An Expanding Neutral Hydrogen Supershell Evacuated by Multiple
  Supernovae in M101
  Several neutral hydrogen (HI) 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 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 HI supershell in the nearby spiral galaxy M101, with
an UV emitting OB association inside it. We measure its size (500 pc) and
expansion velocity (20 km/s) 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 supernovae. The UV emission of the
cluster inside the supershell is compared with simulated spectral energy
distribution of synthetic clusters of the appropriate age (~15 Myr). The
observed UV flux is found to be consistent with an association of the
appropriate mass (~10^5 Solar Mass) 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 over-dense swept up gas is discussed in the context of
propagating star formation.
