Ram Pressure Stripping in Galaxy Clusters

• Ram Pressure Stripping is a hydrodynamic process that removes a galaxy’s interstellar medium as it moves through the dense intracluster medium.
• Observable features include asymmetric gas tails, truncated disks, and enhanced star formation or quenching in affected regions.
• Modeling via both analytic prescriptions and high-resolution simulations reveals dependencies on galaxy mass, orbital dynamics, and ICM properties.

Ram pressure stripping (RPS) is a hydrodynamical process that removes the interstellar medium (ISM) from galaxies as they traverse the hot, diffuse intracluster medium (ICM) of galaxy clusters, profoundly influencing galaxy evolution in dense environments. RPS acts by subjecting galaxies to a dynamical wind pressure that can overcome the gravitational binding of their ISM, resulting in truncated gas disks, altered star formation, metal enrichment patterns, and the development of distinctive gaseous and stellar morphologies. The process manifests across a wide range of galaxy and cluster masses and is observed from the local Universe to redshifts $z>2$.

1. The Physics and Classical Criterion of Ram Pressure Stripping

The core physical principle of ram pressure stripping is the competition between external wind pressure from the ICM and internal gravitational forces binding the gas to the galaxy. The ram pressure is given by the expression: $P_{\rm ram} = \rho_{\rm ICM} v^2$ where $\rho_{\rm ICM}$ is the local ICM density and $v$ is the galaxy’s velocity relative to the ICM (Tiwari et al., 7 Nov 2024, Durret et al., 2022, Tecce et al., 2010). Stripping ensues when $P_{\rm ram}$ exceeds the local gravitational restoring pressure: $P_{\rm grav} \approx 2\pi G \Sigma_* \Sigma_g$ where $G$ is the gravitational constant, $\Sigma_*$ and $\Sigma_g$ are the stellar and gas surface densities at a given disk radius (Tiwari et al., 7 Nov 2024, Luo et al., 2022, Choi et al., 2022). The radius $r_{\rm strip}$ at which the equality $P_{\rm ram}=P_{\rm grav}$ holds defines the truncation of the gas disk. Gas at larger radii is unbound and removed from the galaxy.

Refinements of the classical criterion incorporate disk geometry, the addition of self-gravitating gas, the angle between the wind and disk, and the contribution of non-stellar potential terms (Lee et al., 2022, Wang et al., 2020, Xie et al., 17 Apr 2025). For instance, the anchor force may be modeled as: $$F_r = 2\pi G [\Sigma_*(r) + \Sigma_{\rm HI}(r)] \Sigma_{\rm HI}(r)$$ to account for the self-gravity of H I in the outer disks (Wang et al., 2020).

2. Observable Manifestations and Multiphase Gas Stripping

RPS is most readily identified through one-sided, asymmetric gas or stellar features—truncated H I disks, “jellyfish” morphologies with trailing tails (both ionized and molecular), and compressed gas on the leading side (Luo et al., 2022, Laudari et al., 2021, Ebeling et al., 2019, Xu et al., 27 Mar 2025). The stripped material can extend tens of kpc beyond the stellar disk (e.g., 87 kpc in the case of ESO 137-001 (Luo et al., 2022)) and is frequently detected in multiple phases:

• Neutral and Molecular Gas: Stripped tails and asymmetries are observed in H I and CO, confirmed with ALMA, VLA, and APEX (Laudari et al., 2021, Luo et al., 2022, Xu et al., 27 Mar 2025).
• Warm Ionized Gas: Extended H$\alpha$ emission traces shocked gas and star-forming regions in the tail (Luo et al., 2022, Lee et al., 2022).
• Dust and X-ray Emission: Dust filaments and soft X-ray tails mark the removal and ablation of denser ISM clouds (Laudari et al., 2021).

