Ring Galaxies, the Rarest in the Universe, Lastly Explained

Spirals, ellipticals, and irregulars are all more frequent than ring galaxies. Finally, we know how these ultra-rare things are made.

When we look out into deep space, beyond the confines of the Milky Way, we discover that the Universe is not quite so empty. Galaxies-tiny and huge, near and far, in rich clusters and near-total isolation-fill the abyss of space, with the Milky Way being merely one of approximately two trillion such galaxies within the visible Universe.

Galaxies are collections of ordinary matter, including plasmas, gas, dust, planets, and stars. Through the exam of that starlight, we have learned more about the physical properties of galaxies and could rebuild how they came to be.

Generally, there are four classes of galaxies that we see. Spirals, like the Milky Way, are the most frequent sort of large galaxy in the Universe. Ellipticals, like M87, are the biggest and most common kind of galaxy in the rich, main areas of galaxy clusters. Irregular galaxies are a third omnipresent kind, usually distorted from a prior spiral or elliptical form by gravitational interactions.

However, an extremely rare type is striking and lovely: ring galaxies. They compose just 1-in-10,000 of all the galaxies out there, with the first one, Hoag’s item, discovered in 1950. After more than 70 years, we have figured out how the Universe makes them.

Visually, when you look at a ring galaxy, a set of features highlights as uncommon among galaxies.

• There is a central core to the galaxy, reasonably compact, that is reduced in gas and is composed primarily of older stars. There has been little recent star formation in that central region.
• Surrounding that galaxy is a lacuna: an area of reduced density, with practically no stars, no light, and little gas or neutral matter.
• And then, beyond that, there is another luminous population of stars. This population exists in a bright, luminous ring that surrounds the central core yet is much bluer in color than the core itself. This shows that the stars within the ring formed much more lately and are dominated by warm, brief, blue-colored stars.

Furthermore, when you look at where ring galaxies are located, they are overwhelmingly situated in what astronomers call “the field,” unlike the central locations of rich galaxy groups and clusters. Even though this set of features may seem bizarre and unassociated, they are all cosmic clues to the origins of these features.

There have been a variety of possible explanations presented for these ring galaxies that we are sure are incorrect, as they can not account for the observed characteristics when we analyze them in detail.

• They are not planetary nebulae, which sometimes have rings around them. They are definitively made up of stars, not of gas and various other ejecta stemming from a singular, dying star.
• They are not made from a young galaxy getting stretched and ripped apart into a ring that comes to surround a separate, older, more large galaxy that sits at the center. The ages of the stars in the external rings and the shapes of the rings themselves reveal this can not be the case, as the timescales and angular momentum constraints conflict with this possibility.
• Moreover, they are not examples of gravitational lensing, where a vast, enormous object stretches, distorts and magnifies the background light from luminous things along a similar line of sight. Gravitational lenses do exist and can create ring-like forms under properly aligned conditions. However, these ring galaxies all have the “ring” population and the “central” population happening at the same redshift, eliminating the possibility of a gravitational lens.

Regardless of what we are looking at, we can be positive that these are all examples of a single galaxy with two different populations of stars: an old one in the central region and a young one in the ring region.

We have some examples of these ring galaxies at present instead of simply a unique example. By analyzing their numerous features, we can assemble some of the puzzle pieces, attempt to assemble a coherent understanding of how these things develop and describe why they appear with the characteristics and properties we see.

In April of each year, NASA and the Space Telescope Science Institute always launch an anniversary image from Hubble, celebrating its 1990 launch on April 24. Even though 2022’s picture, commemorating Hubble’s 32nd birthday, is “just” a tightly-knit galaxy group, the picture released for Hubble’s 14th anniversary, back in 2004, gives a series of significant clues.

Shown beneath, galaxy AM 0644-741 discloses a ring that is not entirely circular yet makes a sort-of extended ellipsoid. In theory, this can either be because there is a projection effect, and we are seeing a circular feature as though it is inclined to us or because whatever occurred to form the outer ring happened asymmetrically. As it turns out, both explanations have merit for this one thing; however, other characteristics are worth pointing out.

First, at a distance of only 300 million light-years, it is relatively easy to solve a number of essential properties. The long axis of the blue-colored ring characteristic is around 130,000 light-years, making it comparable in size to the Milky Way, while the central, white/yellow-colored element is much smaller at simply ~ 50,000 light-years.

Second, dusty characteristics are seen silhouetted against the large ringed feature, which shows that ” fuel ” not only exists to provide gas for continued star-formation but indicates that there are unequal regions of density inside. Several darkest patches are regions that ought to develop new stars moving ahead millions of years into the future.

Third, there are pinkish areas littering the blue ring, indicating the presence of ionized hydrogen: a regular feature of new star-forming regions where stars are proactively being generated.

Finally, let us look at a wider-field perspective than the one caught by Hubble. We can even identify the culprit: an intruder galaxy that evidently “punched through” what is currently a ring galaxy. In other words, this ring characteristic occurred out of nowhere but was created by an interloper that led to its formation very lately.

