Jellyfish and bunny ear galaxies have cosmic consequences

Even though the Universe looks like it’s full of tiny islands of light — luminous, star-filled galaxies with only the blank darkness of empty space between them — the reality is that the space between galaxies isn’t empty at all. An isolated galaxy still moves through the abyss of deep space: where a rarified, sparsely populated sea of ions (mostly protons and electrons) still persists. There may only be about one particle per cubic meter populating the intergalactic medium, on average, but considering that galaxies are frequently 100,000 light-years across or even more, they do encounter large numbers of particles, particularly at higher speeds.

This effect gets more and more severe for galaxies that move at faster speeds through the intergalactic medium, and also for environments where the intergalactic medium is denser and more particle-rich. A galaxy that moves within a galactic group (like our own Local Group) experiences both faster speeds and also denser environments than an isolated one in deep intergalactic space, which can begin to strip material out of the fastest-moving galaxies in the densest environments, producing tails of material that, if they’re dense enough, can even form stars behind them. The galaxies with the most prominent tails often have jellyfish-like or bunny ear-like features, with cosmic consequences that aren’t necessarily obvious. Here’s what we found when we examined these systems in detail.

Located within the Norma cluster of galaxies, ESO 137-001 speeds through the intracluster medium, where interactions between the matter in the space between galaxies and the rapidly-moving galaxy itself cause ram pressure-stripping, leading to a new population of tidal streams and intergalactic stars. Sustained interactions such as this can eventually remove all of the gas from within a galaxy, eliminating its ability to form new stars. Phenomena such as this allow us to conclude that the galaxy, the cluster, and the gas within it are all made of matter, not antimatter, while the tidal streams of new stars will contain practically no dark matter at all.

To understand how galaxies behave when they speed through intergalactic space, imagine the following three analogous scenarios all involving firing a bullet from a gun. Imagine that you fired the bullet:

• into a perfect vacuum, where there’s no material, no air, no plasma, no particles, etc.,
• into air at normal, standard temperature and pressure found on Earth’s surface,
• into the air on a foggy, rainy day, where the air isn’t just filled with gaseous molecules like nitrogen and oxygen, but also with droplets and even full-fledged drops of liquid water.

In each of these three scenarios, the bullet won’t just fly through space as an object in motion remaining in constant motion, save for the force of gravity that curves its path, but will encounter some resistance as it encounters resistance (and a drag force) from the particles it runs into.

In all three cases, the bullet — as it flies through space — has the potential to experience effects that arise from its interaction with its environment. In the first case, because its environment is empty, the bullet doesn’t interact with anything at all, and so it remains perfectly intact so long as it’s in flight in a vacuum. In the second case, though, traveling through the air, the bullet might only experience ablation at an atomic level; you would likely need an electron microscope to see how traveling through the air had stripped particles off of the bullet itself. But in the third case, where the bullet runs into drops of liquid water, the damage would likely be more severe, leading to streaks and scarring in the bullet that would be visible with tools as simple as a magnifying glass, or even just the naked eye might be sufficient.

The Coma Cluster of galaxies, as seen with a composite of modern space and ground-based telescopes. The infrared data comes from the Spitzer Space telescope, while ground-based data comes from the Sloan Digital Sky Survey. The Coma Cluster is dominated by two giant elliptical galaxies, with over 1000 other spirals and ellipticals inside. Gas-free, red-and-dead elliptical galaxies are very common, especially toward the cluster center, in large galaxy clusters such as this one. The speed of galaxies within the cluster can be used to help determine the cluster’s total mass, revealing our first evidence for dark matter in the 1930s.

