There... are... THREE... clouds !

Thanks everyone who voted on yesterday's poll (https://plus.google.com/u/0/+RhysTaylorRhysy/posts/h4ePJaDsHXf). The final score was :
1 group : 0%
2 groups : 7%
3 groups : 71 %
4 groups : 16%
More : 7%

... and the correct answer is.... there are FOUR LIGHTS three groups !

This diagram shows the location of galaxies in a particularly complicated part of the Virgo cluster, where, in addition to the main cluster, it's believed there are two other infalling sub-groups. Distance measurements to individual galaxies are hard but velocity (redshift) measurements are much easier. The distance measurements which we do have indicate that these three different groups are at different distances. It's also possible to see this using the velocity measurements, with each group centred on a different overall velocity (though with lots of scatter).

A colleague of mine disputed whether you'd really pick out three distinct groups without already knowing the distances. I thought it looked clear enough, but heck, this is the age of social media so why not test it ? The diagram I showed you yesterday was the same as the one below, just stripped of all distance information and rotated to a random angle. I deliberately gave you absolutely no information on the problem and didn't specify what size features count as significant - I wanted to see what you'd naturally guess without over-thinking what counts as a group.

Turns out the answer's three. Since I told you absolutely nothing, that makes the conclusion much stronger - not only can you pick out the three groups, but that's what most people naturally actually do. So you definitely don't need the distance measurements to realise there are three distinct groups here.

Clusters are messy places, so it's completely understandable that you might think there are more than three groups from this limited information. Even with the distance assignments, you can see that some objects at similar velocities apparently below to different groups. That's probably not actually the case - more likely, the distance assignments just didn't take this velocity information into account. There's clearly a lot of scatter in all cases - these aren't nice spheroidalish distributions - this is the real universe, messy and ugly and with galaxies lying in awkward locations. It's quite possible that a few of them do lie at quite different distances to the others; whether they would constitute separate groups or not is another matter (though not a terribly interesting one).