The presence of additional relativistic particles at the time of recombination can be inferred through their contribution to $\mathrm{\Delta }{N}_{\mathrm{eff}}$. If these species have a finite but low mass (light massive relics—LiMRs), they act as a hot subcomponent of dark matter and impact late-time structure formation. Understanding these effects will be crucial to pin down the underlying particle physics properties of any future $\mathrm{\Delta }{N}_{\mathrm{eff}}$ detection. While their impact has been well studied on linear scales, this work develops the framework for and presents results from the first set of cosmological N-body simulations that can track the effects of LiMRs, as a function of their mass and temperature, down to fully nonlinear scales. Importantly, our simulations model the impact of both the massive Standard Model neutrinos and LiMRs, which will be crucial in disentangling possible degeneracies. We systematically explore the effects of LiMR properties such as mass, temperature, and initial distribution, on various cosmological observables, including the total matter power spectrum, halo mass functions, mass-concentration relation, radial halo profiles, and weak lensing signals around massive clusters. The presence of LiMRs leads to a suppression of $\sim 10\%$ and $\sim 6\%$ in the total matter power spectrum for fermionic and bosonic species, respectively. This suppression is measured relative to a $\mathrm{CDM}+\mathrm{SM}$ neutrino cosmology at $k\sim 0.1h{\mathrm{Mpc}}^{-1}$ for LiMRs fraction ($f_l$) $\sim 0.02$. The framework and simulations developed here will enable detailed followup of the rich phenomenology of LiMR cosmologies.

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• Dark matter
• Evolution of the Universe
• Large scale structure of the Universe
• Particle dark matter
• Astrophysical & cosmological simulations
• Numerical simulations in gravitation & astrophysics