxies, we measured its size to be about 500 pc and expansion velocity of 20 km/s by identifying its approaching and receding components in the position velocity space, using 21 cm emission spectroscopy. This H I cavity provides us with an ideal system to test the theory of supershells driven by the mechanical feedback from multiple Supernovae, advanced by McCray and Kafatos. We compared the UV emission of the cluster inside the supershell with simulated spectral energy distribution of synthetic clusters of the appropriate age (~15 Myr) and found that the observed UV flux is consistent with an association of mass (~10^5 MSun) required by the energy budget of the supershell. Supershell characteristics of this galaxy can be used to infer the neutral hydrogen scale height and mean neutral hydrogen density in the disk of M101. Another UV-emitting stellar association in the overdense swept-up gas in the supershell is found and this will be discussed in the context of propagating star formation. Spatially resolved integral field spectroscopy may determine the metallicity enrichment of the gas and the true age of the stellar population and has the potential to reveal the role of multiple stellar populations within the cavity.