# Co-Interacting Dark Matter

## TL;DR

This paper introduces a two-component dark matter model where a subdominant component influences the dominant one, creating core density profiles in galaxies through a novel interaction mechanism involving ultralight dark photons and Dirac fermions.

## Contribution

It proposes a new mechanism for dark matter interactions that differ from self-interacting dark matter, avoiding certain astrophysical constraints and explaining core profiles in galaxies.

## Key findings

- Subdominant dark matter thermalizes the dominant component in galaxies.
- Ultralight dark photons with mass > 10^{-21} eV can produce core profiles without conflicting with Lyman-alpha constraints.
- The mechanism evades Bullet Cluster bounds due to reduced Bose enhancement.

## Abstract

We propose a novel mechanism with two component dark matter models. The subdominant dark matter can thermalize the dominant one in galaxies, and leads to core density profiles. Using ultralight dark photons and $\mathrm{GeV}$-$\mathrm{TeV}$ Dirac fermions as an example, we couple the two dark matter candidates with a $U(1)$ interaction. This mechanism differs from self-interacting dark matter, due to three effects: (1) higher occupation numbers, (2) forward-backward scattering cancellation, and (3) the multiple scatterings required for the heavy dark matter. Furthermore, the Bullet Cluster bound is evaded due to the reduced Bose enhancement factor. Unlike the fuzzy dark matter solution to the small structure problems which have tension with Lyman-$\alpha$, the ultralight dark photons with mass $\gtrsim 10^{-21}$ eV can have a core profile through interactions with $\psi$ and are not constrained by other astrophysical observations.