Ring galaxies, the rarest galaxy type of all, are finally understood

When we look out into deep space, beyond the confines of the Milky Way, we find that the Universe isn’t quite so empty. An enormous variety of galaxies fill the abyss of space: small and large, near and far, in rich clusters and in near-total isolation. The Milky Way itself represents just one of at least two trillion such galaxies (and probably several times as many) within the observable Universe. Galaxies are collections of both dark matter and normal matter, where the latter includes plasmas, gas, dust, planets, black holes, and — most prominently — stars. After all, it’s through the examination of that starlight that we’ve learned the most about the physical properties of galaxies, and been able to reconstruct how they came to be.

In general, there are four classes of galaxies that we see.

1. Spirals, like the Milky Way, are the most common type of large galaxy in the Universe.
2. Ellipticals, like M87, are the largest and most common type of galaxy in the rich, central regions of galaxy clusters.
3. Irregular galaxies are a third ubiquitous type, usually distorted from a prior spiral or elliptical shape by gravitational interactions.
4. But there’s a very rare type that’s striking and beautiful: ring galaxies.

Ring galaxies make up only 1-in-10,000 of all the galaxies out there, where the first one known to humanity, Hoag’s object, was only discovered in 1950. It’s now more than 70 years since that serendipitous discovery, and we’ve finally figured out how the Universe makes them.

The galaxy NGC 6028 possesses many features common to ring galaxies, with an inner population of older stars in a primarily elliptical configuration with a large, separated population of younger stars in a surrounding ring/halo. The stars are different ages and colors, but are found at the same redshift and distance from us as one another.

Visually, when you look at a ring galaxy, there are numerous features that stick out as highly unusual among galaxies individually.

1. There’s a central core to the galaxy, one that’s relatively compact, that only possesses little-to-no gas and whose stellar population consists primarily of older stars. There has been very little recent star-formation in that central region, if any at all.
2. Surrounding that galaxy, there’s a gap: a region of very low density: low gas density, low dust density, and low stellar density as well. Overall, there are practically no stars, no light, and very little gas or neutral matter, especially when compared to the innermore or outermore regions.
3. And then, beyond that, there’s another luminous population of stars. This population exists in a bright, luminous ring that surrounds the central core, but is much bluer in color than the core itself. This indicates that the stars within the ring formed much more recently, and are dominated by hot, short-lived, blue colored stars.

The fact that these features are all observed alongside one another in ring galaxies makes them even more interesting. It’s not just a profound coincidence, either; it’s a clue towards solving the puzzle of how they’re created and what conditions need to be in place to form them.

When you look at where ring galaxies are located, they’re overwhelmingly located in what astronomers call “the field,” as opposed to the central locations of rich galaxy groups and clusters. Although this set of features might seem bizarre and unrelated, they’re all cosmic clues to the origins of these features.

This two panel image shows ultraviolet (left) and visible light (right) images of the barred ring galaxy NGC 1291. The inner disk and bar persist in the center, where a population of older, cooler stars dominate. In the outer, fainter ring, young blue stars dominate, having formed relatively recently.

There have been a number of possible explanations put forward for these ring galaxies that we’re certain are wrong, as they cannot account for all of the observed features together when we examine these objects in detail.

• They definitively aren’t planetary nebulae, which sometimes possess rings around them. As our observations have taught us, the cores and the rings are composed of stars, not of gases, dusts, plasmas, and other ejecta originating from a single, dying star.
• They aren’t made from a young galaxy getting stretched and ripped apart into a ring that comes to surround a separate, older, more massive galaxy that sits at the center. The ages of the stars in the outer rings and the shapes of the rings themselves show this cannot be the case, as the timescales and angular momentum constraints are in conflict with this scenario.
• And they aren’t examples of gravitational lensing, where a large, massive object stretches, distorts, and magnifies the background light from luminous objects along the same line-of-sight. Gravitational lenses do exist, and can create ring-like shapes under properly aligned conditions, but these ring galaxies all have the “ring” population and the “central” population occurring at the same redshift, rather than one being a foreground and one being a background object. This rules out the possibility of a gravitational lens.

