KECK SPECTROSCOPY OF LYMAN-BREAK GALAXIES AND ITS IMPLICATIONS FOR THE UV-CONTINUUM AND Lyα LUMINOSITY FUNCTIONS AT z > 6

We present Keck spectroscopic observations of z > 6 Lyman-break galaxy (LBG) candidates in the Subaru Deep Field (SDF). The candidates were selected as i'-dropout objects down to z' = 27 AB magnitudes from an ultra-deep SDF z'-band image. With the Keck spectroscopy we identified 19 LBGs with prominent Ly alpha emission lines at 6 <= z <= 6.4. The median value of the Ly alpha rest-frame equivalent widths (EWs) is similar to 50 angstrom, with four EWs > 100 angstrom. This well-defined spectroscopic sample spans a UV-continuum luminosity range of -21.8 <= M-UV <= -19.5 (0.6 similar to 5 L-UV*) and a Ly alpha luminosity range of (0.3-3) x 10(43) erg s(-1) (0.3-3 L-Ly alpha*). We derive the UV and Ly alpha luminosity functions (LFs) from our sample at < z > similar to 6.2 after we correct for sample incompleteness. We find that our measurement of the UV LF is consistent with the results of previous studies based on photometric LBG samples at 5 < z < 7. Our Ly alpha LF is also generally in agreement with the results of Ly alpha-emitter surveys at z similar to 5.7 and 6.6. This study shows that deep spectroscopic observations of LBGs can provide unique constraints on both the UV and Ly alpha LFs at z > 6.