ALMA observations of pre- JWST z ~ 10 galaxy candidates: A CO( J = 9–8) line from a ULIRG at z = 2.54 and revisit of the photometric redshifts with JWST photometry

We present Atacama Large Millimetre/submillimetre Array (ALMA) observations targeting the [O iii] 88\,m line for six z ~ 10 galaxy candidates selected with the Hubble Space Telescope and the Spitzer Space Telescope. We detect a line (4.5σ) and dust continuum emission (30σ) in UDS_18697, while detecting neither robust line nor continuum emission in the remaining five objects. The detected line in UDS_18697 is identified as CO(J = 9–8), because follow-up James Webb Space Telescope (JWST) NIRSpec observations have confirmed the redshift as z = 2.54. UDS_18697 is classified as an ultra luminous infrared galaxy (ULIRG) with far-infrared (FIR) luminosity of LFIR ≈ 1.1 × 1012 L⊙, assuming a dust temperature of Td ≈ 42 K, estimated using a physically-motivated method. We find that UDS_18697 follows the LFIR-LCO relation for local and z > 2 galaxies, albeit being slightly brighter in CO(J = 9–8). Also, based on the follow-up NIRSpec observations and spectral energy distribution fitting using JWST/NIRCam photometry, we found that most of our targets are suggested to be low-z interlopers. Motivated by these redshift misclassifications, we investigate colour–colour selection criteria for high-z galaxies using JWST spectroscopic survey catalogues. We find that elevating a colour threshold tracing the Lyman break is crucial for constructing a robust high-z sample, particularly for wide field surveys such as Euclid Deep Fields and Roman High-Latitude Wide-Area Survey.