The BINGO project: VI. HI halo occupation distribution and mock building

Abstract

© ESO 2022.Context. BINGO (Baryon Acoustic Oscillations from Integrated Neutral Gas Observations) is a radio telescope designed to survey from 980 MHz to 1260 MHz, observe the neutral hydrogen (HI) 21 cm line, and detect the baryon acoustic oscillation signal with the intensity mapping technique. Here we present our method for generating mock maps of the 21 cm intensity mapping signal that cover the BINGO frequency range and related test results. Aims. We would like to employ N-body simulations to generate mock 21 cm intensity maps for BINGO and study the information contained in 21 cm intensity mapping observations about structure formation, HI distribution and HI mass-halo mass relation. Methods. We fit an HI mass-halo mass relation from the ELUCID semianalytical galaxy catalog and applied it to the Horizen Run 4 halo catalog to generate the 21 cm mock map, which is called HOD. We also applied the abundance-matching method and matched the Horizen Run 4 galaxy catalog with the HI mass function measured from ALFALFA, to generate the 21 cm mock map, which is called HAM. Results. We studied the angular power spectrum of the mock maps and the corresponding pixel histogram. The comparison of two different mock map generation methods (HOD and HAM) is presented. We provide the fitting formula of ΩHi, HI bias, and the lognormal fitting parameter of the maps, which can be used to generate similar maps. We discuss the possibility of measuring ΩHi and HI bias by comparing the angular power spectrum of the mock maps and the theoretical calculation. We also discuss the redshift space distortion effect, the nonlinear effect, and the bin size effect in the mock map. Conclusions. By comparing the angular power spectrum measured from two different types of mock maps and the theoretical calculation, we find that the theoretical calculation can only fit the mock result at large scales. At small scales, neither the linear calculation nor the halofit nonlinear calculation can provide an accurate fitting, which reflects our poor understanding of the nonlinear distribution of HI and its scale-dependent bias. We have found that the bias is highly sensitive to the method of populating HI in halos, which also means that we can place constraints on the HI distribution in halos by observing 21 cm intensity mapping. We also illustrate that only with thin frequency bins (such as 2 MHz), we can discriminate the Finger-of-God effect. All of our investigations using mocks provide useful guidance for our expectation of BINGO experiments and other 21 cm intensity mapping experiments.