The Lyman alpha reference sample

XV. Relating ionised gas kinematics with Lyman-α observables

¹ Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune University Campus, Pune 411 007, India
² Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, D-69120 Heidelberg Germany
³ , Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Straße 2, D-69120 Heidelberg, Germany
⁴ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁵ Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
⁶ Department of Astronomy, Stockholm University, AlbaNova University Centre, SE-106 91, Stockholm, Sweden
⁷ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA
⁸ Department of Physics & Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA
⁹ Leibniz Institute for Astrophysics (AIP), An der Sternware 16, 14482 Potsdam, Germany

^⋆ Corresponding author; edmund.herenz@iucaa.in

Received: 6 June 2024
Accepted: 11 September 2024

Abstract

Gas kinematics affect the radiative transfer and escape of hydrogen Lyman-α (Lyα) emission from galaxies. We investigate this interplay empirically by relating the ionised gas kinematics of 42 galaxies in the extended Lyα Reference Sample (eLARS) with their Lyα escape fractions, f_esc^Lyα, Lyα equivalent widths, EW_Lyα, and Lyα luminosities, L_Lyα. To this aim we use PMAS integral-field spectroscopic observations of the Balmer-α line. Our sample contains 18 rotating discs, 13 perturbed rotators, and 13 galaxies with more complex kinematics. The distributions of f_esc^Lyα, EW_Lyα, and L_Lyα do not differ significantly between these kinematical classes, but the largest Lyα observables are found amongst the kinematically complex systems. We find no trends between either f_esc^Lyα or EW_Lyα and kinematic or photometric inclinations. We calculate shearing velocities, v_shear, and intrinsic velocity dispersions, σ₀^obs (empirically corrected for beam-smearing effects), as global kinematical measures for each galaxy. The sample is characterised by highly turbulent motions (30 km s⁻¹ ≲ σ₀^obs ≲ 80 km s⁻¹) and more than half of the sources show dispersion-dominated kinematics. We uncover clear trends between Lyα observables and global kinematical statistics: EW_Lyα and L_Lyα correlate with σ₀^obs, while f_esc^Lyα anti-correlates with v_shear and v_shear/σ₀^obs. Moreover, we find, that galaxies with EW_Lyα ≥ 20 Å are characterised by higher σ₀ and lower v_shear/σ₀^obs than galaxies below this threshold. We discuss the statistical importance of v_shear, σ₀^obs, and v_shear/σ₀^obs for regulating the Lyα observables in comparison to other galaxy parameters. It emerges that σ₀^obs is the dominating parameter for regulating EW_Lyα and that is as important as nebular extinction, gas covering fraction, and ionising photon production efficiency in regulating f_esc^Lyα. A simple scenario where the starburst age is simultaneously regulating turbulence, EW_Lyα, and f_esc^esc is not supported by our observations. However, we show that the small-scale distribution of dust appears to be influenced by turbulence in some galaxies. In support of our observational result, we discuss how turbulence is theoretically expected to play a significant role in modulating f_esc^Lyα.