Whatever it is that we’re looking at, we can be confident that these are all examples of a single galaxy with two distinct populations of stars: an old one in the central region, and a young one in the ring region.

This object isn’t a single ring galaxy, but rather two galaxies at very different distances from one another: a nearby red galaxy and a more distant blue galaxy that’s gravitationally lensed by the foreground galaxy’s mass. These objects are simply along the same line of sight, with the background galaxy’s light gravitationally distorted, stretched, and magnified by the foreground galaxy. The result is a near-perfect ring, which would be known as an Einstein ring if it made a full 360 degree circle. While lensing is more commonly seen from galaxy clusters, individual galaxies can do it if they’re compact enough and if the alignment is right.

Fortunately, we no longer live in the 1950s, with just one example of a ring galaxy. We now have a large number of similar examples of ring galaxies, and we can see that across these ring galaxies, there are many features that appear to be universal or nearly universal to them. By examining these various features, we can put together some of the puzzle pieces, and attempt to assemble a coherent understanding of how these objects form, and explain why they appear with the features and properties that we see.

In April of every year, NASA and the Space Telescope Science Institute always release an anniversary image from Hubble, commemorating its 1990 launch on April 24. This is interesting to longtime fans of Hubble, as the image released for Hubble’s 14th anniversary, back in 2004, provided a series of major clues.

Shown below, that 14th anniversary image showcased galaxy AM 0644-741, and revealed a ring that isn’t in a perfectly circular shape, but rather makes a sort-of elongated ellipsoid. In theory, this could either be because there’s a projection effect, and we’re seeing a circular feature as though it’s inclined to us, or because whatever occurred to form the outer ring happened in an asymmetric fashion. As it turns out, both explanations have merit if we only consider this one object, but other features are worth highlighting and help us determine which one is the better fit to what we observe.

This ellipsoidal ring galaxy, unremarkably named AM 0644-741, consists of a nucleus of old stars, approximately a third the size of the Milky Way, surrounded by a large ring of hot, young, blue stars approximately 130,000 light-years across. For comparison, the diameter of the inner, swirling component of the galaxy is just 50,000 light-years.

First off, at a distance of only 300 million light-years, it’s relatively easy to resolve a number of important properties about this galaxy. The long axis of the blue-colored ring feature is around 130,000 light-years, making it comparable in size to the Milky Way, while the central, white/yellow-colored component is much smaller: at only 50,000 light-years.

Second, there are dusty features seen silhouetted against the large ringed feature, which shows that not only is there “fuel” remaining to supply gas for continued star-formation, but indicate that there are unequal regions of density inside. Many of the patches that appear darkest to our eyes aren’t regions where there are no stars, but rather are regions which are rich in light-blocking dust. Over time, that dust should contract and form new stars, fueling this galaxy even millions of years into the future.

Third, there are pinkish regions littering the blue ring, which indicate the presence of ionized hydrogen: a typical feature of new star-forming regions where stars are actively being born right now.

And finally, if we look at a wider-field view than the one captured by Hubble, we can even find a perfect candidate for being the culprit responsible for this irregularly shaped galactic ring: an intruder galaxy that apparently “punched through” what’s now a ring galaxy. In other words, this ring feature didn’t arise out of nowhere, but was caused by an interloper that led to its formation quite recently.

This X-ray/optical composite image shows the ring galaxy AM 0644-741 along with a wide-field view of its surroundings. Below and to the left of this ring galaxy is a gas-poor ellipsoidal galaxy that may have punched through the ringed galaxy a few hundred million years earlier. The subsequent formation and evolution of a ring of new stars would be expected from the propagation of gas away from the center, like ripples in a pond.