Velocity fields derived from IFU spectroscopy (e.g., MUSE) demonstrate that stripped gas initially retains the kinematics of the parent disk (embedded rotation signatures), with velocity gradients perpendicular to the tail direction that fade at downstream distances (disappearing $\sim$45 kpc downstream in ESO 137-001 (Luo et al., 2022)). Velocity dispersions rise from $\sim$30–35 km s$^{-1}$ in the undisturbed disk to $80$ km s$^{-1}$ in the tail at 20 kpc, a signature of turbulence driven by Kelvin–Helmholtz instabilities and ICM–ISM mixing (Luo et al., 2022, Choi et al., 2022). Cold and warm phases are kinematically mixed, as confirmed by CO and H$\alpha$ LOS velocity differences below 20 km s$^{-1}$ (Luo et al., 2022).

3. Star Formation, Chemical Evolution, and Gas Survival

RPS impacts star formation activity in a temporally and spatially non-uniform manner:

• Triggered (Compression Phase): The leading edge’s ISM is compressed by the incoming wind, temporarily raising the star formation rate (SFR) by up to 30–50% in both simulations and observations (Vulcani et al., 2023, Choi et al., 2022, Lee et al., 2022, Wang et al., 2020). The “starburstiness” $R_{\rm SB} = \mathrm{SFR} / \mathrm{SFR}_{\rm MS}$ correlates positively with cluster velocity dispersion, ICM density, and $P_{\rm ram}$ (Lee et al., 2022).
• Quenching (Stripping Phase): After compression, as gas is stripped, SFR declines. In extreme environments, complete quenching occurs within $\sim200$ Myr, while milder RPS leads to slow strangulation on Gyr timescales (Steinhauser et al., 2016, Choi et al., 2022).
• Disk vs. Tail Star Formation: SFR is often enhanced in the galactic disk before gas is stripped, with the stripped tails hosting spatially separated star-forming knots in some cases (“jellyfish” galaxies), though tails can also be nearly devoid of active SF (e.g., ESO 137-002 (Laudari et al., 2021)).
• Chemical Enrichment: RPS preferentially removes metal-poor outer gas, producing a $+0.2$ dex mean metallicity enhancement at a fixed mass and shallow negative metallicity gradients ($-0.03$ dex/Mpc predicted for RPS alone) (Gupta et al., 2017, Khoram et al., 8 Apr 2025). Metallicity offsets are more pronounced in lower-mass systems, and detailed modeling indicates that additional processes like strangulation (inhibition of further inflow) contribute to observed gradients (Gupta et al., 2017, Khoram et al., 8 Apr 2025).
• Survival of Stripped Gas: The fate of stripped ISM is controlled by the competition between radiative cooling and ICM energy flux. Tails may remain multiphase (H I/H$\alpha$) or become X-ray–bright if mixing is insufficient to allow cooling before escape (Choi et al., 2022, Luo et al., 2022).

4. Parametric Dependence, Regimes, and Environmental Impact

The efficacy and nature of RPS depend on several interdependent cluster and galaxy parameters:

• Cluster and Galaxy Mass: In Coma-like halos (log $M_h$ / $M_\odot$ ∼ 15), nearly all low-mass galaxies ($\log M_\star/M_\odot < 9.5$) suffer strong RPS and lose most of their gas during first pericentric passage. In Virgo-like clusters (log $M_h$ / $M_\odot$ ∼ 14), only about half the galaxies experience strong RPS during first passage, and significant fractions retain much of their gas after pericenter (Xie et al., 17 Apr 2025).
• Location and Phase Space: RPS signatures are most prevalent at small clustercentric radii and high orbital velocities (“recently accreted” orbits). Weak/incipient RPS (“preprocessing”) is already present at and beyond $R_{200}$ in massive clusters, and effects are not confined to the core (Wang et al., 2020, Lee et al., 2022).
• Critical Ram Pressure and Gas Loss Fractions: Quantified in GAEA/TNG, a $P_{\rm ram}$ of $10^{-10.5}\,{\rm g\,cm}^{-1}\,{\rm s}^{-2}$ can strip up to $90\%$ of the cold gas, while $10^{-13.5}\,{\rm g\,cm}^{-1}\,{\rm s}^{-2}$ removes only $\sim$20\% (Xie et al., 17 Apr 2025).
• ICM Density and Velocity Structure: Direct hydrodynamical sampling of the ICM density and velocity field is essential for accurate RPS estimates; analytic models systematically overestimate ram pressure by $\gtrsim50\%$ at $z>0.5$ (Tecce et al., 2010).

