Webb Data Reveal Uncatalogued Gravitational Arc in MACS J0308.9+2645

An independent search of archival James Webb Space Telescope imaging has turned up a previously uncatalogued gravitational arc in the massive galaxy cluster MACS J0308.9+2645. The discovery, reported by astrophysicist Homer Dávila Gutiérrez, illustrates both how much remains to be found in Webb’s public archive and how easily automated analysis can mislead researchers about what they’ve found.

An Independent Search Turns Up a Missed Arc

Dávila Gutiérrez, founder and director of the Costa Rican science outlet SKYCR.ORG and a Fellow of the Royal Astronomical Society, searched 54 public JWST Near-Infrared Camera (NIRCam) fields gathered under Webb’s General Observation program GO-5293, part of the Reionization Lensing Cluster Survey (RELICS). Evaluating 1,591 candidate light sources across those fields, he identified one, designated A1, as a strong candidate for a genuine gravitational arc — light from a distant background galaxy stretched and magnified by the gravity of a foreground cluster.

MACS J0308.9+2645 is a massive, Planck-selected cluster at redshift 0.356 that was among the fields imaged in Webb’s first public data release in July 2022. A1 sits about 51 arcseconds from the cluster’s X-ray center and has an axis ratio of roughly 6.5 to 1, an elongation tangentially aligned with the cluster center to within about a degree — the geometric signature gravitational lensing produces. Dávila Gutiérrez checked A1 against the published RELICS strong-lensing catalog as well as the SIMBAD, NED, and VizieR astronomical databases and found no prior record of it. Contacted directly, the GO-5293 team confirmed the source did not overlap with objects they had already catalogued.

From the Early Universe to Nine Billion Years Ago

Using the EAZY photometric redshift tool, Dávila Gutiérrez’s initial analysis placed A1 at a photometric redshift of roughly 4.4, which would put the light more than 13 billion years old, from a galaxy seen when the universe was under a billion years old. Applying a public cluster mass model built by astrophysicist Ana Acebron and collaborators, he calculated that A1 was being magnified by a factor of about 7.

Follow-up analysis, however, changed the picture. Standard catalog photometry captures only a fraction of the light from an extended, elongated source like A1, which skewed the original redshift estimate. After correcting for that effect, Dávila Gutiérrez found A1 is more likely a galaxy at redshift 1.4 — seen as it appeared roughly 9 billion years ago — lying behind MACS J0308.9+2645 and singly lensed, meaning its light is stretched and modestly magnified rather than split into multiple images. Its projected distance from the cluster center and its elongation are both consistent with that revised interpretation. A definitive answer awaits an updated lens model of the cluster now being built from the new JWST data.

A second, fainter candidate arc, designated A2, was identified at a similar projected distance from the cluster core. Its photometry is even less well constrained than A1’s, and Dávila Gutiérrez has not yet assigned it a redshift pending the same corrected reanalysis.

Why Automated Catalogs Miss Sources Like This

The case underscores a limitation of automated survey photometry: aperture-based catalogs, which measure light within a fixed radius around a source, systematically underestimate the brightness of extended, stretched objects like gravitational arcs. That underestimate can distort derived properties, including redshift, unless researchers go back and remeasure the source by hand. Because A1 does not appear in any existing catalog of the cluster’s known lenses, it was never flagged for that kind of manual follow-up until Dávila Gutiérrez’s independent search.

How Webb’s Public Archive Enabled This Discovery

The GO-5293 imaging of MACS J0308.9+2645 has been publicly available since shortly after its 2022 observation, meaning A1 sat in released data for roughly four years before being flagged. Dávila Gutiérrez’s manuscript describing the discovery and its redshift correction is currently under review at Publications of the Astronomical Society of Japan, and the preprint is posted on arXiv.

What This Means for Cluster Astronomy

Gravitational arcs like A1 serve two purposes for astronomers: they are markers used to build precise mass models of the galaxy clusters that produce them, and in cases where a lensed source really does sit at high redshift, they act as natural telescopes that magnify otherwise-undetectable early-universe galaxies. Even though A1 turned out to be closer than first estimated, the discovery adds a previously missing data point to the cluster’s mass model and demonstrates that Webb’s rapidly growing public archive still holds real, uncatalogued sources accessible to independent researchers.

The episode also reinforces a standard of practice in lensing studies: initial photometric redshifts from automated pipelines require independent verification, particularly for extended or unusual sources, before they are treated as established results. That verification process, still underway for A1, is expected to conclude once the cluster’s updated lens model is complete.

Source: Homer Dávila Gutiérrez (SKYCR.ORG), in collaboration with the Webb GO-5293 team. Published in: Under review, Publications of the Astronomical Society of Japan. Primary source: arxiv.org/abs/2607.12129. Based on reporting by Matthew Williams, Universe Today (CC BY 4.0): universetoday.com.