This week’s OOTW features my OOTD ‘A Dark Secret in Virgo‘ posted on the 11th of December 2010.

On the 17th of March 1781, Pierre Méchain discovered this beautiful galaxy. NGC4254 lurks 50 million light years away in the constellation Coma Berenices. It’s a disturbed spiral, with its right arm jutting out further than the other. So what’s caused this? Let’s zoom out…

There’s nothing to see that could have caused the disruption, right? Well, let’s zoom into this patch of sky in Coma Berenices’ next door neighbour, Virgo:

Is it a black hole? No. Is it a gigantic cloaked alien ship that tugs galaxies?! Nope. It is in fact VIRGOHI21; a HI region 50 million light years away that was first detected by the Lovell telescope. A HI region is a mass of neutral hydrogen, and in this case it has hardly any or no stars. But there’s something more to this object than meets the eye…

This is a screenshot of an animation (my attempts at posting the animation here failed!) which shows a map of VIRGOHI21. According to this website here the larger brighter mass is NGC 4254, and you can see the cloud is cascading down from the disturbed spiral arm in a stream of neutral hydrogen to the centre of the image. Astronomers have calculated that the total mass of this HI region is 2×10^8 M ʘ (solar masses), but the velocity and spin of this object indicates that there is more mass than we can detect and so the object actually has a mass of 10^11 M ʘ ! So where does the rest of the mass come from…?

Dark matter! It is currently thought that VIRGOHI21 is a dark galaxy, which is a starless galaxy made up of mostly dark matter with little else apart from dust and hydrogen. This dark galaxy is interacting with NGC 4254 like any other normal galaxy would!

VIRGOHI21 is currently the best dark galaxy candidate out there, but others include HE0450-2958 which is a quasar that appears to be galaxy-less! Usually quasars have a host galaxy, but this one doesn’t appear to have one that we can see, so it has been proposed that the Quasar is actually part of a dark galaxy.

A survey called AGES uses the Arecibo observatory to find HI regions that are in connection with dark galaxies: http://www.naic.edu/~ages/