What a rare ring galaxy reveals about cosmic history

Ring galaxies like the well-known Cartwheel Galaxy can form for two reasons:

  • First – a spiral galaxy is involved in a collision with another galaxy, which punches through the spiral galaxy, thereby clearing away its center.
  • Second – the bar of a barred spiral galaxy becomes unstable because its rotational velocity becomes too high.

Events like these are rare, so ring galaxies themselves are also rare.

All the more reason for astronomers to celebrate, because with R5519, they have now found another object of this kind. R5519 is 11 billion light-years from Earth. Thus, the galaxy, which has a clearing in its middle that is about 32,000 light-years across, is still in its youth (for comparison: the Milky Way has a diameter of about 170,000-200,000 light-years). R5519 is forming new stars 50 times faster than the Milky Way, even though the two galaxies have approximately the same mass.

For R5519, astronomers believe that the ring was created by a collision. This is because with such a young galaxy, any bar it might have had could not yet have grown big enough to become unstable.

They have also concluded that spiral galaxies must have formed very early in the universe, because this discovery shows that one was involved in a collision and by the time of the incident, it was already fully formed. In contrast, our Milky Way didn’t form until billions of years later.

Artist’s depiction of a ring galaxy (image: James Josephides, Swinburne Astronomy Productions)
Hubble image of R5519 (image: Tiantian Yuan/Hubble Space Telescope)

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