ALMA observations of pre- JWSTz ∼ 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

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ABSTRACT We present Atacama Large Millimetre/submillimetre Array observations targeting the [O iii] $88\, \mu$m line for six $z\sim 10$ galaxy candidates selected with the Hubble Space Telescope and the Spitzer Space Telescope. We detect a line ($4.5\sigma$) and dust continuum emission ($30\sigma$) 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($J=9\text{-}8$), 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 with far-infrared luminosity of $L_\mathrm{FIR}\approx 1.1\times 10^{12}\, \mathrm{L}_\odot$, assuming a dust temperature of $T_\mathrm{d}\approx 42\,$K, estimated using a physically-motivated method. We find that UDS_18697 follows the $L_\mathrm{FIR}-L^{\prime }_\mathrm{CO}$ relation for local and $z\gt 2$ galaxies, albeit being slightly brighter in CO($J=9\text{-}8$). 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.

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