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Starts With A Bang

Astronomers Find A Galaxy Of Unusual Size (G.O.U.S.), And Discover Why It Exists

It’s one thing to find a galaxy that “shouldn’t exist.” It’s quite another to learn why it does.

Above a certain size, spiral galaxies shouldn’t exist. A single major merger — where two galaxies of comparable mass interact to form a larger one — will almost always destroy that spiral structure, producing a giant elliptical instead. The only ultra-large spiral galaxies we typically find are in the process of gravitationally interacting with a neighbor, producing an extended but temporary “grand spiral” structure.

But for every rule, there are remarkable exceptions. One particular galaxy, known unofficially as Rubin’s Galaxy after Vera Rubin’s observations of the rotational properties of UGC 2885, is far larger and quieter than practically any other spiral galaxy known. This is a spiral galaxy of unusual size, a true G.O.U.S., and while it doesn’t quite defy our theories of how galaxies form, it certainly is a challenge to explain. Remarkably, just from observing the right details, astronomers now think they know how this most unusual galaxy formed.

The previous record-holder for largest spiral galaxy, Malin 1, consists of a small core surrounded by extensive, sweeping spiral arms. These extended features were created by gravitational interactions with surrounding nearby galaxies, and led to the belief that there would be no larger spirals that weren’t experiencing such interactions, a belief that was overturned with the discovery and analysis of UGC 2885. (BOISSIER/A&A/ESO/CFHT)

In theory, there are two ways to build up a large spiral galaxy, and they both begin the same way. In the young Universe, a large cloud of matter — both normal matter and dark matter — will begin to collapse under its own gravity. While the dark matter is responsible for the majority of the mass, it only interacts gravitationally, which means it can’t collide, heat up, lose angular momentum, or collapse. The dark matter always remains in a diffuse, “fluffy” halo.

But the normal matter, made out of the same ingredients that we are, interacts with itself. Normal matter doesn’t just experience gravitation, but as it collapses, the different atoms, molecules and other particles collide and interact. They lose angular momentum, and in whichever dimension it collapses first, it goes “splat” and forms a disk, which then rotates. This is the origin of the disk-like structure present in all spiral galaxies.

In general, a cloud of gas that will collapse to form structure (such as a galaxy) in the Universe will begin as an irregularly shaped mass, which will then gravitationally contract along all three axes. The shortest axis will ‘splat’ first, leading to the formation of a plane and a disk that will rotate: a phenomenon that works on scales from large spiral galaxies down to individual stars and planetary systems. (JOSHDIF / WIKIMEDIA COMMONS)

As far as we can tell, galaxies always start out small and then grow in two possible ways.

  1. Intergalactic gas can get gravitationally drawn in from the surrounding, less dense areas of space. This slow, gradual funneling of matter into the galaxy will provide new fuel for new generations of stars, will settle into the disk-and-spiral structure of the existing galaxy, and will cause the galaxy to both become slightly thicker and significantly larger in terms of its radial extent.
  2. Smaller galaxies and proto-galaxies, also from the surrounding, less dense areas of space, can get drawn into the larger galaxy. This process is a little different, since there are already stars and structure inside these objects, and they will get disrupted and torn apart, stretched into debris streams before eventually settling down as part of the larger spiral, also growing it to become both thicker and larger in extent.

Both of these processes are seen to occur in our Universe, with the latter one occurring for dwarf galaxies surrounding our own Milky Way right now.

This artist’s impression shows how intergalactic gas flows and funnels onto galaxies, leading to gradual growth that neither disturbs nor destroys and pre-existing spiral structure. (ESO/L. CALÇADA/ESA/AOES MEDIALAB)

What couldn’t happen, though, is the fastest, most efficient, and most common way to increase a galaxy’s mass: through a major merger. If two galaxies that are comparable in size ever merge together, regardless of the orientation of the merger, an enormous fraction of the gas contained within both galaxies will collapse in a spectacular burst of new star formation. It’s a spectacular astronomical event known as a starburst: where the entire galaxy becomes a giant star forming region.

