Bars Across Time: Tracing Galactic Structures Over 12 Billion Years

Zoe Le Conte and her collaborators set out to understand how the bars seen in spiral galaxies, those bright, elongated bands of stars stretching across their centers, have changed throughout cosmic history. Using the powerful James Webb Space Telescope (JWST), they examined galaxies as far back as 12 billion years ago (redshift z = 4), tracking how common these bars are and how long they appear to be.

The Importance of Bars in Galaxies

In the local Universe, about two-thirds of disc galaxies have bars, which play a crucial role in their internal evolution. Bars help move gas and stars around, fueling star formation in the center and possibly feeding black holes. Because bars can only form once a galaxy’s disc becomes stable and dynamically cold, their presence provides clues about when galaxies first settled into their mature forms.

Earlier studies using the Hubble Space Telescope (HST) suggested that the bar fraction, the fraction of disc galaxies that are barred, drops sharply beyond z ≈ 1, meaning bars were thought to be rare in the early Universe. JWST’s improved infrared sensitivity now allows astronomers to peer further back and detect bars even when hidden by dust.

Data and Sample

Le Conte et al. used observations from the Cosmic Evolution Early Release Science Survey (CEERS), combining images from ten JWST NIRCam fields. From these, they selected over 2400 galaxies with reliable mass and distance estimates between z = 1 and 4, focusing on those massive enough to host visible discs. The sample was filtered carefully: galaxies viewed edge-on were removed, and the remaining ones were visually classified by five independent astronomers. Each galaxy was labeled as a barred disc, unbarred disc, or other morphological type, using both short-wavelength (F200W) and long-wavelength (F356W + F444W) images to account for dust effects and resolution limits.

Bar Fraction Through Time

The team found that the bar fraction decreases steadily with redshift, that is, bars become rarer the farther back in time we look. For the combined dataset, the bar fractions were: 16% at 1 ≤ z < 2, 8% at 2 ≤ z < 3, and 7% at 3 ≤ z ≤ 4. This means that even 11 billion years ago, a noticeable number of galaxies already had bars, implying that disc structures were forming earlier than previously thought. These results confirm and extend earlier JWST findings, showing that bar formation, and therefore internal, secular evolution, was already underway soon after galaxies formed.

Bar Lengths and Galaxy Mass

The study also measured how bar length changes with time. Surprisingly, the average bar length, about 3–4 kiloparsecs (roughly 10,000 light-years), did not evolve significantly from z = 4 to z = 1. Bars seen in the shorter-wavelength images appeared slightly shorter, which the authors attribute to better spatial resolution and to dust obscuration at those wavelengths. When bar length was compared to the overall size of the galaxy disc, the ratio remained constant over the whole 12-billion-year period. This suggests that bars and their host discs grow together, maintaining roughly the same relative size.

Interestingly, at lower redshifts (closer to today), the team found some bars longer than 8 kpc, structures not seen at earlier times, indicating that the largest bars form later as galaxies continue to evolve and their discs expand.

Implications and Conclusions

Le Conte’s results paint a picture of a Universe where bars, and by extension, stable disc galaxies, were already present just a few billion years after the Big Bang. These bars were not only common enough to shape early galactic evolution, but in some cases already as long and strong as modern bars. The findings challenge older models that placed the emergence of bar-driven evolution much later and show that galaxy structure was settling into familiar forms surprisingly early.

Source: Le Conte