External Gas Accretion Processes

• External gas accretion processes are mechanisms whereby systems gain gas from their surroundings, influencing star formation and galaxy structure.
• They encompass diverse channels including cold filament flows, hot halo cooling, galactic fountains, and merger-induced inflows.
• Quantitative metrics like accretion rates, kinematic signatures, and metallicity gradients provide insight into feedback and environmental effects.

External gas accretion processes denote the suite of mechanisms by which astrophysical systems acquire gaseous material from their surroundings, external to the initial reservoir or internal recycling loop. This encompasses the infall of cosmological gas onto galaxies and halos, the mixing of hot and cold phases in galaxy coronae, the accretion of gas via cosmic filaments, dynamical environmental effects that modulate gas supply, torques and instabilities that alter accretion geometries, and feedback-driven processes that can both enhance and suppress the inflow of external gas. These processes are fundamental to the ongoing evolution of galaxies, star clusters, and planetary systems, driving star formation, chemical evolution, and structural transformation.

1. Physical Mechanisms and Accretion Channels

External gas accretion is realized through several physically distinct channels, each operating in varying environments and scales:

1. Cosmic Filament and Cold Mode Accretion: In galaxies, gas accretion occurs as metal-poor material flows along cosmic filaments. This process is especially prominent in low-mass halos and at high redshift (Combes, 2013, Scholz-Diaz et al., 2021). Such flows can be continuous or clumpy, often leading to chemically inhomogeneous disks characterized by metal-poor star-forming regions.
2. Hot Halos and Coronal Cooling: At higher halo masses, infalling gas is shock-heated to the virial temperature ($T \gtrsim 10^6$ K) and forms a quasi-static corona. For gas to accrete onto the disk, it must cool radiatively as quantified by:

$$t_{\mathrm{cool}} = \frac{3 k_B T}{2 n_e \Lambda(T, Z)},$$

where $\Lambda(T, Z)$ is the cooling function dependent on temperature and metallicity (Fraternali, 2010). However, thermal stability mechanisms (buoyancy, conduction) often inhibit direct condensation of cool clouds.

1. Galactic Fountains and Fountain-Driven Accretion: Feedback processes in star-forming disks drive outflows (fountains) of metal-rich gas. These fountain clouds mix with hot, low-metallicity coronal gas in their wakes, greatly reducing the cooling time and triggering condensation—the “fountain-driven accretion” scenario (Fraternali, 2016). The mass gain rate of a fountain cloud follows:

$\frac{dM}{dt} \sim \alpha M_\mathrm{fountain},$

with $\alpha$ an efficiency parameter. This process “hides” much of the accretion since it does not occur as isolated clouds but via turbulent wakes.

1. Tidal Interactions and Minor/Major Mergers: Interactions with companions or minor/major mergers can deliver external gas, disrupt existing gas reservoirs, or drive non-axisymmetric inflows. Merger-driven accretion typically brings in higher metallicity gas compared to pristine cold mode accretion (Conselice et al., 2012, Scholz-Diaz et al., 2021).
2. Ram Pressure and Environmental Modulation: In low-density voids and at void walls, relative velocities between halos and ambient gas can reach regimes where ram pressure halts accretion or even strips existing gas. The stripping condition is set by

$$\rho_\mathrm{amb,gas} v_\mathrm{flow}^2 > \alpha \frac{G M_\mathrm{tot}(R) \rho_\mathrm{int,gas}(R)}{R}$$

(Thompson et al., 2022), where low-mass halos are particularly susceptible.

1. Preventive Feedback from AGN and Magnetic Fields: Giant radio lobes associated with AGN can inject magnetic energy into the IGM, raising the non-thermal pressure to levels that suppress the gravitational accretion of baryons onto neighboring halos (Qiu et al., 4 Sep 2025). Suppression becomes significant when the Alfvén speed approaches the velocity dispersion:

$x \equiv \frac{v_A}{\sqrt{2}\sigma_v} \gtrsim 1$

2. Quantitative Characterization and Observational Diagnostics

Quantitative assessment of external gas accretion relies on direct and indirect tracers:

• Gas Mass Balance Equations: For galaxies, the evolution of cold gas mass is tracked as

$$M_g(t) = M_g(0) + M_{g,\mathrm{M}}(t) + M_{g,\mathrm{A}}(t) - \int \psi dt + \varepsilon M_{*,\mathrm{recy}},$$

where $M_{g,\mathrm{M}}$ is gas from mergers and $M_{g,\mathrm{A}}$ is pure accretion (Conselice et al., 2012).

• Kinematic Signatures: Extra-planar gas rotation lags and radial inflows, such as gradients $\sim 15\,\mathrm{km\,s}^{-1}\,\mathrm{kpc}^{-1}$ in NGC 891, provide evidence for angular momentum loss via mixing with coronae (Fraternali, 2010, Fraternali, 2016).
• Chemical Inhomogeneities: Regions of anomalously low O/H and enhanced N/O ratios beyond closed-box evolutionary relations indicate mixing of disk gas with externally accreted metal-poor material (Luo et al., 2020).
• HI Profiles and Gas-Star Kinematics: Narrowing of HI linewidths and transitions from double-horned to single-peaked profiles in misaligned galaxies reflect central gas inflows resulting from angular momentum redistribution during external accretion (Zhang et al., 15 Sep 2025).
• Simulation-Derived Suppression Fractions: Cosmological MHD simulations quantify feedback-induced reductions in baryon fractions as a function of local $B$-field strength (Qiu et al., 4 Sep 2025): | Halo Mass Range ($M_\odot$) | Baryon Fraction Reduction (%) | |------------------------------|-------------------------------| | $[10^{11}-10^{12})$ | 17 | | $[10^{12}-10^{13})$ | 14 | | $[10^{13}-10^{14})$ | 12 |

