Discovery of a gravitational arc candidate at photometric redshift 1.4 in MACS J0308.9+2645 from a catalogue-based search of JWST imaging
We report a previously uncatalogued gravitational arc candidate (A1) in the massive RELICS cluster MACS J0308.9+2645 (z_l = 0.356), identified in public JWST/NIRCam imaging from programme GO-5293 through a transparent catalogue-based selection of extended, elongated, tangentially aligned sources. A1 has axis ratio a/b ~ 6.5, isophotal extent ~ 5.1 arcsec, and lies 50.9 arcsec from the cluster X-ray centre, tangentially aligned to within 1.4 +/- 3.0 deg. We show that the catalogue aper_total_abmag photometry, designed for unresolved sources, is invalid for this extended arc: the aperture captures only a few per cent of the isophotal flux and, because its radius grows with wavelength, imposes ~ 1.4 mag of spurious red colour that would drive a spurious high-redshift solution. Extended-source photometry (isophotal, fixed matched apertures, curve-of-growth) combined with archival HST/RELICS forced photometry in seven bands yields z ~ 1.4 (range 1.2-1.7). A significant HST/ACS F435W detection independently excludes the high-redshift solution, which would require a complete Lyman-limit dropout. At z ~ 1.4 the public Zitrin-LTM-Gauss lens model places A1 near the tangential critical curve in the single-image regime: full-map ray-tracing predicts no counter-images for z < 2, consistent with the absence of any counterpart, whereas a z = 4.4 source would require a counter-image of m ~ 22.7 where none is observed. The most probable interpretation is a singly lensed galaxy at z ~ 1.4 behind the cluster. The arc's curvature is concave toward the cluster centre (radius ~ 19 arcsec) but of marginal, method-dependent significance once correlated noise is accounted for. A fainter arclet (A2) is retained as a secondary candidate. Spectroscopy and a dedicated lens model are required for confirmation.