Galaxies The Milky Way cannibalized a neighboring galaxy
NASA

Galaxies The Milky Way cannibalized a neighboring galaxy

Ten billion years ago, the Milky Way encountered another galaxy in the vast emptiness of space, and consumed it. Astronomers dubbed this visitor Gaia-Enceladus because it was around one-quarter the size of the Milky Way, and it forever changed the makeup and shape of our home galaxy.

Scientists have had evidence for a while that the Milky Way saw a major merger in its past. Even in the absence of direct evidence in our own galaxy, astronomers know that galaxy collisions are common in the universe. These mergers are the major way that galaxies grow and evolve.

Nevertheless, this is the first time that astronomers have been able to accurately measure the ages of different stellar populations inside the Milky Way in order to determine when this merger occurred and how it affected our home galaxy. Researchers led by Carme Gallart from the Instituto de Astrofísica de Canarias in Spain published their findings in Nature Astronomy.

Galaxies Collide

Astronomers must estimate the ages of different populations and groups of stars within the Milky Way to read its history. This is tricky because measuring stars’ ages is an inexact science. Scientists can’t really just look at a star and tell its age, even with detailed measurements.

They instead examine batches of stars and compare them to model star populations. Stars are typically born in large litters, and astronomers may run the clock backward and gain a more accurate picture of when that birth occurred by collecting details about entire groups of stars.

And thanks to the outpouring of new data from the Gaia mission, which is creating the most accurate stellar map yet, astronomers were able to take a big step forward in this challenge.

Astronomers mapped stars from similar parts of the Milky Way using their new data. They discovered two distinct star populations. Some stars, which appear redder in color, appear to have formed in a larger, more metal-rich galaxy. (It’s worth noting that astronomers refer to any element that’s not hydrogen or helium as a “metal.”) The bluer population of stars should have formed in a smaller, more metal-poor galaxy. The fact that astronomers see these populations mixed together is a sign that the larger galaxy (the early Milky Way) encountered and consumed a smaller galaxy (Gaia-Enceladus) at some point in the past.

Based on prior observations, astronomers suspected such an event, but the new Gaia data provides more clarity. The findings also support astronomers’ estimates that the interloper galaxy was around the size of the original Milky Way.

However, the timeline of this event is contested. The data, however, allowed scientists to estimate the ages of stars in the Milky Way’s halo, a sort of bubble of stars rising above and below the more familiar disk shape, which were all cut off about 10 billion years ago. The reason these stars orbit out of the Milky Way’s disk is because they are moving faster than other stars, and the implication is that some energetic event tossed them to these high speeds.

By combining the ages of the stars with models of galaxy evolution, astronomers can paint a timeline of the Milky Way’s history. The young Milky Way evolved on its own for around 3 billion years, until it collided with the smaller Gaia-Enceladus cluster 10 billion years ago. This encounter tossed some stars into the halo, and also poured gas – the fuel for new star formation – into the Milky Way’s disk, causing a burst of new star formation. This flurry of activity decreased during the next several billion years, yet our galaxy still has enough fuel to continue producing stars at a slower rate.

Thanks to the massive amount of data Gaia collects, scientists receive the findings on a delay. The current findings are based only on the first 22 months of data, which were collected between 2014 and 2016. Gaia will keep collecting data until at least 2022, and probably 2024 if all continues smoothly. Researchers’ understanding of our galaxy can only grow as the study continues to release more measurements.

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