Intragroup Medium (IGrM) Dynamics

• Intragroup Medium (IGrM) is a hot, diffuse baryonic plasma in galaxy groups (halo masses ~10^13–10^14 M⊙) that plays a key role in galaxy evolution and feedback.
• IGrM properties include thermal components (T ~10^6–10^7 K) and nonthermal elements like magnetic fields and turbulence, probed by X-ray spectroscopy, radio, and RM studies.
• Feedback from AGN and supernovae modifies IGrM entropy profiles, lowers baryon fractions, and influences cosmological measurements via its impact on galaxy group dynamics.

The intragroup medium (IGrM) is the hot, diffuse baryonic plasma occupying the gravitational potential wells of galaxy groups, defined operationally as systems with halo masses in the range $M_{500} \sim 10^{13}$--$10^{14}~M_\odot$, intermediate between massive clusters and individual galaxies. The IGrM is critical in regulating galaxy evolution via feedback, hosting substantial baryonic and metal reservoirs, and—due to its dynamics, enrichment, and multiphase structure—acts as a sensitive probe of astrophysical and cosmological processes, including feedback efficiency, magnetic field amplification, turbulence, baryon cycling, and the longstanding "missing baryons" problem.

1. Thermal and Nonthermal Properties

The IGrM is a predominantly hot, ionized plasma with characteristic temperatures $T \approx 10^6$--$10^7$ K, as inferred from X-ray observations and scaling relations. A key thermodynamic diagnostic is the entropy proxy,

$S = \frac{k_B T}{n_e^{2/3}},$

which encapsulates the assembly and feedback history. X-ray spectral analyses of low-mass groups with eROSITA and Chandra (e.g., $kT \approx 0.76$ keV in MRC 0116+111 (Mernier et al., 2019), $T \sim 1~\mathrm{keV}$ in CLoGS systems (Hough et al., 7 Jun 2024)) show that the IGrM typically exhibits higher central entropy, lower gas densities, and lower X-ray luminosities than would be expected from gravitational heating alone. This is notably distinct from intracluster medium (ICM) conditions, due to weaker accretion shocks and the greater relative impact of feedback.

The nonthermal component of the IGrM includes magnetic fields ($B \approx$ a few $\mu$G), cosmic rays, and turbulence. The standard deviation of Faraday Rotation Measures (RMs) from deep POSSUM grids demonstrates that the IGrM within 1--2 splashback radii enhances $\sigma_\text{RRM}$ by $6.9 \pm 1.8~\textrm{rad}~\textrm{m}^{-2}$, corresponding to magnetic field strengths of several $\mu$G (Anderson et al., 29 Jul 2024). Turbulent velocity dispersion $\sigma_\text{turb}$ scales as $\sigma_\text{turb} \propto e^{0.5}$, where $e$ is the specific thermal energy, indicating that the turbulent-to-thermal energy ratio is nearly constant across a wide halo mass range (Schmidt et al., 2021). The plasma beta parameter in the IGrM is often near unity, so that magnetic pressure can be dynamically significant. Radio-bright, X-ray-faint groups (e.g., MRC 0116+111) display exceptionally strong synchrotron emission over regions several times larger than the X-ray-emitting plasma, indicating spatially extended, aging, and likely re-accelerated relativistic electron populations (Mernier et al., 2019).

2. Multiphase Structure and Cool Gas Components

UV QSO absorption-line surveys (COS-IGrM (McCabe et al., 2021), partial Lyman limit systems (Narayanan et al., 2021)) reveal that the IGrM is multiphase, with cool ($T\sim10^4~\mathrm{K}$), warm ($T\sim10^{4.7}$–$10^6~\mathrm{K}$), and hot ($T>10^6~\mathrm{K}$) components. Multiphase gas is detected via various transitions, with Ly$\alpha$ and metal lines (C II, Si II, Si III, N V, O VI) indicating that while a pervasive hot phase fills the group potential, cooler phases exist as embedded clouds, filaments, and boundary layers. The O VI ion traces radiatively cooling gas at $T \sim 10^{5.8}$--$10^6$ K, but its covering fraction is only $\sim 44\%$ of sightlines, indicating that O VI is not a bulk tracer of the volume-filling IGrM but likely marks transitional or interface regions between hot and cool gas (McCabe et al., 2021).

