---
title: "Why a galaxy tail can fall back"
description: "A cluster wind can lift gas that still belongs to its galaxy. Follow the difference between a visible tail, gravitational binding and permanent fuel loss."
canonical_url: "https://sandboxphysics.com/stories/ram-pressure-stripping/why-a-galaxy-tail-can-fall-back/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/ram-pressure-stripping/why-a-galaxy-tail-can-fall-back/"
last_modified: "2026-11-11"
---

[Ram pressure and returning gas](https://sandboxphysics.com/stories/ram-pressure-stripping/)

Ram pressure and returning gas · L1

# Why a galaxy tail can fall back

Sandbox Physics · 2026-11-11

A long gas tail can remain attached to a galaxy by gravity. Wind can push gas outside the disk; escape also depends on its speed and on the gravitational energy still needed to leave the galaxy.

## Before you begin

Everyday force, speed and gravity are enough. The next chapter introduces the pressure and energy equations.

## The galaxy meets a wind it cannot see

Imagine riding beside a spiral galaxy as it moves through a galaxy cluster. Its stars form a bright disk. Much farther apart, the cluster contains very dilute hot gas. That gas seems almost empty by laboratory standards, yet a galaxy crosses an enormous amount of it over a long journey.

In the galaxy’s frame the surrounding gas arrives as a wind. Diffuse gas in the disk presents a broad target: incoming material transfers momentum to it. Stars are compact objects separated by large spaces, so this direct pressure acts very differently on the stellar disk and on the diffuse gas. Both still feel gravity.

Ram pressure is the name for the incoming momentum delivered per unit area and time. A denser surrounding gas or a larger relative speed gives the wind more ability to push. As the galaxy passes through a denser region and returns to thinner surroundings, the pressure rises and falls.

Reading: Köppen, Jáchym, Taylor & Palouš, Ram Pressure Stripping Made Easy, arXiv:1806.05887v1 (2018)

## Lift a ball without throwing it away

Toss a ball upward. It can leave your hand and rise well above your head while still being certain to return. Its height tells you where it is. Its upward speed tells you how much farther it can climb. The same distinction matters for gas lifted above a galaxy disk.

Gravity is spread over the whole galaxy, including its dark-matter halo. The disk edge is therefore not a wall separating gravity from empty space. Gas beyond a chosen disk volume can still be bound: its motion does not contain enough energy to climb all the way out of the gravitational potential.

A passing wind complicates the ball picture because it can continue doing work while the gas moves. Gas that is bound at one instant may gain more energy later. The useful statement is therefore “currently bound”, followed by a record of what the wind does next.

Reading: Köppen, Jáchym, Taylor & Palouš, Ram Pressure Stripping Made Easy, arXiv:1806.05887v1 (2018) · Original fixed-slice and static-restoring-force materials

## Measure one tail in two ways

Specific mechanical energy means kinetic plus gravitational potential energy per unit gas mass. Choose zero gravitational potential at infinity. A negative total means the gas is currently bound when assessed at its present position and velocity; the figure counts positive-energy gas separately from gas outside the disk.

Our reduced calculation follows gas in a specified stellar, bulge and halo potential. Set model time zero at closest approach to the cluster centre. The first slice is about 2.34 million years before that point:26.6% of the remaining gas lies outside a disk volume of radius 16 kiloparsecs and half-height 1 kiloparsec, while none has positive specific mechanical energy.

At 150 million years after closest approach, the outside fraction is still 26.6%, while 6.25% has positive energy. At 800 million years after closest approach, those values are 23.4% and 9.38%. These values belong to this synthetic model; the two measurements answer different physical questions.

A long tail is a position measurement. A mechanical-energy test also uses velocity, which a single image normally does not supply in full. A continuing wind may do more work later, so the binding statement refers to each specified instant.

Original synthetic q0 slices at model times $-2.34375$, $150$ and $800\,\mathrm{Myr}$. All bars share a $0\%$ to $30\%$ scale. Tail height is geometric, while positive specific energy uses velocity and the same external potential. Open the full figure

Reading: Original fixed-slice and static-restoring-force materials

## The fading gust gives some gas a way back

Once the pressure weakens, gravity may reverse the motion of bound gas. In a weaker passage, some gas rises just above the disk and then drops back. That returning material can refill the volume where the disk’s star formation is being counted.

The returning tail is not guaranteed to retrace an earlier shape. Gas has orbital motion around the galaxy as well as motion above the disk, and the model follows both. A sample can cross the disk boundary more than once. Watching the full history is more informative than labelling every point in one tail as permanently removed.

The model treats the incoming wind as uniform across every gas sheet, using its density at the galaxy centre. Real tails also mix, cool and interact with a changing environment. Following position and energy separately shows which gas has been displaced and which remains bound.

Reading: Original fixed-slice and static-restoring-force materials · Gronke & Oh, The growth and entrainment of cold gas in a hot wind, arXiv:1806.02728v2 (2018)

## Exercises and explained answers

1. A gas sample is outside the disk but is slowing down as it rises. What else would you need to decide whether it can escape?

   Read the explanation

   Its full velocity and the gravitational potential at its position, plus the later work of any continuing wind. Its height gives the position; speed and gravity are also needed to judge escape.

2. If the outside fraction falls after the wind peak, has the model created gas?

   Read the explanation

   No. Bound gas can move back across the chosen disk boundary. The all-gas mass and the disk-volume mass answer different questions.

## Try the prediction

### [Follow gas lifted by a cluster wind](https://sandboxphysics.com/labs/ram-pressure-stripping/)

1. Pause the central passage near closest approach. Compare the outside and current unbound fractions.
2. Choose the returning-tail preset and follow the gold probe as its selected gas sheet moves toward the disk.

[Previous: Why cluster galaxies become redder](https://sandboxphysics.com/stories/ram-pressure-stripping/why-cluster-galaxies-become-redder/) [Next: From a wind pulse to a restoring force](https://sandboxphysics.com/stories/ram-pressure-stripping/from-a-wind-pulse-to-restoring-force/)

## Sources and calculation scope

1. [Köppen, Jáchym, Taylor & Palouš, Ram Pressure Stripping Made Easy, arXiv:1806.05887v1 (2018)](https://arxiv.org/abs/1806.05887v1)

   Sections2.1–2.2 and5.4 explain the slow-pressure/impulse limits and re-accretable gas. Our fixed three-component potential, equal-mass sheets and consuming gas are specified independently; the own static comparison does not reproduce every published simulation or observational sample.

2. [Gronke & Oh, The growth and entrainment of cold gas in a hot wind, arXiv:1806.02728v2 (2018)](https://arxiv.org/abs/1806.02728v2)

   Controlled three-dimensional cloud-in-wind cooling study. Mixed-gas cooling, material origin, downstream domain and resolution are the relevant reading questions. It is not a self-gravitating star-forming disk or a calibration of this gas-sheet model.

3. [Original fixed-slice and static-restoring-force materials](https://sandboxphysics.com/stories/ram-pressure-stripping/materials/ram-pressure-science-v1.zip)

   Our five exact synthetic slices with sixty-four sheets and thirteen fields, initial common arrays, two global-light histories/three screens/four-age band subset, independent Python static maxima and mask/energy/colour diagnostics. Original trajectory and native SSP production are distinct earlier calculations; this offline package repeats diagnostics and band ratios. No browser Engine, original FITS/SED, paper, author code or observed-galaxy data is included.