CANDELSz7: a large spectroscopic survey of CANDELS galaxies in the reionization epoch

Abstract

We present the results of CANDELSz7, a European Southern Observatory (ESO) Large Program aimed at spectroscopically confirming a homogeneous sample of z ≃ 6 and z ≃ 7 star forming galaxies. The candidates were selected in the GOODS-South, UDS, and COSMOS fields using the official CANDELS catalogs based on H 160 -band detections. Standard color criteria, which were tailored depending on the ancillary multi-wavelength data available for each field, were applied to select more than 160 candidate galaxies at z ≃ 6 and z ≃ 7. Deep, medium-resolution FORS2 spectroscopic observations were then conducted with integration times ranging from 12 to 20 h to reach a Ly α flux limit of approximately 1 − 3 × 10 −18 erg s −1 cm −2 at 3 σ . We could determine a spectroscopic redshift for about 40% of the galaxies, mainly through the detection of a single emission line that we interpret as Ly α emission, or for some of the brightest objects ( H 160 ≤ 25.5) from the presence of faint continuum and a sharp drop that we interpret as a Ly α break. In this paper we present the redshifts and main properties of 65 newly confirmed high-redshift galaxies. Adding previous proprietary and archival data we assemble a sample of ≃260 galaxies that we use to explore the evolution of the Ly α fraction in Lyman break galaxies and the change in the shape of the emission line between z ∼ 6 and z ∼ 7. We also discuss the accuracy of the CANDELS photometric redshifts in this redshift range.