Galaxies are fairly similar to bullets, at least in this sense. Only, the thing that determines the density of the medium that a galaxy travels through is its location. A galaxy could be located within:

• a cosmic void, where there’s only the sparse occasional ion in the intergalactic medium,
• a small galaxy group, such as our own Local Group, where there might be a few large galaxies accompanied by larger numbers of small ones, where the intergalactic medium contains a much denser plasma and many rogue planets,
• or in a large galaxy cluster, containing thousands of massive galaxies or more, and with a dense intergalactic medium (often called the intracluster medium) that isn’t just filled with a plasma and planets, but also copious populations of stars and even globular clusters.

Sure, such as in the case of a bullet through those media of varying densities, the galaxy that moves through a denser environment will experience a greater amount of disruption than a galaxy moving through a sparser environment. That’s expected.

But there’s also a second effect at play: in the dense environment of a rich cluster of galaxies, there’s a much larger overall mass. That means a larger gravitational potential, and that leads to galaxies moving through those denser environments more quickly: as gravitational potential energy gets turned into the kinetic energy of a galaxy’s motion. Whereas galaxies in deep intergalactic space might only move at a few kilometers-per-second relative to the Hubble flow (or their local standard of rest), a Milky Way-like galaxy will move at a few hundred km/s, and a galaxy in a rich cluster, like the Coma Cluster, is likely to be moving at thousands of km/s through its environment.

A map of neutral hydrogen (in red) overlaid on the galaxy D100 in the Coma Cluster shows how much gas is being quickly stripped from this galaxy as it travels through the cluster. Galaxies found in environments like this one become ‘red-and-dead’ far more quickly than galaxies in less dense regions of space.

These two effects, together — of galaxies moving with varying speeds through environments of varying densities — make rich galaxy clusters the ideal environments to find galaxies that experience the greatest amounts of stripping from within them. Gas-rich disk galaxies, often with spiral structures like our own Milky Way, experience much more stripping than a gas-depleted one or a gas-free, red-and-dead elliptical galaxy/ This makes the:

• spiral and other disk-galaxies,
• that are rich in gas,
• that are found in dense, massive galaxy clusters,
• that speed through the cluster very rapidly, at speeds of thousands of kilometers-per-second,

the best candidates for experiencing the greatest amounts of stripping. They’re the best places to look for phenomena like ram pressure stripping, and also for possessing star-rich tails trailing behind them.

That’s what, in general, a “jellyfish galaxy” is: a gas-rich galaxy speeding through the intracluster medium of a rich galaxy cluster at thousands of kilometers-per-second: up to 2-3% the speed of light! By smashing into the particles and objects present in the intracluster medium, the gas within the galaxy’s disk gets compressed, triggering new episodes of star formation throughout it, and some of that gas even gets stripped out of the galaxy behind it, leading to tails of stars emanating off of them. The faster the galaxy moves, and the richer the cluster environment that it moves through, the more prominent these tails, or jellyfish-like features, are.

This simulation shows how a disk galaxy evolves, dependent on what orientation it moves at through a dense intracluster medium, over time. The different columns show different angles from face-on (left) to edge-on (right), and different steps in time evolution in 100 million year increments. Note the “bunny ear” like structure at the lower right.

But not all galaxies that experience these effects look like jellyfish; some of them look much more like “bunny ears” according to astronomers. If you took a flat disk — something like a solid dinner plate — and you launched the disk face-on, at great speeds, into a dense air chamber, that disk would experience a large amount of ablation on the forward-facing side. For galaxies that are traveling through their intracluster medium in a face-on orientation, they commonly do display these jellyfish-like features, as the material within the galaxy gets stripped out of it in roughly an even fashion: the same in all locations throughout the disk.

However, what if you took that same flat disk, but instead of launching it face-on into a dense air chamber, you launched it edge-on into that air chamber? This time, you wouldn’t expect to see an even amount of stripping throughout the galaxy. The components of the galaxy that were located close to the middle would likely experience an enhanced rate of star-formation throughout them, but because those parts of the galaxy appeared so “thick” when compared to the rarified intracluster medium, less material would be stripped out of them. For the material at the edges of the disk, on the other hand, they’d experience the most stripping, and so would lead to a bright disk with two tails at their edges — or two bunny ear-like features — trailing off of them.

