The intervelocity of galaxy pairs in $\Lambda$CDM -- The observed velocity peak at ~130 km/s is not unique to MOND

Abstract

Observational studies of pairs of galaxies have uncovered that their differential line-of-sight velocities indicate the presence of a peak in their three-dimensional intervelocity distribution at 130-150 km/s. It had been argued that galaxy pairs in the standard model of cosmology, $\Lambda$CDM, should not exhibit such an intervelocity peak, while Modified Newtonian Dynamics (MOND) predicts such a preferred intervelocity for paired galaxies. However, no direct comparison with $\Lambda$CDM applying the same selection criteria and methodology as the observational studies has been performed yet, placing the comparison on unsure footing. To rectify this, we investigate this potential challenge for $\Lambda$CDM by determining whether an analog of the observed intervelocity peak is present in galaxy pairs within the IllustrisTNG-300 cosmological simulation. We identify galaxy pairs following the observational study's selection criteria, measure their projected velocity difference, and analyse both the de-projected as well as the full velocity difference for this galaxy pair sample in the simulation. We recover a deprojected intervelocity peak at ~130 km/s for galaxy pairs selected from the simulation. The full three-dimensional velocity information available for the pairs in the simulation also reveals a clear preference for this intervelocity. The intervelocity peak among galaxy pairs does not appear to be a feature unique to MOND, but is also present in $\Lambda$CDM. It can thus not be claimed as a unique success of either theory over the other. Developing the galaxy pair intervelocity into a test of gravity in the low acceleration regime will require more detailed studies to identify measurable differences in the models.