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 $zsim3.8$ 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 $>5sigma$ significance (at $Planck$). This is the first time that the average IR spectral energy distribution (SED) of a protocluster has been constrained at $zsim4$. 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 $19.3_{-4.2}^{+0.6}times10^{13}~L_{odot}$ and a star formation rate (SFR) of $16.3_{-7.8}^{+1.0}times10^3~M_{odot}$ yr$^{-1}$ are found whereas for the AGN/SFG composite model, $5.1_{-2.5}^{+2.5}times10^{13}~L_{odot}$ and $2.1^{+6.3}_{-1.7}times10^3~M_{odot}$ 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 $zsim4$. Meanwhile, no significant IR flux excess is observed around optically selected QSOs at similar redshifts, which confirms previous results. Our results suggest that the $zsim4$ protoclusters trace dense, intensely star-forming environments that may also host obscured AGNs missed by the selection in the optical.
تحميل البحث