Ring of the Week: Arp 87
“Up on a hill, as the day dissolves
With my pencil turning moments into line
High above in the violet sky
A silent silver plane – it draws a golden chain
One by one, all the stars appear
As the great winds of the planet spiral in
Spinning away, like the night sky at Arles
In the million insect storm, the constellations form
On a hill, under a raven sky
I have no idea exactly what I’ve drawn
Some kind of change, some kind of spinning away
With every single line moving further out in time”
– Brian Eno, “Spinning Away”
Well if you ask me, what you’ve drawn there Brian is a “Polar Ring” galaxy.
Polar ring galaxies, unlike all other galaxies in the Universe, are made up of two distinct parts. In the centre we have a normal galaxy and around the outside we have a “golden chain” of stars and gas clouds. This ring is perpendicular to the “silver plane” of the host galaxy disk, rotating over the poles, and so it’s known as a polar ring.
So how are polar rings formed? Polar rings are thought to form when two galaxies gravitationally interact with each other. We believe that “one by one, all the stars appear” as they are stripped from a passing galaxy and “spiral in” to produce the polar ring we see today. Polar rings, although not quite as rare as smoke rings, are pretty hard to find. According to “New observations and a photographic atlas of polar-ring galaxies”, about 1 in every 200 lenticular galaxies (a type of galaxy between an elliptical and a spiral) have these “golden chains” of stars and gas spinning around them. Below is a selection of some of my favourite polar rings from the Galaxy Zoo:
The Zooites have done fantastically well at finding Polar Rings and you can see all of their incredible finds on the Possible Polar Ring thread on the Galaxy Zoo forum.
My Ring of the Week this week is the stunning pair of interacting galaxies Arp 87. Located in the constellation Leo, approximately 300 million light years away from Earth, Arp 87 gives us a fantastic insight in to exactly how polar ring galaxies are formed. The image on the left is the Galaxy Zoo Arp 87 image and on the right is an image taken by the Hubble Space Telescope. We can clearly see the galaxy on the left gravitationally stripping away the stars and gas from the spiral galaxy on the right.
Unfortunately for Brian Eno, his hypothesis of a “golden chain” of stars that “spiral in” a “silver plane” came a full 23 years after the first polar ring galaxy was identified by J. L. Sérsic in 1967.
However, perhaps someone else had already had a genuine polar ring premonition a full 78 years before Sérsic’s discovery…?
– “Starry Night” 1889, Vincent Van Gogh
The Hubble image is part of a collection of 59 images of merging galaxies released on the occasion of its 18th anniversary on April 24, 2008. (NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University))