Haro 11: Where is the Lyman continuum source?
  Identifying the mechanism by which high energy Lyman continuum (LyC) photons
escaped from early galaxies is one of the most pressing questions in cosmic
evolution. Haro 11 is the best known local LyC leaking galaxy, providing an
important opportunity to test our understanding of LyC escape. The observed LyC
emission in this galaxy presumably originates from one of the three bright,
photoionizing knots known as A, B, and C. It is known that Knot C has strong
Ly$\alpha$ emission, and Knot B hosts an unusually bright ultraluminous X-ray
source, which may be a low-luminosity AGN. To clarify the LyC source, we carry
out ionization-parameter mapping (IPM) by obtaining narrow-band imaging from
the Hubble Space Telescope WFC3 and ACS cameras to construct spatially resolved
ratio maps of [OIII]/[OII] emission from the galaxy. IPM traces the ionization
structure of the interstellar medium and allows us to identify optically thin
regions. To optimize the continuum subtraction, we introduce a new method for
determining the best continuum scale factor derived from the mode of the
continuum-subtracted, image flux distribution. We find no conclusive evidence
of LyC escape from Knots B or C, but instead, we identify a high-ionization
region extending over at least 1 kpc from Knot A. Knot A shows evidence of an
extremely young age ($\lesssim 1$ Myr), perhaps containing very massive stars
($>100$ M$_\odot$). It is weak in Ly$\alpha$, so if it is confirmed as the LyC
source, our results imply that LyC emission may be independent of Ly$\alpha$
emission.
