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

TL;DR

We report the discovery and detailed JWST/NIRSpec IFS analysis of GS18660, the most distant ring galaxy candidate known at $z\approx3.08$. The system features a clumpy ring with intense star formation, a rotating disk plus radial expansion of $\sim200$ km s$^{-1}$, and an intruder galaxy offset by $\sim425$ km s$^{-1}$, consistent with a recent head-on collision that formed the ring within the last $\sim50$ Myr. Multi-wavelength, spatially resolved diagnostics reveal elevated electron densities, a metallicity gradient with a relatively unenriched ring, and young stellar populations concentrated in the ring, all pointing to a collisional origin rather than a resonance ring. This high-redshift case provides a unique laboratory for collision-driven galaxy evolution and demonstrates JWST’s capability to probe the ISM, kinematics, and stellar assembly in the early Universe.

Abstract

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 (R~100) spectroscopy covering ~0.2-1.3um rest-frame, and high-resolution (R~2700) 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.

Figures (19)

• Figure 1: Colour-composite image of GS18660 (the 'target') and its close companions (the 'intruder' and the 'line emitters'), derived from NIRSpec R100. Blue shows the rest-frame UV, green the [O iii]-H$\beta$, and red the optical continuum, as labelled. The circles mark the regions used to extract the integrated spectra analysed in Sect. \ref{['sec:environment']}. The arrow marks a very faint source detected in [O iii] and H$\alpha$ (see also Fig. \ref{['fig:R2700MapsTarget']}, bottom).
• Figure 2: Integrated spectra of GS18660, with best-fit results. Top panel: The black curve represents the R100 spectrum obtained integrating over the entire extension of GS18660 ($r=0.65\arcsec$), with 3$\sigma$ uncertainties in light-grey. The red curve denotes the best-fit model, including both emission lines and continuum, while the dashed orange line represents the continuum alone. Fitted emission lines are marked with grey vertical lines and labelled. Bottom panels: R2700 spectrum integrated over the same region (black curves, with 3$\sigma$ uncertainties in grey) and best-fit results in the vicinity of modelled emission lines: H$\gamma$ and [O iii] $\lambda$4363 (left); H$\beta$ and [O iii] $\lambda\lambda$4960, 5008 (centre); H$\alpha$, [N ii] and [S ii] doublets (right).
• Figure 3: Integrated R100 and R2700 spectra of the intruder galaxy, within a $r=0.15\arcsec$ aperture. See Fig. \ref{['fig:integratedspectraGS18660']} for further details. The [S ii] doublet is likely undetected, as the observed peak in R100 is offset from its expected position and may instead result from continuum fluctuations.
• Figure 4: H$\alpha$ and [O iii] flux distributions, along with the [O iii] moment-1 (velocity) and moment-2 (velocity dispersion) maps. The flux maps reveal the morphology of GS18660, highlighting its ring structure and nuclear region, as well as the elongated morphology of the intruder. The velocity map displays a rotation-like pattern, with an additional high-velocity redshifted component associated with the intruder. The velocity dispersion map shows a relatively uniform distribution, with localized enhancements near the intruder, and in the western part of the ring. The contourns in the first panel trace H$\alpha$ emission; those in the remaining panels trace [O iii] emission. All images are oriented north up, with east to the left.
• Figure 5: GS18660 (top) and intruder (bottom) H$\alpha$ and [O iii] flux distributions, along with [O iii] moment-1 and moment-2 maps. These maps are similar to those in Fig. \ref{['fig:R2700MapsTotal']}, but were generated after removing all Gaussian components not associated with the GS18660 system (top) or the intruder (bottom). The intruder Moment-1 map is computed with respect to its systemic velocity ($\delta v = -430~$km s$^{-1}$ relative to GS18660).