Detailed studies of partial Lyman limit systems at $z \sim 0.8$ indicate that the detected gas is multiphase, photoionized, sub-solar in metallicity (e.g., $[\mathrm{O}/\mathrm{H}] \sim -1$), and shows chemical signatures ($[\mathrm{C}/\mathrm{O}] < 0$) of core-collapse SNe enrichment, suggesting metal release from galaxy outflows and tidal interactions, followed by mixing in the group environment (Narayanan et al., 2021). Observations with MUSE have imaged $>1000~\mathrm{kpc}^2$ Mg II nebulae in group environments, directly linking the IGrM’s spatial extent and kinematic complexity to enrichment via stellar-driven outflows and tidal stripping (Leclercq et al., 2022).

3. Energetics, Feedback, and Entropy Modification

Feedback from member galaxies and active galactic nuclei (AGN) is the dominant non-gravitational determinant of IGrM thermodynamic structure. Cosmological hydrodynamic simulations (e.g., OWLS (McCarthy et al., 2010), SIMBA-C (Hough et al., 7 Jun 2024), SIMBA-C Chem5 (Padawer-Blatt et al., 7 Feb 2025)) and classical entropy analysis (Cavaliere et al., 2016) reveal that while radiative cooling, star formation, and SNe feedback are all necessary, only feedback operating in quasar (high-Eddington) mode efficiently ejects low-entropy gas from low-mass progenitors at $z \sim 2$–4. Specifically, AGN feedback with temperature increments $\Delta T_\mathrm{heat} = 10^8$ K truncates the entropy distribution by preferentially removing lowest-entropy gas, leading to higher median IGrM entropies and suppressing the over-cooling (overproduction of stars) problem (McCarthy et al., 2010). The resulting gas entropy profiles deviate from pure gravitational shock heating, producing flatter slopes ($a\simeq 0.6$–0.8 in $K(r) = K_c + (K_R - K_c)(r/R)^a$) compared to clusters ($a \sim 1.1$) (Cavaliere et al., 2016).

The feedback-induced reduction in the baryon fraction of the IGrM is robustly detected by X-ray stacking in poor groups ($f_\mathrm{gas}\sim6$–8\% compared to the cosmic baryon fraction $f_b \approx 16\%$) (Li et al., 2 Dec 2024) and by comparing FRB dispersion measures to model halos, with group baryon fractions constrained below the expected cosmic value at $R_{500}$ and in tension with earlier X-ray estimates at $R_{200}$ (Lanman et al., 8 Sep 2025). The disparity is a direct consequence of feedback ejecting or preventing gas accretion into the group potential.

4. Enrichment, Metallicity, and Chemical Evolution

Metallicity and elemental abundance patterns in the IGrM constrain feedback models and the timing of enrichment. The IGrM is enriched by metals from type II (core-collapse) and Ia supernovae as well as asymptotic giant branch (AGB) stars, with mass-weighted iron and silicon abundances increasing moderately with group temperature and flattening in the most massive systems (e.g., $[\mathrm{Fe}/\mathrm{H}] \sim -0.9$ at $kT \sim 1~\mathrm{keV}$ (Liang et al., 2015)). Fractionally, $\sim50\%$ of IGrM metals originate from in-group galaxies, with intragroup stars directly contributing up to $30\%$ of Fe.

State-of-the-art simulations with time-resolved (non-instantaneous) enrichment channels (SIMBA-C Chem5 (Padawer-Blatt et al., 7 Feb 2025, Hough et al., 7 Jun 2024)) yield more physical, flatter, and lower-amplitude abundance profiles—particularly for Si, S, Ca, and Fe—than previous instantaneous recycling models. However, in low-mass groups, simulations tend to over-enrich the IGrM, highlighting the need for more physically accurate sub-grid turbulent diffusion and feedback implementations.

