COZMIC. II. Cosmological Zoom-in Simulations with Fractional non-CDM Initial Conditions

Abstract We present 24 cosmological dark matter (DM)-only zoom-in simulations of a Milky Way analog with initial conditions appropriate for scenarios where non-cold dark matter (NCDM) is a fraction of the total DM abundance (f-NCDM models) as the second installment of the COZMIC suite. We initialize our simulations using transfer functions, T f − NCDM ( k ) ≡ P f − NCDM ( k ) / P CDM ( k ) (where P(k) is the linear matter power spectrum), with an initial suppression similar to thermal-relic warm dark matter (WDM) followed by a constant-amplitude plateau. We simulate suppression wavenumbers [22.8, 32.1, 41.8, 52.0, 57.1, 95.3] Mpc−1, corresponding to thermal-relic WDM masses m WDM ∈ [3, 4, 5, 6, 6.5, 10] keV, and plateau amplitudes δ ∈ [0.2, 0.4, 0.6, 0.8]. We model the subhalo mass function in terms of the suppression wave number and δ. Integrating these models into a forward model of the MW satellite galaxy population yields new limits on f-NCDM scenarios, with suppression wavenumbers greater than 46 and 40 Mpc−1 for δ = 0.2 and 0.4, respectively, at 95% confidence. The current data do not constrain δ > 0.4. We map these limits to scenarios where a fraction f WDM of DM behaves as a thermal relic, which yields the following bounds on cosmologies with a mixture of WDM and CDM: m WDM > 3.6, 4.1, 4.6, 4.9, 5.4 keV for f WDM = 0.5, 0.6, 0.7, 0.8, 0.9, respectively, at 95% confidence. The current data do not constrain WDM fractions f WDM < 0.5. Our results affirm that low-mass halo abundances are sensitive to partial suppression in P(k), indicating the possibility of using galactic substructure to reconstruct P(k) on small scales.