2025/04/15 by Lu, Pengjun, Li, Mingyu, Baron, Dalya +3
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2504.11531
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 narrow-band 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 [OII]3726,3729 (∼ 30 kpc) and [OIII]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 [OIII]/[OII] ratio (O32) sets it apart from typical AGN light echoes. For the host galaxy, HST and JWST images reveal a disturbed red disk galaxy with a single blue spiral "arm". Spectral energy distribution (SED) fitting suggests the 2-6× 1010 ~\rm M\odot host galaxy sits above the star-forming main sequence with an ongoing starburst, especially in the "arm", and have <5% luminosity contribution from AGN, consistent with X-ray non-detection and radio continuum. Standard photoionization and shock models struggle to explain simultaneously Oxyster's emission line luminosities, low O32 ratio, and the non-detection of Hβ line. A plausible explanation could involve the combination of a recent (<108 yrs) starburst and a low-luminosity AGN (L_\rmbol ∼ 1×1042 erg/s). While Oxyster's nature awaits future investigation, its discovery highlights the potential of ground-based narrow-band 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.