The influence of primordial magnetic fields on the spherical collapse model in cosmology

Despite the ever growing observational evidence for the existence of the large scale magnetic fields, their origin and the evolution are not fully understood. If the magnetic fields are of primordial origin, they result in the generation of the secondary matter density perturbations and the previous studies show that such density perturbations enhance the number of dark matter halos. We extend the conventional spherical collapse model by including the Lorentz force which has not been implemented in the previous analysis to study the evolution of density perturbations produced by primordial magnetic fields. The critical over-density $\delta_{\rm c}$ characterizing the halo mass function turns out to be a bigger value, $\delta_{\rm c}\simeq 1.78$, than the conventional one $\delta_{\rm c}\simeq 1.69$ for the perturbations evolved only by the gravitational force. The difference in $\delta_{\rm c}$ between our model and the fully matter dominated cosmological model is small at a low redshift and, hence, only the high mass tail of the mass function is affected by the magnetic fields. At a high redshift, on the other hand, the difference in $\delta_{\rm c}$ becomes large enough to suppress the halo abundance over a wide range of mass scales. The halo abundance is reduced for instance by as large a factor as $\sim10^5$ at $z=9$.