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.