Cluster Halo Mass	Typical Fraction of Gas Loss in Low-Mass Satellites	Reference
Virgo-like ($\log M_{h} \sim 14$)	$\lesssim 50\%$ after first pericenter	(Xie et al., 17 Apr 2025)
Coma-like ($\log M_{h} \sim 15$)	$\gtrsim 90\%$ after first pericenter (low-mass galaxies)	(Xie et al., 17 Apr 2025)

5. Modeling and Theoretical Developments

Modeling efforts range from analytic prescriptions to high-resolution numerical simulations:

• Semi-analytic Models (SAMs): Implement the Gunn & Gott criterion to compute instantaneous stripping radii and mass loss at each timestep. Realistic ICM environments can be reconstructed from hydrodynamic simulations, avoiding the overestimations inherent in spherically symmetric analytic profiles (Tecce et al., 2010, Gupta et al., 2017).
• Hydrodynamical Simulations: Adaptive mesh and moving-mesh approaches (e.g., cdFLASH, AREPO) capture multiphase ISM, turbulence, disk–tail kinematics, feedback and mixing. Numerical studies demonstrate the importance of local ISM and ICM structure, cooling, turbulence, and the survival of cold molecular clouds (Steinhauser et al., 2016, Choi et al., 2022, Steyrleithner et al., 2020, Luo et al., 2022).
• Key Uncertainties: The importance of inclination, orbit, local ICM substructure, and the role of secondary mechanisms (tidal forces, starvation, strangulation) complicate the direct interpretation of RPS signatures. There is broad agreement that the analytic momentum-balance criterion works well during active stripping with well-defined ICM, but it fails in pre-/post-peak RPS or where strong non-RPS effects are present (Lee et al., 2022).
• Accretion and AGN Feeding: While RPS can drive central gas inflows via induced pressure torques, observational studies have not found widespread enhancement of AGN activity in RPS galaxies; the duty cycles or luminosity boosts may be too brief or weak for current sensitivity (Tiwari et al., 7 Nov 2024).

6. Broader Implications, Diversity, and Frontiers

RPS is now established as a primary driver of environmental quenching, with broad astrophysical consequences:

• Environmental Quenching and Early-Universe RPS: ALMA and JWST have revealed clear RPS signatures in $z=2.51$ cluster CLJ1001, including elongated, one-sided molecular gas tails in galaxies with undisturbed stellar morphologies, confirming RPS as an important quenching process even at the epoch of cluster formation (Xu et al., 27 Mar 2025).
• Mass–Metallicity Relation and Population Gradients: RPS induces a $0.2$ dex metallicity offset over non-stripped analogs, especially at low $M_\star$, and drives both spatial and population gradients (removal of low-mass, low-metallicity galaxies from the star-forming population near cluster centers) (Khoram et al., 8 Apr 2025, Gupta et al., 2017).
• Jellyfish Galaxies, Disk Truncation, and Tails: The morphological and kinematic properties of RPS galaxies are utilized as probes of cluster collision geometry and merger axes (Ebeling et al., 2019, Luo et al., 2022).

RPS remains a central subject of paper in extragalactic astrophysics, with ongoing work focused on the microphysics of gas–ICM interaction, the fate and cooling of stripped material, the coupling to star-formation and AGN feedback modes, and the statistical quantification of its role in galaxy population evolution across cosmic time.