The correlation of high-redshift galaxies with the thermal Sunyaev-Zel’dovich effect traces reionization

We explore a potential new probe of reionization: the cross-correlation of
high-redshift galaxies with maps of the thermal Sunyaev-Zel’dovich (tSZ)
effect. We consider two types of high redshift galaxies: Lyman break galaxies
(LBGs) and Lyman-$alpha$ emitters (LAEs). LBGs and LAEs will be detected in
large numbers at high redshift ($z approx$ 4 to 7) by ongoing and future
surveys. We consider a LBG sample like that expected from the The Rubin
Observatory Legacy Survey of Space and Time (LSST), and a selection of LAEs
modelled after the Subaru SILVERRUSH program, but covering a much larger sky
fraction. The tSZ effect is sensitive to a line-of-sight integral of the
ionized gas pressure, and can be measured across large patches of the sky using
multi-frequency CMB surveys. We consider forecast tSZ maps from CMB Stage 4 and
more futuristic observations. Using a suite of hydrodynamical simulations, we
show that the high-redshift galaxies are correlated with the tSZ signal from
reionization, with cross-power amplitude of order $10^{-15}$ at $ell sim
1000$. The cross-spectra between LBGs/LAEs with tSZ maps contain information
about the reionization history of the Universe, such as the distribution of
bubble sizes, and could in principle be used to directly measure the timing of
reionization. The amplitude of the signal is small, however, and its
detectability is hindered by low-redshift contributions to tSZ maps and by
instrumental noise. If the the low-redshift contribution to the observed tSZ
signal is suppressed by masking of massive halos, a combination of overlapping
futuristic CMB and galaxy surveys could potentially probe this signal.
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