AN ULTRAVIOLET ULTRA-LUMINOUS LYMAN BREAK GALAXY AT Z = 2.78 IN NDWFS BOÖTES FIELD* , ** , ** *

We present one of the most ultraviolet (UV) luminous Lyman break galaxies (LBGs; J1432+3358) at z = 2.78, discovered in the NOAO Deep Wide-Field Survey Bo?tes field. The R-band magnitude of J1432+3358 is 22.29?AB, more than two magnitudes brighter than typical L* LBGs at this redshift. The deep z-band image reveals two components of J1432+3358 separated by 10 with a flux ratio of 3:1. The high signal-to-noise ratio rest-frame UV spectrum shows Ly? emission line and interstellar medium absorption lines. The absence of N V and C IV emission lines, and the non-detection in X-ray and radio wavelengths and mid-infrared (MIR) colors indicates weak or no active galactic nuclei (<10%) in this galaxy. The galaxy shows a broader line profile, with a FWHM of about 1000?km?s?1 and a larger outflow velocity (500?km?s?1) than those of typical z ~ 3 LBGs. The physical properties are derived by fitting the spectral energy distribution (SED) with stellar synthesis models. The dust extinction, E(B ? V) = 0.12, is similar to that in normal LBGs. The star formation rates (SFRs) derived from the SED fitting and the dust-corrected UV flux are consistent with each other, ~300 M ??yr?1, and the stellar mass is (1.3 ? 0.3) ? 1011 M ?. The SFR and stellar mass in J1432+3358 are about an order of magnitude higher than those in normal LBGs. The SED-fitting results support that J1432+3358 has a continuous star formation history, with a star formation episode of 6.3 ? 108?yr. The morphology of J1432+3358 and its physical properties suggest that J1432+3358 is in an early phase of a 3:1 merger process. The unique properties and the low space number density (~10?7?Mpc?3) are consistent with the interpretation that such galaxies are either found in a short unobscured phase of the star formation or that a small fraction of intensive star-forming galaxies are unobscured.