Constraint on the early-formed dark matter halos using the free-free emission in the Planck foreground analysis

TL;DR

This paper constrains small-scale dark matter density fluctuations by analyzing free-free emission from early dark matter halos, providing the most stringent limits on perturbations below 1 Mpc scales.

Contribution

It introduces a novel method using free-free emission in the microwave range to constrain small-scale density fluctuations in the early universe.

Findings

Constraints on small-scale density fluctuations: $P_\zeta \lesssim 10^{-7}$ for $k \simeq 1-100~\mathrm{Mpc}^{-1}$.

Most stringent limits on perturbations below 1 Mpc.

Method based on diffuse free-free emission from early-formed dark matter halos.

Abstract

We provide a new constraint on the small-scale density fluctuations, evaluating the diffuse background free-free emission from dark matter halos in the dark ages. If there exists a large amplitude of the matter density fluctuations on small scales, the excess enhances the early formation of dark matter halos. When the virial temperature is sufficiently high, the gas in a halo is heated up and ionized by thermal collision. The heated ionized gas emits photons by the free-free process. We would observe the sum of these photons as the diffuse background free-free emission. Assuming the analytical dark matter halo model including the gas density and temperature profile, we calculate the intensity of the diffuse background free-free emission from early-formed dark matter halos in the microwave frequency range. Comparing with the recent foreground analysis on cosmic microwave background, we obtain the constraint on the excess of the density fluctuations on small scales. Our constraint corresponds to $P_\zeta \lesssim 10^{-7}$ for $k \simeq 1-100~\mathrm{Mpc}^{-1}$ with assuming the delta-function-type curvature power spectrum. Therefore, our constraint is the most stringent constraint on the perturbations below $1~\rm Mpc$ scales.