Hubble Tangos with a Dancer in Dorado

The Dorado group membership has fluctuated over the past few decades, as various scientific papers changed its list of constituent galaxies. This is one example of why it is so challenging for astronomers to pin down members of galaxy groups like the Dorado group. One way to better understand this problem is by imagining a photograph of an adult human and a large oak tree. We know the approximate size of the person and the tree, so if we see a photo where the person appears roughly the same size as the tree, then we would assume that, in reality, the person was much closer to the camera than the tree. When astronomers try to figure out which galaxies are members of a galaxy group, they do not necessarily know the size of the individual galaxies. Instead, they have to work out whether the galaxies really are relatively close together in space, or whether some of them are actually much closer or much further away. This process is easier with more sophisticated observation techniques, but it still can present a challenge.