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Constraining isocurvature perturbations with the 21cm emission from minihaloes

We investigate the effects of isocurvature perturbations on the 21cm radiation from minihaloes (MHs) at high redshifts and examine constraints on the isocurvature amplitude and power spectrum using the next generation of radio telescopes such as the Square Kilometre Array. We find that there is a realistic prospect of observing the isocurvature imprints in the 21cm emission from MHs, but only if the isocurvature spectral index is close to $3$ (i.e., the spectrum is blue). When the isocurvature fraction increases beyond $\sim 10\%$ of the adiabatic component, we observe an unexpected decline in the 21cm fluctuations from small-mass MHs, which can be explained by the incorporation small MHs into larger haloes. We perform a detailed Fisher-matrix analysis, and conclude that the combination of future CMB and 21cm experiments (such as CMBPol and the Fast-Fourier-Transform Telescope) is ideal in constraining the isocurvature parameters, but will stop short of distinguishing between CDM and baryon types of isocurvature perturbations, unless the isocurvature fraction is large and the spectrum is blue.