Declining metallicity and extended He II in the outflow of an Epoch of Reionisation analogue galaxy

We present VLT/X-shooter spectroscopy of the extremely metal-poor starburst galaxy SBS 0335-052E, a nearby ( D ∼ 54 Mpc) analogue of high-redshift systems, probing its outflow up to a distance of ∼2.6 kpc. Using direct-method oxygen abundances, we find a complex metallicity profile that generally declines with distance, decreasing by 0.37 dex from the galaxy centre out to 2.4 kpc into the outflow. This implies a metal-loading factor roughly an order of magnitude lower than predictions based on the mass–metallicity relation for low-mass galaxies. We also detect extended He II emission, including a broad, redshifted component beyond 2 kpc, which is distinct from the narrow emission associated with star clusters. No H β emission associated with the broad redshifted component is detected, which implies extremely high He II /H β ratios (0.7–4.8). Such extreme values, combined with the decreasing metallicity, challenge our current models of stellar feedback. The contribution of an intermediate-mass black hole could simultaneously account for the declining metallicity and the unusually extended He II emission.