Planck Far-infrared Detection of Hyper Suprime-Cam Protoclusters at z ∼ 4: Hidden AGN and Star Formation Activity
2019
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
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