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Signatures of Light Massive Relics on nonlinear structure formation

Cosmologies with Light Massive Relics (LiMRs) as a subdominant component of the dark sector are well-motivated from a particle physics perspective, and can also have implications for the $σ_8$ tension between early and late time probes of matter clustering. The effects of LiMRs on the Cosmic Microwave Background (CMB) and structure formation on large (linear) scales have been investigated extensively. In this paper, we initiate a systematic study of the effects of LiMRs on smaller, nonlinear scales using cosmological $N$-body simulations; focusing on quantities relevant for photometric galaxy surveys. For most of our study, we use a particular model of nonthermal LiMRs but the methods developed easily generalize to a large class of models of LiMRs -- we explicitly demonstrate this by considering the Dodelson-Widrow form of the velocity distribution. We find that, in general, the effects of LiMR on small scales are distinct from those of a $Λ$CDM universe, even when the value of $σ_8$ is matched between the models. We show that weak lensing measurements around massive clusters, between $\sim 0.1 h^{-1}$Mpc and $\sim 10 h^{-1}$Mpc, should have sufficient signal-to-noise in future surveys to distinguish between $Λ$CDM and LiMR models that are tuned to fit both CMB data and large (linear) scale structure data at late times. Furthermore, we find that different LiMR cosmologies which are indistinguishable by conventional linear probes can be distinguished by these probes if their velocity distributions are sufficiently different. LiMR models can, therefore, be best tested and constrained by jointly analyzing data from CMB and late-time structure formation on both large \textit{and} small scales.