Stars around the milky way: Cosmic space invaders or victims of galactic eviction?

A compelling evidence.

The Milky Way galaxy, perturbed by the tidal interaction with a dwarf galaxy, as predicted by N-body simulations. The locations of the observed stars above and below the disk, which are used to test the perturbation scenario, are indicated.
The Milky Way galaxy, perturbed by the tidal interaction with a dwarf galaxy, as predicted by N-body simulations. The locations of the observed stars above and below the disk, which are used to test the perturbation scenario, are indicated.

Scientists at the Max Planck Institute for Astronomy (MPIA) have recently investigated the origin of groups of stars around the Milky Way galaxy. These halo stars are assembled together in giant structures that orbit the center of our universe, above and beneath the flat disk of the Milky Way. Scientists figured they may have formed from debris left behind by smaller galaxies that attacked the Milky Way before.

Through this discovery, scientists now have compelling evidence showing that some of these halo structures actually originate from the Milky Way’s disk itself, but were kicked out.

Co-author Judy Cohen, Kate Van Nuys Page Professor of Astronomy at Caltech said, “This phenomenon is called galactic eviction. These structures are pushed off the plane of the Milky Way when a massive dwarf galaxy passes through the galactic disk. This passage causes oscillations, or waves, that eject stars from the disk, either above or below it depending on the direction that the perturbing mass is moving.”

Lead author Maria Bergemann of MPIA said, “The oscillations can be compared to sound waves in a musical instrument. We call this ‘ringing’ in the Milky Way galaxy ‘galactoseismology,’ which has been predicted theoretically decades ago. We now have the clearest evidence for these oscillations in our galaxy’s disk obtained so far!”

This is for the first time, astronomers have presented detailed chemical abundance patterns of these halo stars using the W. M. Keck Observatory on Maunakea, Hawaii.

Scientists investigated 14 stars located in two different halo structures – the Triangulum-Andromeda (Tri-And) and the A13 stellar overdensities. These two structures lie on inverse sides of the Milky way disk; around 14,000 light a very long time above and underneath the Galactic plane. They obtained spectra of the halo stars using Keck Observatory’s High-Resolution Echelle Spectrometer (HIRES) and a spectrum of one additional star taken with the European Southern Observatory’s Very Large Telescope (VLT) in Chile.

Bergemann said, “The analysis of chemical abundances is a very powerful test, which allows, in a way similar to the DNA matching, to identify the parent population of the star. Different parent populations, such as the Milky Way disk or halo, dwarf satellite galaxies or globular clusters, are known to have radically different chemical compositions. So once we know what the stars are made of, we can immediately link them to their parent populations.”

Cohen stated that “The high throughput and high spectral resolution of HIRES were crucial to the success of the observations of the stars in the outer part of the Milky Way. Another key factor was the smooth operation of Keck Observatory; good pointing and smooth operation allow one to get spectra of more stars in only a few nights of observation. The spectra in this study were obtained in only one night of Keck time, which shows how valuable even a single night can be.”

When looking at the chemical compounds of these stars with the ones found in other cosmic structures, the researchers were astonished to find that the chemical compositions are relatively indistinguishable, both inside and between these gatherings, and nearly coordinate the abundant patterns of the Milky Way outer disk stars.

This gives convincing proof that the halo stars no doubt generated from the Galactic thin disk itself.

Co-author Allyson Sheffield of LaGuardia Community College/CUNY said, “We showed that it may be fairly common for groups of stars in the disk to be relocated to more distant realms within the Milky Way – having been ‘kicked out’ by an invading satellite galaxy. Similar chemical patterns may also be found in other galaxies, indicating a potential galactic universality of this dynamic process.”

Scientists are now planning to analyze the spectra of additional stars in the Tri-And and A13 overdensities, as well as stars in other stellar structures further away from the disk. They also plan to determine masses and ages of these stars so they can constrain the time limits of when this galactic eviction took place.

The study is published online in the journal Nature.