Halton Arp observed galaxies that seem to be interacting physically with other galaxies through gas filaments yet the redshift between the two are radically different.
Halton Arp observed galaxies that seem to be interacting physically with other galaxies through gas filaments yet the redshift between the two are radically different.
The standard view is that these are chance alignments between near and far objects, and that the physical connections don't really exist.
It's an interesting question - what would constitute a sufficiently strong case to get you off the fence?
Here's one aspect: AFAIK, the Arp idea requires that 'not one atom' of material have a redshift intermediate between the redshifts of the two galaxies (so any search for intermediate redshift material - somewhere in a bridge, for example - is doomed to failure). Yet, in the Arp idea, all atoms behave just like the atoms here on Earth, in terms of their 'lab transition wavelengths', their motion in magnetic fields, their response to gravity, and so on.
Here's another aspect: the more closely these examples of so-called physical interaction are looked at - in new wavebands, at higher resolutions, etc - the fewer 'real Arpian interactions' seem to remain. Yet somehow new examples always get added - fainter systems, smaller (angular size) systems, etc.
