When Stars Mimic Monsters: Spectral Evidence for an $\eta$ Carinae-like Giant Eruption in SBS 0335$-$052 E

Abstract

SBS 0335$-$052 E is an extremely low-metallicity ($Z\sim 0.04Z_{\odot }$) blue compact dwarf galaxy. An active galactic nucleus has been proposed to explain the broad H$\alpha$ emission and near-infrared (NIR) time variability in super star clusters 1 and 2 (SSCs 1&2). However, Peng et al. discovered broad wings in the forbidden [O III] $\lambda 5007$ emission (up to $\sim 5000\mathrm{km}{\mathrm{s}}^{-1}$), challenging the broad-line region interpretation. We present new KCWI/KCRM integral-field spectroscopy to directly compare spectra across multiple SSCs. The nebula surrounding SSCs 1&2 shows unique features. The Ly$\beta$-pumped O I $\lambda 8446$ emission constrains ${\tau }_{\mathrm{Ly}\alpha }\sim 10^8$. Multiple ionization states of iron are detected from Fe${}^{+}$ to Fe${}^{+4}$. Stellar photoionization models can reproduce the [Fe III]/[Fe II] and [Fe IV]/[Fe III] line ratios at high density ($n_e\sim 10^6\mathrm{c}{\mathrm{m}}^{-3}$), but they fail to account for most of the [Fe V] emission. The broad H$\alpha$ wings exhibit an exponential profile; the asymmetric wings extend from $\sim -5000\mathrm{km}{\mathrm{s}}^{-1}$ to $\sim 10000\mathrm{km}{\mathrm{s}}^{-1}$. Thomson scattering in a radially expanding medium provides a good fit with $v_w\sim 200\mathrm{km}{\mathrm{s}}^{-1}$, optical depth ${\tau }_e\sim 10$, and an outer to inner radius of 10. Enhanced N/O and potentially depleted Fe/O ratios are consistent with CNO-cycled ejecta from massive stars and with dust formation, respectively. We propose that mass loss from a massive star interacting with its circumstellar medium drives a shock that powers the NIR variability, the luminous X-ray point source, and the [Fe V] emission. If confirmed, the proposed stellar eruption would be a distant example of an $\eta$ Carinae-like giant eruption, and the first in an ultra-low metallicity environment.

28 pages, 13 figures, 3 tables. Accepted for publication in ApJ

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