Intervelocity of galaxy pairs in ΛCDM

Abstract

Context. 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 (3D) intervelocity distribution at 130−150 km s−1. It had been argued that galaxy pairs in the standard model of cosmology, ΛCDM, 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 ΛCDM applying the same selection criteria and methodology as the observational studies has been performed at present, placing the comparison on uncertain footing. Aims. To rectify this situation, we investigate this potential challenge to ΛCDM by determining whether an analog of the observed intervelocity peak is present in galaxy pairs within the IllustrisTNG-300 cosmological simulation. Methods. We identified galaxy pairs following the observational study’s selection criteria, measured their projected velocity difference, and analyzed both the de-projected and the full velocity difference for this galaxy pair sample in the simulation. Results. We recovered a deprojected intervelocity peak at ∼130 km s−1 for galaxy pairs selected from the simulation. The full 3D velocity information available for the pairs in the simulation also reveals a clear preference for this intervelocity. Conclusions. The intervelocity peak among galaxy pairs does not appear to be a feature unique to MOND, as it is also present in ΛCDM. Thus, it cannot be claimed to be a unique success characteristic 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 among the models.