SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES): Faint-End Counts at 450
  um
  The SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES) is a three-year JCMT
Large Program aiming at reaching the 450 $\mu$m confusion limit in the
COSMOS-CANDELS region, to study a representative sample of the high-redshift
far-infrared galaxy population that gives rise to the bulk of the far-infrared
background. We present the first-year data from STUDIES. We have reached a 450
$\mu$m noise level of 0.91~mJy for point sources at the map center, covered an
area of 151 arcmin$^2$, and detected 98 and 141 sources at 4.0 and 3.5
$\sigma$, respectively. Our derived counts are best constrained in the 3.5-25
mJy regime using directly detected sources. Below the detection limits, our
fluctuation analysis further constrains the slope of the counts down to 1 mJy.
The resulting counts at 1-25 mJy are consistent with a power law having a slope
of $-2.59$ ($\pm0.10$ for 3.5-25 mJy, and $^{+0.4}_{-0.7}$ for 1-3.5 mJy).
There is no evidence of a faint-end termination or turn-over of the counts in
this flux density range. Our counts are also consistent with previous SCUBA-2
blank-field and lensing cluster surveys. The integrated surface brightness from
our counts down to 1 mJy is $90.0\pm17.2$ Jy deg$^{-2}$, which can account for
up to $83^{+15}_{-16}\%$ of the COBE 450 $\mu$m background. We show that
Herschel counts at 350 and 500 $\mu$m are significantly higher than our 450
$\mu$m counts, likely caused by its large beam and source clustering.
High-angular resolution instruments like SCUBA-2 at 450 $\mu$m are therefore
highly beneficial for measuring the luminosity and spatial density of
high-redshift dusty galaxies.
