Gas-Removal Timescale in Astrophysics

• Gas-removal timescale is a critical duration that quantifies how quickly gas is expelled, consumed, or transformed in astrophysical systems, affecting star, planet, and galaxy evolution.
• This concept integrates models of dynamical feedback, ram pressure stripping, and internal consumption with mathematical formulations to constrain evolutionary processes across various mass regimes.
• Accurate estimation of gas-removal timescales informs simulations and guides observational strategies for studying star formation quenching, cluster survival, and planetary disk evolution.

Gas-removal timescale refers to the characteristic duration over which gas is depleted, expelled, or transformed within a stellar, planetary, or galactic system due to a combination of internal and external physical mechanisms. In astrophysical contexts, this timescale sets the evolution of planet formation, star cluster survival, galaxy quenching, and ISM cycling. The governing physical processes, relevant equations, and outcomes vary widely depending on environment, mass regime, and feedback phenomena, but the timescale itself is always a critical modeling and observational constraint.

1. Key Physical Processes Governing Gas-Removal Timescales

In astrophysical systems, gas removal is driven by several classes of processes, each setting distinct timescales:

• Dynamical Expulsion and Feedback: In star clusters, stellar feedback (winds, supernovae, pulsar winds, hypernovae) imparts momentum and energy, driving gas outward. The balance between feedback energy and gravitational binding energy, often regulated by the cluster compactness parameter $C_5$, determines both the possibility and timescale for rapid, "explosive" gas expulsion (Krause et al., 2015, Bobrick et al., 2 Sep 2025).
• Ram Pressure and Environmental Stripping: Galaxies traversing a dense intracluster or intragroup medium may lose their diffuse atomic (HI) and, to a lesser degree, molecular gas through ram pressure stripping, with characteristic timescales $\lesssim1.5$ Gyr, often shorter than internal consumption timescales (Boselli et al., 2014, Rodríguez et al., 2022).
• Internal Consumption and Feedback: In galaxies, the conversion of gas into stars (astration), removal via supernova (SN) feedback, and regulation by AGN feedback dominate gas depletion. The absence of replenishment (starvation or "overconsumption") sets depletion and quenching timescales for isolated or accretion-shutoff satellites (Visser-Zadvornyi et al., 19 Mar 2025).
• Heating and Ionization: In early-type galaxies, ionization by evolved low-mass stars (post-AGB), SNe Ia, and AGN feedback act in tandem to transition the ISM from cold, star-forming phases to hot, inert states, producing coordinated exponential declines in molecular, atomic, and dust mass (Michałowski et al., 9 Jan 2024).
• Planet Formation Disks: Protoplanet accretion and atmospheric recycling are limited either by the disk dissipation timescale or the comparative rates of Bondi/Hill accretion versus Kelvin–Helmholtz contraction, where typical runaway growth occurs on $\sim10^5$ yr, much shorter than the surrounding solid core's assembly, but contingent on the persistence of nebular gas (Machida et al., 2010, Ormel et al., 2014).

2. Characteristic Timescales Across Different Astrophysical Environments

The measured or simulated gas-removal timescales span several orders of magnitude across astrophysical systems, reflecting varying physical scales and feedback modalities:

System/Context	Gas-Removal Timescale	Dominant Mechanism(s)
Protoplanet runaway accretion (gas giants)	$10^5$ yr	Gravitational capture, disk dissipation
Embedded protoplanet atmosphere recycling	$10^4$ yr ($t_\mathrm{replenish}$)	Nebular inflow-outflow, modified Bondi cycling
Low-mass star-forming galaxies	$100$–$500$ Myr	SN-driven HI turbulence, energy dissipation
Dwarf spheroidal galaxies (e.g. Ursa Minor)	$\sim0.6$ Gyr (central), $\sim3$ Gyr (global)	Type II+Ia SNe, environmental stripping
Clustered star-forming regions/clusters	Crossing time ($0.2$–$5$ Myr), up to 10⁹ yr (globular clusters)	Feedback (winds/SNe), feedback-limited accretion
Spiral galaxies (ram pressure in clusters)	$0.1$–$1.5$ Gyr (HI), $~1–3$ Gyr (total gas)	Ram pressure stripping, star formation
Quenching in satellite galaxies	Peak at $M_\star\sim10^{9.5}M_\odot$ (typically 1–5 Gyr), faster outside this mass	Overconsumption (star formation + feedback)
Early-type galaxies (ISM and dust removal)	$2.0$–$2.5$ Gyr (cold gas/dust), SFR decline at $1.8$ Gyr	Internal ionization, SNe Ia, AGN, suppression of fragmentation
High-redshift galaxies	$\sim0.4$ Gyr (dust; proxy for gas)	AGN outflows, SNe shocks, efficient astration
Globular clusters (GCs)	$\lesssim10^9$ yr	Accretion onto stars/compact objects, feedback

These timescales critically set the window for planet assembly, star and cluster survival, the shut-down of star formation (quenching), and the emergence of multiple populations or morphological transformations in galaxies.

3. Mathematical Formulations of Gas-Removal and Depletion

The physical modeling of gas-removal frequently relies on governing equations tailored to system geometry and relevant processes:

• Energy and Momentum Balance in Clusters:

$$\frac{\partial}{\partial t}({\cal M} v) = p A - {\cal M} g$$

where ${\cal M}$ is the shell mass, $v$ its velocity, $p$ the internal pressure, $A$ the area, and $g$ the gravitational field (Krause et al., 2015).

