Resolving Ionisation and Metallicity on Parsec Scales Across Mrk 71 with
  HST-WFC3
  Blue Compact Dwarf (BCD) Galaxies in the nearby Universe provide a means for
studying feedback mechanisms and star-formation processes in low-metallicity
environments in great detail. Due to their vicinity, these local analogues to
young galaxies are well suited for high-resolution studies that would be
unfeasible for primordial galaxies in the high-redshift universe. Here we
present HST-WFC3 observations of one such BCD, Mrk 71, one of the most powerful
local starbursts known, in the light of [O II], He II, Hb, [O III], Ha, and [S
II]. At D=3.44 Mpc, this extensive suite of emission line images enables us to
explore the chemical and physical conditions of Mrk 71 on ~2 pc scales. Using
these high spatial-resolution observations, we use emission line diagnostics to
distinguish ionisation mechanisms on a pixel-by-pixel basis and show that
despite the previously reported hypersonic gas and super-bubble blow out, the
gas in Mrk 71 is photoionised, with no sign of shock-excited emission. Using
strong-line metallicity diagnostics, we present the first 'metallicity image'
of a galaxy, revealing chemically inhomogeneity on scales of <50 pc. We
additionally demonstrate that while chemical structure can be lost at large
spatial scales, metallicity-diagnostics can break down on spatial scales
smaller than a HII region. HeII emission line images are used to identify up to
six Wolf-Rayet stars in Mrk 71, three of which lie on the edge of blow-out
region. This study not only demonstrates the benefits of high-resolution
spatially-resolved observations in assessing the effects of feedback
mechanisms, but also the limitations of fine spatial scales when employing
emission-line diagnostics. Both aspects are especially relevant as we enter the
era of extremely large telescopes, when observing structure on ~10 pc scales
will no longer be limited to the local universe.
