Ring galaxies, the rarest in the Universe, finally explained
Universe

Ring galaxies, the rarest in the Universe, finally explained

Spirals, ellipticals, and irregulars are all more common than ring galaxies. At last, we know how these ultra-rare objects are made.

Almost all galaxies can be classed as spiral, elliptical, or irregular. Ring galaxies are the most rare of all, accounting for only one in every 10,000 galaxies. The first ring, Hoag’s object, was discovered in 1950, with a dense core of old stars and a circular or elliptical ring of bright, blue, young stars. We finally know where these items come from after decades of wondering how they form. We’ve seen enough of them, capturing them at various phases of evolution.

When we look out into deep space, beyond the bounds of the Milky Way, we discover that the Universe isn’t nearly so empty. Galaxies – small and vast, close and far, in rich clusters and in near-total isolation — fill the void of space, with the Milky Way being just one of around two trillion such galaxies in the observable Universe. Galaxies are collections of ordinary matter such as plasmas, gas, dust, planets, and, most notably, stars. We’ve learned the most about the physical attributes of galaxies and how they formed by studying starlight.

There are four types of galaxies that we see in general. The Milky Way is the most frequent form of big galaxy in the Universe. Ellipticals, such as M87, are the largest and most common kind of galaxy in galaxy clusters’ rich, core regions. Irregular galaxies are a third common kind, generally distorted by gravitational interactions from a previous spiral or elliptical shape. However, there is one very rare variety that is both stunning and beautiful: ring galaxies. They account for only one in every 10,000 galaxies, with the first, Hoag’s object, identified in 1950. We’ve finally found out how the Universe creates them after more than 70 years of research.

ring galaxy
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.(Credit: Sloan Digital Sky Survey)

When looking at a ring galaxy, there are several traits that stand out as uncommon among galaxies.

  • The galaxy has a very compact central core that is low in gas and mostly composed of older stars. In that core region, there has been virtually little recent star formation.
  • There is a gap surrounding that galaxy: an area of very low density, with nearly no stars, no light, and very little gas or neutral matter.
  • Then there’s another brilliant group of stars beyond that. This population lives in a brilliant, luminous ring that surrounds the center core but is much bluer 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.

Furthermore, ring galaxies are primarily found in what astronomers refer to as “the field,” as opposed to the central areas of rich galaxy groupings and clusters. Although this collection of traits appears strange and unrelated, they are all cosmic clues to their beginnings.

ring galaxy
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.(Credit: NASA/JPL-Caltech/SSC)

A number of alternative explanations for these ring galaxies have been proposed, all of which we believe are incorrect since they fail to account for the observed properties when examined in depth.

  • They aren’t planetary nebulae, which can have rings around them, because they’re made up of stars, not gas and other ejecta from a single dying star.
  • They are not formed by a young galaxy being stretched and ripped apart into a ring that eventually surrounds a separate, older, more massive galaxy in the center. The ages of the stars in the outer rings, as well as the shapes of the rings themselves, indicate that this is not the case, as timeframes and angular momentum limitations contradict this idea.
  • They’re also not examples of gravitational lensing, which occurs when a large, massive object stretches, distorts, and magnifies background light from luminous objects in the same line of sight. Gravitational lenses do exist and can produce ring-like forms when properly aligned, but all of these ring galaxies have the “ring” and “central” populations occuring at the same redshift, ruling out the possibility of a gravitational lens.

Whatever we’re looking at, we can be certain that it’s a single galaxy with two unique populations of stars: an old one in the center and a youthful one in the ring.

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. They’re simply along the same line of sight, and the background galaxy is getting gravitationally lensed 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. It is visually stunning, but physically unrelated to ring galaxies.(Credit: ESA/Hubble & NASA)

Fortunately, we now have a lot of examples of these ring galaxies, rather than just one. We may piece together some of the puzzle pieces and try to piece together a coherent understanding of how these items arise and why they emerge with the traits and properties that we see by investigating their varied features.

Every April, NASA and the Space Telescope Science Institute release an anniversary image from Hubble, honoring its April 24, 1990 launch. Although the image for Hubble’s 32nd birthday in 2022 is “merely” a tightly-knit galaxy group, the image for Hubble’s 14th anniversary in 2004 presents a number of key indications.

Galaxy AM 0644-741 exhibits a ring that isn’t entirely round, but rather resembles an elongated ellipsoid. In principle, this may be due to a projection effect, in which we view a circular feature as if it were inclined to us, or it could be due to whatever happened to generate the outer ring occurring in an asymmetric pattern. As it turns out, both explanations have merit for this one object, but other features are worth pointing out as well.

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.(Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI))

To begin with, at a distance of only 300 million light-years, a number of important qualities are rather easy to resolve. The blue-colored ring feature has a long axis of roughly 130,000 light-years, making it equivalent in size to the Milky Way, whereas the central, white/yellow-colored component is much smaller at only 50,000 light-years.

Second, dusty features can be observed silhouetted against the massive ringed structure, indicating that there is not just “fuel” remaining to supply gas for continued star formation, but also that there are unequal regions of density inside. Many of the darkest areas represent regions where new stars should emerge millions of years in the future.

Third, there are pinkish regions scattered within the blue ring, indicating the presence of ionized hydrogen, which is a common feature of new star-forming regions where stars are actively developing right now.

Finally, a wider-field view than the one recorded by Hubble reveals the culprit: an interloper galaxy that appears to have “punched through” what is now a ring galaxy. In other words, this ring feature did not appear out of nowhere, but was caused by an interloper who caused it to form relatively recently.

ring galaxy
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.(Credit: X-ray: NASA/CXC/INAF/A. Wolter et al; Optical: NASA/STScI)

How would this happen? Even in present times, there are vast stores of gas inside almost every spiral galaxy. Gas is stripped and depleted, mainly within rich galaxy clusters, resulting in “red and dead” galaxies.

