A large stream of flowing stars is evidence of the violent history of the Milky Way

When you go out and look at the sky on a clear night, it seems like nothing else could be so serene and peaceful. But many of those seemingly peaceful stars are evidence of a troubled history, and now we are learning to uncover their secrets.

Using data from the Gaia Milky Way mapping survey, astronomers have discovered a large stream of stars that they believe to be the remnants of a massive dwarf galaxy that was dragged into the galactic disk before being ripped apart. They have called the Nyx stream, for the Greek goddess of the night.

According to the standard model From the evolution of the Universe, galaxies grow by merging and absorbing smaller galaxies, the process of accretion. In fact, there is plenty of evidence that this occurs in the Milky Way: Several currents have been identified that have been linked to dwarf galaxies and globular clusters disrupted by the galaxy’s tidal forces.

Then the Gaia satellite appeared. It was released in 2013, and since then it has been collecting data to produce the most accurate 3D map of the Milky Way to date. You are carefully studying the proper motions, radial velocities, and distances of the stars to determine where everything is and how it moves.

This reveals in unprecedented detail the history of the Milky Way’s altercations with other star bubbles, such as Antlia 2, the dwarf spheroidal galaxy of Sagittarius and, um, the sausage Gaia. Not to mention the Pisces-Eridanus star current, which is believed to be what’s left of a star cluster.

But all of this has been identified by looking for things that move and are constructed differently. It is much more difficult to identify a crushed galaxy that fits. Stars that move with the rotation of the galactic disk and have chemical compositions similar to the stars that formed here could be overlooked.

So theoretical physicist Lina Necib of Caltech and her colleagues applied a neural network to build a catalog of stars from Gaia’s second data launch that had been dragged into the galaxy, rather than being born here.

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

When they extracted the stars that the safest neural network had been accumulated, the team found a group of 232 stars that moved all together in a programmed movement, that is, with the rotation of the galaxy, and with similar chemical compositions. This group had not previously been associated with any other stellar currents.

When they simulated the orbits of these stars a billion years ago in the past, the team discovered that they had different orbital properties from stars in both the thick and thin discs of the Milky Way.

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

Stellar groups moving together can be created by other means, such as resonances generated by galactic bar disturbance or density waves in the spiral arms, but these do not conform to Nyx. Simulations of these phenomena could not produce the Nyx delay without causing other effects that have not been identified in the data.

The team believes the best option for their data is a dwarf galaxy that, at some point during the Milky Way’s long history, slurped up and then stretched when stars began to orbit the center of the Milky Way.

And when the researchers repeated their study with slightly relaxed certainty, they found another group of stars that almost exactly matched the Nyx current. The programmed galactic orbit and chemical compositions were the same, but the second group had an opposite radial velocity. This is also consistent with the dwarf galaxy model, as simulations showed that the second group could be debris from a separate passage from the same dwarf galaxy.

Nyx is likely to contain stars that were not identified in this study, because they fell outside the strict parameters fed into the neural network. But future research could help shed light on this event: when it happened, how it happened, and how massive that dwarf galaxy was.

And since there is also evidence that accumulated star populations correlate with clusters of dark matter believed to have been entrained in the merger along with the stars, Nyx can help us understand how such mergers contribute to the dark matter disk. of a galaxy.

“If Nyx is really the result of such a merger, then it would provide evidence of accumulated programmed stars and, potentially, an accompanying dark matter component in a current or disk,” the researchers wrote.

“The presence of such a dark matter component would substantially alter our current understanding of the local phase-space distribution of dark matter, and would have important ramifications for terrestrial searches for the dark matter particle.”

The research has been published in Nature.