Massive Stream of Stars Proves Milky Way’s Violent History

When looking at the sky on a clear night, you might believe there is nothing else that can be as serene and peaceful as the view unfolding before your eyes. However, many of the seemingly peaceful stars are proof of a violent life, and now scientists are finally starting to glimpse into their secrets. 

Using data collected by the Gaia Milky Way mapping survey, astronomers have found an expanded stream of stars that they believe are the leftovers of a massive dwarf galaxy that got towed into the galactic disk before being shattered in all directions. They have named the stream Nyx, after the Greek goddess of night.  

Stellar Streams 

As per the standard model of the evolution of the Universe, galaxies develop by merging with and consuming smaller galaxies, also known as an accretion process. There is actually a massive amount of evidence of this taking place in the Milky Way; a number of streams have been found and associated with dwarf galaxies and globular clusters distorted by the galaxy’s tidal forces. 

The Gaia satellite was then launched in 2013 and has been gathering information ever since to piece together the most precise 3D map yet of the Milky Way. It is carefully analyzing the proper motions, radial velocities, and distances of the stars in order to figure out where everything is, and how it is moving around. 

Stellar streams. [Wikipedia]
The data is unveiling unique and new details regarding the history of the Milky Way’s altercations with other places with stars, such as Antlia 2, the Sagittarius Dwarf Spheroidal Galaxy and the Gaia Sausage. We should also mention the Pisces-Eridanus stellar stream, believed to be the remnants of a star cluster. 

Theoretical physicist Lina Necib of Caltech and her colleagues used a neural network to create a catalog of stars from the second Gaia data launch that had been absorbed into the galaxy, instead of being born here. 

“The network,” the researchers explained in their paper, “takes as inputs the five-dimensional kinematics of each star (two angular coordinates, two proper motions, and parallax) and then outputs a score associated with the probability that the star is accreted.” 

There is an Explanation 

When it selected the stars that the neural array was most definitely had been accreted, the team discovered a group of 232 stars all moving together in a low-grade pattern (with the orbit of the galaxy) and with similar chemical elements. This particular group has not previously been linked to any other stellar stream. 

When the scientists simulated the rotations of these stars one billion into the past, they discovered that the objects had orbital properties that were different from the stars found in the Milky Way’s dense disk and thin disk. 

“Coupling this observation with the fact that Nyx lags behind the disk by ~90kms−1 and has a substantial radial velocity component makes a strong case that it is the result of a satellite merger,” the researchers wrote

The Milky Way galaxy. [Image: solarsystem.nasa.gov]
Stellar groups that move in an identical rhythm can be created by other means, including resonances produced by perturbations from the galactic bar or density waves in the spiral arms – however, these do not fit Nyx. Simulations of these events couldn’t trigger Nyx’s lag without causing other impacts that have not been found in the data. 

Another Group Resembling Nyx 

Still, the best explanation, the team says, is a dwarf galaxy that, at some point during our Milky Way’s long record, was absorbed in and then got stretched out as the stars began to rotate around the core of the Milky Way. 

When the scientists repeated their analysis with a bit of relaxed certainty, they found another group of stars that matched the Nyx stream almost to the last details. The prograde galactic orbit and chemical elements were similar, but the second group had opposite radial velocity. This also matches the dwarf galaxy model, as simulations depicted that the second group could be leftovers from another passage of the same dwarf galaxy. 

It can be possible for Nyx to host stars that were not detected in this research due to the fact that they fell outside the rigid parameters fed into the neural network. However, future studies could help shed some light on this phenomenon and reveal when and how it happened, as well as how massive that dwarf galaxy was. 

“If Nyx is indeed the result of such a merger, then it would provide evidence for accreted prograde stars, and potentially, an accompanying dark matter component in a stream or disk,” the researchers wrote. “The presence of such a dark matter component would substantially alter our current understanding of the local dark matter phase-space distribution, and have important ramifications for terrestrial searches for the dark matter particle.” 

The research has been published in Nature.

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