How could this have occurred? We have a coherent explanation ready to go. Inside pretty much every disk galaxy, which includes all spiral galaxies, even at these relatively late cosmic times, there are copious reservoirs of gas. When disk/spiral galaxies travel through a rich medium of space, like the medium found inside galaxy clusters, the gas gets stripped and depleted. When the gas is fully exhausted, no new stars can form any longer. This leads to a species of galaxy where only the older, longer-lived stars survive: that we call “red and dead” galaxies.

The converse of that is that whenever new episodes of star-formation occur, those newly formed stars span the full gamut of colors and masses: from hot, blue, and high mass stars to cool, red, low mass ones. However, the hottest, bluest, most massive stars burn through their fuel the fastest, so they’re the first to die. As a stellar population ages, the color of those stars transitions from blue to white to yellow to orange to red. The greater the amount of time that’s elapsed since its most recent star-formation episode, the redder the stellar population will be. If there’s no gas left to form new stars, it’s not just red, it’s also “dead,” at least in an astronomical sense.

This is why, we think, we primarily find ring galaxies in the field, rather than in clusters. We need a gas-rich spiral galaxy to start with, and then when an interloping galaxy passes through its center, that collision will then create outward-moving ripples in the gas, which trigger star formation and create the notorious ring-like shape.

The Hubble vs. JWST views of the Cartwheel galaxy (and its surroundings) showcase some spectacular differences. JWST data, at longer wavelengths of light, reveals features that Hubble could never see. Note the presence of a central nucleus of old stars and a bright ring of young stars that are connected by a series of thin bridges of gas and stars. The bright, star-forming galaxy to the upper left of the Cartwheel is thought to have punched through it, creating the Cartwheel’s ring.

Another example of a ring galaxy, and one that’s clearly in a less-fully-evolved state, is the Cartwheel galaxy, shown above. On the right, you can not only see the dense, older core of a pre-existing gas-rich spiral galaxy surrounded by a bright blue ring of hot, young stars, but also a series of filaments between the core and the ring. Those filaments themselves are dotted with blue and white stars, although of a much lower brightness than either the main core or the ring itself.

Could this have formed in the same fashion: from an interloping galaxy that punched through the center of what’s now a ring galaxy, causing gas to ripple outwards, compress and rarify in turn, and form new stars?

Not only is that the best explanation, but there’s a “smoking gun” signature that provides excellent evidence for this possibility. Just to the left of the Cartwheel galaxy and slightly to the upper portion of the image above, you can find a smaller, irregular galaxy that itself is rich in young, blue, glittering stars. Again, this was likely the galaxy that just recently punched through the larger galaxy’s center. In this particular system, not only was the Cartwheel galaxy a gas-rich spiral, but so, quite likely, was the interloper, which became irregular owing to the recent interaction.

This unusual ring galaxy appears to be lacking a central nucleus, despite having a bright ring rich in not only new stars, but also bright pink star-forming regions. At the upper left of the ring, the original nucleus likely persists, although the particular interaction dynamics to produce this feature have not been perfectly reconstructed yet, based on insufficient available data to do so.

Some ring galaxies, like Zwicky II 28, shown above, are atypical in some fashion. In some cases, the interloper galaxy is nowhere to be found. This is part of the reason why the original ring galaxy — Hoag’s object — still remains so mysterious, even a full 75 years after its discovery. Others, like the one you see here, appear to lack a central, old core of stars.

However, we have to remember, when we look at any one particular object, we’re constrained by our particular perspective. In the case of Zwicky II 28, the asymmetry of the ring is key; the “brighter” part at the top left appears to house the central core, while the “darker” part at the bottom right is antipodal to the core.

This has a potentially simple explanation: the orientation of the collision and the galaxy to our own line-of-sight matter a great deal!

Of course, orientation isn’t the only matter of importance; it’s also possible for the entire galaxy, itself, to get stretched into a ring owing to a collision. Generally, this occurs when you have a collision between two comparably massive galaxies, where only one of them initially had a relatively low number of stars inside of it. It’s under those conditions that a collision can lead to both a ring and also to the gravitational disruption of the original galaxy itself, allowing both the precursor galaxy and the ring itself to occupy the same region in space. That, rather than a simple displaced core, is likely the cause of at least some coreless ring galaxies, including the coreless ring found in Arp 147, below.