Abundance ratios such as $[\mathrm{Si}/\mathrm{O}]$ and $[\mathrm{Si}/\mathrm{Fe}]$ capture the balance between core-collapse and Type Ia contributions, with simulated values increasingly consistent with X-ray measurements as models incorporate separate enrichment channels and more nuanced feedback coupling (Hough et al., 7 Jun 2024).

5. Dynamics, Magnetic Fields, and Nonthermal Phenomena

Magnetization is a dynamically significant component—RM grid studies show that the IGrM typically hosts magnetic fields of several $\mu$G, with plasma beta $\beta \sim 1$ (magnetic pressure comparable to thermal pressure) (Anderson et al., 29 Jul 2024). IGrM magnetic pressure could be more dynamically important than in more massive ICMs or the low-density WHIM, potentially regulating gas cooling and outflow confinement. In exceptional systems (e.g., MRC 0116+111), past AGN activity likely generated turbulence and buoyant bubbles, resulting in extraordinarily high radio-to-X-ray ratios and extended, bright synchrotron haloes (Mernier et al., 2019). Bow shocks and supersonic galaxy motions can both compress and re-energize magnetic fields and relativistic electrons, as evidenced in detailed studies of head-tail radio galaxies like NGC 742 in NGC 741, where shock heating injects $2$–$5\times10^{57}$ erg into the central $10$ kpc IGrM cooling core and drives the formation of vortex rings and radio filaments that trace dynamical IGrM responses (Rajpurohit et al., 27 Aug 2024).

6. Observational Signatures and Baryon Census

Robust detection of the IGrM relies on X-ray spectroscopy, UV absorption-line studies, deep radio, and RM/DM integration techniques. X-ray surface brightness stacking with eROSITA has for the first time conclusively detected hot IGrM in optically-selected poor groups; fits to $\beta$-models constrain both spatial extent and core properties (Li et al., 2 Dec 2024). However, even in the best current samples, the total hot gas content falls short of accounting for the expected baryon budget. Upcoming RM/DM measurements from large FRB samples (CHIME/FRB Outrigger, POSSUM) will provide temperature-independent mapping of diffuse, ionized gas, giving direct, model-free constraints on baryon fractions and gas distribution. These approaches will allow systematic testing of gas retention, ejection, and redistribution in feedback-regulated group environments (Lanman et al., 8 Sep 2025, Anderson et al., 29 Jul 2024).

In the low-density, warm IGrM ($n \sim 10^{-4}-10^{-5}$ cm$^{-3}$, $T \sim 10^{5-6}$ K), features such as Odd Radio Circles (ORCs) can arise as supernova remnants expanding in the group medium, as the low density allows SNR shells up to several hundred parsecs in size with faint radio surface brightnesses (Omar, 2022). UV-optical searches for star formation in the IGrM using ultra-deep U-band imaging set stringent upper limits on the contribution of star formation to the intragroup light ($<1\%$), implying that the atomic gas in the IGrM is generally too diffuse to ignite significant star formation and that the IGrM is a passive baryon reservoir in typical environments (McCabe et al., 2023).

7. Evolution, Environmental Variation, and Cosmological Significance

The IGrM is sensitive to the assembly history, feedback, and environmental context of its host group. Its entropy structure, multiphase character, and degree of chemical and thermodynamic regulation vary with mass, accretion rate, and cosmic environment. In regions with ongoing accretion and strong feedback, entropy profiles are flatter and group baryon fractions lowest. Groups in poor environments or at low redshift may show entropy profile flattening or bending due to "starvation" of fresh inflow (Cavaliere et al., 2016).

In galaxy evolution, the IGrM acts as both a source and sink for baryonic mass, metals, and angular momentum—modulating fueling, star formation, and AGN/stellar feedback (Johnson et al., 2018, Oppenheimer et al., 2021). Cosmologically, IGrM physics governs corrections to the matter power spectrum at non-linear scales, and uncertainties in feedback strengths and baryon partitioning directly impact cosmological parameter estimation from lensing and LSS surveys (Oppenheimer et al., 2021).

The IGrM thus emerges as a highly structured, enriched, and energetically regulated baryonic component, whose properties record the interplay of gravitational dynamics, feedback, magnetic fields, multiphase structure, and environmental processes in the most common galaxy environments in the Universe.