These diagrams show, from left-to-right in time evolution, the development of an asymmetric tail (appearing sometimes like twin tails) that form as a result of a highly inclined ram pressure stripping interaction. The top row shows a general diagram, while the lower two rows show the evolution in gas density from simulations, showing all galaxies rotating clockwise with a wind pointing upwards.

Now, let’s add in one more effect: the fact that disk galaxies often rotate, or spin, in one particular direction. This causes us to modify our previous analysis by a little bit. Large spiral galaxies typically rotate at a few hundred kilometers per second: fast, but much slower than the speed at which a galaxy moves through the intracluster medium. However, if this galaxy is oriented edge-on to the medium it’s traveling through, then one side of the disk will be rotating “into” the direction of motion, causing that side to experience a more rapid relative velocity than the average of the whole galaxy, while the opposite side will be rotating “away from” the direction of motion, causing that side to experience a smaller relative velocity than average with respect to the intracluster medium.

Remember: it’s the gas-rich galaxies with the fastest relative speeds through the densest of media that produce the greatest amounts of stripping, and that display the longest, most star-rich trails behind them. A galaxy that rotates and that moves edge-on through the intracluster medium is going to have one side — the side rotating into the direction-of-motion — experience the greatest relative velocities, and hence, display the greatest effects of stripping and have the longest tails. Meanwhile, the opposite side will experience the opposite: lower relative velocities, and therefore less stripping and a shorter, perhaps even more asymmetric (and less straight) tail.

This image, acquired with the Hubble Space Telescope in 2018, shows the giant elliptical galaxy NGC 4860 alongside its passing neighbor, the rapidly-moving spiral NGC 4858. Both of these galaxies are located in the Coma Cluster, but NGC 4858 is special: it’s fast-moving, at 5600 km/s through the intracluster medium, and speeding through it in an edge-on fashion. The evidence for ram pressure stripping, including trails of newly formed stars in its wake, leads to this being known as a type of jellyfish galaxy.

That’s precisely the explanation behind the “bunny ears” effect, as displayed beautifully by galaxy NGC 4858, as shown above. This galaxy displays all of the features that are consistent with this analogy. It’s a disk galaxy in the Coma Cluster, moving at a whopping 5600 km/s relative to the intracluster medium (about 1.9% the speed of light), oriented edge-on to the intracluster medium and rotating while it moves through the cluster. The “bunny ears” appearance of this galaxy allows us to determine, just by a visual inspection, which way the galaxy is rotating: the longer “ear” is the side where the galaxy is rotating into the direction of the galaxy’s motion through the cluster, while the shorter “ear” is where the galaxy rotates away from the direction of motion.

Now, here’s where it gets interesting, and perhaps a little bit surprising as well. Whenever you have ram pressure stripping, you expect to see blue features: features whose hallmark is a population of bright, young, newly-formed massive stars, the kind of stars that only live for a short time. That’s what we see, and that’s normal. But if we measure the relative velocities of the individual components of the galaxy, including the tails (or bunny ears), we’d expect that the “stripped” material would be moving away from the galaxy. Most of it is, and that’s good. But the surprise is that, in one prominent location along the base of the shorter bunny ear, there appears to be stripped material that’s moving towards the galaxy instead!

These color-coded maps of NGC 4858 show, at left, the absolute velocity of the different galaxy components as determined spectroscopically, the middle panel shows a model of the galaxy’s mean speed that accounts for rotational motion, and then the right panel shows the motion of the galactic material relative to that overall model. Note, in red at right, the evidence for material falling back onto the galaxy, instead of moving away from it in greens, blues, and violets.

What could that be due to? The implication is that there is gas that was pushed out of the disk of the galaxy some time ago, but that gas wasn’t stripped with great enough pressure to be gravitationally removed from the galaxy entirely. Instead some of this pushed-out gas must now be falling back into the galaxy itself.

