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 ( 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 . 3 H 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 H I > 2000 M pc −2 . This is several orders of magnitude abo v e the highest measured peak values ( 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 10 11 M ), as well as strongly lensed, the latter having a typical probability of the order of 1 in 10 3 –10 4 . We conclude that the claimed detection is spurious.