Galactic thin/thick disk riddle solved

Thick disk galaxies contain young as well as old stars according to a new report, potentially ending a long-running astronomical anomaly.

The Milky Way analog galaxy, NGC 891. The colours show flares from groups of stars with similar ages. The stars combined create a disk of uniform thickness, shown by the white lines. Image Credit: Adam Block, Mt. Lemmon SkyCenter, University of Arizona / Ivan Minchev, AIP
Published: April 27, 2015 at 12:00 pm

The thick disc in a galaxy is composed not only of old stars, but also of younger stars located far from the centre, astronomers have found.


The stars within galaxies create a ‘fluffy’ thick disk enshrouding a thin disk.

Prior to this latest research, it was thought that the stars in the thick disk were the oldest.

But after running simulations it appears that this is not the case.

Simulations have shown that groups of stars of the same age will always flare up when their galaxy is bombarded by smaller galaxies.

This flaring puffs up the galactic disk and forms what is known as the thick disk.

The research found that any collection of stars of the same age group will constitute a disk with flared edges.

And because the oldest stars are formed in the inner regions of a galaxy, their flaring will occur closer to the centre, while younger stars will flare in the outer edges.

This flaring from different collections of stars combines to produce the uniform thick disk.

“We were able to show for the first time that galactic thick disks are not composed only of old stars but must also contain young stars at larger distances from the galactic centre”, explains Ivan Minchev from the Leibniz Institute for Astrophysics Potsdam, who led the research.

“The flaring seen in groups of stars with the same age is caused mostly by the bombardment of small satellite galaxies.

These cosmological car crashes pummel the young disk and cause it to swell and flare.“

Minchev’s report points to the Milky Way as a means to explain the nature of the problem.

If the Milky Way were the same as many other galaxies and had a thick disk composed of older and more central stars, then we might expect its thick disk to be short.

However, we know that in other galaxies the thick disk can be as extended as the entire galaxy itself.

Minchev’s research solves this problem by requiring that thick disk stars are younger in the disk outskirts.

“With our new understanding of the formation of, and interplay between, galactic thin and thick disks, we have moved much closer to solving one of the most fundamental problems of Galactic astrophysics,” Minchev says.


“Our predictions will soon be tested with data from the Gaia space mission and using high precision instruments, such as MUSE on the Very Large Telescope.”


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.

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