Tuesday, March 16, 2021

The mistery of the Ring galaxies

Ring galaxies are a rare type of galaxy with a very characteristic shape.

These galaxies are in fact made up of two main components. At the center there is a nuclear region that is usually dim and made up of old yellow stars. This is then surrounded by a very bright ring made up of very massive young stars of blue color. The two regions are separated by a space that appears to be completely empty.

Astronomers think that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. The space between stars in a galaxy is vast, so when galaxies collide, the stars don’t actually crash into each other. Instead, it’s their gravity that makes a mess. In this situation, it’s thought that the smaller galaxy slices right through the disk of the larger galaxy. The gravity of the smaller galaxy collapses vast clouds of gas and dust, and creates a burst of star formation around the edge of the larger galaxy.

The change in gravity drastically affects the orbit of the stars in the larger galaxy. They orbit outward and bunch up into the bright starforming ring. This blue ring is continuing to expand outward, and astronomers believe that it only lasts for a few hundred million years before it begins disintegrating. Eventually only the bright galaxy core will remain.

In fact, supposing that the small galaxy passes perpendicularly through the center of the larger galaxy, we can explain the observed shapes. The shock waves generated during the collision would in fact be able to sweep away the spiral arms, making them take on the ring shape.

However, the ring is an unstable structure. It is believed that after a period of some tens of millions of years it disintegrates to resume the shape of the spiral arms.

The most classic example of a ring galaxy is the Hoag Object, shown in the figure. 600 million light years away from Earth, this galaxy caused a lot of astonishment at the time of its discovery in 1950.

The Hoag Object also has a unique feature: it is in fact aligned on the same line of sight as another ring galaxy, which is clearly visible in this image as well.

Credit: NASA, The Hubble Heritage Team, Ray A. Lucas (STScI/AURA).

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