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 mu m observations in the COSMOS-CANDELS region as part of the James Clerk Maxwell Telescope Large Program SCUBA-2 Ultra Deep Imaging EAO Survey. Our maps at 450 and 850 mu m cover an area of 450 arcmin(2). We achieved instrumental noise levels of sigma(450) = 0.59 mJy beam(-1) and sigma(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 sigma (3.5 sigma) threshold, we detected 360 (479) sources at 450 mu m and 237 (314) sources at 850 mu m. We derive the deepest blank-field number counts at 450 mu m, covering the flux-density range of 2-43 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 mu m at the R = 6 ' scale is consistent with Poisson noise, so there is no evidence of strong 2D clustering at this scale. Additionally, we derive the integrated surface brightness at 450 mu m down to 2.1 mJy to be 57.3(-6.2)(+1.0) Jy deg(-2), contributing to 41% +/- 4% of the 450 mu m extragalactic background light (EBL) measured by Cosmic Background Explorer and Planck. Our results suggest that the 450 mu m EBL may be fully resolved at 0.08(-0.08)(+0.09) mJy, which extremely deep lensing-cluster observations and next-generation submillimeter instruments with large aperture sizes may be able to achieve.