Unveiling large-scale rotational motions in the intragroup medium at z~1 through gravitational-arc tomography
We present the first spatially resolved characterisation of the cool intragroup medium (IGrM) in a spectroscopically confirmed galaxy group at z=1.167. Using 30 independent sightlines towards the gravitationally lensed galaxy SGAS J0033+02, we combine 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 the group. We detect prominent MgII, FeII, CaII, and MgI absorption extending up to 62 kpc from a massive star-forming spiral galaxy 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 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 provides a good match. Beyond individual galaxy envelopes, the data are consistent with a group-scale structure that co-rotates in concert with the galaxies. Assuming dynamical equilibrium, we estimate a total (cool+warm+hot) gas mass of 1.3-2.5x10^11 Msol, with large systematic uncertainties, corresponding to approximately 50% of all baryons, within one-quarter of the group's virial radius. These results point to a multiphase IGrM in which cool (~10^4 K) clouds are embedded within a dynamically coherent, group-wide halo. The gas appears gravitationally bound to the group rather than reaccreting onto individual galaxies.