• Exponential Decay for ISM/Dust in Galaxies:

$M_{\mathrm{gas}}(t)/M_* = A \, e^{-t/\tau}$

where $\tau$ is the characteristic removal timescale (Michałowski et al., 2019, Michałowski et al., 9 Jan 2024, Leśniewska et al., 27 May 2025).

• Analytic Models of Overconsumption:

$$\frac{d M_{\mathrm{gas}}}{dt} = -\mathrm{SFR}(t) - \mathrm{Outflow}(t)$$

SFR may take the form $\mathrm{SFR}(t) = G_{SF} M_{\mathrm{gas}}$, where $G_{SF}$ is the star-formation efficiency; outflow includes feedback processes (Visser-Zadvornyi et al., 19 Mar 2025, Semenov et al., 2017).

• Recycling and Cycling Timescale Models:

$$t_{\mathrm{dep}} = (1+\xi+t_{\mathrm{nsf}}/t_{\mathrm{sf}}) t_{\mathrm{sf}}$$

where $\xi$ is a mass-loading parameter, and $t_\mathrm{sf}$ and $t_\mathrm{nsf}$ are times spent in the star-forming and non-star-forming ISM, respectively (Semenov et al., 2017).

• Planetary Disk Gas Accretion:

$$\frac{dM_p}{dt} = \begin{cases} 0.83\, \left(r_{\rm H}^3\right)^{3/2} & r_{\rm H}^3 < 0.3 \ 0.14 & r_{\rm H}^3 > 0.3 \ \end{cases}$$

and

$$t_\mathrm{replenish} \simeq 10^4\,\mathrm{yr}\cdot(\chi_\mathrm{atm}/f_{\mathrm{cover}}^*)(M_p/M_\oplus)^{-2}(a/1~\mathrm{AU})^{2.75}$$

for atmospheric recycling (Machida et al., 2010, Ormel et al., 2014).

4. Feedback, Turbulence, and Regulation

Stellar and AGN feedback, as well as turbulence, are central in both directly removing ISM gas and lowering its star formation efficiency:

• Supernova Feedback: SNe inject mechanical energy over timescales of $10^7$–$10^8$ yr, with the decay of ISM turbulence sustaining elevated line widths and delaying gas cooling/fragmentation (Hunter et al., 25 Jul 2025, Rodríguez et al., 2022, Caproni et al., 2017).
• AGN Feedback: Particularly at higher redshifts, AGNs rapidly eject, heat, or ionize ISM and dust, reducing cold gas content and accelerating the removal timescale to sub-Gyr values (Leśniewska et al., 27 May 2025).
• Morphological Quenching / Turbulent Support: In early-type galaxies, even with normal gas fractions, high velocity dispersion, bulge-dominated morphology, or strong magnetic fields stabilize the ISM against collapse, suppressing star formation before the ultimate removal or transformation of the cold phase (Michałowski et al., 9 Jan 2024).

5. Environmental and Structural Dependencies

The efficiency and timescale of gas removal are highly sensitive to environmental and internal structure:

• Cluster Compactness ($C_5$): Gas expulsion in young and massive stellar clusters occurs rapidly (on the crossing time) only if the compactness index $C_5$ is sufficiently low; for high $C_5$ (as in most GCs), gas removal is more protracted and gentle, mediated by longer-term feedback (Krause et al., 2015, Bobrick et al., 2 Sep 2025).
• Galaxy Mass Dependence: Satellite quenching (gas removal) timescales exhibit a non-monotonic (peaked) dependence on stellar mass, the maximum corresponding to minimal total feedback efficiency and maximal star formation efficiency (Visser-Zadvornyi et al., 19 Mar 2025).
• ISM Phase Hierarchy: In cluster galaxies, atomic hydrogen (HI) is stripped on much shorter timescales than the denser molecular phase, supporting an outside-in removal picture (Boselli et al., 2014).

6. Comparative and Cross-Scale Perspectives

The gas-removal timescale is a unifying concept across a wide range of systems, from protoplanetary disks to galactic environments and dense stellar systems. Recent work emphasizes that:

• Quenching of star formation and ISM/dust removal track similar exponential timescales, typically a few Gyr at low redshift but declining with increasing cosmic epoch to hundreds of Myr at $z>3$ due to more efficient feedback (Michałowski et al., 2019, Leśniewska et al., 27 May 2025).
• In low-mass galaxies, measurable HI turbulence and its decay reflect energy input from star formation episodes up to several hundred Myr in the past (Hunter et al., 25 Jul 2025).
• In protoplanetary environments, the timescale for atmospheric gas-recycling can be much shorter than cooling or contraction, halting rapid gas growth and shaping planetary architectures (Ormel et al., 2014).
• In globular clusters, a combination of gas accumulation via stellar winds, accretion onto luminous stars and compact objects, and eventual feedback clears the cluster of gas within $\lesssim10^9$ yr, with this timescale directly reflected in the masses and abundance patterns of multiple stellar populations (Bobrick et al., 2 Sep 2025).

7. Significance for Astrophysical Models and Observations

The precise quantification of gas-removal timescales informs models of galaxy and stellar evolution, planet formation, chemical enrichment, and environmental transformation. Empirically-derived timescales anchor feedback prescriptions in hydrodynamical simulations, set expectations for the duration of star-forming or planet-accreting phases, and constrain the mechanisms responsible for rapid versus slow quenching phenomena. Disentangling the contribution of feedback-driven, environmental, and structural processes—and resolving the timescales associated with each—remains central for predictive theoretical frameworks and for interpreting the observed diversity in galactic and stellar system properties.