JWST's PEARLS: A clumpy ring galaxy at $z = 4.0148$

Abstract

Ring galaxies are an uncommon class of galaxies whose morphology is closely related to dynamical processes that govern galaxy evolution. Some ring galaxies, known as "collisional ring galaxies", are thought to form as a consequence of head-on collisions between galaxies, and a number of high-redshift collisional ring galaxies have been discovered and/or studied in the era of the James Webb Space Telescope (JWST). In this paper, we present HST/ACS, JWST/NIRCam, and JWST/NIRSpec observations of a candidate ring galaxy at $z_{\rm spec} = 4.0148$, previously identified as a potential gravitational lens. The galaxy exhibits a complex morphology, including three bright clumps along an apparent ring with radius $\approx 0.25$" $\simeq 1.8$ kpc. It has a total SFR $= 140^{+20}_{-30}$ ${\rm M}_{\rm \odot}$ yr$^{-1}$ and $\log(M_\ast/{\rm M}_\odot) = 10.41^{+0.11}_{-0.13}$, making it similar to other high-redshift collisional ring galaxies. Although we argue strongly in favor of the collisional ring explanation, we cannot entirely rule out a galaxy-galaxy strong lensing explanation for the system's morphology, in which a foreground galaxy at $z \simeq 1.7$ lenses a galaxy at $z \simeq 4.0$ into an Einstein ring-like configuration; to confirm the nature of this source, we require kinematic information via high spectral resolution observations. We suggest that current and future gravitational lens surveys should consider high-redshift ring galaxies as possible but significant contaminants.

Figures (7)

• Figure 1: Left: 4 arcmin$^2$ pseudo-RGB image of the MACS0416 cluster, using JWST NIRCam F115W + F150W as blue, F200W + F277W as green, and F356W + F444W as red. The location of the ring galaxy is indicated in a magenta box. Right:$2\hbox{$^{\prime\prime}$}$ cutout of the candidate ring galaxy in JWST F150W imaging (top) and $2\hbox{$^{\prime\prime}$}$ re-scaled RGB cutout of the CRG (bottom). In the top panel, we show the $\sim 3\sigma$ ALMA detection from the ALCS survey fujimoto2024 as a red contour. The JWST imaging of the CRG clearly reveals a ring morphology; we have scaled the channels of the RGB cutout to emphasize the clumpy blue ring surrounding the red center of the system.
• Figure 2: 2$\hbox{$^{\prime\prime}$}$ cutouts of the ring galaxy as observed by HST ACS (F606W and F814W) and JWST NIRCam (F090W, F115W, F150W, F200W, F277W, F356W, and F444W) at their native resolutions. The magenta box shows the region from which the NIRSpec PRISM data were taken. These data products (2D spectrum and integrated 1D spectrum) are shown in the bottom half of the figure. We have scaled the contrast in the cutouts and 2D spectra to highlight important features, showing a red continuum exhibiting strong emission lines from a source at $z = 4.0148$, with emission lines that appear to encompass the spatial extent of the continuum source. The vertical dashed lines indicate the location of expected emission lines at $z = 4.0148$, and are included with annotations to demonstrate a match with the spectroscopic redshift of the galaxy.
• Figure 3: Top: 2$\hbox{$^{\prime\prime}$}$ cutouts of HST ACS and JWST NIRCam imaging, after matching the spatial resolution to F444W. Five apertures are shown and labeled in each cutout, corresponding to the three clumps A (red), B (blue) and C (green), the center (orange), and the total flux (black, dashed line). Bottom: Flux density versus observed wavelength within each aperture, as color coded in the top panel. A 5% flux uncertainty was multiplied in quadrature to each photometric point before SED fitting. The best-fit SED models from bagpipes, assuming $z = 4.0148$ for all apertures, are shown as solid lines, demonstrating excellent agreement between the photometry and the the spectroscopic redshift from CANUCS sarrouh2025.
• Figure 4: Left: Best-fit SEDs (median and 1$\sigma$ error) to photometry of two companions on the outskirts of the CRG; these clumps have photometric redshifts that are similar to the spectroscopic redshift of the CRG. Right: A model-subtracted image of F277W imaging, with the bulk of emission modeled as a 2D Sèrsic profile. Photometric redshift fits for all five companions are shown in different colors; the two companions that roughly agree with $z_{\rm spec} = 4.0148$ are located along an arm-like tidal feature that appears to extend from the edge of the ring. We have tuned the image scaling to highlight the nearby companions and tidal feature.
• Figure 5: Left: Flux density versus observed wavelength, plotting photometric points from the two HST and seven JWST wide-filter images in the green squares. These data are taken from an aperture corresponding to the "center" shown in Figure \ref{['fig:photometry']} (and the green circle in the right panel). The orange and blue spectra represent bagpipes models of a galaxy clump at $z=4.0148$ versus a foreground gravitational lens with a best-fit redshift of $z = 1.66$. The $z \simeq 4$ solution better describes the photometry, and the predicted source SED at $z \simeq 1.7$ exhibits a number of lines between $3.2 - 5.0$$\mu$m that are not found in the NIRSpec data (see Figure \ref{['fig:stamps']}). Right: A scaled version of F182M imaging subtracted from the F200W imaging; we calculate a "corrective factor", accounting for diffuse starlight near the "center" of the galaxy, from the flux enclosed within the magenta aperture.