We perform a stacking analysis of Planck, AKARI, Infrared Astronomical Satellite (IRAS), Widefield Infrared Survey Eplorer (W ISE), and Herschel images of the largest number of (candidate) protoclusters at z ∼ 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 µm wavelength range is successfully detected with > 5σ significance (at P lanck). 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 nuclei (AGN)/SFG composites. For the pure star-forming model, a total IR (from 8 to 1000 µm) luminosity of 19.3 +0.6 −4.2 × 10 13 L and a star formation rate (SFR) of 16.3 +1.0 −7.8 × 10 3 M yr −1 are found whereas for the AGN/SFG composite model, 5.1 +2.5 −2.5 × 10 13 L and 2.1 +6.3 −1.7 × 10 3 M 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 ∼ 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.

We present SCUBA-2 850-µm observations of 13 candidate starbursting protoclusters selected using Planck and Herschel data. The cumulative number counts of the 850-µm sources in 9/13 of these candidate protoclusters show significant overdensities compared to the field, with the probability <10 −2 assuming the sources are randomly distributed in the sky. Using the 250-, 350-, 500-and 850-µm flux densities, we estimate the photometric redshifts of individual SCUBA-2 sources by fitting spectral energy distribution (SED) templates with an MCMC method. The photometric redshift distribution, peaking at 2 < z < 3, is consistent with that of known z > 2 protoclusters and the peak of the cosmic star-formation rate density (SFRD). We find that the 850µm sources in our candidate protoclusters have infrared luminosities of L IR 10 12 L and star-formation rates of SFR=(500-1,500)M yr −1 . By comparing with results in the literature considering only Herschel photometry, we conclude that our 13 candidate protoclusters can be categorised into four groups: six of them being high-redshift starbursting protoclusters, one being a lower-redshift cluster/protocluster, three being protoclusters that contain lensed DSFG(s) or are rich in 850-µm sources, and three regions without significant Herschel or SCUBA-2 source overdensities. The total SFRs of the candidate protoclusters are found to be comparable or higher than those of known protoclusters, suggesting our sample contains some of the most extreme protocluster population. We infer that cross-matching Planck and Herschel data is a robust method for selecting candidate protoclusters with overdensities of 850-µm sources.

Dusty star-forming galaxies (DSFGs) detected at z > 4 provide important examples of the first generations of massive galaxies. However, few examples with spectroscopic confirmation are currently known, with Hershel struggling to detect significant numbers of z > 6 DSFGs. NGP6 D1 is a bright 850 µm source (12.3 ± 2.5 mJy) with no counterparts at shorter wavelengths (a SPIRE dropout). Interferometric observations confirm it is a single source, with no evidence for any optical or NIR emission, or nearby likely foreground lensing sources. No > 3σ detected lines are seen in both LMT RSR and IRAM 30m EMIR spectra of NGP6 D1 across 32 GHz of bandwidth despite reaching detection limits of ∼ 1mJ y/500km s −1 , so the redshift remains unknown. Template fitting suggests that NGP6 D1 is most likely between z = 5.8 and 8.3. SED analysis finds that NGP6 D1 is a ULIRG, with a dust mass ∼ 10 8 -10 9 M and a SFR of ∼ 500 M yr −1 . We place upper limits on the gas mass of NGP6 D1 of M H2 < (1.1 ± 3.5) × 10 11 M , consistent with a gas-to-dust ratio of ∼ 100 -1000. We discuss the nature of NGP6 D1 in the context of the broader submm population, and find that comparable SPIRE dropouts account for ∼ 20% of all SCUBA-2 detected sources, but with a similar flux density distribution to the general population.

We present SCUBA-2 and SMA follow-up observations of four candidate high redshift Dusty Star-Forming Galaxies, selected as sources with rising SEDs in the 250, 350 and 500 μm Herschel SPIRE bands. Previous SMA observations showed no counterparts to these sources, but in our deeper sub-mm observations we detect counterparts to all four 500 μm risers, with three resolving into multiple systems. For these three multiple systems, the SMA 345 GHz (≈870 μm) observations recover $123 \pm 73\%$, $60 \pm 15\%$ and $19 \pm 4\%$ of the integrated 850 μm flux density from SCUBA-2, indicating that there may be additional sources below our SMA detection limit making up a dense, protocluster core. The fourth 500 μm riser was observed at a lower frequency and so we cannot make a similar comparison. We estimate photometric redshifts based on FIR/sub-mm colours, finding that 3/4 likely lie at z ≥ 2. This fits with the interpretation that the 500 μm riser selection criterion selects both intrinsically red, individual galaxies at z &gt; 4, and multiple systems at more moderate redshifts, artificially reddened by the effects of blending. We use the SCUBA-2 850 μm maps to investigate the environments of these 500 μm risers. By constructing cumulative number counts and estimating photometric redshifts for surrounding SCUBA-2 detections, we find that one of our 500 μm risers could plausibly reside in a z ≥ 2 protocluster. We infer that bright 500 μm risers with faint 850 μm flux densities are typically multiple systems at z ≥ 2 that may reside in overdensities of bright sub-mm galaxies.