This generally uses up most of the gas present in the new galaxy, forms a whole slew of stars all at once, and then star formation ceases. These stars form over a large volume of space, creating an elliptical structure rather than a spiral one, and then — as the galaxy ages — the most massive stars die and only the smaller, cooler, redder stars remain. Elliptical galaxies are notorious for having very few instances of star formation past the initial burst arising from their creation, and are far and away the largest and most massive galaxies of all.

Galaxies that have formed no new stars in billions of years and have no gas left inside them are considered ‘red-and-dead.’ A close look at NGC 1277, shown here, reveals that it may be the first such galaxy in our own cosmic backyard. (NASA, ESA, M. BEASLEY (INSTITUTO DE ASTROFÍSICA DE CANARIAS), AND P. KEHUSMAA)

To find a spiral as large as the one we see here — Rubin’s galaxy (UGC 2885) — implies that there were no major mergers. The fact that we still see:

  • a spiral structure,
  • with dusty arms,
  • with the pink signatures of ionized hydrogen (from new star formation),
  • with blue stars dotting the arms (indicating recent episodes of newly forming stars),
  • and an undisturbed, flat, even disk,

tell us that this spiral grew by either gas accretion, minor mergers, or both, but via no other processes.

Even if it’s a cosmic rarity that a galaxy would form this way, though, a good scientist always wants to know exactly how it happened. Fortunately, there’s a very clever way to tell: by looking at the globular clusters present within the galaxy.

The globular cluster Messier 69 is highly unusual for being both incredibly old, at just 5% the Universe’s present age, but also having a very high metal content, at 22% the metallicity of our Sun. The brighter stars are in the red giant phase, just now running out of their core fuel, while a few blue stars are these unusual blue stragglers. The globular clusters within the Milky Way display a variety of ages and colors, but the majority of them, like Messier 69, formed 12 or 13 billion years ago. (HUBBLE LEGACY ARCHIVE (NASA / ESA / STSCI), VIA HST / WIKIMEDIA COMMONS USER FABIAN RRRR)

Whenever you get a big burst of star formation, you don’t just produce new stars evenly throughout the galaxy, although you do produce copious amounts of them over a wide area. What happens is that the largest, most concentrated areas of gas result in an enormous, dense collection of stars — from tens of thousands of stars all the way up to millions of new stars — all contained within just a few dozen light-years: a globular cluster.

Each galaxy has its own unique population of globular clusters found distributed all throughout its halo, which are formed during episodes of extreme star formation. If all the extreme star forming episodes happened at once, we expect the globular clusters to all be the same age in the galaxy, indicative of at least a medium-sized merger at a specific period in time. On the other hand, if there were many mergers of small galaxies or a build-up of gas to form the one we see at the present day, we expect globular clusters to come in a variety of ages. Both scenarios are eminently possible, but good enough observations of the globular clusters themselves should be able to determine which one is true from the colors of the stars within them.

This is a blink comparison that plots the location of the red stars and blue stars that dominate the globular clusters in galaxies NGC 1277 and NGC 1278. It shows that NGC 1277 is dominated by ancient red globular clusters. This is evidence that galaxy NGC 1277 stopped making new stars many billions of years ago, compared to NGC 1278, which has more young blue star clusters. The number and colors of globular clusters can shed light on the parent galaxy’s star formation history. (NASA, ESA, AND Z. LEVAY (STSCI))

In our own Milky Way, for example, the majority of the globular clusters we find are extremely old, formed some 12 or 13 billion years ago. This component of the globulars indicate that the main component of our Milky Way was formed early on by gravitational collapse and a potential merger, leading to an extreme burst of star formation that occurred over just a brief period of time. However, alongside those, we also find globular clusters that are much younger, indicating that smaller galaxies and the inflow of gas, which caused new bursts of star formation and the formation of new globular clusters at various times, occurred gradually over time.

