On the implausible physical implications of a claimed lensed neutral hydrogen detection at redshift z = 1.3

The Square Kilometre Array mid-frequency array will enable high-redshift detections of neutral hydrogen (H I) emission in galaxies, providing important constraints on the evolution of cold gas in galaxies over cosmic time. Strong gravitational lensing will push back the H I emission frontier towards cosmic noon (z ∼ 2), as has been done for all prominent spectral lines in the interstellar medium of galaxies. Chakraborty & Roy report a z = 1.3H I emission detection towards the well-modelled, galaxy-scale gravitational lens, SDSS J0826+5630. We carry out H I source modelling of the system and find that their claimed H I magnification, μH _I = 29 ± 6, requires an H I disc radius of ≲1.5 kpc, which implies an implausible mean H I surface mass density in excess of Σ_{H I} > 2000 M_⃠ pc⁻². This is several orders of magnitude above the highest measured peak values (Σ_{H I} ∼ 10 M_⃠ pc⁻²), above which H I is converted into molecular hydrogen. Our re-analysis requires this to be the highest H I mass galaxy known (M_{H I} ∼ 10¹¹ M_⃠), as well as strongly lensed, the latter having a typical probability of the order of 1 in 10³–10⁴. We conclude that the claimed detection is spurious.