When Bars Take Shape: Tracing the History of Galactic Bars Across Cosmic Time

In the journey to understand how galaxies evolve, one of the key internal structures under scrutiny is the stellar bar — a long, linear structure of stars that cuts across many spiral galaxies, including our own Milky Way. These bars are not just eye-catching features; they play a critical role in how galaxies evolve over time through internal (or secular) processes. In this study, de Sá-Freitas et al. (2025) present, for the first time, a large-scale observational effort to determine when bars formed in nearby galaxies — an important step in figuring out when galaxies started evolving due to their internal dynamics rather than external influences like collisions.

A Novel Method for Measuring Bar Ages

To determine bar formation epochs, the authors used observations from the TIMER survey, which targeted 24 nearby galaxies with the MUSE instrument on the Very Large Telescope (VLT). They ultimately analyzed 20 galaxies that contained a specific structure known as a nuclear disc. These are dense, disc-like regions near the galactic center that are thought to form soon after the stellar bar does. Since it’s difficult to date bars directly — because they’re made of stars that predate the bar itself — the authors instead focus on these nuclear discs. By analyzing the ages of the stars in the nuclear discs and comparing them to the stars in the surrounding disc, they can estimate when the bar first formed and began channeling gas into the center to build the nuclear disc.

Separating the Stars to Read the Past

To pull this off, the authors used a detailed method that separates the star formation history (SFH) of the nuclear disc from the main disc. Using advanced data processing techniques and spectral analysis tools, they calculated the moment in each galaxy’s history when the nuclear disc began forming stars at a faster rate than the surrounding disc — a telltale sign that a bar had begun funneling gas inward. They applied this technique to the TIMER sample, along with two additional galaxies from earlier work, bringing their total to 20 galaxies — the largest sample of bar age measurements to date.

Bars Born Across Cosmic Time

The results reveal a wide range of bar formation times, from as recent as 1 billion years ago to as long as 13 billion years ago — essentially covering almost the entire history of the Universe. Interestingly, galaxies with younger bars tend to have more active star formation in their centers, while galaxies with older bars appear more "quenched," meaning they form fewer new stars. This supports the idea that bars contribute to quenching by funneling gas inward, which may initially trigger star formation but eventually depletes the gas reservoir, slowing star birth.

Growing Older, Growing Stronger

Another significant finding is that older bars tend to be longer and stronger, and their nuclear discs more massive — providing observational evidence that bars grow and evolve over time. However, the study found no clear link between a galaxy’s mass and the age of its bar, challenging the idea of “downsizing” — a theory which predicts that more massive galaxies form bars earlier. This suggests that other factors, like internal dynamics or interactions with nearby galaxies, may play a larger role in determining when a bar forms.

Reconstructing the History of Bars in the Universe

Lastly, by combining their bar age measurements with assumptions about galaxy evolution, the authors estimate the historical bar fraction — the proportion of disc galaxies with bars at different points in the Universe's history. Their reconstructed curve agrees well with both simulations and distant galaxy observations, reinforcing the idea that bars began forming very early in cosmic history and continue to form even today.

A New Chapter in Galactic Archaeology

This work marks a major step forward in observational galactic archaeology. By offering a new method to time the formation of stellar bars, it opens a fresh window into understanding the internal evolution of galaxies — when it started, how it's progressed, and how structures like bars shape galaxies as we see them today.

Source: de Sá-Freitas