For this reason, measuring the ages of the globular clusters within Rubin’s galaxy — a true G.O.U.S. — will reveal whether there were significant mergers in the past that resulted in bursts of star formation and the creation of new globulars all at once, or whether they formed at many different times, indicating only a gradual accretion of gas without any significant galactic mergers (and large bursts of star formation) to speak of. When a team of scientists turned the Hubble Space Telescope’s eye on Rubin’s galaxy, they were able to uncover something unprecedented.

The inner regions of UGC 2885, Rubin’s galaxy, shows the ionized hydrogen (red) that occurs when you have new star formation, as well as a clearly visible population of young, blue stars along the arms. The globular clusters found throughout it, all 1600 of them, show a variety of colors and ages, but this number is very small for a galaxy this large and massive. (NASA, ESA, AND B. HOLWERDA (UNIVERSITY OF LOUISVILLE))

First off, all of the globular clusters they did find showed a variety of colors, which is a great indication that they were formed at a variety of epochs from gradually inflowing gas. Perhaps most interestingly, there isn’t a large set of globulars that all seemed to form at around the same time, indicating that there weren’t any major or medium-sized mergers in the history of Rubin’s galaxy. This piece of evidence, all on its own, is a point in favor of the “gradual accretion of gas” scenario, rather than an accretion and merger of surrounding, smaller galaxies.

But a second piece of evidence is even stronger: the number of globular clusters found in this behemoth of a spiral galaxy is tiny for its mass, indicating that there were realistically no major bursts of intense star formation since very early times that were triggered by mergers or gravitational interactions.

The outskirts of UGC 2885, hundreds of thousands of light-years from its center, still display sweeping arms and young stars, showing the enormous extent of it: 800,000 light-years across, making it the largest spiral galaxy to date. (NASA, ESA, AND B. HOLWERDA (UNIVERSITY OF LOUISVILLE))

When we look at the environment surrounding this G.O.U.S., there are neither nearby massive structures nor disturbed internal structures that would account for the large, extended spiral structure of this galaxy. Rubin’s galaxy really is this massive cosmic outlier, likely formed only by the gradual accretion of matter.

According to the study’s Principal Investigator, Benne Holwerda, the most comparable galaxy to Rubin’s galaxy in our own local neighborhood is the quiet, small spiral: M83, the southern pinwheel galaxy. It is:

  • relatively isolated,
  • with no massive galaxies in its neighborhood,
  • with only one stable nucleus,
  • undergoing stable, quiet, slow star-formation along its spiral arms,

all of which point to a quiet, slow accretion of gas. However, Rubin’s galaxy is enormous, making it the first galaxy with these combined properties to date.

The spiral galaxy M83, also known as the Southern Pinwheel Galaxy, bears many similarities to UGC 2885 in terms of its isolation, globular cluster population, morphology and star formation rate and history. But UGC 2885 is approximately 16 times larger in diameter and contains about 40 times as many stars. (NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA); ACKNOWLEDGEMENT: WILLIAM BLAIR (JOHNS HOPKINS UNIVERSITY))

At 800,000 light-years across and with some 4 trillion stars inside, this is one of the largest spiral galaxies ever discovered: a true cosmic outlier. At just 230 million light-years away, it’s also close enough that we can image and identify its globular clusters and star formation rate. The fact that a galaxy this large and massive is so regularly shaped, with such low levels of star formation and so few globular clusters (1600) for its incredible size really does make this a cosmic unicorn.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

This galaxy of unusual size really is a first-of-its-kind, and not just for being so beautifully symmetric and quiet, but for growing to this enormous magnitude without a single major disruptive event throughout its history. In all the Universe, there may not be another like it, but the odds are far better that this is just the first discovery of a new type of spiral galaxy: a G.O.U.S.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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