FAINT SUBMILLIMETER GALAXY COUNTS AT 450 μm

We present the results of SCUBA-2 observations at 450 mu m and 850 mu m of the field lensed by the massive cluster A370. With a total survey area > 100 arcmin(2) and 1 sigma sensitivities of 3.92 and 0.82 mJy beam(-1) at 450 and 850 mu m, respectively, we find a secure sample of 20 sources at 450 mu m and 26 sources at 850 mu m with a signal-to-noise ratio (S/N) > 4. Using the latest lensing model of A370 and Monte Carlo simulations, we derive the number counts at both wavelengths. The 450 mu m number counts probe a factor of four deeper than the counts recently obtained from the Herschel Space Telescope at similar wavelengths, and we estimate that similar to 47%-61% of the 450 mu m extragalactic background light resolved into individual sources with 450 mu m fluxes greater than 4.5 mJy. The faint 450 mu m sources in the 4 sigma sample have positional accuracies of 3 arcsec, while brighter sources (S/N > 6 sigma) are good to 1.4 arcsec. Using a deep radio map (1 sigma similar to 6 mu Jy) we find that the percentage of submillimeter sources having secure radio counterparts is 85% for 450 mu m sources with intrinsic fluxes > 6 mJy and 67% for 850 mu m sources with intrinsic fluxes > 4 mJy. We also find that 67% of the > 4s 450 mu m sources are detected at 850 mu m, while the recovery rate at 450 mu m of > 4s 850 mu m sources is 54%. Combined with the source redshifts estimated using millimetric flux ratios, the recovered rate is consistent with the scenario where both 450 mu m and 20 cm emission preferentially select lower redshift dusty sources, while 850 mu m emission traces a higher fraction of dusty sources at higher redshifts. We identify potential counterparts in various wavelengths from X-ray to mid-infrared and measure the multiwavelength photometry, which we then use to analyze the characteristics of the sources. We find three X-ray counterparts to our robust submillimeter sample (S/N > 5), giving an active galactic nucleus fraction for our 450 (850) mu m sample of 3/8 (3/9) or 38% (33%). We also find a correlation between the K-s band and the 850 mu m/20 cm flux ratio.