Title: 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

Abstract: The velocity offset of Ly$\alpha$ emission from a galaxy's systemic redshift is an excellent tracer of conditions that enable the escape of Ly$\alpha$ photons from the galaxy, and potentially hydrogen ionizing Lyman continuum photons. However at $z\geq6$, Ly$\alpha$ is often heavily attenuated by the neutral intergalactic medium. Here we investigate the utility of Ciii],$\lambda\lambda1907,1909$ emission, usually the brightest UV line after Ly$\alpha$, as a proxy estimating the Ly$\alpha$ velocity offset ($\Delta v_{\rm{Ly}\alpha}$). 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 $-18.5 < M_{\rm{UV}} < -22.0$, with a sample average value of EW(Ciii]) $=5.3$\,{\AA}. We find a slight increase of EW(Ciii]) with increasing EW(Ly$\alpha$), but find a large range of EW(Ciii]) $\sim$1-13\,{\AA} particularly at EW(Ly$\alpha$) $<10$\,{\AA}. Using the Ciii] line peak as the systemic redshift, we calculate $\Delta v_{\rm{Ly}\alpha}$ and recover the previously reported trend of decreasing $\Delta v_{\rm{Ly}\alpha}$ with increasing EW(Ly$\alpha$). Interestingly, we find an anti-correlation between $\Delta v_{\rm{Ly}\alpha}$ and EW(Ciii]), which also displays a dependence on the UV absolute magnitude. We derive a multi-variate fit to obtain $\Delta v_{\rm{Ly}\alpha}$ using both EW(Ciii]) and $M_{\rm{UV}}$, finding that $\Delta v_{\rm{Ly}\alpha}$ is more strongly dependent on EW(Ciii]), with a weaker but non-negligible dependence on $M_{\rm{UV}}$. We find that for a fixed EW(Ciii]), UV-bright Ly$\alpha$ emitting galaxies show smaller values of $\Delta v_{\rm{Ly}\alpha}$,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 $z>10$.