Large 21 cm signals from AGN-dominated reionization
  We present here predictions for the spatial distribution of 21 cm brightness
temperature fluctuations from high-dynamic-range simulations for AGN-dominated
reionization histories that have been tested against available Lyman-alpha and
CMB data. We model AGN by extrapolating the observed M-sigma relation to high
redshifts and assign them ionizing emissivities consistent with recent UV
luminosity function measurements. We assess the observability of the predicted
spatial 21 cm fluctuations by ongoing and upcoming experiments in the late
stages of reionization in the limit in which the hydrogen 21 cm spin
temperature is significantly larger than the CMB temperature. Our AGN-dominated
reionization histories increase the variance of the 21 cm emission by a factor
of up to ten compared to similar reionization histories dominated by faint
galaxies, to values close to 100 mK^2 at scales accessible to experiments (k <
1 h/cMpc). This is lower than the sensitivity claimed to have been already
reached by ongoing experiments by only a factor of about two or less. When
reionization is dominated by AGN, the 21 cm power spectrum is enhanced on all
scales due to the enhanced bias of the clustering of the more massive haloes
and the peak in the large scale 21 cm power is strongly enhanced and moved to
larger scales due to bigger characteristic bubble sizes. AGN dominated
reionization should be easily detectable by LOFAR (and later HERA and SKA1) at
their design sensitivity, assuming successful foreground subtraction and
instrument calibration. Conversely, these could become the first non-trivial
reionization scenarios to be ruled out by 21 cm experiments, thereby
constraining the contribution of AGN to reionization.
