17 February 2022The mystery of the formation of the Milky Way

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An international team of astrophysicists, including researchers from the Observatoire astronomique
de Strasbourg, have detected six “merging galaxies” of our Milky Way – those small “foreign” galaxies
that sequentially merged into the Milky Way during its long history of 12 billion years and led to its
formation. To uncover these mergers, the team implemented a unique search strategy using
astrophysical data from European Space Agency’s Gaia satellite, and from the surveys of SDSS,
LAMOST, APOGEE, among others. Revelation of one of the six mergers is a new discovery, and the
team named it “Pontus”. Furthermore, they found that another detected “LMS-1/Wukong” merger
represents the most metal-poor merger – i.e. , the stars of LMS-1/Wukong, in a way, are 2500 times
deficient in heavy metals than the Sun. This suggests that LMS-1/Wukong must have formed very early
in the Universe, perhaps some 3 billion years after the Big Bang. These new results explain the genesis
of our Galaxy, by revealing the ‘family tree’ of the Milky Way.

A modern astronomy mystery is – How do galaxies, like the Milky Way, form in our Universe? Our current understanding is that since the Milky Way’s formation, it has been growing in mass and size through a series of “mergers” – drawing in smaller galaxies and clusters of stars, and making these foreign stars its own. These mergers take place in the “halo” of the Milky Way, that vastly surrounds our Galaxy. But detecting these mergers is generally a daunting task because – when a foreign galaxy merges with the Milky Way, great gravitational forces pull it apart, forcing the infalling galaxy to get disrupted and its stars get scattered throughout the halo and no clear signature is visible. Nonetheless, it is important to find and study these mergers to reveal the ‘family tree’ of smaller foreign galaxies that helped in the formation of the Milky Way.

The Galactic map showing 257 objects used in the study, namely 170 globular clusters (denoted by ‘star’ markers), 41 stellar streams (denoted by ‘dot’ markers) and 46 satellite galaxies (denoted by ‘square’ markers). These objects are colored by their heliocentric distances (“blue”=closer, “red”=distant).

Furthermore, as it can be seen in the figure, the Milky Way harbours a vast population of globular star
, star streams and small “satellite” galaxies; ~257 objects in total. These objects represent the most ancient structures of our Galaxy, and their star members contain very low fraction of “heavy” elements; less than 10% than that present in the Sun. Among these objects, the streams “C-19”, “Sylgr” and “Phoenix” are the three most metal-deficient structures of our Galaxy, containing less than 0.1% of the elements present in the Sun. In this regard, an interesting question to ask is – were these extremely metal-deficient streams born in the Milky Way itself, or were they brought into our Galaxy inside some merging galaxy? The team answers all of these questions in this new research work.

Metallicity of stars
The Sun is composed of 98.5% of the two lightest atomic elements in the universe: hydrogen and helium. The sum of all the other heavier atomic elements (carbon, oxygen, iron, etc.) represents only 1.5% of the total mass of our star. The vast majority of these heavy elements is produced within massive stars. As these reach the finals stages of their evolution, they give these elements to the interstellar gas through winds or as they explode as supernovae. New stars, like the Sun 4.5 billion years ago, are born from this now enriched interstellar gas. This implies that the first generations of stars were very poor in heavy elements.

This groundbreaking work, published in the Astrophysical journal, demonstrates the power of processing the astrophysical datasets of the Milky Way using the state-of-the-art algorithms. This international team, led by Khyati Malhan (Max-Planck-Institut für Astronomie, Heidelberg), found that our Galaxy was built by a series of six massive merging galaxies. They found this by using the kinematics data for all the 257 objects, and plotting them according to their energy and momentum to look for those “groups” representing the merging galaxies. For this, they made use of the ENLINK algorithm, and coupled it with a statistical procedure to detect high significant “groups”.

These six groups included the previously known mergers “Sagittarius”, “Cetus”, “Gaia-Sausage/Enceladus”, “LMS-1/Wukong”, “Arjuna/Sequoia/I’itoi” and one new merger that they call “Pontus”. In Greek mythology, “Pontus” (meaning “the Sea”) is the name of one of the first children of the Gaia deity (the Greek goddess of the Earth). The team also suggested the possibility of a seventh merger. Furthermore, they found that 25% of the aforementioned 257 objects fell inside one of these six mergers.

They also found that the detected merger “LMS-1/Wukong” was the natal galaxy of the metal-deficient
streams “C-19”
, “Sylgr” and “Phoenix”. This renders “LMS-1/Wukong” as the most metal-poor merger of
our Galaxy. Since “C-19” is estimated to have 2500 times less metallicity than the Sun, and the fact that it was likely formed some 3 billion years after the Big Bang, it indicates that its natal galaxy “LMS-1/Wukong” must have also formed with similar metallicity and around the same time.

The different stellar structures associated with the newly discovered “Pontus” merger are colored in purple on this sky map.

This study has two broad implications. First, it informs about the number of massive merging events (i.e., N= 6) that our Galaxy experienced during its long history of 12 billion years. Secondly, we now understand that which particular set of globular star clusters, streams and satellites were brought into the Milky Way inside which particular natal galaxy. These exciting results have advanced our knowledge about the formation of the Milky Way, by revealing those ancient mergers that helped make our Galaxy what it is today.

Article: Malhan, K., et al. 2022, ApJ, 926, 107; DOI: 10.3847/1538-4357/ac4d2a https://iopscience.iop.org/article/10.3847/1538-4357/ac4d2a

Science contact : Nicolas Martin nicolas.martin@astro.unistra.fr
Communication contact : communication@astro.unistra.fr