The VIMOS Ultra Deep Survey: The role of HI kinematics and HI column density on the escape of Lya photons in star-forming galaxies at 2 < z < 4

Aims. We wish to assess the role of kinematics and neutral hydrogen column density in the escape and distribution of Lya photons. Methods. We selected a sample of 76 Lya emitting galaxies from the VIMOS Ultra Deep Survey (VUDS) at 2 = z = 4. We estimated the velocity of the neutral gas flowing out of the interstellar medium as the velocity offset, δv, between the systemic redshift (zsys) and the center of low-ionization absorption line systems (LIS). To increase the S/N of VUDS spectra, we stacked subsamples defined based on median values of their photometric and spectroscopic properties. We measured the systemic redshift from the rest-frame UV spectroscopic data using the CIII]1908 nebular emission line, and we considered SiII1526 as the highest signal-to-noise LIS line. We calculated the Lya peak shift with respect to the zsys, the EW(Lya), and the Lya spatial extension, Ext(Lya-C), from the in the 2D stacked spectra. Results. The galaxies that are faint in the rest-frame UV continuum, strong in Lya and CIII], with compact UV morphology, and localized in an underdense environment are characterized by outflow velocities of the order of a few hundreds of km s-1. The subsamples with smaller δv are characterized by larger Lya peak shifts, larger Ext(Lya-C), and smaller EW(Lya). In general we find that EW(Lya) anti-correlates with Ext(Lya-C) and Lya peak shift. Conclusions. We interpret these trends using a radiative-transfer shell model. The model predicts that an HI gas with a column density larger than 1020 cm-2is able to produce Lya peak shifts larger than >300 km s-1. An ISM with this value of NHI would favour a large amount of scattering events, especially when the medium is static, so it can explain large values of Ext(Lya-C) and small EW(Lya). On the contrary, an ISM with a lower NHI, but large velocity outflows would lead to a Lya spatial profile peaked at the galaxy center (i.e. low values of Ext(Lya-C)) and to a large EW(Lya), as we see in our data. Our results and their interpretation via radiative-transfer models tell us that it is possible to use Lya to study the properties of the HI gas. Also, the fact that Lya emitters are characterized by large δv could give hints about their stage of evolution in the sense that they could be experiencing short bursts of star formation that push strong outflows