This suggests a much more complex picture for the evolution of these bunny ear systems. Material isn’t just stripped out of these fast-moving, gas-rich galaxies, where it gets removed from the galaxy and forms stars. Instead, it looks like that stripping is only permanent at great enough relative velocities, and where there’s only a “thin” amount of material. If you have a lower relative velocity and/or a thick amount of material, the stripping effect is much lower in magnitude, and that translates into a transition zone:

• where on one side of the zone, material gets stripped out of the galaxy and escapes from it, forming new stars in a tail behind it,
• but on the other side of the zone, material gets stripped out of the galaxy only barely, but not by a great enough amount to escape from it, and at some later moment, the gas falls back onto the galaxy it was initially stripped from.

These four different panels all show Subaru (left) and Hubble (right) imaging of galaxy NGC 4858, with overlays showing the LOFAR 144 MHz radio continuum tail (a), Balmer alpha emission contours (b), carbon monoxide transitions from ALMA (c), and those same carbon monoxide transitions again (d). Note the bunny ear structures visible in some of the images and overlays.

It’s important to recognize, even though this creates large features that might look like an extension of the galaxy’s internal spiral arms, it isn’t the spiral arms themselves that get stretched and extended into these bunny ear features that we see. Instead, it’s stripped gas — gas that was compressed from the ram pressure stripping — that newly forms those blue-appearing stars, not stripped stars or stripped spiral arms. There’s a preference for stripping on one side of the galaxy, and that leads to the gas on the “into the direction-of-motion” side getting stripped very efficiently.

But over on the other side of the galaxy, the side that’s moving against the direction-of-motion of the galaxy through the intracluster medium, the ram pressure stripping effects are lower in magnitude, and therefore the stripping is less effective and less efficient. Less material gets stripped out on that side, forming fewer stars in a shorter tail, and where the material is stripped at low enough relative velocities, some of it can even fall back onto the main galaxy. Although some of the gas that gets stripped did originate within the spiral arms themselves, the tails and bunny ears are unrelated to the stars that are found within the main galaxy’s spiral arms.

This image showcases the massive, distant galaxy cluster Abell S1063, imaged as part of the Hubble Frontier Fields program. The diffuse, bluish-white light shown here is actual intracluster starlight, which was captured for the first time only in 2018. This light at least partially arises from material stripped out of galaxies within the cluster that have collapsed to form stars. Those stars then follow the cluster’s overall gravity, tracing out the underlying dark matter distribution.

Perhaps remarkably, one can perform wind tunnel simulations that model the effects of a rotating disk-like structure moving at remarkable speeds into a low-density medium, and the same “bunny ear” features for motions arise within them. On the faster-moving side, stripping is the most efficient, and the longest tails are produced with the least amount of material falling back onto the main object itself. But on the slower-moving side, the tails are shorter, and large amounts of material wind up falling back onto the main, original object.

You might think to ask, as a follow-up question, what role dark matter might play in these sorts of systems? It’s an interesting question, and it turns out that dark matter does have an important effect, but only for the underlying galaxy itself. In a sense, the effects of dark matter’s distribution will be “baked into” the rotation curve of the galaxy, which has important effects on the relative speeds of the different sides of the galaxy relative to the intracluster medium. However, when it comes to ram pressure stripping, because of its non-interacting nature, dark matter likely plays no role; the stripping has no effect on the dark matter.

However, many of these rich galaxy clusters are illuminated with intracluster light: light that arises from stars that were stripped out of the various galaxies present within the cluster, that now trace out the cluster’s underlying dark matter distribution. With the presence of these jellyfish and bunny ear galaxies, we now know where these stars come from, showcasing the full suite of cosmic consequences that arise from the galaxies, found in galaxy clusters, that move fast and — as a result — lose their gas quickly as well.