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_-4.2 ^+0.6 × 10¹³ L_⊙ and a star formation rate of 16.3_-7.8 ^+1.0 × 10³ M_⊙ yr^-1 are found, whereas for the AGN/SFG composite model, 5.1_-2.5 ^+2.5 × 10¹³ L_⊙ and 2.1_-1.7 ^+6.3 × 10³ 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.