Galactic evictions may have supplied the Milky Way's stellar halo
According to a newly published paper, the distant halo of stars that orbits the Milky Way could include parts of our own galaxy that were forcefully ejected following a dramatic collision or close pass with a smaller dwarf galaxy.
The majority of the stars that make up the Milky Way rotate in a colossal disk around the galactic center, which itself plays host to the supermassive black hole Sagittarius A*. However, a significant population of stars orbit beyond the outer edges of this disk, forming the Galactic halo.
It had previously been thought that the halo was comprised of stars stolen from smaller dwarf galaxies as they passed close to, or even merged with the Milky Way. However, the recently published study supports a different theory, which asserts that some of the stars that make up the halo were in fact created in the outer regions of our galaxy, and subsequently cast out.
According to the theory, as dwarf galaxies interact with, or pass through the Milky Way's galactic disk, they create oscillations, or waves, in its structure. These perturbations would be capable of expelling groups of stars from the galactic plane, throwing them out to orbit in the halo. This phenomenon is referred to as a "galactic eviction."
"The oscillations can be compared to sound waves in a musical instrument," said lead author Maria Bergemann of the Max Planck Institute for Astronomy, located in Heidelberg, Germany. "We call this 'ringing' in the Milky Way galaxy 'galactoseismology,' which has been predicted theoretically decades ago."
The new research focussed on two relatively dense structures of stars orbiting in the Galactic halo, known as Triangulum-Andromeda (Tri-And) and A-13. The stellar groupings lie either side of the disk, positioned roughly 14,000 light-years above and below the Milky Way's galactic plane
The astronomers used the High-Resolution Echelle Spectrometer (HIRES) instrument mounted aboard the W.M. Keck Observatory, located on Maunakea, Hawaii, to analyze the chemical makeup of 14 Stars that belonged to the two cosmic structures. The HIRES observations were supplemented with data from the European Southern Observatory's Very Large Telescope, detailing the characteristics of one additional star.
Knowing a star's chemical makeup can be vital to understanding where it came from. A star created in the outer disk of the Milky Way would most likely have a drastically divergent chemical fingerprint to one that had formed in a dwarf galaxy, due to the differing compositions of the stellar nurseries that spawned them.
By using an instrument like HIRES to measure the spectrum of electromagnetic radiation emitted by a star, astronomers can figure out what that celestial body is made of. They can then narrow down where it formed by comparing its composition with that of other candidate stellar populations. If a close match is discovered, there is a good chance that the star in question belongs to the same parent population as its counterpart.
In this case, the international team of scientists involved in the study discovered that the compositions of stars in both the Tri-And and A-13 structures closely matched not only each other, but also the chemical abundance patterns of stars in the outer disk of the Milky Way. In other words, the target stars were most likely not the remnant of an alien galaxy that had fallen victim to the Milky Way's gravitational influence, but instead a displaced part of our own galaxy.
According to its authors, the results of the study provide the clearest evidence to date for oscillations due to a galactic merger or close pass disrupting the Milky Way to populate the galactic halo.
By learning about the halo, astronomers can gain insights regarding the tumultuous history of our galaxy, and at the same time gain a greater understanding as to what will happen to our home when it collides with other galaxies in the distant future. It also stands as proof of the changes that smaller galactic mergers can cause to the structure of the Milky Way.
Moving forward, the researchers intend to examine the compositions, masses, and ages of the stars involved in the study, as well as those of additional stars in the Tri-And and A13 structures, and other stellar bodies orbiting even farther away from the disk of the Milky Way. The team members hope that the follow up study will allow them to narrow down when the galactic evictions took place.
A paper detailing the research has been published in the journal Nature.
Source: W. M. Keck Observatory