Gaia Discovers New Fossil Galaxy Hidden in Milky Way’s Halo

Using data from ESA’s Gaia spacecraft, astronomers have looked for the remains of ancient dwarf galaxies in the Milky Way’s halo, which surrounds the disk of younger stars and central bulge of older stars that comprise the more luminous parts of our Galaxy.

This image shows the Milky Way Galaxy as seen by ESA’s Gaia spacecraft; the squares represent the location of globular clusters, the triangles the location of satellite galaxies, and the small dots are stellar streams; the dots and squares in purple are objects brought into the Milky Way by the Pontus merging galaxy. Image credit: ESA / Gaia / DPAC / CC BY-SA 3.0 IGO.

Our Milky Way Galaxy began forming around 12 billion years ago. Since then, it has been growing in both mass and size through a sequence of mergers with smaller galaxies.

Perhaps most exciting is that this process has not quite finished, and by using Gaia data, astronomers can see it taking place.

“When a foreign galaxy falls into our own, great gravitational forces known as tidal forces pull it apart,” said Dr. Khyati Malhan of the Max-Planck-Institut für Astronomie and colleagues.

“If this process goes slowly, the stars from the merging galaxy will form a vast stellar stream that can be easily distinguished in the halo.”

“If the process goes quickly, the merging galaxy’s stars will be more scattered throughout the halo and no clear signature will be visible.”

“But the merging galaxy may contain more than just stars. It could also be surrounded by a population of globular clusters and small satellite galaxies.”

So, the astronomers looked for these in the data from the Gaia’s early third data release (EDR3).

In total, they studied 170 globular clusters, 41 stellar streams and 46 satellites of the Milky Way.

Plotting them according to their energy and momentum revealed that 25% of these objects fall into six distinct groups.

Each group is a merger taking place with the Milky Way. There was also a possible seventh merger in the data.

Five — Sagittarius, Cetus, Gaia-Sausage/Enceladus, LMS-1/Wukong, and Arjuna/Sequoia/I’itoi — had been previously identified on surveys of stars.

But the sixth was a newly-identified merger event. The authors called it Pontus, meaning the sea.

Based upon the way Pontus has been pulled apart by the Milky Way, they estimate that it probably fell into our Galaxy some 8-10 billion years ago.

Four of the other five merger events likely also took place around this time as well.

But the sixth event, Sagittarius, is more recent. It might have fallen into the Milky Way sometime in the last 5-6 billion years.

As a result, the Milky Way has not yet been able to completely disrupt it.

“The global dynamical atlas of Milky Way mergers that we present provides a present-day reference for galaxy formation models,” the researchers said.

Their paper was published in the Astrophysical Journal.

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Khyati Malhan et al. 2022. The Global Dynamical Atlas of the Milky Way Mergers: Constraints from Gaia EDR3-based Orbits of Globular Clusters, Stellar Streams, and Satellite Galaxies. ApJ 926, 107; doi: 10.3847/1538-4357/ac4d2a