When you go out and look at the sky on a clear night, it seems that nothing else can be as calm and peaceful. But many of these seemingly peaceful stars are evidence of a turbulent history – and now we are learning to unlock their secrets.
Using data from the Gaia Milky Way mapping survey, astronomers have discovered a large stream of stars that they believe are the remains of a massive dwarf galaxy that hit the galactic disc before being torn apart. They have called the stream Nyx after the Greek nightdress.
According to standard model of the universe’s development, galaxies grow by merging with and absorbing smaller galaxies – the accretion process. In fact, there is plenty of evidence that this is happening in the Milky Way: a number of streams have been identified that have been linked to dwarf galaxies and bullet clusters disturbed by the galaxy̵
Then the Gaia satellite came along. It was launched in 2013 and has since collected data to produce the most accurate 3D map yet on the Milky Way. It carefully studies the right motions, radial velocities and distances from the stars to determine where everything is and how it moves.
This reveals in unrivaled detail the history of Milky Way changes with other stars of stars – such as Antlia 2, Sagittarius Dwarf Spheroidal Galaxy and, um, Gaia sausage. Not to mention the Pisces-Eridanus stellar stream, thought to be what remained of a star cluster.
But these have all been identified by looking for things that move and are built differently. It is much more difficult to identify a shredded galaxy that fits. Stars that move with the rotation of the galactic disk and have similar chemical compositions as stars formed here may be overlooked.
So theoretical physicist Lina Necib from Caltech and her colleagues used a neural network to build a directory of stars from the second Gaia data release that had been slurped into the galaxy, rather than being born here.
“The network,” the researchers explained in their paper, “takes as input the five-dimensional kinematics of each star (two angled coordinates, two real motions and parallax) and then outputs a score associated with the probability that the star is accredited.”
When the extracted stars that the neural network was most likely to have encountered, the team found a group of 232 stars that all move together in a strong motion – that is, with the rotation of the galaxy – and with similar chemical compositions. This group had not previously been associated with any other star current.
When they simulated the orbits of these stars 1 billion years into the past, the team found that they had orbital properties that differed from the stars on both Milky Way’s thick slice and thin slice.
“Pair this observation with the fact that Nyx is behind the disc by ~ 90 km-1 and having a significant radial velocity component makes a strong case that it is the result of a satellite fusion, “the researchers wrote.
Star groups moving together can be created in other ways, such as resonances generated by interference from the galactic bar, or density waves in the spiral arms, but these do not fit Nyx. Simulations of these phenomena could not cause Nyx delay without causing other effects that have not been identified in the data.
The best fit for their tasks, the team believes, is a dwarf galaxy that at some point in the long history of the Milky Way was blurred and then stretched out when the stars began to orbit the Milky Way.
And when the researchers repeated their study with some relaxed certainty, they found another group of stars that almost exactly matched the Nyx current. The galactic orbit rate and chemical composition were the same – but the other group had the opposite radial velocity. This is also consistent with the dwarf galaxy model, as simulations showed that the other group could be debris from a separate passage of the same dwarf galaxy.
It is likely that Nyx contains stars that were not identified in this study, as they fell outside the strict parameters entered into the neural network. But future research can help shed light on this event – when it happened, how it happened, and how massive the dwarf galaxy was.
And since there is also evidence that populations of associated stars correlate with clumps of dark matter that are believed to have been blurred in the merger with the stars, Nyx can help us understand how such mergers contribute to a galaxy’s dark matter disk.
“If Nyx really is the result of such a merger, it would provide evidence for accredited progression stars, and possibly an associated component of dark matter in a stream or disk,” the researchers wrote.
“The presence of such a component of dark matter would significantly alter our current understanding of the local phase-space distribution of dark matter and have important ramifications for the ground-based search of particles with dark matter.”
The research has been published in Nature.