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 ≳ 6.5 galaxies have revealed a marked decline with increasing redshift in the detectability of Ly α 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, CIII] λ1909 Å, seen in spectroscopic exposures of a star-forming galaxy at z_Lyα = 6.029. We also report tentative detection of CIII] λ1909 Å in a galaxy at z_Lyα = 7.213. The former 3.3σ detection is based on a 3.5 h XShooter spectrum of a bright (J₁₂₅ = 25.2) gravitationally-lensed galaxy behind the cluster Abell 383. The latter 2.8σ detection is based on a 4.2 h MOSFIRE spectra of one of the most distant spectroscopically confirmed galaxies, GN-108036, with J₁₄₀ = 25.2. Both targets were chosen for their continuum brightness and previously-known redshift (based on Ly α), 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 α 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 α equivalent widths in the context of those found at z ≃ 2 from earlier work and discuss the motivation for using lines other than Ly α to study galaxies in the reionization era.