Can We Detect the Anisotropic Shapes of Quasar HII Regions During Reionization Through The Small-Scale Redshifted 21cm Power Spectrum?

Light travel time delays distort the apparent shapes of HII regions surrounding bright quasars during early stages of cosmic reionization. Individual HII regions may remain undetectable in forthcoming redshifted 21 cm experiments. However, the systematic deformation along the line of sight may be detectable statistically, either by stacking tomographic 21cm images of quasars identified, for example, by JWST, or as small-scale anisotropy in the three-dimensional 21cm power spectrum. Here we consider the detectability of this effect. The anisotropy is largest when HII regions are large and expand rapidly, and we find that if bright quasars contributed to the early stages of reionization, then they can produce significant anisotropy, on scales comparable to the typical sizes of HII regions of the bright quasars (approx. 30 Mpc and below). The effect therefore cannot be ignored when analyzing future 21cm power spectra on small scales. If 10 percent of the volume of the IGM at redshift z=10 is ionized by quasars with typical ionizing luminosity of S= 5 x 10^{56} photons/second, the distortions can enhance by more than 10 percent the 21cm power spectrum in the radial (redshift) direction, relative to the transverse directions. The level of this anisotropy exceeds that due to redshift-space distortion, and has the opposite sign. We show that on-going experiments such as MWA should be able to detect this effect. A detection would reveal the presence of bright quasars, and shed light on the ionizing yield and age of the ionizing sources, and the distribution and small-scale clumping of neutral intergalactic gas in their vicinity.