Bullseye! Galaxy with nine rings may also reveal dark matter secrets
A team of international researchers recently discovered a galaxy with nine rings. They called it a 'serendipitous discovery' because previous ringed galaxies have only displayed two or three rings at best.
Using the Hubble space telescope, the team confirmed the presence of eight rings, while data from the W.M. Keck Observatory in Hawaii confirmed the existence of the ninth ring.
This unusual galaxy has been named LEDA 1313424 but its common name is more memorable: the Bullseye Galaxy. The story of how its rings could have formed may render it more memorable.
A thin trail of evidence
When the researchers analysed the data from the space telescope and the observatory, they found signs that a blue dwarf galaxy, located at the immediate centre-left in an image, had passed through the centre of the Bullseye Galaxy about 50 million years ago. They have said this interaction gave the Bullseye Galaxy its unique shape.
As evidence of this interaction, the team reported a thin trail of gas connecting the two galaxies even though they are separated by 130,000 lightyears (or 1.22 billion billion km). This is all the more remarkable considering the Bullseye Galaxy is nearly 2.5-times larger than the Milky Way with a diameter of 250,000 light-years.
On cosmic timescales, galaxies crash or barely miss one another relatively often. But even then it is very rare for one galaxy to literally dart through the core of another. The blue dwarf galaxy's straight path through the Bullseye Galaxy caused gas in the latter to ripple back and forth in waves, creating new places of star formation. The interaction didn't alter the orbits of individual stars but it caused groups of stars to pile up and form the distinct rings over millions of years.
The Bullseye Galaxy will continue to evolve and, as a result, will have these star-filled rings only for a short interval of time. This means the astronomers captured an intriguing image of a multi-ring galaxy in a special moment.
For other astronomers, however, the intrigue may run even deeper: the Bullseye Galaxy also contains signs that it could one day evolve into a giant low surface brightness (GLSB) galaxy, which are important in the study of dark matter.
Their findings were published in February in The Astrophysical Journal Letters.
Cosmic oddballs
Low surface-brightness galaxies have a shortage of elements heavier than hydrogen and helium. They also have very little star formation despite possessing large disks filled with hydrogen, the primary fuel for new stars. Scientists haven't been able to explain this paradox. These galaxies are also believed to be filled with dark matter, making them excellent targets of study if scientists are to understand this still-mysterious form of matter.
These galaxies also have a more uniform distribution of mass near their centres — which is at odds with the standard model of cosmology, which predicts the centres of galaxies to be a lot more dense. This discrepancy is another challenge scientists are trying to overcome with more data and better theories.
Giant low surface brightness (GLSB) galaxies are the largest of the low surface-brightness galaxies. All GLSB galaxies are truly colossal. Their most famous member, called Malin 1, is roughly 6.5-times wider than the Milky Way and one of the largest spiral galaxies known.
'GLSB galaxies are spiral galaxies that possess very diffuse or low surface density stellar disks, yet they are embedded within large neutral hydrogen gas disks,' said Mousumi Das, a professor at the Indian Institute of Astrophysics, Bengaluru, who specialises in low-surface brightness galaxies among other areas.
She added that the mass of black holes at the centers of these galaxies is also lower than usual, meaning they aren't fully evolved.
Given the various ways in which LSBs deviate from the patterns that unite other galaxies, astronomers have struggled to understand them. Their simulations, which are based on the standard model of cosmology, predict these galaxies' hydrogen content, the surface brightness of their disks, and their density profiles — only for them to be at odds with what astronomers see in the data collected by telescopes and observatories.
In science, such a disagreement means the scientific theories are in some way incomplete.
There are some ideas to resolve the disagreement. Das offered one example: 'Previous studies have indicated that the dark-matter halos surrounding these galaxies spin more rapidly than expected,' a process she likened to 'how clay behaves on a spinning potter's wheel. As the wheel turns, the clay spreads and thins out. In the same way, as these dark-matter halos rotate rapidly, the material within them expands outward, creating a large, disk-like shape with a low density,' and eventually GLSB galaxies.
'And their disks are not dense enough to easily form stars.'
Studying GLSB galaxies in more detail could help check whether this idea, and others like it, could be true.
This in turn makes confirming whether newfound galaxies like the Bullseye could be GLSB galaxies in future more important.
New insight
The international team of researchers reported that the size of the Bullseye Galaxy's extended disk and hydrogen content relative to its stellar mass are comparable to that of other GLSB galaxies, and that it's likely to become one in future. But in their paper they were also careful to add that more analysis is still required.
Das said, 'The transition from a ringed galaxy to a GLSB galaxy is still a theory that is being explored' and that 'past simulations have demonstrated that some GLSB galaxies may have formed from head-on collisions between disk galaxies', as in the Bullseye Galaxy's case. But she also said most GLSB galaxies are found in isolation — i.e. not surrounded by other galaxies nearby — making it less likely for them to have experienced such collisions.
In other words, confirming Bullseye's candidacy as a pre-GLSB galaxy is complicated.
A 10th ring?
At this moment, astronomers have an opportunity to obtain the first direct observational evidence of a collisional ring galaxy turning into a GLSB galaxy — or not.
Das expressed optimism because, she said, the new study offers compelling evidence of an evolutionary link between the Bullseye Galaxy and GLSB galaxies.
The authors of the new study have also said the Bullseye Galaxy may once have had a 10th ring that has since faded. They added that over billions of years following the collision, the nine rings will also slowly drift out and fade away, leaving behind a GLSB galaxy.
Das added that more information about the formation of GLSB galaxies could reveal new insights into the distribution of dark matter in the universe. Eventually, 'if the [current] theoretical models are correct, we should see GLSB-like galaxies in the results of cosmological simulations.'
Shreejaya Karantha is a freelance science writer and a content writer and research specialist at The Secrets of The Universe.
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