Why are galaxies shaped like spirals

Astronomers discover oldest spiral galaxy

Toronto (Canada) - A pronounced spiral galaxy just three billion years after the Big Bang: this find has now surprised Canadian astronomers. Because when the first galaxies formed, the universe was still very wild, most galaxies were misshapen clusters. A well-formed spiral galaxy like our Milky Way, however, was not previously known from this early phase of the universe. The researchers were therefore very astonished to find an eleven billion old galaxy on their images that has pronounced and clearly structured spiral arms. From over 300 observed galaxies from this time this was the only spiral galaxy, the astronomers report in the journal "Nature". They conclude that either the mechanism of formation is very rare or the spiral structure is extremely short-lived.

"As far as we know today, pronounced spiral galaxies should not actually exist at such an early point in the history of the universe," says astronomer David Law from the University of Toronto. His colleague Alice Shapley adds: "Galaxies from this period look really strange, lumpy and irregular, not symmetrical." The early galaxies only grew into the structures we know today through mutual collisions and the influx of intergalactic gas. The nicely symmetrically shaped galaxy with the unpretentious name Q2343-BX442 is a special case that causes some headaches for astronomers.

One explanation could be that its companion, a dwarf galaxy, helped it develop this spiral structure. Combined measurements with the Hubble space telescope and the Osiris spectrograph on the extinct Mauna Kea volcano in Hawaii have revealed this companion. As the researchers calculated using simulations, its gravity could have caused the spiral structure. At some point this dwarf galaxy will be devoured by the big one; just as our Milky Way has incorporated some smaller companions. Astronomers therefore suspect that spiral galaxies are primarily formed through mergers.

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source: "High velocity dispersion in a rare grand-design spiral galaxy at redshift z = 2.18", David R. Law et al., Nature, doi: 10.1038 / nature11256