Reconstructing the radial velocity distribution of the Milky Way’s circumgalactic medium with HESTIA

¹ Institute für Physik und Astronomie, University of Potsdam, Karl Liebknecht Straße 24/25, 14476 Potsdam, Germany
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
⁴ Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
⁵ Instituto de Astronomía y Física del Espacio (IAFE, CONICET-UBA), CC 67, Suc. 28, 1428 Buenos Aires, Argentina
⁶ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁷ Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel
⁸ Univ. Lille, CNRS, Centrale Lille, UMR 9189 CRIStAL, 59000 Lille, France
⁹ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
¹⁰ Tartu Observatory, University of Tartu, Observatooriumi 1, 61602 Tõravere, Estonia
¹¹ Estonian Academy of Sciences, Kohtu 6, 10130 Tallinn, Estonia

^★ Corresponding author: fruenger@astro.physik.uni-potsdam.de

Received: 2 July 2024
Accepted: 30 May 2025

Abstract

The accretion and processing of neutral and ionized gas play substantial roles in the evolution of the Milky Way. From the position of the Sun, circumgalactic gas flows in the Milky Way halo are known to span a large range of radial velocities, but the complex kinematics of the circumgalactic medium (CGM) cannot be fully reconstructed from observations because of the blending with foreground interstellar gas in the Milky Way disk. For this paper we used three zoom-in magnetohydrodynamic simulations of the Milky Way and the Local Group from the HESTIA project to systematically investigate the radial velocity distribution of neutral hydrogen (H I) clouds in the CGM in the (simulated) Local Standard of Rest (LSR) velocity frame. Our three simulations, which exhibit substantial differences in their global CGM properties, reveal that 48–65 percent of the extraplanar H I at z > 2 kpc above the plane is confined to a velocity range |v_LSR| ≤ 100 km s⁻¹, implying that the gas is (at least partly) corotating with the underlying disk. In the two most realistic Milky Way realizations, the CGM velocity distribution is skewed toward negative velocities (in particular for H I clouds at vertical distances z > 10 kpc), indicating a net accretion of neutral gas. These results are in line with the statistics from UV absorption-line measurements of the Milky Way CGM, and we also find broad agreement with the Illustris TNG50 simulation. Our study supports a scenario in which a substantial fraction of the Milky Way’s CGM resides close to the disk at |v_LSR| ≤ 100 km s⁻¹, where it is hiding from observations as its spectral signatures are covered by foreground interstellar gas features. We furthermore find that 97 percent of the clumps live in the Milky Way halo and are not associated with satellite galaxies. The clumps are magnetized with a magnetic pressure often dominating over the thermal pressure.