Oxyster: A Circumgalactic Low-ionization Oxygen Nebula next to a Starburst Galaxy at $z\sim1$

Pengjun Lu, Mingyu Li, Dalya Baron, Minghao Yue, Song Huang, Zheng Cai

First listed 2025-04-15 | Last updated 2025-11-03

Abstract

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 ($\sim 30$ kpc) and [OIII]5007 ($\sim 20$ 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 $2-6\times 10^{10} ~\rm M_{\odot}$ 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 H$β$ line. A plausible explanation could involve the combination of a recent ($<10^8$ yrs) starburst and a low-luminosity AGN ($L_{\rm{bol}} \sim 1\times10^{42}$ 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.

Short digest

Subaru/HSC narrow-band NB718/NB973 imaging plus Magellan LDSS3 follow-up reveal “Oxyster,” a CGM oxygen nebula next to a disturbed, interacting starburst at z=0.924, extending ~30 kpc in [O II] and ~20 kpc in [O III]. On the luminosity–size plane it outscales low‑z ENLRs and evokes a high‑z Voorwerp analog, but shows uniformly low O32 and no Hβ. HST/JWST imaging and SED fitting place the 2–6×10^10 Msun host above the main sequence with <5% AGN contribution and no X-ray/radio AGN, while standard photoionization/shock models cannot reproduce the luminosities, low O32, and Hβ non‑detection. The authors favor a recent (<10^8 yr) starburst plus a low‑luminosity AGN (~10^42 erg s^-1), underscoring narrow-band searches as a route to CGM mapping around non‑AGN systems near z~1.

Key figures to inspect

• Fig. 1: Compare [O II] (green) and [O III] (white) contours to the JWST/NIRCam morphology to gauge the nebula–galaxy offset, anisotropy, and whether the blue spiral arm aligns with the brightest oxygen emission.
• Fig. 2: Inspect Prospector SED fits for the whole galaxy vs. “core” and “arm” to quantify the bursty SF episode, stellar mass range (2–6×10^10 Msun), and the <5% AGN light fraction that matches the multiwavelength non-detections.
• Fig. 3: Use the LDSS3 long‑slit spectra to verify redshifted [O II] and [O III] detections, measure their spatial extent along the slit, and read off the stringent Hβ upper limit that drives the low O32/Hβ tension.
• Fig. 4: Examine the smoothed O32 map for its uniform, low ionization state and the radial surface‑brightness profiles showing [O II] more extended than [O III], constraining the hardness and reach of the ionizing source(s).