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.
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)