A Galaxy Merged With Milky Way In Recent Past. And Some More Are Advancing Towards Us: Study

Astronomers from the Harvard-Smithsonian Center for Astrophysics recently tried to piece together the history of the Milky Way's stars in detail to determine the nature of the galaxy's last merger.

The tip of the "wing" of the Small Magellanic Cloud galaxy dazzling in this view from NASA's Great Observatories. The Small Magellanic Cloud, or SMC, is a small galaxy about 200,000 light-years way that orbits our own Milky Way spiral galaxy. (Photo: NASA)

New Delhi: According to modern cosmology, galaxies evolve via a hierarchical process of colliding and merging with other systems. Our own Milky Way Galaxy, which is 13.61 billion years old, provides the clearest view of this build-up.

A team of astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) recently tried to piece together the history of the Milky Way's stars in unprecedented detail to determine the nature of the galaxy's last merger. 

Their findings were recently published in The Astrophysical Journal.

Currently, two nearby dwarf galaxies, the Large and Small Magellanic Clouds, are falling towards us. The Large and Small Magellanic Clouds have about one per cent and 0.7 per cent of the stellar mass of the Milky Way, respectively. 

Also, streams of globular clusters encircle the Milky Way, and this marks the effects of prior mergers.

Astronomers Used Data From The Gaia Spacecraft

The CfA astronomers used results from the Gaia spacecraft, which was launched in 2013, with the goal of making a precise three-dimensional map of the Milky Way by surveying one per cent of its approximately 100 billion stars, to conduct this study. 

The astronomers combined the Gaia results with a new survey (H3 Survey of Stars) of the outer reaches of our galaxy with the 6.5 metre MMT Telescope in Arizona. 

A Dwarf Galaxy Had Merged With The Milky Way In The Past

A single dwarf galaxy, known as Gaia-Sausage-Enceladus (GSE) had merged with the Milky Way about eight to ten billion years ago. The stellar motions and compositions of the stars in the inner halo of GSE help scientists infer what is left of the object. 

However, it is not certain whether GSE had a head-on collision with the Milky Way, or it orbited the Milky Way before gradually merging.

In order to get the answers to these questions, the astronomers modelled Gaia's measured halo stars with a set of numerical simulations coupled with a comparison to the stellar ages and compositions. 

The researchers observed that GSE contained about half a billion stars, and did not orbit the Milky Way but approached it moving in a retrograde direction, which means opposite it approached the Milky Way in a direction to the galaxy's rotational motion. 

Half Of Milky Way's Stars Descended From The Dwarf Galaxy

They concluded that around 50 per cent of the Milky Way's current stellar halo, and about 20 percent of its dark matter halo have descended from the dwarf galaxy called GSE. The stellar halo of the Milky Way is the spherical distribution of stars, has a diameter of around hundred thousand light-years, and is older than about 10 to 12 billion years.

Some stars in the Milky Way are around 13 billion years old, and may have been captured by the galaxy after it was formed. 

The completion of the research will help scientists account for almost the entire growth of the Milky Way over the past 10 billion years.

Some Galaxies Are Advancing Towards The Milky Way

The Small Magellanic Cloud is a nearby dwarf galaxy that is merging with the Milky Way, the study said. 

Also, the Sagittarius dwarf galaxy, a neighbour of the Milky Way, is undergoing tidal disruption (an astronomical phenomenon in which a star approaches close to a supermassive black hole and is torn apart by the black hole's tidal force).

The record of ancient mergers can be extracted from the positions and motions of stars in the Milky Way's stellar halo.
The Andromeda Galaxy, which is our nearest large neighbouring galaxy, is about ten times farther away than the dwarf galaxies. A merger of the Milky Way and Andromeda is expected in another five billion years.