Neutral hydrogen in galaxy clusters: impact of AGN feedback and implications for intensity mapping

By means of zoom-in hydrodynamic simulations, we quantify the amount of neutral hydrogen (H I) hosted by groups and clusters of galaxies. Our simulations, which are based on an improved formulation of smoothed particle hydrodynamics, include radiative cooling, star formation, metal enrichment and supernova feedback, and can be split into two different groups, depending on whether feedback from active galactic nuclei (AGN) is turned on or off. Simulations are analysed to account for HI self-shielding and the presence of molecular hydrogen. We find that the mass in neutral hydrogen of dark matter haloes monotonically increases with the halomass and can be well described by a power law of the form M-H I (M, z) proportional to M-3/4. Our results point out that AGN feedback reduces both the total halo mass and its HI mass, although it is more efficient in removing HI. We conclude that AGN feedback reduces the neutral hydrogen mass of a given halo by similar to 50 per cent, with a weak dependence on halo mass and redshift. The spatial distribution of neutral hydrogen within haloes is also affected by AGN feedback, whose effect is to decrease the fraction of HI that resides in the halo inner regions. By extrapolating our results to haloes not resolved in our simulations, we derive astrophysical implications from the measurements of Omega(H) (I)(z): haloes with circular velocities larger than similar to 25 km s(-1) are needed to host HI in order to reproduce observations. We find that only the model with AGN feedback is capable of reproducing the value of Omega(HI)b(HI) derived from available 21 cm intensity mapping observations.