The Nature of 500 Micron Risers II: Sub-mm Faint Dusty Star Forming Galaxies

Abstract

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 ± 73%, 60 ± 15% and 19 ± 4% respectively 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 > 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.

A galaxy group candidate at z ≈ 3.7 in the COSMOS field

“…We measured its integrated far-IR (FIR) to radio photometry by performing the superdeblending technique as described in Jin et al (2018) at the fixed position of HPC1001.a. The Herschel colors (S 250 µm < S 350 µm < S 500 µm ) indicate that it is a 500 µm riser which is even redder than the cluster CL J1001 at z = 2.5, supporting its z > 2 nature (Riechers et al 2017;Donevski et al 2018;Cairns et al 2022).…”

The SCUBA-2 Large eXtragalactic Survey: 850um map, catalogue and the bright-end number counts of the XMM-LSS field

“…Assuming sources are randomly distributed the probability of finding two 10 mJy sources separated by <15 arcsec is less than 0.1 per cent, so we are unlikely to overlook any bright sources by imposing this separation limit. However, we note that if bright sources are clustered (as seen at fainter fluxes e.g., Cairns et al 2022;Greenslade et al 2020) or are part of merging systems, imposing this separation limit may potentially exclude some sources. To mitigate this we can check for additional bright sources by visually inspecting the S2LXS XMM-LSS science map at the position of the sources in the survey catalogue, searching for elongated sources or clearly separated > 5𝜎 signal-to-noise ratio peaks within the SCUBA-2 beam (noting that confirmation of multiple galaxies within the SCUBA-2 beam requires follow up by high resolution interferometers).…”