Probing the kinematics of the Local Group with chemically enriched gas in the HESTIA simulations

¹ Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Física Buenos Aires, Argentina
² Instituto de Astronomía y Física del Espacio (IAFE, CONICET-UBA), CC 67 Suc. 28 1428 Buenos Aires, Argentina
³ Institut für Physik und Astronomie, Universität Potsdam Karl-Liebknecht-Str. 24/25 14476 Golm, Germany
⁴ Leibniz-Institut für Astrophysik An der Sternwarte 16 14482 Potsdam, Germany
⁵ New York University Abu Dhabi, Department of Physics PO Box 129188 Abu Dhabi, UAE
⁶ Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi PO Box 129188 Abu Dhabi, UAE
⁷ Department of Physics, Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology Cambridge MA 02139, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 June 2025
Accepted: 24 November 2025

Abstract

We present a study of the gas kinematics within the HESTIA project, a state-of-the-art set of simulations of the Local Group, with a particular focus on the velocity patterns of different ions and the large-scale motion of gas and galaxies towards the Local Group’s barycentre. Using two of the HESTIA high-resolution runs, we examined the distribution and velocities of H I, C IV, Si III, O VI, O VII, and O VIII and their imprints on sightlines observed from the Sun’s location in different reference frames. To mimic observational strategies, we assessed the contribution of rotating disc gas, assuming simple kinematic and geometrical considerations. Our results indicate that local absorption features in observed sightlines most likely trace material in the circumgalactic medium of the Milky Way. Some sightlines, however, show that intragroup material could be more easily observed towards the barycentre, which defines a preferred direction in the sky. In particular, H I, Si III, and C IV roughly trace cold gas inside Milky Way and Andromeda haloes, as most of their mass flux occurs within the virial region of each galaxy, while oxygen high ions mostly trace hot halo and intragroup gas, with comparable mass fluxes in the Local Group outskirts and the circumgalactic medium of the two main galaxies. Additionally, we find that pressures traced by different ionic species outside the Milky Way’s halo show systematically higher values towards the barycentre direction in contrast to its antipode in the sky. Kinematic imprints of the global motion towards the barycentre can be seen at larger distances for all ionic species as the Milky Way rams into material in the direction of Andromeda, with gas towards the anti-barycentre lagging behind.