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

¹ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: antonello.calabro@inaf.it
² INAF – Osservatorio Astronomico di Bologna, via P. Gobetti 93/3, 40129 Bologna, Italy
³ University of Bologna – Department of Physics and Astronomy ‘Augusto Righi’ (DIFA), via Gobetti 93/2, 40129 Bologna, Italy
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán 1305, La Serena, Chile
⁶ Departamento de Física y Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena, Chile
⁷ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen Ø, Denmark
⁸ European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany
⁹ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹² Department of Astronomy, University of Geneva, 51 Chemin Pegasi, 1290 Versoix, Switzerland

Received: 27 June 2022
Accepted: 19 August 2022

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 (v_IS) is globally in outflow, with a v_IS 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 v_IS 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 v_max 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 v_IS 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 r_T⁵⁰), higher concentration (C_T), and higher SFR surface density Σ_SFR. From the spectral stacks, v_max 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 Ṁ_out 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.