Spatially Extended 21 cm Signal from Strongly Clustered UV and X-Ray
  Sources in the Early Universe
  We present our prediction for the local 21 cm differential brightness
temperature ($\delta T_{b}$) from a set of strongly clustered sources of
Population III (Pop III) and II (Pop II) objects in the early Universe, by a
numerical simulation of their formation and radiative feedback. These objects
are located inside a highly biased environment, which is a rare, high-density
peak ("Rarepeak") extending to $\sim7$ comoving Mpc. We study the impact of
ultraviolet (UV) and X-ray photons on the intergalactic medium (IGM) and the
resulting $\delta T_{b}$, when Pop III stars are assumed to emit X-ray photons
by forming X-ray binaries very efficiently. We parameterize the rest-frame
spectral energy distribution (SED) of X-ray photons, which regulates X-ray
photon-trapping, IGM-heating, secondary Lyman-alpha pumping and the resulting
morphology of $\delta T_{b}$. A combination of emission ($\delta T_{b}>0$) and
absorption ($\delta T_{b}<0$) regions appears in varying amplitudes and angular
scales. The boost of the signal by the high-density environment
($\delta\sim0.64$) and on a relatively large scale combine to make Rarepeak a
discernible, spatially-extended ($\theta\sim10'$) object for 21 cm observation
at $13\lesssim z\lesssim17$, which is found to be detectable as a single object
by SKA with integration time of $\sim1000$ hours. Power spectrum analysis by
some of the SKA precursors (LOFAR, MWA, PAPER) of such rare peaks is found
difficult due to the rarity of these peaks, and the contribution only by these
rare peaks to the total power spectrum remains subdominant compared to that by
all astrophysical sources.
