The Milky Way collided with a smaller galaxy billions of years ago and in the process gobbled up the dwarf galaxy and made the latter’s stars its own, say astronomers, adding that this event took place about 10,000 million years ago.
The study, published in the journal Nature Astronomy, reveals the first stages of our cosmic history with unprecedented detail. It states that the remains of the dwarf galaxy, known as Gaia-Enceladus, eventually became a part of the halo of the present-day Milky Way.
The Milky Way is the galaxy we live in, and it is made up of billions of stars. Astronomers have known that 13,000 million years ago, stars were forming at a very rapid rate in the universe, creating the first dwarf galaxies, whose mergers gave rise to the more enormous present-day galaxies, including the Milky Way.
However, until now, the exact chain of the events that led to the formation of the present day Milky Way was not known.
Researchers from the Instituto de Astrofísica de Canarias (IAC), Spain, used the Gaia space telescope to calculate precise measurements of the position, brightness, and distance of nearly a million stars of our galaxy within 6,500 light-years of the sun.
Gaia is a space telescope of the European Space Agency, which was launched in 2013. It is part of an ambitious mission of the same name to create the largest and most precise three-dimensional map of our galaxy, the Milky Way, by studying “one percent of the galaxy’s population of 100 billion stars”.
Milky Way’s cannibal past decoded
The current findings show that 13,000 million years ago, stars started to form in two different stellar systems, which then merged: one was the dwarf galaxy Gaia-Enceladus, and the other was the primary ancestor of our galaxy, which was about four times more massive and with a larger proportion of metals.
The researchers say some 10 billion years ago, there was a violent collision between the more massive system or the primitive Milky Way and Gaia-Enceladus. As a result, some of its stars, and those of Gaia-Enceladus were set into “chaotic motion”, and eventually formed the halo of the present Milky Way. After that, says the research team, there were violent bursts of star formation until 6,000 million years ago, when the gas settled into the disc of the galaxy and produced what is known as the “thin disc”.
“A primitive Milky Way had been forming stars over a period of some 3Gyr when a smaller galaxy, which had been forming stars on a similar timescale but was less chemically enriched owing to its lower mass, was accreted into it. This merger heated some of the existing stars in the main progenitor into a stellar halo-like configuration. A ready supply of infalling gas during the merger ensured the maintenance of a disk-like configuration, with the thick disk continuing to form stars at a substantial rate. Subsequently, around 8–6Gyr ago, the gas settled into a thin disk that has continued to form stars until the present day,” says the study.
According to previous studies, the Galactic halo showed clear signs of being made up of two distinct stellar components, one dominated by bluer stars than the other. Analyzing the data from Gaia, astronomers ascertained the distribution of the ages of the stars in both components and found that the two are formed by equally old stars, which are older than those of the thick disc. Put simply, the finding indicates that both components were formed at the same time, dating back to 10,000 million years ago.
“The movement of the stars in the blue component allowed us to identify it as the remains of a dwarf galaxy which impacted onto the early Milky Way,” says the paper. Both the red and the blue groups live in the Milky Way’s halo.
“We identify the red-sequence stars as the first stars formed within the Milky Way progenitor,” says the study.
However, what helped the researchers differentiate between the two components was the quantity of metals present in each.
“The stars in the blue component have a smaller quantity of metals than those of the red component. The difference in the amount of metals in the red and blue sequences indicates that the accreted Gaia-Enceladus galaxy had about 30% of the mass of stars in the main progenitor of the Milky Way, although we stress that this ratio remains quite uncertain,” says the research team.