Rapid Secular Evolution: A Bar Matures in a Galaxy 7.6 Billion Years Ago ✦
The James Webb Space Telescope captured an image of a galaxy at redshift 0.92 (when the universe was 6.2 billion years old, i.e., 7.6 billion years ago). It already shows an X-shaped bulge, a nuclear stellar disk, and an extended stellar disk. The geometry and estimated bar size are indistinguishable from those of modern barred spiral galaxies. This indicates that the main evolutionary phase involving the bar was completed when the universe was less than half its current age. The result shows that mature structures can form much earlier than previously thought.
Context
Even Edwin Hubble noted that many spiral galaxies harbor bars, and Vera Rubin discovered that rotation curves stay flat due to substantial dynamical mass. According to Hubble’s law, galaxy UDS 12999 is receding at a speed corresponding to a redshift of 0.923, meaning its light traveled 7.6 billion years—we see it as it was when the Universe was 6.2 billion years old. Disk galaxies undergo a prolonged internal evolution: the disk forms a bar, the bar funnels gas inward triggering star formation in a nuclear disk, and then thickens into an X-shaped bulge. However, this process was thought to finish only at late epochs; fresh JWST data now allow us to check how quickly secular evolution can proceed.
Methods
The researchers used images from the PRIMER survey taken with JWST/NIRCam and the spectrograph NIRSpec to precisely measure the redshift (z=0.92331±0.00003). The galaxy was observed in eight filters from 0.9 to 4.4 µm. Multi-component image modeling included a disk with a vertical profile, a central Sérsic component, and an X-shaped structure with an m=4 mode. A dust lane linked to interstellar dust was masked, and empirical PSF models were used to account for the point spread function. The model was fitted to all wavelengths simultaneously, yielding photometry and structural parameters for each component. The stellar population was analyzed using the Prospector code, in comparison with numerical simulations of cosmological evolution.
Results
Galaxy UDS 12999 showcases a fully formed X-shaped bulge, a nuclear stellar disk with a radius of ~800 pc, and a disk with a scale length of 6.1 kpc. The geometry of the X-structure (axis ratio and normalized length) falls exactly on the local sequence of edge-on galaxies. The estimated bar size of 11.8 (+5.5/-4.0) kpc matches the size–mass relation for present-day galaxies. The star formation history reveals a vigorous growth phase over 13 billion years ago: half of the stellar mass (7.7×10^10 M☉) was already assembled by 11.4–13.0 billion years ago (z~2.7–6.9). After that, star formation dropped to 8 M☉/yr, and in the X-bulge it ceased ~270 million years ago, while the disk continued forming stars. The time budget is extremely tight: only 3.8–5.4 billion years remained for disk formation, bar development, and thickening, demanding an early assembly of the disk and rapid dynamics.
Implications
The results imply that massive disk galaxies can wrap up the main phase of secular evolution as early as 6.2 billion years after the Big Bang, corroborating JWST findings of mature bars at z~3. This challenges models in which X-shaped bulges only form by z~0.5–0.7. Early formation of a cold disk and rapid bar growth require baryonic mass to dominate in the center, a natural consequence of vigorous early star formation. The dynamics described by the law of universal gravitation can, under such conditions, accelerate the evolution. UDS 12999 thus becomes a bridge between observations and numerical simulations.
Future development
Further statistics from the JWST PRIMER survey and the upcoming Euclid telescope will enable targeted searches for edge-on galaxies at high redshifts. High-resolution spectroscopy with ELT will yield stellar and gas kinematics, probing the longevity of bars. Improved high-resolution cosmological simulations with baryonic physics should reproduce early X-bulge formation. It will also be exciting to explore the link between dark matter properties and the pace of secular evolution.
Impact
The discovery will impact galaxy formation models, stellar system dynamics, and cosmology, calling for a revision of disk evolution timescales and the role of bars in mass redistribution.
Next steps
Searching for and confirming more examples of X-bulge galaxies at z>0.9; performing detailed spectroscopic kinematics to directly test the presence of a long-lived bar.
Key open problems
The results directly address the problem of disk formation and their dynamical cooling timescale, as well as the question of how quickly cosmic structures reach maturity within a hierarchical picture of mergers and accretion.
🎯 Our very own Milky Way also sports an X-shaped bulge, discovered by the infrared telescope WISE; its formation likely wrapped up about 8 billion years ago.
Key numbers
- Redshift: 0.92331
- Lookback time: 7.6 billion years
- Stellar mass of the galaxy: 7.7×10^10 M☉
- Bar size estimate: 11.8 kpc
- Star formation rate: 8 M☉/yr