2023MNRAS.519.4074C

We report the first 5σ detection of H I 21 cm emission from a star-forming galaxy at redshift z ∼ 1.3 (nearly 9 billion years ago) using upgraded Giant Metrewave Radio Telescope (uGMRT). This is the highest redshift H I detection in emission from an individual galaxy to date. The emission is strongly boosted by the gravitational lens, an early-type elliptical galaxy, at redshift z ∼ 0.13. The measured H I mass of the galaxy is $M_{\rm H\, \small {\rm I}} = (0.90 \pm 0.14 \pm 0.05) \times 10^{10}\, \mathrm{M}_{\odot }$, which is almost twice the inferred stellar mass of the galaxy, indicating an extended structure of the H I gas inside the galaxy. By fitting 2D Gaussian to the H I signal at the peak of the spectral line, we find the source to be marginally resolved with the position angle consistent with the emission being tangential to the critical curve of the lens mass distribution. This indicates that the solid angle of the approaching H I line flux comes very close to the inner lens caustic and results in very high magnification. These results, for the first time, demonstrate the feasibility of observing high-redshift H I in a lensed system with the modest amount of telescope time and open up exciting new possibilities for probing the cosmic evolution of neutral gas with existing and upcoming low-frequency radio telescopes in the near future.