Meet Zhúlóng: The oldest known spiral galaxy in the universe

When the Webb telescope launched, astronomers expected new surprises. But few predicted the discovery of something as striking as Zhúlóng – a massive, elegant spiral galaxy that had already formed just one billion years after the Big Bang.

In a cosmos that should still have been chaotic and unstructured at that time, this galaxy stands out as an anomaly. Not only is it huge and distant, but it also resembles modern spirals like our own Milky Way, which were long believed to be latecomers in cosmic history.

The discovery, detailed in a new study from the PANORAMIC Survey team, is shaking up long-held ideas about how galaxies grow and evolve. Using JWST’s cutting-edge imaging, astronomers spotted Zhúlóng nestled in the ancient Universe.

At a redshift of z ∼ 5.2, meaning that the light has been travelling from this galaxy for 12.5 billion years, Zhúlóng is now the most distant spiral galaxy known – a record-breaker and a puzzle rolled into one.

Zhúlóng: The “Torch Dragon”

‘‘We named this galaxy Zhúlóng, meaning ‘Torch Dragon’ in Chinese mythology,” explained Dr. Mengyuan Xiao, postdoctoral researcher at the Department of Astronomy at the University of Geneva, and lead author of the study.

“In the myth, Zhúlóng is a powerful red solar dragon that creates day and night by opening and closing its eyes, symbolizing light and cosmic time.’’

Zhúlóng is massive, red, and shockingly mature considering that it formed at a very early time in cosmic history. It sports a well-defined classical bulge and a sweeping disk with spiral arms that stretch across 62,000 light-years.

This structure is not common among galaxies formed in the early universe, which usually appear clumpy and irregular in deep space images.

The PANORAMIC Survey that uncovered Zhúlóng used JWST’s “pure parallel mode,” a method that allows one camera to collect additional images while another conducts its primary task. This clever approach captured extra slices of the early universe, increasing the chances of turning up rare finds.

“Wide-area surveys are necessary to discover rare, massive galaxies. We were hoping to discover massive and bright galaxies across the earliest epochs of the universe to understand how massive galaxies form and evolve, which helps to interpret the later epochs of their evolution that will be observed with the LSST,” noted Christina Williams, co-leader of the study.

Zhúlóng looks like a modern galaxy

Zhúlóng looks nothing like the galaxies typically found at this redshift. Most galaxies formed in early epochs are distorted, compact, or undergoing chaotic starbursts. Instead, this one features a calm, balanced structure – a red, quiescent core surrounded by a vast disk filled with young stars.

Using JWST’s deep infrared data, astronomers revealed a stellar mass of about 10¹¹ solar masses. That places it on par with the Milky Way.

The galaxy has already transitioned its core into a quiet state that no longer forms stars. Surrounding this core, however, the disk still supports some ongoing star formation, albeit at a relatively modest rate of around 66 solar masses per year.

Zhúlóng’s specific star formation rate is surprisingly low for a galaxy formed so early in the history of the universe.

This suggests the galaxy is entering a transitional phase – moving from active formation into cosmic retirement, long before most galaxies of its kind.

Mature spiral born in chaos

The elegance of Zhúlóng’s spiral arms has drawn intense attention. These arms are not faint or broken but stretch cleanly from the central bulge outward, resembling the structure of well-formed spirals like the Milky Way.

For decades, theorists believed such arms required many billions of years to develop, as they rely on organized disk dynamics and stable conditions.

Yet Zhúlóng contradicts this.

“It is really exciting that this galaxy resembles a grand-design spiral galaxy like our Milky Way,” says Williams.

“It is generally thought that it takes billions of years for this structure to form in galaxies, but Zhúlóng shows that this could also happen in only one billion years.”

What’s more, the arms appear in residual maps after subtracting the galaxy’s main light components, which confirms their reality. The spiral pattern extends across an unusually large face-on disk that is nearly 20 kiloparsecs wide.

Its half-light radius reaches up to 3.9 kiloparsecs, and when using a non-parametric method, 80% of its stellar light spreads out to nearly 10 kiloparsecs. This is immense by any standard, let alone for a galaxy so early in cosmic history.

Rare find or tip of the iceberg?

Could Zhúlóng be unique, or is it the first of many such ancient spirals? The answer may lie in future wide-area JWST surveys and improved models of galaxy formation.

So far, only a handful of spiral-like galaxies have been identified at redshift z > 3. Zhúlóng stands as the most distant example.

Its large disk, mature bulge, and red spiral arms all point to a level of organization that is rarely expected in the early cosmos.

The galaxy also avoids heavy dust obscuration, unlike some other massive galaxies discovered with submillimeter telescopes. Instead, it offers a clear view into the dynamics of early disk galaxies.

The rarity of spiral arms in early epochs could mean they are short-lived. In the high-density, merger-rich universe of that time, such delicate structures might often be destroyed before reaching our telescopes. Zhúlóng may represent a fleeting moment of cosmic symmetry.

Why does Zhúlóng matter?

Zhúlóng challenges the narrative of slow galaxy growth. It suggests that some massive galaxies, perhaps under specific conditions, can grow their bulges and disks rapidly.

It also reinforces that the classic Hubble sequence – with spirals, ellipticals, and bulge-disk galaxies – may have begun forming far earlier than once thought.

In comparing Zhúlóng to the Milky Way, the similarities become startling. The size, mass, and structure closely match our home galaxy, but Zhúlóng achieved this structure in only a tenth of the time.

This forces a reevaluation of the timing and mechanics behind spiral galaxy formation.

Despite the turbulent environment of the early universe, some galaxies found a path to order and elegance. Zhúlóng is one such case – not just a cosmic oddity but a glimpse into what the first billion years could produce when conditions aligned.

What happens next?

Zhúlóng’s discovery opens the door to a new class of early galaxies.

As the Webb telescope continues scanning the deep universe and surveys like COSMOS-Web expand, more such spirals may emerge. If they do, astronomers will need to refine their theories about when – and how – galaxies become mature.

This galaxy is a striking example of cosmic maturity arriving far earlier than theory allows. Its red bulge, youthful arms, low star formation rate, and ultra-massive profile defy expectations.

But it is only one data point. Future spectroscopy and deep imaging from JWST and ALMA will help decode its origin story.

For now, Zhúlóng stands as a dragon in the deep, carrying light, time, and mystery from an age we are only just beginning to understand.

The study is published in the journal Astronomy and Astrophysics.

Image Credit: NOIRLab, NSF, AURA, NASA, CSA, ESA, M. Xiao (University of Geneva), G. Brammer (Niels Bohr Institute), D. de Martin & M. Zamani (NSF NOIRLab)

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