Decaying particles and the reionization history of the Universe

Abstract

We investigate the possibility that the Universe is reionized by the decay products of heavy particles. In particular we study under which circumstances this decay may produce a significant reionization at high redshift ($z \simeq 20$), as requested by the WMAP result.Extra sources of reionization at high redshifts increase the recombination rate. As a result, the number density of decaying particles required to produce a high level of ionization at $z \simeq 10-20$ is significantly higher ($\simeq 300$ times) than previously estimated. Moreover, the ionization produced by decay particles implies a high optical depth even if the maximum level of ionization ever produced is of the order of $10^{-2}$. In order to match the cosmic microwave background polarization power spectrum, other significant sources of reionization are needed at low redshift. Decaying particles producing a ionization fraction $x \simeq 0.5$ at $z \simeq 20$ would imply an optical depth much too high to fit the cosmic microwave background spectra at $l \ge 30$. For decay times $t_{dec} < 2 \times 10^{16} {\rm s}$, recent cosmic microwave background and large scale structure data limits the primordial abundance of the decaying particles to $\Omega_x \le 21 \times 10^{-9} (1+z)^3$ at 95 % C.L., and favors long decay times. The data do not favor this reionization model versus the instantaneous reionization one. We also find that the decay of these heavy particles significantly heats up the medium, bringing the expected $y$ distortion to unobservable levels.