When new stars emerge, they come in a variety of hues and masses, ranging from hot, blue, and heavy to cool, red, and light. The hottest, bluest, and most massive stars, on the other hand, burn up their fuel the fastest, and so die first. The color of a stellar population changes as it ages, from blue to white to yellow to orange to red, and the longer it has been since its previous star-formation cycle, the redder it is.  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 believe, ring galaxies are more common in the field than in clusters. To begin, we require a gas-rich spiral galaxy, and then when an interloping galaxy passes through its center, the collision causes outward-moving ripples in the gas, which causes star formation and the infamous ring-like shape.

The Cartwheel galaxy, shown at right, is a stunning example of an imperfect ring galaxy, where a central nucleus of old stars and a bright ring of young stars are connected by a thin bridge of gas and stars throughout it. The cause of this ring, an interloping galaxy that smashed through the Cartwheel, is at the top left of the image, itself forming new stars as a result of the interaction.(Credit: ESA/Hubble & NASA)

The Cartwheel galaxy, shown above, is another example of a ring galaxy that is definitely in a less-evolved condition. On the right, you can see not just the dense, older core of a pre-existing gas-rich spiral galaxy, but also a network of filaments connecting the core and the ring. Those filaments are sprinkled with blue and white stars, however they are much fainter than the main core or the ring itself.

Could this be the result of an interloping galaxy punching through the center of what is now a ring galaxy, forcing gas to ripple outwards, compress and rarify, and generate new stars?

Not only is that the most reasonable theory, but there’s a “smoking gun” close to the left of the Cartwheel galaxy: a smaller, irregular galaxy rich in young, blue, glittering stars. In other words, not only was the Cartwheel galaxy a gas-rich spiral in this case, but so was the interloper, which became irregular as a result of the recent contact.

ring galaxy
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.(Credit: ESA/Hubble and NASA; Acknowledgment: Judy Schmidt)

Some ring galaxies, such as Zwicky II 28, are unusual in some way. In certain circumstances, the interloper galaxy is nowhere to be discovered, which contributes to the mystery surrounding the original ring galaxy — Hoag’s object. Others, like this one, appear to be missing a fundamental, ancient core. However, we must keep in mind that when we gaze at any particular item, we are limited by our particular perspective. The asymmetry of the ring is important in the case of Zwicky II 28; the “brighter” component at the top left looks to house the central core, while the “darker” part at the bottom right is antipodal to the core.

In other words, orientation is important!

However, it is also feasible for the entire galaxy to be stretched into a ring as a result of a collision. In general, this happens when two very massive galaxies collide, but one of them was initially very low in the amount of stars it held. A collision can therefore result in both a ring and the gravitational disruption of the galaxy itself, allowing both the precursor galaxy and the ring to inhabit the same location in space. This, rather than a simple displaced core, is most likely the cause of at least some coreless ring galaxies, such as the one discovered in Arp 147.

ring galaxy
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.(Credit: NASA, ESA, and M. Livio (STScI))

Of course, this is a pretty wonderful narrative, but are we sure it’s correct?

There is only one way to test it. In theory, if our picture is correct, we should discover:

  • pair of galaxies speeding towards each other and poised to collide,
  • a few such pairs in which one comes in at just the correct angle to “punch through” the exact center of the other,
  • resulting in the formation of new stars in a ring outside the main galaxy
  • includes the possibility of partial or complete displacement of the original core,
  • If our sample size is large enough, this will be followed by additional evolution into a variety of ring-like shapes.

Simulations can duplicate this, but we need to locate examples of all the many stages of this process in the Universe to confirm it. When we study the Universe, the timescale of human civilization is too short to observe this process developing; we can only take pictures. There are numerous examples of interacting pairs of galaxies with features that could lead to a ring, particularly in the field (rather than in clusters). And there are numerous examples of rings forming from a post-collisional state.

However, some objects, like as Mayall’s, show the precise vital time we’d like to recognize. When it was discovered in 1940, it was assumed to be a “question mark,” but it is now recognized to be the collision of two galaxies in the process of forming a ring galaxy.

Mayall's object
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 ine one that got “punched” is having its gas propagate outward in waves, triggering new star formation on its way towards creating an overall ring-like shape.(Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University))

Even while we now understand how ring galaxies originate in general, Hoag’s object — the original ring — remains an outlier that stubbornly refuses to be described by any single straightforward scenario. The ring and the core have nearly identical velocities, indicating that if the ring was constructed by an interloper, it was a very quiet operation. Surprisingly, there is no indication of a possible interloper galaxy anywhere in its vicinity, nor are there any galaxy fragments. The scenario cannot be saved by shifting the collision further back in time, because the outer ring of stars is too young. The inner core, instead of being a spiral, is a gas-poor elliptical.

Nonetheless, it is a great accomplishment to be able to explain the process by which the rarest class of all major galaxy types, ring galaxies, form. If you have a gas-rich spiral galaxy and another galaxy comes along and punches right through the center, your internal gases will ripple outward, smashing into the pre-existing gas along the way, triggering new waves of star formation on the outskirts, all while depleting the matter in the galactic core. The remaining riddles may yet be answered with improved data over more wavelengths. Still, it’s worth noting how far we’ve come in our understanding of not only what’s out there in the Universe, but also how what’s out there came to be.

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MICHAEL
MICHAEL
8 months ago

I look at the pic, and it seems I understand this. Thank you for the comfirmation.
. 🌌🤠.

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