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 Lyα emission lines at 6 ≤ z ≤ 6.4. The median value of the Lyα rest-frame equivalent widths (EWs) is 50 , with four EWs >100 . This well-defined spectroscopic sample spans a UV-continuum luminosity range of -21.8 ≤ M _UV ≤ -19.5 (0.6 5 L*_UV) and a Lyα luminosity range of (0.3-3) × 10⁴³ erg s^-1 (0.3-3 L*_Lyα). We derive the UV and Lyα luminosity functions (LFs) from our sample at 〈z〉 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 Lyα LF is also generally in agreement with the results of Lyα-emitter surveys at z 5.7 and 6.6. This study shows that deep spectroscopic observations of LBGs can provide unique constraints on both the UV and LyαLFs at z > 6.