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

TL;DR

This study uses high-resolution HST-WFC3 observations of Mrk 71, a nearby blue compact dwarf galaxy, to analyze ionization and metallicity on parsec scales, revealing chemical inhomogeneity and the presence of Wolf-Rayet stars.

Contribution

First high-resolution metallicity map of a galaxy showing chemical inhomogeneity on <50 pc scales, and analysis of ionization mechanisms at parsec resolution.

Findings

Gas in Mrk 71 is photoionized, not shock-excited.

Metallicity varies on scales smaller than HII regions.

Identified six Wolf-Rayet stars, three near blow-out regions.

Abstract

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.