SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES). V. Confusion-limited Submillimeter Galaxy Number Counts at 450 μm and Data Release for the COSMOS Field

We present confusion-limited SCUBA-2 450-μm observations in the COSMOS-CANDELS region as part of the JCMT Large Program, SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES). Our maps at 450 and 850 μm cover an area of 450 arcmin^2. We achieved instrumental noise levels of σ_450= 0.59 mJy beam^-1 and σ_850= 0.09 mJy beam^-1 in the deepest area of each map. The corresponding confusion noise levels are estimated to be 0.65 and 0.36 mJy beam^-1. Above the 4 (3.5) σ threshold, we detected 360 (479) sources at 450 μm and 237 (314) sources at 850 μm. We derive the deepest blank-field number counts at 450 μm, covering the flux-density range of 2 to 43 mJy. These are in agreement with other SCUBA-2 blank-field and lensing-cluster observations, but are lower than various model counts. We compare the counts with those in other fields and find that the field-to-field variance observed at 450 μm at the R=6^' scale is consistent with Poisson noise, so there is no evidence of strong 2-D clustering at this scale. Additionally, we derive the integrated surface brightness at 450 μm down to 2.1 mJy to be 57.3^+1.0_-6.2 Jy deg^-2, contributing to (41±4)% of the 450-μm extragalactic background light (EBL) measured by COBE and Planck. Our results suggest that the 450-μm EBL may be fully resolved at 0.08^+0.09_-0.08 mJy, which extremely deep lensing-cluster observations and next-generation submillimeter instruments with large aperture sizes may be able to achieve.