Boosting the 21 cm forest signals by the clumpy substructures

TL;DR

This paper investigates how small-scale substructures within halos can significantly enhance the 21 cm forest signals, especially for larger host halos, thereby improving prospects for detection.

Contribution

It quantifies the boost in 21 cm optical depth due to subhalos and highlights their importance in signal detection, which was previously underestimated.

Findings

Subhalo contribution can increase optical depth by an order of magnitude for larger halos.

The 21 cm absorption line abundance can be enhanced by up to 10% due to substructures.

Larger host halos benefit more from subhalo effects, aiding in 21 cm forest detection.

Abstract

We study the contribution of subhalos to the 21 cm forest signal. The halos can host the substructures and including the effects of those small scale clumps can potentially boost the 21 cm optical depth in favor of detecting the 21 cm forest signals. We estimate the boost factor representing the ratio of the optical depth due to the subhalo contribution and that due to the host halo alone (without subhalos). Even though the optical depth boost factor is negligible for a small host halo with the mass of order $10^5 M_{\odot}$, the subhalo contribution can enhance the optical depth by an order of magnitude for a host halo of order $10^7 M_{\odot}$. The resultant 21 cm absorption line abundance which is obtained by integrating over the halo mass range relevant for the 21 cm forest signal can be enhanced by up to of order $10\%$ due to the substructures. The larger boost factor for a larger host halo would be of particular interest for the 21 cm forest detection because the the contribution of the larger host halos to the 21 cm forest signals is smaller due to their higher temperature and less abundance than the smaller host halos. The subhalos hence can well help the larger host halos more important for the signal estimation which, without considering the subhalos, may not give appreciable contribution to 21 cm forest signals.