3. Implications for Galaxy Evolution and Star Formation

External gas accretion sustains star formation and drives structural evolution:

• Maintenance of Star Formation: Gas accretion rates inferred from fountain-driven models ($\sim 3\,M_\odot\,\mathrm{yr}^{-1}$ in NGC 891) match observed SFRs (Fraternali, 2010, Fraternali, 2016). At high redshift, the inflow rates required to sustain SFRs in massive ($M_*>10^{11}\,M_\odot$) galaxies are $$\dot{M}_\mathrm{acc} = 96 \pm 19\,M_\odot\,\mathrm{yr}^{-1}$$ (Conselice et al., 2012).
• Chemical Evolution and Metallicity Gradients: Inflows of low-metallicity gas dilute disk metallicities, modulate abundance gradients, and can trigger star formation in external annuli or central regions (Luo et al., 2020, Egorova et al., 2018, Combes, 2013).
• Bar and Spiral Structure Renewal: Gas accretion rejuvenates spiral arms, bars, and drives inside-out growth of disks (Combes, 2013).
• Early-Type and Quiescent Galaxies: In S0 and E/S0 systems, external accretion is the primary route for late-time gas replenishment. Observed counter-rotating cores and misaligned gas reservoirs indicate past accretion and merger episodes that may reactivate nuclear activity (AGN) and reshape kinematics (Raimundo, 2021, Davis et al., 2015).
• Impact of Feedback and Environmental Suppression: AGN-driven radio lobes can reduce baryon accretion, providing a preventive feedback channel that must be accounted for in galaxy formation models (Qiu et al., 4 Sep 2025).

4. Physical Limits, Dependencies, and Environmental Effects

Multiple environmental and internal parameters modulate the efficacy of external gas accretion:

• Halo Mass and Temperature: Fountain-driven condensation loses efficiency in halos of $M_\mathrm{halo}\gtrsim10^{12}\,M_\odot$ due to high coronal temperatures (T > 4 × 10⁶ K), limiting star formation in massive systems (Fraternali, 2016).
• Gas Compressibility and Cooling: The equation of state and cooling function $\Lambda(T, Z)$ set the transition between efficient, nearly isothermal inflow (favorable for accumulation) and adiabatic regimes where heating suppresses accretion (Naiman et al., 2011).
• Relative Velocities and Ram Pressure: In void wall environments, ram pressure and high flow velocities truncate gas reservoirs, forming jellyfish-like morphologies and suppressing external accretion in low-mass halos (Thompson et al., 2022).
• Feedback-Driven Suppression: Radio lobe–seeded magnetic fields elevate non-thermal pressure, quantified via $v_A/\sqrt{2}\sigma_v$, above unity to suppress acrretion. The effect is inhomogeneous and correlated with the presence of powerful AGN sources (Qiu et al., 4 Sep 2025).

5. External Gas Accretion beyond Galaxies: Protostars, Planets, and Star Clusters

• Protostellar Clouds: External, subsonic inflow reduces the protostellar accretion rate to a fraction $\sim 2u_\infty/c_s$ of the classical inside-out collapse rate, matching low observed accretion luminosities and slow infall dynamics (1207.1453).
• Circumplanetary Accretion: In the planetary context, the accretion rate onto embedded gas giants is modulated by the angular momentum transport efficiency in the circumplanetary disk (CPD). Low-viscosity (“dead zone”) CPDs can bottleneck gas inflow, stretching mass doubling timescales to $5$–$10$ Myr and regulating final planet mass (Rivier et al., 2012, Nelson et al., 2022).
• Star Clusters: Young clusters embedded in molecular clouds accrete mass along dense filaments (efficient) and from the ambient medium (less efficient, with possible net mass loss if drift velocities are high). Mass gain and loss are governed by SPH simulations tracking the balance between filamentary accretion and unbinding (Karam et al., 2023).

6. Open Questions and Future Directions

While recent observational and simulation advances have refined the physical models and provided stringent constraints, several open issues remain:

• Direct Detection Limitations: The “hidden” nature of much accreted gas (e.g., through mixing/condensation rather than direct HI detection) poses continued challenges (Fraternali, 2010, Fraternali, 2016).
• Chemical vs. Kinematic Diagnostics: Integrating spatially resolved metallicity and HI maps (linked to star formation and inflow rates) will sharpen indirect signatures of accretion (Luo et al., 2020, Zhang et al., 15 Sep 2025).
• Modeling Feedback Interplay: Next-generation simulations must couple internal (stellar and AGN) and external (jet-lobe, environment) feedback with accretion, particularly including global magnetic field effects (Qiu et al., 4 Sep 2025).
• Evolutionary Transitions: Understanding how external accretion evolves from being the dominant driver at high redshift to a modulated, environment-dependent process at low redshift remains a central pursuit (Conselice et al., 2012, Scholz-Diaz et al., 2021).
• Star Formation and Quenching Pathways: Disentangling the transition from HI-rich, actively star-forming galaxies to HI-deficient, quenched systems, especially in the context of angular momentum redistribution and feedback-driven suppression, is an ongoing area of research (Zhang et al., 15 Sep 2025, Davis et al., 2015).

External gas accretion is thus established as a multifaceted and indispensable set of processes across cosmic environments and evolutionary stages, governing the regulation of baryonic matter in structure formation and the lifecycle of astrophysical systems.