LBT/LUCI SPECTROSCOPIC OBSERVATIONS of z≃7 Galaxies GALAXIES

Abstract

We present deep near-infrared spectroscopic observations of 13 luminous z ≃ 7 Lyman break galaxies (LBGs) (MUV ≃ -21) and a z ≃ 9.6 lensed LBG candidate, MACS1149-JD1, using the LBT/LUCI spectrograph in the multi-object mode and long-slit mode, respectively. The z ∼ 7 galaxies are selected in one of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey/WIDE survey observations, the UKIRT Infrared Deep Sky Survey Ultra Deep Survey field, and MACS1159-JD1 is selected from the Cluster Lensing And Supernova survey with Hubble survey observations. With ∼ 4-5 hr integrations, the LBT spectra are sensitive to Ly α emission, with rest-frame equivalent width greater than 55 ł (EW0 > 55 ł) at z ≃ 7 and 90 (EW0 > 90 ł) at z ∼ 9.5. No Ly α emission is observed in any of the z ≃ 7 LBGs. After correcting the spectroscopic incompleteness, our measurements place an upper limit on the Ly α emission fraction o XLya (EW0 > 55 ł, z ≃ 7) < 9.8% in luminous LBGs at z ≃ 7. This result is consistent with the lack of Lyα emission in z >7 LBGs in previous studies. Together with other measurements of the Lyα emission fraction in LBGs at z ≃7, this study puts a strong constraint on the fraction of luminous z ≃ 7 LBGs with large EWXLyα, X (EW 055 ł, z ≃ 7) 2.6+ 3.2-2.6 %. We estimate the expected Lyα emission fraction at z≃7 by extrapolating the evolution of the fraction at low redshifts and find that the observed Lyα emission fraction is lower than the expected Lyα emission fraction at the 89% significance?level, consistent with expectations if Lyα emission in z≃7 LBGs has been suppressed by neutral hydrogen in the intergalactic medium or optically thick absorbers around the galaxies. We do not find any strong Lyα emission feature at the redshift range of z≃7.3-9.7 in the MACSJ1159-JD1 spectrum, either. This is consistent with the conclusion that Lyα emission in the galaxy has been highly suppressed.