Cosmic microwave background constraints on a physical model of reionization

We study constraints on allowed reionization histories by comparing predictions of a physical seminumerical model with secondary temperature and polarization anisotropies of the cosmic microwave background (CMB). Our model has four free parameters characterizing the evolution of ionizing efficiency ζ and the minimum mass M_min of haloes that can produce ionizing radiation. Comparing the model predictions with the presently available data of the optical depth τ and kinematic Sunyaev-Zeldovich signal, we find that we can already rule out a significant region of the parameter space. We limit the duration of reionization $\Delta z= 1.30^{+0.19}_{-0.60}$ (Δz < 2.9 at $99{{\ \rm per\ cent}}$ C.L.), one of the tightest constraints on the parameter. The constraints mildly favour M_min ≳ 10⁹ M_⊙ (at $68{{\ \rm per\ cent}}$ C.L.) at z ∼ 8, thus indicating the presence of reionization feedback. Our analysis provides an upper bound on the secondary B-mode amplitude $D_{l=200}^{BB} \lt 18$ nK² at $99{{\ \rm per\ cent}}$ C.L. We also study how the constraints can be further tightened with upcoming space- and ground-based CMB missions. Our study, which relies solely on CMB data, has implications not only for upcoming CMB surveys for detecting primordial gravitational waves but also redshifted 21 cm studies.