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, $T_{\mathrm{f}-\mathrm{NCDM}}(k)\equiv \sqrt{P_{\mathrm{f}-\mathrm{NCDM}}(k)/P_{\mathrm{CDM}}(k)}$ (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 $[22.8,32.1,41.8,52.0,57.1,95.3]{\mathrm{Mpc}}^{-1}$, corresponding to thermal-relic WDM masses $m_{\mathrm{WDM}}\in [3,4,5,6,6.5,10]\mathrm{keV}$, and plateau amplitudes $\hat{I}´\in [0.2,0.4,0.6,0.8]$. We model the subhalo mass function in terms of the suppression wavenumber and $\hat{I}´$. 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 $40{\mathrm{Mpc}}^{-1}$ for $\hat{I}´=0.2$, $0.4$, respectively, at $95\%$ confidence. The current data do not constrain $\hat{I}´>0.4$. We map these limits to scenarios where a fraction $f_{\mathrm{WDM}}$ of DM behaves as a thermal relic, which yields the following bounds on cosmologies with a mixture of WDM and CDM: $m_{\mathrm{WDM}}>3.6,4.1,4.6,4.9,5.4\mathrm{keV}$ for $f_{\mathrm{WDM}}=0.5,0.6,0.7,0.8,0.9$, respectively, at $95\%$ confidence. The current data do not constrain WDM fractions $f_{\mathrm{WDM}}<0.5$. 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.

21 pages, 14 figures, 1 table; updated to published version. Data is available at https://doi.org/10.5281/zenodo.14663119