The ALPINE-ALMA [CII] survey: Small Lya-[CII] velocity offsets in main-sequence galaxies at 4.4 < z < 6

Context. The Lyman- line in the ultraviolet (UV) and the [CII] line in the far-infrared (FIR) are widely used tools to identify galaxies in the early Universe and to obtain insights into interstellar medium (ISM) properties in high-redshift galaxies. By combining data obtained with ALMA in band 7 at 320 GHz as part of the ALMA Large Program to INvestigate [CII] at Early Times (ALPINE) with spectroscopic data from DEIMOS at the Keck Observatory, VIMOS and FORS2 at the Very Large Telescope, we assembled a unique sample of 53 main-sequence star-forming galaxies at 4:4 < z < 6 in which we detect both the Lyman- line in the UV and the [CII] line in the FIR. Aims. The goal of this paper is to constrain the properties of the Ly emission in these galaxies in relation to other properties of the ISM. Methods. We used [CII], observed with ALMA, as a tracer of the systemic velocity of the galaxies, and we exploited the available optical spectroscopy to obtain the Ly -[CII] and ISM-[CII] velocity offsets. Results. We find that 90% of the selected objects have Ly -[CII] velocity offsets in the range 0 < vLy �����[CII] < 400 km s�����1, in line with the few measurements available so far in the early Universe, and significantly smaller than those observed at lower redshifts. At the same time, we observe ISM-[CII] offsets in the range �����500 < vISM�����[CII] < 0 km s�����1, in line with values at all redshifts, which we interpret as evidence for outflows in these galaxies. We find significant anticorrelations between vLy �����[CII] and the Ly rest-frame equivalent width EW0(Ly ) (or equivalently, the Ly escape fraction fesc(Ly )): galaxies that show smaller vLy �����[CII] have larger EW0(Ly ) and fesc(Ly ). Conclusions. We interpret these results in the framework of available models for the radiative transfer of Ly photons. According to the models, the escape of Ly photons would be favored in galaxies with high outflow velocities, producing large EW0(Ly ) and small vLy �����[CII], in agreement with our observations. The uniform shell model would also predict that the Ly escape in galaxies with slow outflows (0 < vout < 300 km s�����1) is mainly determined by the neutral hydrogen column density (NHI) along the line of sight, while the alternative model by Steidel et al. (2010) would more highly favor a combination of NHI at the systemic velocity and covering fraction as driver of the Ly escape. We suggest that the observed increase in Ly escape that is observed in the literature between z 2 and z 6 is not due to a higher incidence of fast outflows at high redshift, but rather to a decrease in average NHI along the line of sight, or alternatively, a decrease in HI covering fraction