21 cm signal from cosmic dawn: Imprints of spin temperature fluctuations
  and peculiar velocities
  The 21 cm brightness temperature $\delta T_{\rm b}$ fluctuations from
reionization promise to provide information on the physical processes during
that epoch. We present a formalism for generating the $\delta T_{\rm b}$
distribution using dark matter simulations and an one-dimensional radiative
transfer code. Our analysis is able to account for the spin temperature $T_{\rm
S}$ fluctuations arising from inhomogeneous X-ray heating and Ly$\alpha$
coupling during cosmic dawn. The $\delta T_{\rm b}$ power spectrum amplitude at
large scales ($k \sim 0.1$ Mpc$^{-1}$) is maximum when $\sim 10\%$ of the gas
(by volume) is heated above the CMB temperature. The power spectrum shows a
"bump"-like feature during cosmic dawn and its location measures the typical
sizes of heated regions. We find that the effect of peculiar velocities on the
power spectrum is negligible at large scales for most part of the reionization
history. During early stages (when the volume averaged ionization fraction
$\lesssim 0.2$) this is because the signal is dominated by fluctuations in
$T_{\rm S}$. For reionization models that are solely driven by stars within
high mass ($\gtrsim 10^9\, \rm M_{\odot}$) haloes, the peculiar velocity
effects are prominent only at smaller scales ($k \gtrsim 0.4$ Mpc$^{-1}$) where
patchiness in the neutral hydrogen density dominates the signal. The
conclusions are unaffected by changes in the amplitude or steepness in the
X-ray spectra of the sources.
