Impact of dark matter decays and annihilations on reionization

TL;DR

This paper investigates how decays and annihilations of dark matter affect reionization, gas temperature, and CMB signatures. The authors compute energy injection rates for four DM candidates, modify RECFAST and CMBFAST to track ionization history and CMB spectra, and compare to observational constraints. They find that light DM (1–10 MeV) and sterile neutrinos (2–8 keV) can drive early partial reionization with small Thomson optical depths, while gravitinos and neutralinos have negligible impact; CMB spectra remain essentially unchanged. The work shows that reionization history, especially when complemented by future 21 cm data, could help discriminate light DM scenarios from heavier DM.

Abstract

One of the possible methods to distinguish among various dark matter candidates is to study the effects of dark matter decays. We consider four different dark matter candidates (light dark matter, gravitinos, neutralinos and sterile neutrinos), for each of them deriving the decaying/annihilation rate, the influence on reionization, matter temperature and CMB spectra. We find that light dark matter particles (1-10 MeV) and sterile neutrinos (2-8 keV) can be sources of partial early reionization (z<~100). However, their integrated contribution to Thomson optical depth is small (<~0.01) with respect to the three year WMAP results (tau_e=0.09+/-0.03). Finally, they can significantly affect the behavior of matter temperature. On the contrary, effects of heavy dark matter candidates (gravitinos and neutralinos) on reionization and heating are minimal. All the considered dark matter particles have completely negligible effects on the CMB spectra.

Figures (5)

• Figure 1: Ionized fraction (bottom panel), Thomson optical depth (central panel) and matter temperature (upper panel) as a function of redshift due to decaying LDM of masses 1 (thick dotted line), 5 (dashed) and 10 MeV (solid). The thin solid line represents, from bottom to top, the relic fraction of free electrons, their contribution to Thomson optical depth and the matter temperature without particle decays.
• Figure 2: Ionized fraction (bottom panel), Thomson optical depth (central panel) and matter temperature (upper panel) as a function of redshift due to decaying gravitinos of masses 10 (dashed) and 100 MeV (solid). The thin solid line is the same as in Fig. 1.
• Figure 3: Ionized fraction (bottom panel), Thomson optical depth (central panel) and matter temperature (upper panel) as a function of redshift due to neutralinos for $\langle{}\sigma{}\,{}v\rangle{}=2\times{}10^{-26}$ (thick dashed line) and $10^{-24}$ cm$^3$ s$^{-1}$ (solid). In both the cases the neutralino mass is 100 GeV. The thin solid line is the same as in Fig. 1.
• Figure 4: Ionized fraction (bottom panel), Thomson optical depth (central panel) and matter temperature (upper panel) as a function of redshift due to radiatively decaying sterile neutrinos of masses 2 (thick dotted line), 4 (dashed) and 8 keV (solid). The thin solid line is the same as in Fig. 1.
• Figure 5: Temperature-temperature (top panel), polarization-polarization (central panel) and temperature-polarization (bottom panel) spectra. Thick lines indicate the CMB spectrum derived assuming Thomson optical depth $\tau{}_e=0.09$ and a sudden reionization model (consistent with the three year WMAP data); thin lines indicate the CMB spectrum derived assuming $\tau{}_e=0$. Dashed (solid) lines indicate the CMB spectrum obtained (without) taking into account the decays of 10 MeV LDM particles. The two thick lines, solid and dashed, appear superimposed, because the contribution of decaying particles (the dashed line) is completely hidden by the stronger effect of a sudden reionization with $\tau_e=0.09$. Open circles in all the panels indicate the three year WMAP data (Hinshaw et al. 2006; Page et al. 2006; Spergel et al. 2006).