Elucidating the impact of massive neutrinos on halo assembly bias

Massive neutrinos have non-negligible impact on the formation of large-scale structures. We investigate the impact of massive neutrinos on the halo assembly bias effect, measured by the relative halo bias b̂ as a function of the curvature of the initial density peak ŝ, neutrino excess ϵν, or halo concentration ĉ, using a large suite of ΣMν = 0.0 and 0.4 eV simulations with the same initial conditions. By tracing dark matter haloes back to their initial density peaks, we construct a catalogue of halo ‘twins’ that collapsed from the same peaks but evolved separately with and without massive neutrinos, thereby isolating any effect of neutrinos on halo formation. We detect a 2 per cent weakening of the halo assembly bias as measured by b̂(ε _ν) in the presence of massive neutrinos. As there exists a significant correlation between ŝ and ϵν (rcc = 0.319), the impact of neutrinos persists at a reduced level (0.1 per cent) in the halo assembly bias measured by b̂(ŝ). However, we do not detect any neutrino-induced impact on b̂(ĉ), consistent with earlier studies and the lack of correlation between ĉ and ϵν (rcc = 0.087). We also discover an analogous assembly bias effect for the ‘neutrino haloes’, whose concentrations are anticorrelated with the large-scale clustering of neutrinos.