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.

Radius enclosing 90% of disk gas $7.01\,\mathrm{kpc}$

Gas outside the disk volume $26.6\,\%$

Currently unbound gas $0\,\%$

Star formation inside the disk $0.186\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}$

Choose a path through the cluster

One of five computed paths

0

b/r_c

Time relative to closest approach

-2.34

t/\mathrm{Myr}

Drag the galaxy to switch paths; drag the gold probe to select gas. The background turns the camera.

Reading the complete representative gas state.

Time, views and the model boundary

The time slider selects native saved frames. The camera follows the galaxy centre. Changing a path switches its entire computed encounter; it does not drive a new orbit.

$t=-2.34\,\mathrm{Myr},\quad T=498\,\mathrm{Myr}$

The local view omits distant gas without changing its mass or motion. Full view fits every current sheet at its physical scale.

$V=1300\,\mathrm{km}\,\mathrm{s}^{-1},\quad \tau_\star=2\,\mathrm{Gyr},\quad r=0$

The same centre-sampled incident wind acts on the far tail. The model uses fixed external gravity and fixed-area sheets; gas pressure, mixing and finite-ICM reaction are not evolved.

Pale gas arrows show the current velocity multiplied by $10\,\mathrm{Myr}$ . The two arrows beside the disk indicate wind direction.

Include the whole native gas record in export

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 diskBound gas outside diskCurrently 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.

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 misconceptionEvery tail point has escaped.

Better mental modelCompare 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 misconceptionA falling disk rate proves the whole galaxy has shut down.

Better mental modelThe 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

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.
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.
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.

Sources and model context

Köppen et al. (2018) — 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) — differences between stripping algorithms and exposure histories; published medians are not individual galaxy trajectories.
BC03 / EzGal, fixed teaching input — 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.

Fixed distribution and dependency notices

EzGal

MIT License

Copyright (c) 2016 cmancone

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

Speclite

Copyright (c) 2015-2025, Speclite Developers <dkirkby@uci.edu>
All rights reserved.

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright notice, this
  list of conditions and the following disclaimer in the documentation and/or
  other materials provided with the distribution.
* Neither the name of the Speclite Team nor the names of its contributors may be
  used to endorse or promote products derived from this software without
  specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

fflate

MIT License

Copyright (c) 2026 Arjun Barrett

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

Three.js

The MIT License

Copyright © 2010-2026 three.js authors

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.