Planck far-infrared detection of Hyper Suprime-Cam protoclusters at $\mathbf{z}\sim 4$: hidden AGN and star formation activity

Abstract

We perform a stacking analysis of {\it Planck}, {\it AKARI}, Infrared Astronomical Satellite ($IRAS$), Wide-field Infrared Survey Eplorer ($WISE$), and {\it Herschel} images of the largest number of (candidate) protoclusters at $z\sim 3.8$ selected from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Stacking the images of the $179$ candidate protoclusters, the combined infrared (IR) emission of the protocluster galaxies in the observed $12-850\mu$m wavelength range is successfully detected with $>5\sigma$ significance (at $Planck$). This is the first time that the average IR spectral energy distribution (SED) of a protocluster has been constrained at $z\sim 4$. 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 nuclei (AGN)/SFG composites. For the pure star-forming model, a total IR (from 8 to 1000 $\mu$m) luminosity of $19.3_{-4.2}^{+0.6}\times 10^{13}{L}_{\odot }$ and a star formation rate (SFR) of $16.3_{-7.8}^{+1.0}\times 10^3{M}_{\odot }$ yr${}^{-1}$ are found whereas for the AGN/SFG composite model, $5.1_{-2.5}^{+2.5}\times 10^{13}{L}_{\odot }$ and $2.1_{-1.7}^{+6.3}\times 10^3{M}_{\odot }$ yr${}^{-1}$ are found. Uncertainty remaining in the total SFRs; however, the IR luminosities of the most massive protoclusters are likely to continue increasing up to $z\sim 4$. Meanwhile, no significant IR flux excess is observed around optically selected QSOs at similar redshifts, which confirms previous results. Our results suggest that the $z\sim 4$ protoclusters trace dense, intensely star-forming environments that may also host obscured AGNs missed by the selection in the optical.

26 pages, 12 figures, Accepted for publication in ApJ

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