--- title: "Ram Pressure Stripping and Returning Gas Interactive Lab | Sandbox Physics" description: "Can gas leave a galaxy disk and still fall back? A cluster wind can lift disk gas without giving it enough energy to escape." canonical_url: "https://sandboxphysics.com/labs/ram-pressure-stripping/" language: "en" page_type: "lab" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/labs/ram-pressure-stripping/" last_modified: "2026-11-12" --- A galaxy meets the cluster wind # Ram Pressure Stripping and Returning Gas **Can gas leave a galaxy disk and still fall back?** A cluster wind can lift disk gas without giving it enough energy to escape. As the pressure pulse fades, some bound gas falls back. Measure displacement and binding separately: a long visible tail does not by itself tell you how much fuel the galaxy has permanently lost. Follow gas stripped from a galaxy on five computed cluster paths. Compare gas outside the disk, gravitational binding and fuel for star formation. > Interactive simulation: open this page in a web browser to change the parameters and run the model. Model assumptions This reduced model follows 64 axisymmetric gas sheets in a fixed stellar, bulge and halo potential. A constant-speed, face-on straight galaxy path samples one tapered beta atmosphere at its centre. That single incident wind acts on every sheet, including the far tail; no local ICM force field or hydrodynamics is solved. Fixed-area sheets consume gas on a two-billion-year timescale with zero mass return. Disk star formation counts gas currently inside the stated cylindrical volume; the global rate is identical for all five paths. Gas outside that volume may remain bound and return. The separate global-light exercise convolves specified histories with a fixed solar-metallicity Salpeter SSP and an optional foreground dust screen. It has no disk/tail colours, newborn-star trajectories or wind-derived global shutdown. The displayed 129 native frames come from independently checked trajectories; path and field interpolation are disabled. Bound gas inside disk Bound gas outside disk Currently positive energy Which initial gas sheets were lifted? The horizontal coordinate is the initial radius. Select one sheet with the gold probe to follow its saved trajectory. $i=51$ Where the gas is still forming stars Gold: disk volume. Blue: every gas sheet. The global curve is identical for all five paths under the specified consumption law. The incident pulse sampled at the galaxy centre Every sheet receives this same incident density and wind speed. Its actual force also depends on its own normal velocity and remaining gas column. Global stellar light: a separate history exercise These global bands combine an initially ten-billion-year-old population with the specified young-star history. They stay identical across impact paths. The optional shutdown at650Myr is an input counterfactual, not an outcome of the wind. Foreground dust screen Gold: the shared continuing history. Rose: the separately specified shutdown. Both use solar-metallicity Salpeter BC03 spectra. SSP mass return changes the population light model; it does not feed the zero-return gas calculation. No disk or tail colours are assigned. Mass, work and the selected source identity The external reservoir supplies work and dissipated energy. No finite ICM reaction, thermal spectrum or newborn-star spatial distribution is inferred. Physics tutorial ## Gas outside the disk can still belong to the galaxy **Background** A galaxy crossing a cluster meets hot, dilute gas. In the galaxy frame that gas arrives as a wind. The wind pushes on the diffuse gas disk, while stars and dark matter supply gravity. **Why it matters** The gas responds to the whole pressure history. Its position near peak pressure and its eventual binding answer different questions. Start with the essentials - **Focus question**: Does leaving the disk mean escaping the gravitational potential? - **One-sentence intuition**: Displacement is a position test. Binding also requires velocity and gravitational potential. ### Core mathematical model #### The pressure scale of the incident wind $$ P_{\rm inc}(t)=\rho_{\rm COM}(t)V^2,\qquad F_z=A\rho_{\rm COM}|V-v_z|(V-v_z) $$ The incoming speed $V$ is fixed. Each sheet has area $A$ and normal velocity $v_z$. The density is sampled once at the galaxy centre and used for every sheet. The finite pulse follows a straight path through a tapered beta profile. #### Position and binding $$ \mathcal D=\{R\leq16\,\mathrm{kpc},\ |z|\leq1\,\mathrm{kpc}\},\qquad e=\tfrac12|\mathbf v|^2+\Phi_{\rm ext} $$ The blue cylinder marks the finite disk volume. Gas outside it can still have negative specific energy. Positive