A Costa Rican Astrophysicist Found a Hidden Galaxy in Public JWST Data

A Costa Rican astrophysicist has uncovered a hidden galaxy in publicly available data from the James Webb Space Telescope (JWST), demonstrating that important discoveries can still be made by combing through archival observations with a careful eye.

What Was Discovered

Homer Dávila Gutiérrez, founder of SKYCR.ORG, the first Costa Rican elected Fellow of the Royal Astronomical Society, and a member of the European Astronomical Society, identified a previously uncatalogued gravitational arc candidate . Designated A1, the object is a distant background galaxy whose light has been gravitationally lensed — bent, magnified, and stretched into a luminous arc — by the massive foreground galaxy cluster MACS J0308.9+2645 (at redshift z_l = 0.356) .

Dávila found A1 by searching through 54 public JWST/NIRCam fields and individually evaluating 1,591 candidate objects .

How the Arc Was Identified

A1 stood out due to three key characteristics :

1. Extreme elongation: An axis ratio of approximately 6.5, far more stretched than typical galaxies.
2. Tangential alignment: The arc aligns to within about 1 degree of the cluster center — exactly the geometry that gravitational lensing produces.
3. Total absence from catalogs: The object does not appear in SIMBAD, NED, VizieR, or the published strong-lensing inventory for this cluster.

The isophotal extent of A1 is roughly 5.1 arcseconds, and it lies 50.9 arcseconds from the cluster's X-ray center. A fainter secondary candidate, A2, was also noted and remains under evaluation .

The Redshift Measurement Challenge

Determining A1's redshift was the central difficulty of the study. The standard JWST catalog photometry (aper_total_abmag) is designed for unresolved point sources and gave invalid results for this extended arc. The fixed aperture captured only a few percent of the total flux, and because its radius grows with wavelength, it imposed a spurious ~1.4 magnitudes of red color that would falsely suggest a very high redshift [arXiv].

Dávila instead used extended-source photometry — isophotal fluxes, fixed matched apertures, and curve-of-growth methods — combined with archival HST/RELICS forced photometry across seven bands [arXiv].

A critical piece of evidence came from a clear HST/ACS F435W detection, which independently rules out a high-redshift solution (which would require a complete Lyman-limit dropout in that band) [arXiv].

The resulting photometric redshift is z ~ 1.4 (with a plausible range of 1.2–1.7), meaning the galaxy existed when the universe was roughly 4–5 billion years old — not primordial, but still far in the past [arXiv].

Scientific Significance

A1 is most likely a singly lensed galaxy at z ~ 1.4, located near the tangential critical curve in the single-image regime of the public Zitrin-LTM-Gauss lens model. Full-map ray-tracing predicts no counter-images for z < 2, consistent with the absence of any visible counterpart. A source at z = 4.4 would require a counter-image of magnitude ~22.7 where none is observed [arXiv].

The work demonstrates that public archival search methods — combining JWST data with existing catalogs — can still uncover objects missed by the original survey teams. It also serves as a cautionary case study: standard catalog photometry can produce severely misleading results for spatially resolved or gravitationally lensed sources [arXiv].

Current Status

The study is published on arXiv (arXiv:2607.12129, v2 revised August 7, 2026) and is undergoing peer review for official journal publication in the Publications of the Astronomical Society of Japan (PASJ) [arXiv]. The GO-5293 team has confirmed via email that no one had investigated this object before, verifying Dávila as the discoverer .

Spectroscopic follow-up and a dedicated lens model are explicitly required for full confirmation and to pin down the precise redshift [arXiv].