The Grism Lens-Amplified Survey from Space (GLASS). XI. Detection of CIV in Multiple Images of the z=6.11 Lyα Emitter behind RXC J2248.7-4431

The C III] and C IV rest-frame UV emission lines are powerful probes of the ionization. states of galaxies. They have furthermore been suggested as alternatives for spectroscopic redshift confirmation of objects at the epoch of reionization (z > 6), where the most frequently used redshift indicator, Ly alpha, is attenuated by the high fraction of neutral hydrogen in the intergalactic medium. However, currently only very few confirmations of carbon UV lines at these high redshifts exist, making it challenging to quantify these claims. Here, we present the detection of C IV lambda lambda 1548, 1551 angstrom in Hubble Space Telescope slitless grism spectroscopy obtained by GLASS of a Ly alpha emitter at z = 6.11 multiply imaged by the massive foreground galaxy cluster RXC J2248.7-4431. The C IV emission is detected at the 3 sigma-5 sigma level in two images of the source, with marginal detection in two other images. We do not detect significant C III]lambda lambda 1907, 1909 angstrom emission implying an equivalent width EWC (III]) < 20 angstrom (1 sigma) and C IV/C III > 0.7(2 sigma). Combined with limits on the rest-frame UV flux from the He II lambda 1640 angstrom emission line and the O III]lambda lambda 1661, 1666 angstrom doublet, we put constraints on the metallicity and the ionization state of the galaxy. The estimated line ratios and equivalent widths do not support a scenario where an AGN is responsible for ionizing the carbon atoms. SED fits, including nebular emission lines,. imply a source with a mass of log(M/M-circle dot) similar to 9, SFR of around 10M(circle dot) yr-(1), and a young stellar population < 50 Myr old. The source shows a stronger ionizing radiation field than objects with detected C IV emission at z < 2 and adds to the growing sample of low-mass (log(M/M-circle dot) less than or similar to 9) galaxies at the epoch of reionization with strong radiation fields from star formation.