Categories: Astronomy

Gaia Finds Ancient Satellite Galaxy Pontus Embedded in Milky Way

A recent study looked at stellar streams hidden in Gaia data, to uncover evidence of an ancient remnant dubbed Pontus.

Our home galaxy the Milky Way is a monster with a ravenous past. In its estimated 12 billion years of existence, our galaxy has swallowed smaller satellite galaxies whole, with collisions resulting in massive rounds of star formation. We see threads of these remnant mergers as streams of stars and clusters, strung out around the Milky Way.

Star streams from past collisons looked at in the study. Credit: ESA.

Most of these phantoms streams remained hidden, until now. Finding these ancient streams was very much a problem of seeing the ‘forest through the trees’ sort of dilemma, on account of the swarms of hundreds of billions of stars in the Milky Way Galaxy. Now, a recent study published in The Astrophysical Journal on February 17th, 2022 points to signs of known past mergers, plus edvince for a few that were previously unknown. The study used the very latest astrometric data from Gaia to look at the distance and motion of 170 globular clusters, 41 star streams and 46 satellite galaxies bound to the Milky Way. The study found evidence supporting five previously known mergers, with one newly discovered merger and the potential for a seventh.

Enter Gaia

The data comes courtesy of the European Space Agency’s illustrious Gaia spacecraft. Launched on December 19th, 2013 atop a Soyuz rocket from the Kourou Space Center, Gaia sits at the L2 Sun-Earth point (yes, along with the James Webb Space Telescope) and is an astrometry mission, refining the position/parallax and the distance to over two billion stars. To date, there has been two major data releases (DR1 in 2016 and another, DR2 in 2018) with another data release DR3 expected this summer. We’ve also already seen an early preview (EDR3) in 2020.

High speed ‘sprinting stars’ seen in Gaia’s DR2 release. Credit: ESA.

Stars and Streams

The five known streams were Sagittarius, Cetus, Gaia-Sausage/Enceladus, LMS-1/Wukong, and Arjuna/Sequoia/I’itoi.

The sixth new stream is dubbed ‘Pontus,’ meaning ‘sea in Greek. The name is fitting, as Pontus was the first child of Gaia, the Greek Goddess of the Earth. This sixth new stream points at a possible satellite galaxy devoured long ago, perhaps 8-10 billion years in the past. For context, our solar system formed 4.5 billion years ago, and has journeyed only about 18 times around the galactic center in its once every quarter of a billion year orbit.

Anatomy of a collision: how the Milky Way can absorb a small satellite galaxy like Pontus over time. Credit: ESA simulation labs.

These remnant streams span the sky on a dark night. Messier objects M92 and M13 in Hercules are cited in the study as possible members of the Pontus stream. Both of these famous globular clusters are visible in an amateur telescope.

What’s Next for Gaia

Of course, we’re looking forward to the full Data Release DR3 this year, which was delayed due to the global pandemic. Gaia has thus far lasted well beyond its five year nominal mission and has exceeded expectations. Gaia’s detectors are in good health, and it has more than enough fuel for L2 station-keeping well past late 2024.

Gaia builds on knowledge gathered from the Hipparcos astrometry mission and may one day be followed by the proposed GaiaNIR (Near Infrared) mission.

The discovery of a sixth remnant stream is an amazing find, and is an impressive example of astronomical forensics. This intriguing discovery also serves as a mere hint of what other strange finds await.

David Dickinson

David Dickinson is an Earth science teacher, freelance science writer, retired USAF veteran & backyard astronomer. He currently writes and ponders the universe as he travels the world with his wife.

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