This newly discovered, 11.5-billion-year-old galaxy shares a striking feature with our own

Research led by Daniel Ivanov, a physics and astronomy graduate student in the Kenneth P. Dietrich School of Arts and Sciences at Pitt, uncovered a contender for one of the earliest observed spiral galaxies containing a stellar bar, a sometimes-striking visual feature that can play an important role in a galaxy’s evolution.

This finding helps astronomers get a better handle on when such bars could have first emerged in the universe. Analysis of light from the galaxy, called COSMOS-74706, places it on the cosmic timeline at about 11.5 billion years ago.

“This galaxy was developing bars 2 billion years after the birth of the universe," Ivanov said. “Two billion years after the big bang.”

The findings were presented at the 247th meeting of the American Astronomical Society on Jan. 8.

The making of a stellar bar

A stellar bar isn’t an object, but rather a “linear feature at the center of the galaxy,” Ivanov said. A dense collection of stars and gas, these bars are aligned in such a way that in images taken from above or below a galactic plane, there appears to be a bright line splitting the galaxy in two. Our galaxy, the Milky Way, also has a stellar bar.

Despite appearing like a permanent feature, stellar bars are in motion, waves of more dense material that form as the result of instability in the galaxy. It’s something that happens to a galaxy even without any changes from the outside. “Stellar bars are expected to emerge, over certain timescales, on their own,” Ivanov said. They likely appear and disappear multiple times.

But they can also be the result of “tidal perturbations,” gravitational forces caused by something outside of the galaxy. “If you have a close interaction with a nearby galaxy, that can actually trigger the global instability that leads to the formation of a stellar bar,” Ivanov said. 

Stellar bars can play a role shaping their galaxy’s evolution by funneling gas inward from the outer reaches of a galaxy, feeding the supermassive black hole in the center. 

This process supports the model of inside-out evolution, which posits that the inside of a galaxy forms and begins creating stars first, followed by stellar creation in the outer parts of the galaxy, which is driven by the buildup of interstellar gas. “There’s a large burst of star formation near the center of the galaxy, early in its history,” Ivanov said, but not much star formation farther out. “So, you end up with a much older core of the galaxy than in the outer disk.”

There are a few ways a stellar bar can contribute to this inside-out evolution. For instance, as it funnels gas from the outer edges of a galaxy into its interior, it slows star formation in the outer disk while promoting star formation in the galactic center.

Discovering the galaxy

The team made their discovery as they were developing a catalog of barred and non-barred galaxies in a particular region of space. It was during this work that a couple of galaxies were flagged for their unusually high redshifts, an indication of how long the light had been traveling and, therefore, how long ago it was emitted.

Other researchers have reported earlier barred spiral galaxies, but the analyses of those are less conclusive because the methods used to analyze the lights’ redshifts are not as definitive as spectroscopy, which was used to validate COSMOS-74706. In other cases, the galaxy’s light was distorted as it passed by a massive object, a phenomenon known as gravitational lensing.

In essence, Ivanov said, “It's the highest redshift, spectroscopically confirmed, unlensed barred spiral galaxy.”

He wasn’t necessarily surprised to find a barred spiral galaxy so early in the universe’s evolution. In fact, some simulations suggest bars forming at redshift 5, or about 12.5 billion years ago. But, Ivanov said, “In principle, I think that this is not an epoch in which you expect to find many of these objects. It helps to constrain the timescales of bar formation. And it’s just really interesting.” 

Read more about Ivanov’s work in IFL Science, Universe Today, and Space Daily.