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 arcmin². We achieved instrumental noise levels of σ₄₅₀= 0.59 mJy beam⁻¹ and σ₈₅₀= 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.