C iii] λ1909 emission as an alternative to Ly α in the reionization era: the dependence of C iii] and Ly α at 3 < z < 4 from the VANDELS survey

Abstract

ABSTRACT The velocity offset of Ly α emission from a galaxy’s systemic redshift is an excellent tracer of conditions that enable the escape of Ly α photons from the galaxy, and potentially the all-important hydrogen ionizing Lyman continuum photons. However at z ≥ 6, Ly α is often heavily attenuated by the neutral intergalactic medium. Here we investigate the utility of C iii] λλ1907, 1909 emission, usually the brightest ultraviolet (UV) line after Ly α, as a proxy estimating the Ly α velocity offset (ΔvLy α). To do so, we use analogues of reionization era galaxies based upon 52 star-forming galaxies with robust C iii] detections drawn from the VANDELS survey. Our sample spans a broad UV magnitude range of −18.5 < MUV < −22.0, with a sample average value of EW(C iii]) = 5.3 Å. We find a slight increase of EW(C iii]) with increasing EW(Ly α), but find a large range of EW(C iii]) ∼ 1–13 Å particularly at EW(Ly α) < 10 Å. Using the C iii] line peak as the systemic redshift, we calculate ΔvLy α and recover the previously reported trend of decreasing ΔvLy α with increasing EW(Ly α). Interestingly, we find an anticorrelation between ΔvLy α and EW(C iii]), which also displays a dependence on the UV absolute magnitude. We derive a multivariate fit to obtain ΔvLy α using both EW(C iii]) and MUV, finding that ΔvLy α is more strongly dependent on EW(C iii]), with a weaker but non-negligible dependence on MUV. We find that for a fixed EW(C iii]), UV-bright Ly α emitting galaxies show smaller values of ΔvLy α,which suggests that such galaxies may be undergoing more bursty star-formation compared to the UV-fainter ones, akin to a population of extremely UV-bright galaxies identified at z > 10.