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

Zixuan Peng, Crystal L. Martin, Jiamu Huang, Nikolaus Z. Prusinski, Chenliang Huang, Zhuyun Zhuang, Yuan Li, Tin Long Sunny Wong, Jiayang Yang, Joseph F. Hennawi

Abstract

SBS 0335$-$052 E is an extremely low-metallicity ($Z\sim0.04\,Z_{\odot}$) blue compact dwarf galaxy. An active galactic nucleus has been proposed to explain the broad H$α$ 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] $\lambda5007$ emission (up to $\sim5\,000\,\rm{km\,s^{-1}}$), 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$β$-pumped O I $\lambda8446$ emission constrains $τ_{\rm\,Lyα}\sim10^8$. 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 ($n_e\sim10^6\,\rm{cm^{-3}}$), but they fail to account for most of the [Fe V] emission. The broad H$α$ wings exhibit an exponential profile; the asymmetric wings extend from $\sim-5\,000\,\rm{km\,s^{-1}}$ to $\sim10\,000\,\rm{km\,s^{-1}}$. Thomson scattering in a radially expanding medium provides a good fit with $v_w\sim200\,\rm{km\,s^{-1}}$, optical depth $τ_e\sim10$, 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 $η$ Carinae-like giant eruption, and the first in an ultra-low metallicity environment.

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

Abstract

SBS 0335052 E is an extremely low-metallicity () blue compact dwarf galaxy. An active galactic nucleus has been proposed to explain the broad H 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] emission (up to ), 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-pumped O I emission constrains . Multiple ionization states of iron are detected from Fe to Fe. Stellar photoionization models can reproduce the [Fe III]/[Fe II] and [Fe IV]/[Fe III] line ratios at high density (), but they fail to account for most of the [Fe V] emission. The broad H wings exhibit an exponential profile; the asymmetric wings extend from to . Thomson scattering in a radially expanding medium provides a good fit with , optical depth , 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 Carinae-like giant eruption, and the first in an ultra-low metallicity environment.

Paper Structure

This paper contains 32 sections, 8 equations, 13 figures.

Table of Contents

  1. Introduction
  2. Observation and Data Reduction
  3. KCWI/KCRM Observation
  4. Data Reduction
  5. Nebular Properties of SSCs
  6. $n_e$, $T_e$, and $\log U$ Constraints of Different Ionization Zones
  7. Gas-phase Abundances of $N$, $O$, and $Fe$
  8. Signatures of Giant Eruption in SSCs 1&2
  9. A Coherent Giant Eruption Picture
  10. High-Density Gas
  11. Low and High Ionization Fe Lines
  12. Nitrogen and Iron Abundances: Comparison with Literature Values
  13. $\rm{N/O}$ Abundance Ratio
  14. $\rm{Fe/O}$ Abundance Ratio
  15. Asymmetric Broad Wings in Balmer Lines
  16. ...and 17 more sections

Figures (13)

  • Figure 1: The two KCWI/KCRM observation pointings (green rectangles) are overlaid on the HST FR656N image of J0337-0502. The zoom-in panel shows the color-composite image (purple: F220W; blue: F330W; green: F435W; red: FR656N), which indicates the positions of the SSCs, including the luminous X-ray point source Thuan_2004, the apertures used to extract the 1D spectra, and the $2\farcs5$HST/COS circular aperture. The black solid contours demarcate H$\alpha$ isophotes at SB$_{\rm{H}\alpha}$$\simeq$ {2.5, 5.0} $\times 10^{-16} \ \rm{erg \ s^{-1} \ cm^{-2} \ arcsec^{-2}}$.
  • Figure 2: Extracted 1D spectra of KCWI (blue; second row) and KCRM (red; third row) for SSCs 1&2. First-row panels provide zoomed views around the detections of high-density gas indicators (Section \ref{['subsec:high_den']}), such as O I$\lambda8446$, [Fe II] $\lambda5262$, and [Fe II] $\lambda8617$, as well as the high and very-high ionization lines like [Fe IV] $\lambda5234$ and [Fe V] $\lambda4227$ (Section \ref{['sec:nebular_prop']}).
  • Figure 3: $T_e$ and $n_e$ solutions for SSCs 1&2. The best-fit $T_e$ and $n_e$ for each ionization zone are defined as the intersection of the temperature and density diagnostics. The estimated $\log U$ value for each ionization zone is also shown in the panel.
  • Figure 4: The [Fe II] $\lambda 5262$/[Fe II] $\lambda 8617$ ratio for SSCs 1&2 (solid red) and SSC 3 (dashed red). Predictions of the [Fe II] $\lambda5262$/[Fe II] $\lambda8617$ ratio across different $n_e$ and $T_e$ are shown as dashed bluish lines. The rough $n_e$ constraints for gas traced by [Fe II] and [Fe III] in the Weigelt knots of $\eta$ Carinae are shown as dotted lines.
  • Figure 5: Illustration of the proposed giant eruption mechanism (adapted from the $\eta$ Carinae picture; Mehner_2011phdDavidson_2020Gull_2022) in SSCs 1&2. The high-density CSM initially forms from pre-eruption stellar winds (Phase I). Subsequent shock interaction between the giant eruption ejecta (Phase II; near periastron if the system is binary) and the CSM naturally accounts for the observed spectroscopic signatures discussed in Section \ref{['sec:lbv_signs']}. We illustrate the proposed picture in Section \ref{['subsec:picture']}.
  • ...and 8 more figures