Unveiling large-scale rotational motions in the intragroup medium at z ∼ 1 through gravitational-arc tomography^★

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001 Santiago, Chile
² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306 Santiago, Chile
³ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059 Valparaíso, Chile
⁴ Department of Physics and Astronomy, Camosun College, 3100 Foul Bay Road, Victoria, B.C., V8P 5J2, Canada
⁵ Centre de Recherche Astrophysique de Lyon, UMR5574, 9 avenue Charles André, 69230 Saint-Genis-Laval, France
⁶ Instituto de Estudios Astrofísicos, Universidad Diego Portales, Av. Ejército Libertador 441 Santiago, Chile
⁷ STAR Institute, Quartier Agora, Allée du Six Août, 19c, B-4000 Liège, Belgium
⁸ Institut d’Astrophysique de Paris, UMR7095, 98bis boulevard Arago, 75014 Paris, France
⁹ French-Chilean Laboratory for Astronomy, IRL3386, CNRS and U. de Chile, Casilla 36-D, Santiago, Chile

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

Received: 11 November 2025
Accepted: 12 March 2026

Abstract

Context. The circumgalactic medium (CGM) is a crucial interface between galaxies and their large-scale environment, regulating gas accretion and feedback processes. Yet its physical and kinematic properties within galaxy groups, where most galaxies reside, remain poorly constrained.

Aims. We present the first spatially resolved characterisation of the cool intragroup medium (IGrM) in a spectroscopically confirmed galaxy group at z ≃ 1.167 using absorption-line spectroscopy along multiple sightlines.

Methods. Using 30 independent sightlines towards the gravitationally lensed galaxy SGAS J003341.5+024217, we combined background light from an extended gravitational arc and various sources in the field to map the distribution and kinematics of diffuse metal-enriched gas pertaining to this group.

Results. We detected prominent Mg II, Fe II, Ca II, and Mg I absorption extending up to a projected distance of 62 kpc from a massive (log M_★ = 11.0 ± 0.1 M_⊙) star-forming spiral and its interacting companion. Together with four other members, these form a compact group with a virial radius of 313 kpc. Down-the-barrel, blueshifted absorption indicates outflows. The distribution and two-dimensional kinematics of this gas suggest the influence of both tidal stripping and star formation-driven winds. Intervening absorption across the field partly traces internal galaxy motions. A simple superposition of individual discs cannot reproduce the velocity field at large impact parameters or in counter-rotating regions, while a global IGrM halo with a rotational velocity of ≈130 km s⁻¹ provides a good match. Beyond individual galaxy envelopes, we find the data to be consistent with a group-scale structure that co-rotates in concert with the galaxies. Assuming dynamical equilibrium, we estimated a total (cool+warm+hot) gas mass of 1.3 − 2.5 × 10¹¹ M_⊙, with large systematic uncertainties, corresponding to roughly 50% of all baryons within one-quarter of the group’s virial radius.

Conclusions. These results point to a multiphase IGrM in which cool (∼10⁴ K) clouds are embedded within a dynamically coherent group-wide halo. The gas appears to be gravitationally bound to the group rather than reaccreting onto individual galaxies. High-redshift strong Mg II absorbers may thus trace shared metal-enriched halos shaped by galaxy interactions and feedback, with stripped and outflowing gas accumulating in the IGrM over time.