What Did the Early Milky Way Look Like?

In the very early days of our Universe, just over 13 billion years ago, there was very little structure. There were stars, and they were forming at a rapid rate, kicking off what’s known as the Stelliferous Era. But the enormous, majestic galaxies that we see today, including our Milky Way galaxy, hadn’t formed yet.

The first galaxies to form were dwarf galaxies, and over time, they merged together to build the types of spiral galaxies that we see today. Astronomers know that’s what happened, but the exact timeline for the Milky Way has been unclear. Now a new study published in Nature Astronomy has revealed some of the detail in the formation of our home galaxy.

The new study, titled “Uncovering the birth of the Milky Way through accurate stellar ages with Gaia” is based on data from the ESA’s Gaia spacecraft. Gaia’s mission is to map the stars in the Milky Way. It won’t map all of them, but it’ll accurately measure the position and motion of just one percent of the galaxy’s 100 billion stars. That sample reveals an overall picture of the galaxy.

The motion of a star is imparted to that star at the time of its formation. Gaia creates a 3D map of the Milky way by measuring this motion. Essentially, that map allows astronomers to look back in time, by tracing the star’s motion backward. That’s why the Gaia data is such a powerful tool for understanding the history of the Milky Way.

A team of astronomers from the Instituto de Astrofisica de Canarias (IAC) used this data to examine the history of the Milky Way and find out what it looked like in the past. The lead author of the article is Carme Gallart, a researcher at the IAC. In a press release, Gallart said, “We have analyzed, and compared with theoretical models, the distribution of colours and magnitudes (brightnesses) of the stars in the Milky Way, splitting them into several components; the so-called stellar halo (a spherical structure which surrounds spiral galaxies) and the thick disc (stars forming the disc of our Galaxy, but occupying a certain height range.)”

The structure of the Milky Way galaxy. Image Credit: ESA

Astronomers have studied the Milky Way’s galactic halo and found two distinct populations of stars there. One of those populations is dominated by blue stars. The motion of those stars told astronomers that they are the remnants of a dwarf galaxy that merged with the Milky Way. That ancient dwarf galaxy is named Gaia-Enceladus. The other population in the halo is made up of red stars. The history of those stars, and the timeline of the Milky Way / Gaia-Enceladus merger, was never well-understood.

Thanks to the Gaia mission and the work of these astronomers, we’re now getting a better understanding of the merger.

“Analyzing the data from Gaia has allowed us to obtain the distribution of the ages of the stars in both components and has shown that the two are formed by equally old stars, which are older than those of the thick disc,” says IAC researcher and co-author Chris Brook.

But that begs another question: If both populations of stars are the same age, how are they different? Mostly it boils down to their metallicity.

“The final piece of the puzzle was given by the quantity of “metals” (elements which are not hydrogen or helium) in the stars of one component or the other,” explained Tomás Ruiz Lara, an IAC researcher and co-author. “The stars in the blue component have a smaller quantity of metals than those of the red component.”

These findings, with the addition of the predictions of simulations which are also analyzed in the article, have allowed the researchers to complete the history of the formation of the Milky Way.

The Milky Way 10 billion years ago, with the Gaia-Enceladus dwarf galaxy in the process of merging. Image Credit: Gabriel Pérez Díaz, SMM (IAC)

The results of this work tell a story of star formation and galactic merging and growth that results in the present-day Milky Way.

This story starts 13 billion years ago, several hundred million years after the Big Bang, when stars were forming in two separate systems. One was the Gaia-Enceladus dwarf galaxy, and the other was the progenitor of our Milky Way. The early Milky Way was about 4 times more massive than the dwarf galaxy, and was made up of younger, higher metallicity stars.

The Milky Way now, with the magnitude and brightness of the two distinct stellar populations in the halo. Image Credit: Gabriel Pérez Díaz, SMM (IAC)

About 10 billion years ago, there was a violent collision between Gaia-Enceladus and the early Milky Way. That event set some stars from the dwarf galaxy and some from the larger Milky Way into chaotic motion, and eventually they formed the halo. Then there was a long period of chaotic outbursts of stellar formation, until things settled down about 6 billion years ago. Then, the gas settled into the disc of the galaxy, and gave us what we call the thin disc.

“Until now all the cosmological predictions and observations of distant spiral galaxies similar to the Milky Way indicate that this violent phase of merging between smaller structures was very frequent,” explains Matteo Monelli, a researcher at the IAC and a co-author of the article. “Now we have been able to identify the specificity of the process in our own Galaxy, revealing the first stages of our cosmic history with unprecedented detail.”

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Evan Gough

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