energy describes the current fixed potential; the wind may continue doing work later. #### A specified gas-consumption law $$ \dot m=-\frac m{\tau_\star},\quad\tau_\star=2\,\mathrm{Gyr},\qquad {\rm SFR}_{\mathcal D}=\sum_{i\in\mathcal D}\frac{m_i}{\tau_\star} $$ Every sheet keeps forming stars at the specified rate wherever it moves, with zero returned mass. The disk-volume rate changes as gas crosses the boundary. Summing every sheet gives the same global rate for every path. #### Where force work and dissipated energy go $$ \dot W=F_zv_z,\qquad\dot Q=F_z(V-v_z)\geq0,\qquad\dot E_{\rm in}=F_zV=\dot W+\dot Q $$ The external wind is a prescribed reservoir. Its input splits into mechanical work and relative-motion dissipation; this does not evolve the finite cluster gas or predict radiation. The saved birth-energy inventory also follows mass removed from the gas. ### Common difficulties #### A long tail and permanent loss **Typical misconception** Every tail point has escaped. **Better mental model** Compare the outside fraction with the currently unbound fraction. Then watch A returning tail. Bound gas can cross the disk boundary twice. #### Star formation inside a chosen volume **Typical misconception** A falling disk rate proves the whole galaxy has shut down. **Better mental model** The model keeps the same consumption law outside the disk. Its global rate is deliberately wind-independent. The disk measurement reveals fuel transport, not a calculated global shutdown. ### Run the experiment 1. 01 #### Follow the strongest encounter Start with Through the centre. Pause near closest approach and drag the gold probe into the tail. **What to observe:** The probe selects a real radial sheet. Its position, velocity, binding and saved energy all refer to the same time. 2. 02 #### Watch gas return Choose A returning tail, then replay from the common start. Compare its disk rate before and after the pressure peak. **What to observe:** Gas can be lifted beyond the disk boundary and later return. A recovery of the disk rate follows the same actual gas positions. 3. 03 #### Change the path, keep the camera separate Drag the galaxy to switch among five computed paths before releasing the pointer. Drag the background to turn the camera. **What to observe:** Changing the path selects another complete trajectory. Camera rotation reveals depth and leaves every physical value unchanged. Physical conclusion A tail records transport. Position, binding and the location of star formation must be measured separately. ## Sources and model context - [Köppen et al. (2018)](https://arxiv.org/abs/1806.05887v1) — finite ram-pressure pulses, impulse and fallback. The trajectories here are independent computations with the stated fixed-potential and fuel assumptions. - [Xie et al. (2025)](https://arxiv.org/abs/2504.12863v1) — differences between stripping algorithms and exposure histories; published medians are not individual galaxy trajectories. - [BC03 / EzGal, fixed teaching input](https://github.com/cmancone/easyGalaxy/tree/133c62b383fae964701b673e62517293161b41d5) — Bruzual and Charlot population models; EzGal by Conor Mancone and Anthony Gonzalez. Only our band integrals are included; original spectra are fetched separately. The fixed distribution notice is retained with this feature. 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[Article **From a wind pulse to a restoring force**](https://sandboxphysics.com/stories/ram-pressure-stripping/from-a-wind-pulse-to-restoring-force/) 4. [Article **Follow fuel across the disk boundary**](https://sandboxphysics.com/stories/ram-pressure-stripping/follow-fuel-across-the-disk-boundary/) 5. [Article **Recompute five fixed gas slices**](https://sandboxphysics.com/stories/ram-pressure-stripping/recompute-five-fixed-gas-slices/) 6. [Article **Why global colour needs a history**](https://sandboxphysics.com/stories/ram-pressure-stripping/why-global-colour-needs-a-history/) 7. [Article **Read exposure histories and multiphase tail physics**](https://sandboxphysics.com/stories/ram-pressure-stripping/read-exposure-and-multiphase-tail-physics/) Fields & matter · Starter path ## From a field to an electron microscope Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface. [Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/) 1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/) 2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/) 3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/) 4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/) 5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/) 6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/) [View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)