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Submillimeter sources behind the massive lensing clusters A370 and A2390

We report 850 micron Submillimeter Array (SMA) observations of four gravitationally lensed submillimeter galaxies (SMGs), A370-2, A2390-1, A2390-3 and A2390-4, which were originally discovered with the Submillimeter Common-User Bolometer Array (SCUBA). Our SMA detection of A370-2 with a submillimeter flux of 7.95 +/- 0.60 mJy unambiguously identifies the counterparts to this source at optical and radio wavelengths. A2390-1 is an ultraluminous infrared galaxies with a submillimeter flux of 5.55 +/- 0.92 mJy and a redshift of 1.8 +/- 0.2 computed from submillimeter/radio flux ratio analysis. We resolve A2390-3 into two components, A2390-3a and A2390-3b, with fluxes of 3.15 +/- 0.63 mJy and 1.92 +/- 0.60 mJy, respectively. The structure of the system could be consistent with morphological distortion by gravitational lensing. The lack of counterparts in the optical and infrared indicates a heavily dust-enshrouded nature, and a non-detection in the radio implies that these two sources probably lie at z > 4.7, which would make them among the most distant SMGs known to date. Our non-detection of A2390-4 suggests either that there are multiple fainter submillimeter sources within the SCUBA beam or that the SCUBA detection may have been false. Our precise positions allow us to determine accurate amplifications and fluxes for all our detected sources. Our new results give a shallower power-law fit (-1.10) to the faint-end 850 micron cumulative number counts than previous work. We emphasize the need for high-resolution observations of single-dish detected SMGs in order to measure accurately the faint end of the 850 micron counts.