Planck far-infrared detection of hyper suprime-cam protoclusters at z ∼ 4 : hidden AGN and star formation activity.

We perform a stacking analysis of Planck, AKARI, Infrared Astronomical Satellite, Wide-field Infrared Survey Explorer, and Herschel images of the largest number of (candidate) protoclusters at z ∼ 3.8 selected from the Hyper Suprime-Cam Subaru Strategic Program. Stacking the images of the 179 candidate protoclusters, the combined infrared (IR) emission of the protocluster galaxies in the observed 12–850 μm wavelength range is successfully detected with >5σ significance (at Planck). This is the first time that the average IR spectral energy distribution (SED) of a protocluster has been constrained at z ∼ 4. The observed IR SEDs of the protoclusters exhibit significant excess emission in the mid-IR compared to that expected from typical star-forming galaxies (SFGs). They are reproduced well using SED models of intense starburst galaxies with warm/hot dust heated by young stars, or by a population of active galactic nucleus (AGN)/SFG composites. For the pure star-forming model, a total IR (from 8–1000 μm) luminosity of - ´  + 19.3 10 4.2 L 0.6 13 and a star formation rate of - ´  16.3 10 + 7.8 1.0 3 M yr−1 are found, whereas for the AGN/SFG composite model, - ´  + 5.1 10 2.5 L 2.5 13 and - ´  2.1 10 + 1.7 6.3 3 M yr−1 are found. Uncertainty remains in the total SFRs; however, the IR luminosities of the most massive protoclusters are likely to continue increasing up to z ∼ 4. Meanwhile, no significant IR flux excess is observed around optically selected QSOs at similar redshifts, which confirms previous results. Our results suggest that the z ∼ 4 protoclusters trace dense, intensely star-forming environments that may also host obscured AGNs missed by the selection in the optical