Impact of astrophysical scatter on the epoch of reionization [H i]21 bispectrum

It is believed that the first star-forming galaxies are the main drivers of cosmic reionization. It is usually assumed that there is a one-to-one relationship between the star formation rate (SFR) inside a galaxy and the host halo mass in semi-analytical/numerical modeling of large-scale ionization maps during the epoch of reionization. However, more accurate simulations and observations suggest that the SFR and ionizing luminosity in galaxies may vary considerably even if the host halo mass is the same. This astrophysical scatter can introduce an additional non-Gaussianity in the [H i]21cm signal, which might not be captured adequately in the power spectrum. In this work, we have studied the impact of the scatter on the [H i]21cm bispectrum using semi-numerical simulations. We find that the scatter primarily affects small ionized regions, whereas the large ionized bubbles remain largely unaffected. Although, the fractional change in the [H i]21cm bispectra due to the scatter is found to be more than a factor of 10 at large scales (k1 ≲ 1 Mpc-1) for z=7.4, it is found to be statistically insignificant. However, at small scales (k1 ~ 2.55 Mpc-1), we have found the impact due to the scatter to be high in magnitude (|〈Δ B 〉/Bno-scatter| ~ 1) and statistically significant (|〈Δ B〉/σΔB| ≳ 5) at neutral fraction, x̅HI ~ 0.8 for z=7.4. The impact due to scatter is found to be even more prominent (|〈Δ B 〉/Bno-scatter| ≳ 10) at small scales for z=10 and x̅HI ~ 0.8, but with reduced statistical significance to some extent (|〈Δ B〉/σΔB| ~ 3), compared to z=7.4 at the same neutral fraction. We have also found that in the most optimistic scenario, SKA1-Low might be able to detect these signatures of astrophysical scatter, at ~ 3σ and ~ 5σ detection significance for x̅HI ~ 0.8 and 0.9 respectively, for the equilateral [H i]21cm bispectrum at z=7.4.