Inhomogeneous reionization acts as a source of arcminute-scale anisotropies in the cosmic microwave background ( CMB ) , the most important of which is the kinetic Sunyaev-Zel ’ dovich ( kSZ ) effect .
Observational efforts with the Atacama Cosmology Telescope ( ACT ) and the South Pole Telescope ( SPT ) are poised to detect this signal for the first time , with projected 1 \mu { K ^ { 2 } } -level sensitivity to the dimensionless kSZ power spectrum around a multipole of l = 3000 , [ \Delta _ { l 3000 } ] ^ { 2 } .
Indeed , recent SPT measurements place a bound of [ \Delta _ { l 3000 } ] ^ { 2 } < 2.8 ~ { } \mu { K ^ { 2 } } at 95 % C.L. , which degrades to [ \Delta _ { l 3000 } ] ^ { 2 } < 6 ~ { } \mu { K ^ { 2 } } if a significant correlation between the thermal Sunyaev-Zel ’ dovich ( tSZ ) effect and the cosmic infrared background ( CIB ) is allowed .
To interpret these and upcoming observations , we compute the kSZ signal from a suite of \approx 100 reionization models using the publicly available code 21CMFAST .
Our physically motivated reionization models are parameterized by the ionizing efficiency of high-redshift galaxies , the minimum virial temperature of halos capable of hosting stars , and the ionizing photon mean free path – a parameterization motivated by previous theoretical studies of reionization .
We predict the contribution of patchy reionization to the l = 3000 kSZ power to be [ \Delta ^ { patchy } _ { l 3000 } ] ^ { 2 } = 1.5–3.5 \mu { K ^ { 2 } } .
Therefore , even when adopting the lowest estimate in the literature for the post-reionization signal of [ \Delta ^ { OV } _ { l 3000 } ] ^ { 2 } \approx 2 ~ { } \mu { K ^ { 2 } } , none of our models are consistent with the aggressive 2 \sigma SPT bound that does not include correlations .
This implies that either : ( i ) the early stages of reionization occurred in a much more homogeneous manner than suggested by the stellar-driven scenarios we explore , such as would be the case if , e.g. , very high energy X-rays or exotic particles contributed significantly ; and/or ( ii ) that there is a significant correlation between the CIB and the tSZ .
The later is perhaps not surprising , as massive halos should host both hot gas and star forming galaxies .
On the other hand , the conservative SPT bound of [ \Delta _ { l 3000 } ] ^ { 2 } \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1 % .0 pt \hbox { $ < $ } } 6 ~ { } \mu { K ^ { 2 } } is compatible with all of our models and is on the threshold of constraining physically motivated reionization models .
The largest patchy kSZ signals correspond to an extended reionization process , in which the sources of ionizing photons are abundant and there are many recombinations ( absorptions in sinks ) .
We point out that insights into the astrophysics of the early Universe are encoded in both the amplitude and shape of the kSZ power spectrum .