The Thick Disk: Galactic Construction Project or Galactic Rejects?

[/caption]

The disk of spiral galaxies is comprised of two main components: The thin disk holds the majority of stars and gas and is the majority of what we see and picture when we think of spiral galaxies. However, hovering around that, is a thicker disk of stars that is much less populated. This thick disk is distinct from the thin disk in several regards: The stars there tend to be older, metal deficient, and orbit the center of the galaxy more slowly.

But where this population of the stars came from has been a long standing mystery since its identification in the mid 1970’s. One hypothesis is that it is the remainder of cannibalized dwarf galaxies that have never settled into a more standard orbit. Others suggest that these stars have been flung from the thin disk through gravitational slingshots or supernovae. A recent paper puts these hypothesis to the observational test.

At a first glance, both propositions seem to have a firm observational footing. The Milky Way galaxy is known to be in the process of merging with several smaller galaxies. As our galaxy pulls them in, the tidal effects shred these minor galaxies, scattering the stars. Numerous tidal streams of this sort have been discovered already. The ejection from the thin disk gains support from the many known “runaway” and “hypervelocity” stars which have sufficient velocity to escape the thin disk, and in some cases, the galaxy itself.

The new study, led by Marion Dierickx of Harvard, follows up on a 2009 study by Sales et al., which used simulations to examine the features stars would take in the thick disk should they be created via these methods. Through these simulations, Sales showed that the distribution of eccentricities of the orbits should be different and allow a method by which to discriminate between formation scenarios.

By using data from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), Dierickx’s team compared the distribution of the stars in our own galaxy to the predictions made by the various models. Ultimately, their survey included some 34,000 stars. By comparing the histogram of eccentricities to that of Sales’ predictions, the team hoped to find a suitable match that would reveal the primary mode of creation.

The comparison revealed that, should ejection from the thin disk be the norm there were too many stars in nearly circular orbits as well as highly eccentric ones. In general, the distribution was too wide. However, the match for the scenario of mergers fit well lending strong credence to this hypothesis.

While the ejection hypothesis or others can’t be ruled out completely, it suggests that, at least in our own galaxy, they play a rather minor role. In the future, additional tests will likely be employed, analyzing other aspects of this population.

Jon Voisey

Jon has his Bachelors of Science in Astronomy from the University of Kansas (2008). Since graduation, he has taught high school, worked in antique jewelry, and now works as a data analyst. As a hobby, he does medieval re-creation and studies pre-telescopic astronomy focusing. His research can be found at jonvoisey.net/blog.

Recent Posts

Purple Bacteria — Not Green Plants — Might Be the Strongest Indication of Life

Astrobiologists continue to work towards determining which biosignatures might be best to look for when…

8 hours ago

See the Southern Ring Nebula in 3D

Planetary nebula are some of nature's most stunning visual displays. The name is confusing since…

8 hours ago

Hubble Has Accidentally Discovered Over a Thousand Asteroids

The venerable Hubble Space Telescope is like a gift that keeps on giving. Not only…

9 hours ago

NASA Restores Communications with Voyager 1

The venerable Voyager 1 spacecraft is finally phoning home again. This is much to the…

1 day ago

Will We Know if TRAPPIST-1e has Life?

The search for extrasolar planets is currently undergoing a seismic shift. With the deployment of…

1 day ago

Astronaut Food Will Lose Nutrients on Long-Duration Missions. NASA is Working on a Fix

Astronauts on board the International Space Station are often visited by supply ships from Earth…

2 days ago