Simulating the impact of x ray heating during the cosmic dawn
Upcoming observations of the 21 cm signal from the Epoch of Reionization will soon provide us with the first direct detection of this era. This signal is influenced by many as trophysical effects, including long range X ray heating of the intergalactic gas. During the preceding Cosmic Dawn era the impact of this heating on the 21 cm signal is particularly prominent, especially before spin temperature saturation. We present the largest volume (244 h 1 Mpc=349Mpc comoving) full numerical radiative transfer simulations to date of this epoch, including the effects of helium and multi frequency heating, both with and without X ray sources. We show that X ray sources can contribute significantly to early heating of the neutral intergalactic medium and, hence, to the corresponding 21 cm signal. The inclusion of hard, energetic radiation yields an earlier, extended transition from absorption to emission compared to the stellar only case. The presence of X ray sources decreases the absolute value of the mean 21 cm differential brightness temperature. These hard sources also significantly increase the 21 cm fluctuations compared the common assumption of temperature saturation. The power spectrum is initially boosted on large scales before decreasing on all scales. Compared to the case of the cold, unheated intergalactic medium, the signal has lower rms fluctuations and increased non Gaussianity, as measured by the skewness and kurtosis of the 21 cm probability distribution functions. Images of the 21 cm signal with resolutions around 11 arcmin still show fluctuations well above the expected noise for deep integrations with the SKA1 Low, indicating that direct imaging of the X ray heating epoch could be feasible.