Not interacting

https://www.legacysurvey.org//viewer/?ra=241.3565&dec=54.9965&layer=hsc-dr2&zoom=14

I presume these galaxies that look as if they should be interacting must be displaced from one another in distance from us, but I can’t find any z-shift data for them.

They’re in a 2009 catalogue as a non-merging pair.

Is it because they are separated along the line of sight by more distance than would allow them to merge?

I suspect they’re not close enough for gravitational forces to cause any tidal activity. Not sure though. Wouldnt surprise me if they merge eventually.

You may have seen the photo-zs for them – 0.072 ± 0.008 and 0.063 ± 0.006 – so within error bars of being at the same redshift. There are also a number of nearby galaxies with SDSS spectroscopic redshifts around z=0.063.

Yes, but how much distance is covered by those error bars? It seems probable that they are separated by a significant distance along the line of sight, such that their mutual gravitational attraction is not affecting them. If they were that close and both actually in the plane of the image, it’s almost a dead cert that they would be interacting, no? …after all, their halos are overlapping.

Well, the redshift measurements are overlapping within the error bars, so they’re consistent with being at the same redshift, but the range is fairly broad. But clusters are full of ellipticals like this; it’s not as though their appearance changes as soon as their halos touch. Maybe this is a grazing pass or a first encounter. And the strong tidal effects happen when the two galaxies have quite different masses (where the change in gravitational force across the smaller galaxy is larger than its internal gravity). Or maybe, like you say, it’s just projection.