Oxyster: A Circumgalactic Low-ionization Oxygen Nebula next to a Starburst Galaxy at z ∼ 1

Extended emission line nebulae around galaxies or active galactic nuclei (AGNs) provide a unique window to investigate the galactic ecosystem through the circumgalactic medium (CGM). Using Subaru Hyper Suprime-Cam narrowband imaging and spectroscopic follow-up, we serendipitously discover “Oxyster”—a large ionized nebula next to an interacting starburst galaxy at z = 0.924. The nebula is traced by extended [O ii ] λλ 3726, 3729 (∼30 kpc) and [O iii ] λ 5007 (∼20 kpc) emission lines. On the nebula luminosity–size plane, Oxyster surpasses the extended narrow-line regions around low- z AGNs, resembling a higher- z analog of “Hanny’s Voorwerp.” However, its uniformly low [O iii ]/[O ii ] ratio (O32) sets it apart from typical AGN light echoes. For the host galaxy, Hubble Space Telescope and JWST images reveal a disturbed red disk galaxy with a single blue spiral “arm.” Spectral energy distribution fitting suggests the (2–6) × 10 10 M ⊙ host galaxy sits above the star-forming main sequence with an ongoing starburst, especially in the “arm,” and has <5% luminosity contribution from AGNs, consistent with X-ray nondetection and the radio continuum. Standard photoionization and shock models struggle to explain simultaneously Oxyster’s emission line luminosities, low O32 ratio, and the nondetection of the H β line. A plausible explanation could involve the combination of a recent (<10 8 yr) starburst and a low-luminosity AGN ( L bol ∼ 1 × 10 42 erg s −1 ). While Oxyster’s nature awaits future investigation, its discovery highlights the potential of ground-based narrowband imaging to uncover extended emission line nebulae around non-AGN systems, opening new avenues for studying the CGM of normal galaxies in the early Universe.