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

TL;DR

Oxyster is a giant, low-ionization oxygen nebula extending up to ~30 kpc around a starburst galaxy at z ≈ 0.924, discovered via Subaru/HSC narrow-band imaging. Multiwavelength data (HST, JWST, CFHT, VLA, Chandra) and Magellan spectroscopy reveal [O II] and [O III] emission with L([O II]) ≈ 9.2×10^41 erg s−1 and L([O III]) ≈ 1.5×10^41 erg s−1, and a CCSM host with M* ≈ (2–6)×10^10 M⊙ and SFR ≈ 100–150 M⊙ yr−1; O32 < 1 across the nebula and Hβ remains undetected. Ionization modeling with CLOUDY and shock libraries shows that neither pure AGN photoionization nor pure starburst models fully explain the data, though a low-luminosity AGN and/or starburst-driven ionization, possibly aided by shocks, can reproduce the observed line ratios and luminosities within uncertainties. The host’s asymmetric, one-armed morphology and a nearby companion suggest a merger-driven scenario fueling a large CGM reservoir that ionizes Oxyster. This study demonstrates the power of ground-based narrow-band surveys to uncover CGM phenomena around normal star-forming galaxies at z~1 and motivates systematic searches to build statistical samples for CGM studies in the early Universe.

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.

Figures (9)

• Figure 1: Subaru/HSC and JWST/NIRCam imaging of Oxyster. Left panel: Fake RGB images from Subaru HSC r, i, z, NB718 and NB973 bands (noted in lower left corner of each subpanel) of the emission line nebula with [O$\;$] marked in green (upper) and [O$\;$] marked in red (lower) aligned with nebula contours. Middle panel: JWST NIRCam imaging overlaid with [O$\;$] in green solid line and [O$\;$] in white dashed line. Right panel: COSMOS-Web JWST/NIRCam fake RGB images of the galaxy. The substructures of the galaxy (core and arm) are annotated. Bands in different colors are demonstrated in the lower left corners. The PSF sizes of HSC and JWST images are shown in the upper right of each panel.
• Figure 2: The SED of the entire galaxy and substructures. Left Panel: The SED of the entire galaxy. Colored dots represent photometric data from various telescopes, with corresponding filter transmission curves shown below (details in Section \ref{['subsec:photometry']}). The orange and blue curves indicate the best fits obtained with Prospector for delayed-$\tau$ and logM SFH priors, respectively, with their star formation histories (SFHs) shown in the inset. The top subpanel displays the $\chi^2$ values for the fitting between the observed photometry and the model. HST and JWST cutouts are shown in the lower panel, with the aperture used for the SED analysis marked in red. Right Panel: Similar to the left panel but for specific substructures of the galaxy, namely the "arm" and "core." Their best-fit spectra are represented by solid and dashed lines, and photometric data are denoted by dots and triangles, corresponding to the pink and dark red apertures shown in the cutouts below.
• Figure 3: Long-slit spectra from Magellan LDSS3, reduced by Pypeit, as described in Section \ref{['subsec:spectra']}. The upper panel shows the 2D spectra of [O$\;$], non-detected H$\beta$ and [O$\;$] separately. The main figures in the middle row represent the 1D spectra (black solid line) and 1-sigma error (green solid line) along with the fitted 1D Gaussian curve (red solid line). Line centers at the host galaxy's redshift($z=0.924$) are demonstrated in vertical blue dashed lines, and the Gaussian centers are shown in red dashed lines. Residuals of the Gaussian fitting are shown in the lower panel with black dots. The horizontal red dashed line corresponds to the zero level.
• Figure 4: 2D line ratio map and 1D surface brightness radial profile of Oxyster. Left panel: Smoothed [O$\;$]$\lambda 5007$/[O$\;$]$\lambda\lambda 3726,3729$ map from the narrow bands subtraction described in Section \ref{['subsec:surface']}. Cyan and yellow solid lines indicate [O$\;$] and [O$\;$] contours that are 2-sigma above the background after Gaussian smoothing. Elliptical annuluses are shown in black solid lines. Right panel: Radial profiles of two emission lines. The surface brightness $\mu$ is measured inside the intersection between the [O$\;$] nebula contours and the annulus regions. The "distance" refers to each bin's middle point along the elliptical annulus's minor axis.
• Figure 5: Size and luminosity of extended [O$\;$] emission line regions around (fading) AGNs measured from relative works: Sun-2018, Kang-2018, Sun-2017, Schweizer-2013, Keel-2015, Baron-2018. Nebulae with active AGN hosts are marked in squares, while fading AGN hosts are in circles. The gray solid line and the gray region represent the size-luminosity relation and the scattering for extended emission-line regions of obscured AGN from samples in Sun-2018. The blue solid line represents the relation described in Kang-2018. The blue solid squares refer to the kinematic sizes and open squares to the photoionized sizes of the former samples. The red dot represents Oxyster, which is close to the famous Hanny's Voorwerp.