X-ray stellar feedback in low-metallicity starbursts

Insights from the template starburst galaxy ESO 338-IG04 and its halo^★

¹ Institute for Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
² Department of Physics, University of Arkansas, 226 Physics Building, 825 West Dickson Street, Fayetteville, AR 72701, USA
³ Arkansas Center for Space and Planetary Sciences, University of Arkansas, 332 N. Arkansas Avenue, Fayetteville, AR 72701, USA
⁴ Department of Astronomy, The Oskar Klein Centre, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
⁵ Centro de Astrobiología (CAB), INTA–CSIC, Madrid, Spain
⁶ Department of Astronomy, U. Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
⁷ ESA, ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 September 2025
Accepted: 5 February 2026

Abstract

Context. The X-ray output of low-metallicity starburst galaxies is a key component of stellar feedback, tracing the processes responsible for gas ionization and chemical enrichment. The integrated X-ray luminosity (L_X) from high-mass X-ray binaries in star-forming galaxies scales with the star formation rate (SFR) and the host galaxy metallicity Z. Due to the inverse correlation between L_X/SFR and Z, the contribution of X-ray binaries to the ionizing photon budget is expected to be enhanced in metal-poor systems. Their radiation can potentially ionize He II in the surrounding interstellar medium, powering nebular He IIλ4686 Å emission. However, detailed studies of the X-ray emission in individual low-Z starburst galaxies are rare, and their X-ray properties are not well explored.

Aims. The blue compact dwarf galaxy ESO 338-IG04 (ESO 338-4 hereafter) serves as a nearby template for studying stellar feedback and X-ray emission in low-metallicity starbursts. It combines vigorous recent star formation, a low metallicity (12 + log(O/H)≈7.9, or 12% solar), and a rich population of massive stellar clusters. Extensively observed in optical and UV wavelengths with HST and VLT MUSE, ESO 338-4 is ideally suited for multiwavelength feedback studies. We characterize the X-ray emission of ESO 338-4 and its galactic halo using new deep observations obtained with the Chandra X-ray Observatory (Chandra) and XMM-Newton.

Methods. We analyzed X-ray spectra, light curves, and images of ESO 338-4 to constrain the nature of its X-ray sources. Additionally, we employed photoionization modeling to assess the significance of X-ray sources to the observed nebular He IIλ4686 Å emission.

Results. We identified five ultraluminous X-ray sources (ULXs) and diffuse hot gas surrounding ESO 338-4. Two of the ULXs are spatially associated with stellar clusters. The total galactic X-ray luminosity exceeds 10⁴¹ erg s⁻¹. The brightest point source, ULX1, is variable on timescales of days and is not associated with a stellar cluster. Last, our modeling demonstrates that X-ray sources significantly affect the ionizing photon budget of the galaxy. Photoionization modeling with ULX1 as the ionizing source predicts a high nebular He IIλ4686 Å line luminosity of approximately 10³⁹ erg s⁻¹.