GA-NIFS: the highest-redshift ring galaxy candidate from a head-on collision

Collisional ring galaxies are a rare class of interacting systems, making up only 0.01% of galaxies in the local Universe. Their formation is typically attributed to a head-on collision of a massive galaxy with a compact satellite (intruder), triggering density waves that, propagating outward, produce the characteristic ring morphology. Here, we present the discovery and detailed analysis of GS18660, the most distant ring galaxy known to date, at z=3.08, identified in JWST/NIRSpec IFS observations as part of the GA-NIFS programme. This work aims to characterise the physical and dynamical properties of GS18660 and shed light into the formation of its ring. Specifically, we analyse the ionized gas properties, stellar populations, and gas kinematics of the system, and use the observed geometry to constrain the timescale of the collision. Our analysis is based on NIRSpec IFS data, including low-resolution (R100) spectroscopy covering 0.2-1.3um rest-frame, and high-resolution (R2700) spectroscopy covering 0.4-0.8um rest-frame. Multi-wavelength techniques are applied to derive nebular gas conditions and stellar population properties. Gas kinematic analysis reveals that GS18660 exhibits a rotating disk component with an additional radial expansion velocity of ~200 km/s, consistent with a propagating collisional wave. Nebular line diagnostics indicate intense star formation (SFR ~100 Msun/yr) along the ring and in the nucleus. Stellar population analysis shows that the most recent star formation episode, occurring within the last ~50 Myr, predominantly took place in the ring. We also identify a close companion, the intruder galaxy responsible for the collision, moving away with a relative velocity of ~425 km/s. The evidence strongly favours a collisional origin for the ring in GS18660, though the presence of a recently formed bar (and hence a resonance ring) cannot be completely excluded.

💡 Research Summary

This paper reports the discovery and comprehensive analysis of GS18660, the most distant collisional ring galaxy candidate known to date, at a spectroscopic redshift of z = 3.0766. The object was identified in JWST/NIRSpec integral‑field spectrograph (IFS) observations obtained as part of the GA‑NIFS (Galaxy Assembly with NIRSpec IFS) programme. The authors combine low‑resolution (R ≈ 100) spectra covering 0.6–5.3 µm (≈2000–13000 Å rest‑frame) with high‑resolution (R ≈ 2700) spectra spanning 1.7–3.1 µm (≈4000–7700 Å rest‑frame) to map the spatial distribution of ionised gas, stellar populations, and kinematics across the galaxy.

Data reduction follows the JWST Science Calibration pipeline (v1.17.1) with several custom steps: bias level equalisation across dithered frames, 1/f noise removal, masking of cosmic‑ray hits and MSA‑leakage artefacts, and drizzle‑based sub‑sampling to 0.05″ × 0.05″ spaxels. A systematic wavelength offset of +24 ± 5 Å in the R100 mode is measured and corrected, ensuring consistent velocity measurements between the two spectral resolutions.

The reconstructed three‑colour image (rest‑frame UV,