PHYSICAL PROPERTIES AND MORPHOLOGY OF A NEWLY IDENTIFIED COMPACT z=4.04 LENSED SUBMILLIMETER GALAXY IN ABELL 2218

We present the identification of a bright submillimeter (submm) source, SMM J163555.5+661300, detected in the lensing cluster Abell 2218, for which we have accurately determined the position using observations from the Submillimeter Array (SMA). The identified optical counterpart has a spectroscopic redshift of z = 4.044+/-0.001 if we attribute the single emission line detected at lambda = 6140 angstrom to Ly alpha. This redshift identification is in good agreement with the optical/near-infrared photometric redshift as well as the submm flux ratio S(450)/S(850) similar to 1.6, the radio-submm flux ratio S(1.4)/S(850) < 0.004, and the 24 mu m to 850 mu m flux ratio S(24)/S(850) < 0.005. Correcting for the gravitational lensing amplification of similar to 5.5, we find that the source has a far-infrared luminosity of 1.3x10(12) L(circle dot), which implies a star formation rate (SFR) of 230 M(circle dot) yr(-1). This makes it the lowest-luminosity submillimeter galaxy (SMG) known at z > 4 to date. Previous CO(4-3) emission line observations yielded a non-detection, for which we derived an upper limit of the CO line luminosity of L'(CO) = 0.3 x 10(10) K km s(-1) pc(-2), which is not inconsistent with the L'(CO)-L(FIR) relation for starburst galaxies. The best-fit model to the optical and near-infrared photometry give a stellar population with an age of 1.4 Gyr and a stellar mass of 1.6x10(10) M(circle dot). The optical morphology is compact and in the source plane the galaxy has an extent of similar to 6x3 kpc with individual star-forming knots of <500 pc in size. J163556 is not resolved in the SMA data, and we place a strict upper limit on the size of the starburst region of 8 kpc x 3 kpc, which implies a lower limit on the SFR surface density of 12 M(circle dot) yr(-1) kpc(2). The redshift of J163556 extends the redshift distribution of faint, lensed SMGs, and we find no evidence that these have a different redshift distribution than bright SMGs.