Astronomers Solve 'Thick Disk' Riddle of Galaxies

Scientists may have solved a long-standing puzzle regarding the nature of disk galaxies. The scientists have discovered that groups of stars with the same age always flare as the result of massive galactic collisions, and that these flares puff up the disk and make up what astronomers call the "thick" disk.

In this latest study, the researchers ran numerical simulations on super computers and examined the structures of simulated galaxies. They grouped stars by common age and looked at where they were located. What they found was that stars of a given age group constituted a disk with flared edges, much like the mouth of a trumpet. This flaring is unavoidable since it's caused by galaxy collisions. The flaring occurs closer to the center for older stars, while for younger stars it occurs at the periphery of the galaxy. Altogether, the combined flaring produces the 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 center," said Ivan Minchev, one of the researchers, in a news release. "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."

Galaxy stars can be separated into two components: a fluffy thick disk that enshrouds a thin disk. Before not, scientists believed that the stars in the thick disk were the oldest. However, the recent observations revealed 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," said Minchev. "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."

The findings are published in the Astrophysical Journal Letters.

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