Distortions of the Cosmic Microwave Background through cooling lines during the epoch of Reionization

Abstract

By using N-body hydrodynamical cosmological simulations in which the chemistry of major metals and molecules is consistently solved for, we study the interaction of metallic fine-structure lines with the CMB. Our analysis shows that the collisional induced emissions in the OI 145 $\mu$m and CII 158 $\mu$m lines during reionization introduce a distortion of the CMB spectrum at low frequencies ($\nu < 300$ GHz) with amplitudes up to $\Delta I_{\nu}/B_{\nu}(T_{\rm CMB})\sim 10^{-8}$-$10^{-7}$, i.e., at the $\sim 0.1$ percent level of FIRAS upper limits. Shorter wavelength fine-structure transitions (OI 63 $\mu$m, FeII 26 $\mu$m, and SiII 35 $\mu$m) typically sample the reionization epoch at higher observing frequencies ($\nu > 400$ GHz). This corresponds to the Wien tail of the CMB spectrum and the distortion level induced by those lines may be as high as $\Delta I_{\nu}/B_{\nu}(T_{\rm CMB})\sim 10^{-4}$. The angular anisotropy produced by these lines should be more relevant at higher frequencies: while practically negligible at $\nu=145 $GHz, signatures from CII 158 $\mu$m and OI 145 $\mu$m should amount to 1%-5% of the anisotropy power measured at $l \sim 5000$ and $\nu=220 $GHz by the ACT and SPT collaborations (after assuming $\Delta \nu_{\rm obs}/\nu_{\rm obs}\simeq 0.005$ for the line observations). Our simulations show that anisotropy maps from different lines (e.g., OI 145 $\mu$m and CII 158 $\mu$m) at the same redshift show a very high degree ($>0.8$) of spatial correlation, allowing for the use of observations at different frequencies to unveil the same snapshot of the reionization epoch. Finally, our simulations demonstrate that line-emission anisotropies extracted in narrow frequency/redshift shells are practically uncorrelated in frequency space, thus enabling standard methods for removal of foregrounds that vary smoothly in frequency, just as in HI 21 cm studies.