Known colloquially as a “perfect 10,” Arp 147 features two interacting galaxies where each one features a ring, almost certainly as the aftermath of a center-on-center collision between the two precursors. The dusty reddish knot at the lower left of the blue ring probably marks the location of the original nucleus of the galaxy that was hit.

All of this is a very nice story, of course, but are we sure that it’s correct?

There’s one way to put this ring galaxy formation scenario to the test. In theory, if our picture is correct, then we should find:

• pairs of galaxies that speed towards each other and are about to interact,
• a few such pairs where one of them comes in at just the right angle to “punch through” the precise center of the other,
• leading to new stars forming in a ring outside the main galaxy,
• including the possible displacement of part or even all of the original core,
• followed by further evolution into a variety of ring-like shapes, particularly if our sample is large enough.

Simulations can reproduce this, and that sets our expectations. However, we also need to confirm and validate these predictions, and that demands that we find examples of all these different predicted stages of this process out there in the Universe.

When we observe the Universe, the timescale of human civilization is too short to watch this process unfold in any single galactic system; we can only acquire snapshots of how things are right at this very moment, when the light from across the Universe arrives. We do see plenty of examples of interacting pairs of galaxies, particularly in the field (rather than in clusters), with properties that could lead to a ring. And we see many examples of rings themselves, arising from a post-collisional state.

But there are also objects that show the exact critical moment we’d hope to identify, such as Mayall’s object. Originally thought to be a nebula with a “question mark” shape when it was first identified in 1940, it is now known to be the collision of two galaxies in the process of creating a ring galaxy.

This Hubble Space Telescope image of Mayall’s object, also known as Arp 148, shows two galaxies in the process of collision. As one galaxy punches through the center of the other, stars form in both galaxies, but the one that got “punched” is having its gas propagate outward in waves, triggering new star formation on its way toward creating an overall ring-like shape. While they interact and merge, galaxies can take on many fascinating and peculiar shapes.

At last, we have a validated scenario for creating each and every one of the ring galaxies that have been observed. That’s a huge success, of course, but there’s still farther to go.

Despite the fact that we now know how ring galaxies form in general, Hoag’s object — the original ring — is still an outlier that stubbornly refuses to be explained by any one simple scenario. The ring and the core of Hoag’s object have almost identical velocities, indicating that if there was an interloper that formed the ring, it was a very quiet process: one that didn’t significantly disrupt the structure of the galaxy. There’s no evidence anywhere in the vicinity of Hoag’s object for a candidate interloper galaxy, which is surprising, nor do we observe any galaxy fragments nearby.

You can’t save the scenario by pushing the collision farther back into the past, as the outer ring of stars is too young. And the inner core, rather than being a spiral, is instead shows the shape of a gas-poor elliptical. One still-viable potential explanation is that the interloping galaxy punched through Hoag’s object and is currently on the far side of it, perfectly aligned with our line-of-sight, behind the elliptical center. But there is no evidence for this; it just hasn’t been ruled out by current observations.

Still, it’s a remarkable achievement to, on the whole, be able to explain the process by which the rarest class of all major galaxy types, the ring galaxies, form. If you have a gas-rich spiral galaxy and another galaxy then comes along and punches right through your center, your internal gases will ripple out towards the edges, smashing into the pre-existing gas along the way, triggering new waves of star-formation on the outskirts, all while depleting the normal matter in the galactic core. With better data across more wavelengths, which is a huge advantage now that we’ve entered the JWST era, the remaining mysteries may yet on the precipice of being solved. Still, it’s always important to take stock of where we are, and to appreciate just how far we’ve come in our understanding of not only what’s out there in the Universe, but how it came to be.

This article was first published in April of 2022. It was updated in November of 2025.