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Blue Compact Dwarf (BCD) Galaxies in the nearby Universe provide a means forstudying feedback mechanisms and star-formation processes in low-metallicityenvironments in great detail. Due to their vicinity, these local analogues toyoung galaxies are well suited for high-resolution studies that would beunfeasible for primordial galaxies in the high-redshift universe. Here wepresent HST-WFC3 observations of one such BCD, Mrk 71, one of the most powerfullocal starbursts known, in the light of [O II], He II, Hb, [O III], Ha, and [SII]. At D=3.44 Mpc, this extensive suite of emission line images enables us toexplore the chemical and physical conditions of Mrk 71 on ~2 pc scales. Usingthese high spatial-resolution observations, we use emission line diagnostics todistinguish ionisation mechanisms on a pixel-by-pixel basis and show thatdespite the previously reported hypersonic gas and super-bubble blow out, thegas in Mrk 71 is photoionised, with no sign of shock-excited emission. Usingstrong-line metallicity diagnostics, we present the first 'metallicity image'of a galaxy, revealing chemically inhomogeneity on scales of <50 pc. Weadditionally demonstrate that while chemical structure can be lost at largespatial scales, metallicity-diagnostics can break down on spatial scalessmaller than a HII region. HeII emission line images are used to identify up tosix Wolf-Rayet stars in Mrk 71, three of which lie on the edge of blow-outregion. This study not only demonstrates the benefits of high-resolutionspatially-resolved observations in assessing the effects of feedbackmechanisms, but also the limitations of fine spatial scales when employingemission-line diagnostics. Both aspects are especially relevant as we enter theera of extremely large telescopes, when observing structure on ~10 pc scaleswill no longer be limited to the local universe.

Resolving Ionisation and Metallicity on Parsec Scales Across Mrk 71 with HST-WFC3