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

We present the results of CANDELSz7, a European Southern Observatory (ESO) Large Program aimed at spectroscopically confirming a homogeneous sample of z similar or equal to 6 and z similar or equal to 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 similar or equal to 6 and z similar or equal to 7 . Deep, medium-resolution FORS2 spectroscopic observations were then conducted with integration times ranging from 12 to 20 h to reach a Ly alpha flux limit of approximately 1-3 x 10(-18) erg s(-1) cm(-2) at 3 sigma. 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 alpha 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 alpha 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 similar or equal to 260 galaxies that we use to explore the evolution of the Ly alpha fraction in Lyman break galaxies and the change in the shape of the emission line between z similar to 6 and z similar to 7. We also discuss the accuracy of the CANDELS photometric redshifts in this redshift range.