Search

We present a detection of the unnormalized skewness induced by the thermal Sunyaev-Zel'dovich (tSZ) effect in filtered Atacama Cosmology Telescope (ACT) 148 GHz cosmic microwave background temperature maps. Contamination due to infrared and radio sources is minimized by template subtraction of resolved sources and by constructing a mask using outlying values in the 218 GHz (tSZ-null) ACT maps. We measure = -31 +- 6 \mu K^3 (measurement error only) or +- 14 \mu K^3 (including cosmic variance error) in the filtered ACT data, a 5-sigma detection. We show that the skewness is a sensitive probe of sigma_8, and use analytic calculations and tSZ simulations to obtain cosmological constraints from this measurement. From this signal alone we infer a value of sigma_8= 0.79 +0.03 -0.03 (68 % C.L.) +0.06 -0.06 (95 % C.L.). Our results demonstrate that measurements of non-Gaussianity can be a useful method for characterizing the tSZ effect and extracting the underlying cosmological information., Comment: 9 pages, 5 figures. Replaced with version accepted by Phys. Rev. D, with improvements to the likelihood function and the IR source treatment; only minor changes in the results

Motivated by recent interest in redshifted 21 cm emission of intergalactic hydrogen, we investigate the 8.7 GHz $^{2}\mathrm{S}_{1/2}$ $F=0\ensuremath{-}1$ hyperfine transition of $^{3}\mathrm{He}^{+}$. While the primordial abundance of $^{3}\mathrm{He}$ relative to hydrogen is ${10}^{\ensuremath{-}5}$, the hyperfine spontaneous decay rate is 680 times larger. Furthermore, the antenna temperature is much lower at the frequencies relevant for the $^{3}\mathrm{He}^{+}$ transition compared to that of $zg6$ 21 cm emission. We find that the spin temperature of this 8.7 GHz line in the intergalactic medium is approximately the cosmic microwave background temperature, such that this transition is best observed in absorption against high-redshift, radio-bright quasars. We show that intergalactic 8.7 GHz absorption is a promising, unsaturated observable of the ionization history of intergalactic helium (for which $\mathrm{He}\text{ }\mathrm{II}\ensuremath{\rightarrow}\mathrm{He}\text{ }\mathrm{III}$ reionization is believed to complete at $z\ensuremath{\sim}3$) and of the primordial $^{3}\mathrm{He}$ abundance. Instruments must reach $\ensuremath{\sim}1\text{ }\text{ }\ensuremath{\mu}\mathrm{Jy}$ RMS noise in bands of 1 MHz on a 1 Jy source to directly resolve this absorption. However, in combination with H i $\mathrm{Ly}\ensuremath{\alpha}$ forest measurements, an instrument can statistically detect this absorption from $zg3$ with $30\text{ }\text{ }\ensuremath{\mu}\mathrm{Jy}$ RMS noise in 0.1 MHz spectral bands over 100 MHz, which may be within the reach of present instruments.