How would this occur? There are copious gas reservoirs inside practically every spiral galaxy, even in contemporary times. Gas gets stripped and depleted, mainly inside rich galaxy clusters, resulting in what we name “red and dead” galaxies.

Whenever new stars are developed, those new stars cover the complete gamut of colors and masses: from warm, blue, and weighty to cool, red, and light. The hottest, bluest, most massive stars burn through their fuel the fastest, so they are the primary ones to pass away. As the stellar population ages, it goes from blue to white to yellow to orange to red, and the longer it has been since its last star-formation episode, the redder it is. If no gas is stored to create new stars, it is not merely red. It is also “dead,” at least in an astronomical sense.

This is why, we believe, we primarily find ring galaxies in the field instead of in clusters. We need a gas-rich spiral galaxy, to begin with. Afterward, when an interloping galaxy passes through its center, that collision creates outward-moving ripples in the gas, which activate star formation and create the notorious ring-like shape.

Another example of a ring galaxy, which is clearly in a less-fully-evolved state, is the Cartwheel galaxy, shown above. On the right, you can see the dense, older core of a pre-existing gas-rich spiral galaxy surrounded by a bright blue ring of warm, young stars and a series of filaments between the core and the ring. Those filaments are dotted with blue and white stars, although of a much-reduced shine than the principal core or the ring itself.

Could this have developed in the same fashion: from an interloping galaxy that punched through the center of what is currently a ring galaxy, causing gas to undulated outwards, compress and rarify in turn, and form new stars?

Not merely is that the ideal description, but there is a “smoking gun” merely to the left of the Cartwheel galaxy: a smaller, irregular galaxy that itself is rich in young, blue, glittering stars. In other words, in these circumstances, not simply was the Cartwheel galaxy a gas-rich spiral, but so, very likely, was the interloper, which came to be irregular due to the recent interaction.

Some ring galaxies, like Zwicky II 28, shown above, are atypical in some fashion. In some cases, the trespasser galaxy is no place to be found, which becomes part of why the initial ring galaxy– Hoag’s item– stays so mysterious. Others, like this one, seem to lack a central, old core. We must remember that when we look at any particular thing, we are constrained by our particular perspective. In the case of Zwicky II 28, the asymmetry of the ring is crucial; the “brighter” component on the top left appears to house the main core, while the “darker” part at the bottom right is antipodal to the core.

Simply put, orientation issues!

It is not only orientation; it is also possible for the whole galaxy to get stretched into a ring owing to an impact. Usually, this occurs when you have an impact between two massive galaxies, yet one of them initially was relatively low in the number of stars it possessed. It is then that an impact can trigger both a ring and the gravitational disruption of the galaxy itself, permitting both the precursor galaxy and the ring itself to occupy the same area in space. Instead of a straightforward displaced core, it is likely the root of at least some coreless ring galaxies, including the one found in Arp 147 below.

Of course, this is a beautiful story, but are we sure it is correct?

There is one method to place it to the test. Theoretically, if our picture is correct, then we ought to find:

1. pairs of galaxies that speed towards each other and are about to interreact,
2. a few such pairs where one of them comes in at simply the right angle to “punch through” the precise center of the other,
3. carrying new stars developing in a ring outside the main galaxy,
4. including the possible displacement of part or perhaps all of the original core,
5. followed by additional evolution into a variety of ring-like forms, specifically if our specimen is big sufficient.
   Simulations can reproduce this, but if we wish to confirm it, we need to find cases of all the stages of this process available in the Universe. When we observe the Universe, the timescale of human civilization is too short to observe this process unfold; we can only get snapshots. We see many examples of interacting pairs of galaxies, specifically in the field (as opposed to in clusters), with properties that might cause a ring. Furthermore, we see numerous examples of rings themselves, emerging from a post-collisional state.

However, there are also objects that show the specific critical moment we would hope to pinpoint, such as Mayall’s item. Initially thought to be a “question mark” when it was initially identified in 1940, it is now known to be the collision of two galaxies in the process of producing a ring galaxy.

Still, although that we now know how ring galaxies create in general, Hoag’s object– the original ring– is still an outlier that stubbornly rejects to be explained by any one simple circumstance. The ring and the core have almost similar velocities, indicating that if there was an interloper that developed the ring, it was a tranquil process.

There is no evidence of any place in its vicinity for a candidate interloper galaxy, which is unexpected, nor are there any galaxy fragments. You can not conserve the scenario by pushing the impact back into the past, as the outer ring of celebrities is too young. As well as the internal core, instead of being a spiral, is instead a gas-poor elliptical.

Still, it is a remarkable success to be able to clarify the process through which the rarest class of all major galaxy types, the ring galaxies, form. If you have a gas-rich spiral galaxy and another galaxy can come along and punch right through your center, your inner gases will ripple out towards the edges, crushing into the pre-existing gas along the way, causing new waves of star-formation on the outskirts, all while depleting the matter in the galactic core.

The remaining mysteries may be addressed with better data across more wavelengths. Still, it is always important to appreciate how far we have come in our understanding of not just what is around in the Universe but how what is out there came to be.

Read the original article on Big Think.