NGC 4550 A Multi-Spin Galaxy

[Bill S]

I always think that a lot of the pleasure of observing astronomical objects is learning about them as well as seeing them. Sometimes it's reading about them that is the spur to having a look.

In most galaxies almost all the stars are rotating in the same direction. There are some galaxies where this is not the case. NGC 4550 is a lenticular (i.e. lens-shaped) S0 galaxy. Approximately half its stars are rotating one way and half the other.

This paper talks about the discovery of this by Vera Rubin and her co-workers.

http://articles.adsabs.harvard.edu/pdf/1992ApJ...394L...9R

Over the years it was suggested that two galaxies rotating in opposite directions merged to give this multi-spin result. It is not believed this is the case. Rather the galaxy originally consisted of stars rotating in just one direction and then a stream of gas rotating the other way merged with it and new stars formed as a separate population in this counter rotating part.

Mike Merrifield discussed this in this YouTube video:

https://www.youtube.com/watch?v=0oie90j989k

Anyway, this is what it looked like to me:

NGC 4550 is the one in the middle. The bright galaxy up and to the left in this snapshot is NGC 4551, which is an E2 elliptical galaxy.

Best regards

Bill

 

[Martin Meredith]

That is a fascinating object. Thanks for the back-story! It amazes me how so much variety can emerge from a few simple physical principles.

Martin

[Mike JW]

Hi Bill, I never knew a galaxy could have stars orbiting in two directions. Very much enjoyed the video - very well explained. Mike