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

Abstract

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.

Figures (12)

• Figure 1: Field of SGAS J0033$+$02 observed with HST in the optical (F814W) and near-infrared (F105W and F140W) bands. The gravitational arc formed by the lensed galaxy at $z\simeq 2.39$ (called the 'Arc') and its counter-images ('Ci1' and 'Ci2') are indicated. The bluish object located $7\farcs 3$ southwest of the arc is a Galactic star. The main absorbing galaxy at $z\simeq 1.17$, labelled 'G1', is shown along with its closest two companions at the same redshift: 'G2' to the northeast, and 'G3' to the west. Redshifts for these and additional sources at $z>1.16$ are indicated next to each label.
• Figure 2: Brightest sources at $z>1.16$ near the centre of the foreground galaxy cluster observed in HST F140W imaging. Black contours trace the flux distribution in the image. The upper panel shows the image plane, while the lower panel displays the delensed absorber plane at $z=1.17$. The elliptical aperture around G1 includes its eastern region, which is stretched by gravitational lensing.
• Figure 3: Left panel: Rest-frame equivalent width of Mgii $\lambda$2796 as a function of the projected distance from the group's barycentre, normalised by the group's virial radius. Our measurements are compared to the galaxy group sample from Cherrey24. The overall best-fit relation for interveningMgii absorption is displayed as a grey dashed line along with its $1\sigma$ confidence region. The original relation from Cherrey24 is shown in green. Right panel: Velocity span of the Mgii profiles, $\Delta v\pm \sigma_v$, normalised by the group's escape velocity, $v_{\rm esc}=601$ km s$^{-1}$, as a function of the normalised projected distance from the group's barycentre. The velocities of the individual galaxies are shown in red. The reference velocity, $v_{\rm sys}$, is the mass-weighted mean redshift of the group ($z_{\rm sys}=1.16663$).
• Figure 4: Superposition model of extended rotating discs ( upper panels) and rotating IGrM model ( lower panels). Left panels: Modelled velocity maps with the observed intervening Mgii mean velocities overlaid for each of the four delensed sources. The inset shows the zoom-in on a region encompassing Ci1 and the quasar. Right panels: Model-predicted velocities (orange) at the positions of the Arc, Ci1, Ci2, and the quasar, as a function of impact parameter relative to the barycentre, marked by a plus sign in the left panels (superposition model: barycentre of G1 to G3; IGrM: G1 to G5). The observed Mgii velocities are displayed in black for the Arc, Ci1, and Ci2, and in blue for the quasar.
• Figure 5: Schematic of the orientation and kinematics of the intragroup medium in the compact galaxy group in the field of SGAS J0033$+$02. Each region is colour-coded based on the mean velocity offset of the observed Mgii (or [Oii]) gas relative to the mass-weighted mean redshift of G1 to G5 ($z=1.16648$). The axes (black arrows) and directions (blue and/or red arrows) of rotation of G1, G2, G3, and the intragroup medium are indicated. The shaded, overlapping circles denote the extent of one virial radius around each galaxy.