First Identification of a CMB Lensing Signal Produced by 1.5 Million Galaxies at z ∼4 : Constraints on Matter Density Fluctuations at High Redshift

We report the first detection of the dark matter distribution around Lyman break galaxies (LBGs) at high redshift through the cosmic microwave background (CMB) lensing measurements with the public Planck PR3 κ map. The LBG sample consists of 1 473 106 objects with the median redshift of z ∼4 that are identified in a total area of 305 deg² observed by the Hyper Suprime-Cam Strategic Survey Program survey. After careful investigations of systematic uncertainties, such as contamination from foreground galaxies and cosmic infrared background, we obtain the significant detection of the CMB lensing signal at 5.1 σ that is dominated by 2-halo term signals of the LBGs. Fitting a simple model consisting of the Navarro-Frenk-White profile and the linear-bias model, we obtain the typical halo mass of M_h=2. 9_-2.5^+9.5×10¹¹ h^-1 M_⊙ . Combining the CMB lensing and galaxy-galaxy clustering signals on the large scales, we demonstrate the first cosmological analysis at z ∼4 that constrains (Ω_{m 0},σ₈) . We find that our constraint on σ₈ is roughly consistent with the Planck cosmology, while this σ₈ constraint is lower than the Planck cosmology over the 1 σ level. This study opens up a new window for constraining cosmological parameters at high redshift by the combination of CMB and high-z galaxies, as well as studying the interplay between galaxy evolution and large-scale structure at such high redshift, by upcoming CMB and optical and near-infrared imaging surveys.