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We use separate universe simulations with the IllustrisTNG galaxy formationmodel to predict the local PNG bias parameters $b_\phi$ and $b_{\phi\delta}$ ofatomic neutral hydrogen, ${\rm H_I}$. These parameters and their relation tothe linear density bias parameter $b_1$ play a key role in observationalconstraints of the local PNG parameter $f_{\rm NL}$ using the ${\rm H_I}$ powerspectrum and bispectrum. Our results show that the popular calculation based onthe universality of the halo mass function overpredicts the $b_\phi(b_1)$ and$b_{\phi\delta}(b_1)$ relations measured in the simulations. In particular, ourresults show that at $z \lesssim 1$ the ${\rm H_I}$ power spectrum is moresensitive to $f_{\rm NL}$ compared to previously thought ($b_\phi$ is morenegative), but is less sensitive at other epochs ($b_\phi$ is less positive).We discuss how this can be explained by the competition of physical effectssuch as that large-scale gravitational potentials with local PNG (i) acceleratethe conversion of hydrogen to heavy elements by star formation, (ii) enhancethe effects of baryonic feedback that eject the gas to regions more exposed toionizing radiation, and (iii) promote the formation of denser structures thatshield the ${\rm H_I}$ more efficiently. Our numerical results can be used torevise existing forecast studies on $f_{\rm NL}$ using 21cm line-intensitymapping data. Despite this first step towards predictions for the local PNGbias parameters of ${\rm H_I}$, we emphasize that more work is needed to assesstheir sensitivity on the assumed galaxy formation physics and ${\rm H_I}$modeling strategy.

The local PNG bias of neutral Hydrogen, ${\rm H_I}$