Probing the HI distribution at small scales using 21-cm Intensity Mapping at large scales

Abstract: Neutral hydrogen (HI) 21-cm Intensity Mapping (IM) holds the potential to map the large-scale structures in the Universe over a wide redshift range $(z \lesssim 5.5)$, measure cosmological parameters, and shed light on the nature of dark energy. In addition, the signal is also sensitive to how the HI is distributed among the dark matter haloes, this being quantified through the HIHM relation, which relates the HI mass to the halo mass. In this work, we investigate whether measurements of the 21-cm power spectrum (PS) and bispectrum (BS) at large scales can be used to estimate the HIHM relation, which quantifies the HI distribution at small scales. As a proof of concept, we consider the simulated 21-cm IM signal at $z=1$. We find that the measured 21-cm PS and BS at large scales $(k \le k_{ul} = 0.32 \, {\rm Mpc}^{-1})$ are well modeled using perturbation theory, with only two free parameters namely $[\Omega_{\rm HI} b_1]$ and $\gamma = b_2/b_1$. Combining the measured 21-cm PS and BS with an independent measurement of $\Omega_{\rm HI} $, we show that it is possible to estimate the three parameters that quantify the HIHM relation. We expect observational estimates of the HIHM relation to shed light on galaxy formation and the evolution of the ISM. Our preliminary analysis ignores redshift space distortion and the system noise in IM observations, which we plan to address in future work.