Declining metallicity and extended HeII in the outflow of an epoch of reionization analogue galaxy

TL;DR

This study probes the outflow of the extremely metal-poor, nearby analogue SBS 0335-052E using VLT/X-shooter to $\sim$2.4 kpc, enabling direct, $T_e$-based metallicity measurements along the wind. A declining metallicity profile is found (center to $\sim$2.4 kpc) with a local peak at an H$\alpha$ arc and a minimum at the wind's edge, yielding a metal-loading factor $\zeta$ of about $5.75$, far below expectations from the low-mass MZR. In addition, extended HeII $\lambda$4686 emission shows a broad, redshifted component at large radii, with extremely high HeII/H$\beta$ ratios that are difficult to reconcile with purely stellar or shock ionization. The combination suggests additional feedback, possibly from an intermediate-mass black hole (IMBH), contributing to both metal transport and the ionization state of the outflow. These findings imply that metal enrichment and wind physics in extremely metal-poor galaxies may differ from standard models, with implications for chemical enrichment of the early universe.

Abstract

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

Figures (3)

• Figure 1: Left panel: X-shooter slit position overlaid on an HST/WFC continuum-subtracted H$\alpha$ image. We show the 1 division of the slit and mark relevant positions with blue letters. Right panel: The metallicity profile as a function of distance to the brightest H$\alpha$ emission in the galaxy. The red dotted line shows the R$_{90}$ measured from the HST F550n band. Note the rise in metallicity corresponds to the H$\alpha$ emission arc.
• Figure 2: Metal loading vs. stellar mass. The red star shows SBS 0335-052E, black dots show results from Chisholm2018, and the yellow triangle and diamond show results from Cameron2021 and Hamel-Bravo2024, respectively. Blue dotted and dashed lines show models from Peeples2011 to reproduce the MZR from Denicol2002 and Tremonti2004, respectively.
• Figure 3: Spectra around the HeII $\lambda$4686 emission line, in velocity scale, for the four spatial elements marked in blue in Fig. \ref{['fig_1']}. v = 0 corresponds to the redshift of the galaxy, z = 0.01352. Black shows the data and blue shows the Gaussian fit.