21-cm power spectrum and ionization bias as a probe of long-mode modulated non Gaussian sky

Abstract: The observed hemispherical power asymmetry in cosmic microwave background radiation can be explained by long wavelength mode (long-mode) modulation. In this work we study the prospect of the detection of this effect in the angular power spectrum of 21-cm brightness temperature. For this task, we study the effect of the neutral Hydrogen distribution on the angular power spectrum. This is done by formulating the bias parameter of ionized fraction to the underlying matter distribution. We also discuss the possibility that the long mode modulation is companied with a primordial non-Gaussianity of local type. In this case, we obtain the angular power spectrum with two effects of primordial non-Gaussianity and long mode modulation. Finally, we show that the primordial non-Gaussianity enhances the long mode modulated power of 21-cm signal via the non-Gaussian scale-dependent bias up to four orders of magnitude. {Accordingly, the observation of 21-cm signal with upcoming surveys such as the Square Kilometer Array (SKA) is probably capable of detecting hemispherical power asymmetry in the context of the long mode modulation.