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

Abstract

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 108 to 1011 M⊙, and star formation rates (SFRs) from 1 to 500 M⊙ yr−1 in the redshift range between 2 and 5. We find that the bulk ISM velocity along the line of sight (vIS) is globally in outflow, with a vIS of −60 ± 10 km s−1 for low-ionisation gas traced by Si IIλ1260 Å, C IIλ1334 Å, Si IIλ1526 Å, and Al IIλ1670 Å absorption lines, and a vIS of −160 ± 30 and −170 ± 30 km s−1 for higher ionisation gas traced respectively by Al IIIλλ1854-1862 Å and Si IVλλ1393-1402 Å. 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 vmax for the average population, which is of the order of ∼ − 500 and ∼ − 600 km s−1 for the lower and higher ionisation lines, respectively. Comparing vIS to the host galaxies properties, we find no significant correlations with stellar mass M⋆ or SFR, and only a marginally significant dependence (at ∼2σ) on morphology-related parameters, with slightly higher velocities found in galaxies of smaller size (probed by the equivalent radius rT50), higher concentration (CT), and higher SFR surface density ΣSFR. From the spectral stacks, vmax shows a similarly weak dependence on physical properties (at ≃2σ). 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 Ṁout that are comparable to the SFRs of the galaxies (hence a mass-loading factor η 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.