Properties of the interstellar medium in star-forming galaxies at redshifts 2 ≤ z ≤ 5 from the VANDELS survey

Gaseous flows inside and outside galaxies are key to understanding galaxy evolution, as they regulate their star formation activity and chemical enrichment across cosmic time. We study the interstellar medium (ISM) kinematics of a sample of 330 galaxies with C III] or He II emission using far-ultraviolet (far-UV) ISM absorption lines detected in the ultra deep spectra of the VANDELS survey. These galaxies span a broad range of stellar masses from 10(8) to 10(11) M-circle dot, and star formation rates (SFRs) from 1 to 500 M-circle dot yr(-1) in the redshift range between 2 and 5. We find that the bulk ISM velocity along the line of sight (v(IS)) is globally in outflow, with a v(IS) of -60 +/- 10 km s(-1) for low-ionisation gas traced by Si II lambda 1260 angstrom, C II lambda 1334 angstrom, Si II lambda 1526 angstrom, and Al II lambda 1670 angstrom absorption lines, and a v(IS) of -160 +/- 30 and -170 +/- 30 km s(-1) for higher ionisation gas traced respectively by Al III lambda lambda 1854-1862 angstrom and Si IV lambda lambda 1393-1402 angstrom. Interestingly, we notice that BPASS models are able to better reproduce the stellar continuum around the Si IV doublet than other stellar population templates. For individual galaxies, 34% of the sample has a positive ISM velocity shift, almost double the fraction reported at lower redshifts. We additionally derive a maximum outflow velocity v(max) for the average population, which is of the order of similar to -500 and similar to -600 km s(-1) for the lower and higher ionisation lines, respectively. Comparing v(IS) to the host galaxies properties, we find no significant correlations with stellar mass M-star or SFR, and only a marginally significant dependence (at similar to 2 sigma) on morphology-related parameters, with slightly higher velocities found in galaxies of smaller size (probed by the equivalent radius r(T)(50)) higher concentration (C-T), and higher SFR surface density Sigma(SFR). From the spectral stacks, v(max) shows a similarly weak dependence on physical properties (at similar or equal to 2 sigma). Moreover, we do not find evidence of enhanced outflow velocities in visually identified mergers compared to isolated galaxies. From a physical point of view, the outflow properties are consistent with accelerating momentum-driven winds, with densities decreasing towards the outskirts. Our moderately lower ISM velocities compared to those found in similar studies at lower redshifts suggest that inflows and internal turbulence might play an increased role at z > 2 and weaken the outflow signatures. Finally, we estimate mass-outflow rates (M) over dot(out) that are comparable to the SFRs of the galaxies (hence a mass-loading factor eta of the order of unity), and an average escape velocity of 625 km s(-1), suggesting that most of the ISM will remain bound to the galaxy halo.