Spectroscopic detections of C III] λ1909 Å at z ≃ 6-7: a new probe of early star-forming galaxies and cosmic reionization

Deep spectroscopic observations of z greater than or similar to 6.5 galaxies have revealed a marked decline with increasing redshift in the detectability of Ly alpha emission. While this may offer valuable insight into the end of the reionization process, it presents a challenge to the detailed spectroscopic study of bright photometrically-selected distant sources now being found via deep Hubble Space Telescope imaging, and particularly those highly magnified sources viewed through foreground lensing clusters. In this paper, we demonstrate the validity of a newway forward via the detection of an alternative diagnostic line, C III] lambda 1909 angstrom, seen in spectroscopic exposures of a star-forming galaxy at z(Ly alpha) = 6.029. We also report tentative detection of CIII] lambda 1909 angstrom in a galaxy at z(Ly alpha) = 7.213. The former 3.3 sigma detection is based on a 3.5 h XShooter spectrum of a bright (J(125) = 25.2) gravitationally-lensed galaxy behind the cluster Abell 383. The latter 2.8 sigma detection is based on a 4.2 h MOSFIRE spectra of one of the most distant spectroscopically confirmed galaxies, GN-108036, with J(140) = 25.2. Both targets were chosen for their continuum brightness and previously-known redshift (based on Ly alpha), ensuring that any C III] emission would be located in a favourable portion of the near-infrared sky spectrum. Since the availability of secure Ly alpha redshifts significantly narrows the wavelength range where CIII] is sought, this increases confidence in these, otherwise, low-signal-to-noise ratio detections. We compare our CIII] and Ly alpha equivalent widths in the context of those found at z similar or equal to 2 from earlier work and discuss the motivation for using lines other than Ly alpha to study galaxies in the reionization era.