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A Pilot Study on X-ray Ionized Nebulae in Nearby Metal-poor Dwarf Galaxies

Presentation #107.13 in the session Stellar/Compact Objects - Poster Session.

Published onMay 03, 2024
A Pilot Study on X-ray Ionized Nebulae in Nearby Metal-poor Dwarf Galaxies

For several decades, some metal-poor galaxies in the local Universe have been observed to display a harder than expected ultraviolet continuum, compared to expectations from the stellar populations within each galaxy. The physical source of ultraviolet emission remains a puzzle, the solution to which will provide insight into sources of high-energy radiation in analogously metal-poor environments in the high-redshift Universe. Suggested ideas for the excess ultraviolet emission range from rapidly spinning stars, to radiative shocks, to accreting compact objects. Here, we present results from a pilot Chandra survey to investigate if compact objects contribute substantially. Here, we present results on 22 metal-poor, star-forming galaxies (<50 Mpc) with nebular HeII λ4686 emission (which implies a strong ultraviolet continuum above the HeII line’s ionization potential of 54 eV). We find X-ray sources in 30% of galaxies, suggesting that compact objects could be producing the excess ultraviolet emission in some, but not all, metal-poor galaxies. We also highlight one galaxy in our sample that had previously been classified as an active galactic nucleus in a metal-poor dwarf galaxy, using low spatial-resolution XMM-Newton observations. We performed multi-wavelength X-ray (Chandra), optical (HST), and radio (VLA) follow-up on this galaxy that demonstrate it instead hosts two rapidly accreting stellar mass black holes, highlighting the importance of high spatial resolution observations when examining X-ray populations in low-mass galaxies. We conclude by discussing alternative explanations for how metal-poor galaxies might be producing their hard ultraviolet continua, as well as the efficacy of using the HeII emission line to help discover X-ray sources in low-mass galaxies.

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