Ciii]$\lambda1909$ emission as an alternative to Ly$α$ in the reionization era: the dependence of Ciii] and Ly$α$ at $3<z<4$ from the VANDELS survey

Mark H Cunningham; Aayush Saxena; Richard S Ellis; Laura Pentricci

doi:10.1093/mnras/stae939

paper

Ciii] $λ 1909$ emission as an alternative to Ly $α$ in the reionization era: the dependence of Ciii] and Ly $α$ at $3 < z < 4$ from the VANDELS survey

arXiv:2309.02227 · doi:10.1093/mnras/stae939

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 hydrogen ionizing Lyman continuum photons. However at $z \geq 6$ , Ly $α$ is often heavily attenuated by the neutral intergalactic medium. Here we investigate the utility of Ciii], $λλ 1907, 1909$ emission, usually the brightest UV line after Ly $α$ , as a proxy estimating the Ly $α$ velocity offset ( $Δ v_{Ly α}$ ). To do so, we use analogues of reionization era galaxies based upon 52 star-forming galaxies with robust Ciii] detections drawn from the VANDELS survey. Our sample spans a broad UV magnitude range of $- 18.5 < M_{UV} < - 22.0$ , with a sample average value of EW(Ciii]) $= 5.3$ \,Å. We find a slight increase of EW(Ciii]) with increasing EW(Ly $α$ ), but find a large range of EW(Ciii]) $\sim$ 1-13\,Å particularly at EW(Ly $α$ ) $< 10$ \,Å. Using the Ciii] line peak as the systemic redshift, we calculate $Δ v_{Ly α}$ and recover the previously reported trend of decreasing $Δ v_{Ly α}$ with increasing EW(Ly $α$ ). Interestingly, we find an anti-correlation between $Δ v_{Ly α}$ and EW(Ciii]), which also displays a dependence on the UV absolute magnitude. We derive a multi-variate fit to obtain $Δ v_{Ly α}$ using both EW(Ciii]) and $M_{UV}$ , finding that $Δ v_{Ly α}$ is more strongly dependent on EW(Ciii]), with a weaker but non-negligible dependence on $M_{UV}$ . We find that for a fixed EW(Ciii]), UV-bright Ly $α$ emitting galaxies show smaller values of $Δ v_{Ly α}$ ,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 $z > 10$ .

11 pages, 7 figures

Ciii]λ1909 emission as an alternative to Lyα in the reionization era: the dependence of Ciii] and Lyα at 3<z<4 from the VANDELS survey

Abstract

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Ciii] $λ 1909$ emission as an alternative to Ly $α$ in the reionization era: the dependence of Ciii] and Ly $α$ at $3 < z < 4$ from the VANDELS survey