The Oldest X-shaped Structure in a Galaxy: JWST Peers Back 7.6 Billion Years
Disk galaxies like the Milky Way evolve through a sequence of internal processes: the stellar disk forms a bar, the bar funnels gas to the center, creating a nuclear stellar disk, and then the bar vertically thickens into an X-shaped bulge. It was long thought that this evolution takes up most of cosmic history. However, observations from the James Webb Space Telescope revealed a galaxy at redshift 0.92 (7.6 billion years ago) that already has an X-shaped bulge, a nuclear stellar disk, and an extended disk. Its geometry and estimated bar size are indistinguishable from those of modern barred galaxies. Thus, the main phase of bar-induced evolution was completed when the universe was less than half its current age, imposing tight constraints on the timescales of dynamical galaxy evolution.
Context
Disk galaxies, like our Milky Way, follow a complex evolutionary path: gas, mostly hydrogen, and stars settle into a thin rotating disk, then gravitational instability gives rise to an elongated bridge — a bar. The bar, like a giant mixer, churns the material, funneling gas to the center and triggering the formation of a nuclear stellar disk. Over time, as the bar grows sufficiently massive, it loses stability: its inner part “buckles” and takes on the distinctive X-shape seen edge-on. This process, known as secular evolution, was thought to take billions of years and to be completed only in the mature universe. However, the exact timing remained unclear. Long ago, Edwin Hubble showed that redshift lets us look back in time, and now, thanks to dark energy accelerating the expansion, we know that 7.6 billion years ago dark matter had already clumped enough matter to form such structures.
Methods
The scientists used images from the JWST space telescope in eight near-infrared filters from the PRIMER survey, as well as archival data from Hubble and high-resolution spectroscopy with the NIRSpec instrument to precisely measure the redshift (z=0.92331±0.00003). Galaxy UDS 12999 is seen almost exactly edge-on, which made it possible to resolve the vertical structure and estimate the disk’s thickness. To separate the X-shaped bulge from the disk and central component, the researchers applied multicomponent modeling: from images in each filter they subtracted a model of an exponential disk and a compact core, and described the remaining X-shaped structure with a Sérsic profile plus an azimuthal m=4 mode, creating four symmetric arms. Modeling the spectral energy distribution with the Prospector code yielded the star formation history and accumulated stellar mass.
Results
The analysis showed that galaxy UDS 12999, with a mass of about 7.7×10^10 solar masses — already comparable to the Milky Way at that time — has all the signs of completed secular evolution. The parameters of the X-shaped bulge (axis ratio 0.72 and major axis length 4.5 kpc) and the estimated bar size (11.8+5.5−4.0 kpc) fit closely the local relations derived for modern galaxies. At the center, a nuclear stellar disk was found with a radius of 812 parsecs and an ellipticity of 0.72, typical for the products of gas inflow along the bar. The stellar populations of the X-shaped structure and the outer disk are similar, but star formation in the bulge ceased about 270 million years ago, supporting the scenario of its formation from disk stars "kicked" onto stable orbits by bar resonances. The star formation history indicates a burst of star formation at a rate of up to 50 M☉/year more than 13 billion years ago, followed by a decline: the galaxy quickly assembled half its mass within 3.8–5.4 billion years after the Big Bang, after which it entered a quenched state.
Implications
The discovery shows that key stages of secular evolution — formation of a bar, nuclear disk, and X-shaped bulge — can be completed much earlier than previously thought. It’s independent confirmation that massive disk systems can quickly stabilize and form long-lived bars already in the early universe, consistent with recent simulations and observations of bars at z~3, but adds a critical piece — the vertical structure. Such a rapid evolution requires baryon dominance in the inner regions, which amplifies gravitational instability and speeds up the dynamics, as predicted by the works of Vera Rubin on galaxy rotation, which highlighted the importance of mass distribution.
Future development
Future JWST observations in deep fields, like those that once inspired the pioneer of extragalactic astronomy Edwin Hubble, will allow finding even more distant X-shaped bulges, and high-resolution spectroscopy on current and future telescopes will measure the kinematics of stars and gas to directly confirm the presence of a bar and its age. A joint analysis with gravitational lensing data and numerical simulations on supercomputers will help refine the physics of disk instabilities and the role of dark matter in this process. Perhaps we are on the verge of revising the timeline of galaxy evolution, where the active bar-driven evolution phase wraps up just a few billion years after the Big Bang.
Impact
The results will impact theories of galaxy formation, cosmological models of large-scale structure evolution, and the interpretation of JWST data, as well as our understanding of the history of the Milky Way, which is thought to have undergone a similar evolution around the same epoch.
Next steps
Searching for similar galaxies at even higher redshifts (z > 1.5) and obtaining high-resolution spectroscopy for detailed kinematic modeling of UDS 12999.
Key open problems
This result is directly connected to the problem of the epoch of disk galaxies formation and the onset of secular evolution, as well as the role of dark matter and baryonic physics in the early universe. It challenges standard scenarios where X-shaped bulges appear only after z~0.7, and refines predictions of the cosmological model that traces back to the hypothesis of the primeval atom by Georges Lemaître.
🎯 An X-shaped bulge is more than just a shape: stars inside it move along complex orbits that resemble an "hourglass". If we could see the Milky Way edge-on, we would also see such a structure — it was discovered in our Galaxy in the 2010s using data from the WISE infrared telescope. And did you know that our Milky Way also has a bar about 4–5 kpc long, and its age might be comparable to that of UDS 12999?
Key numbers
- Redshift: 0.92331 ± 0.00003
- Time after Big Bang: 6.2 billion years
- Galaxy mass: 7.7 × 10^10 M☉
- Bar size: 11.8 (+5.5, -4.0) kpc
- Peak star formation rate: up to 50 M☉/year