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

Abstract

The Square Kilometre Array mid-frequency array will enable high-redshift detections of neutral hydrogen (HI) emission in galaxies, providing important constraints on the evolution of cold gas in galaxies over cosmic time. Strong gravitational lensing will push back the HI emission frontier towards cosmic noon ($z\sim2$), as has been done for all prominent spectral lines in the interstellar medium of galaxies. Chakraborty & Roy (2023, MNRAS, 519, 4074) report a $z=1.3$ HI emission detection towards the well-modelled, galaxy-scale gravitational lens, SDSS J0826+5630. We carry out HI source modelling of the system and find that their claimed HI magnification, $\mu_{\rm HI} = 29 \pm 6$, requires an HI disk radius of $\lesssim 1.5$ kpc, which implies an implausible mean HI surface mass density in excess of $\Sigma_{\rm HI} > 2000$ M$_\odot$ pc$^{-2}$. This is several orders of magnitude above the highest measured peak values $(\Sigma_{\rm HI} \sim 10 \, {\rm M}_\odot\,{\rm pc}^{-2})$, above which HI is converted into molecular hydrogen. Our re-analysis requires this to be the highest HI mass galaxy known (M$_{\rm HI}~\sim 10^{11}$M$_\odot$), as well as strongly lensed, the latter having a typical probability of order 1 in 10$^{3-4}$. We conclude that the claimed detection is spurious.