GW170817: Is its Host Galaxy an Interloper?

Observation of the GW170817 NS-NS merger presented the first opportunity to measure H₀ using gravitational waves; furthermore, it offered a method for resolving the Hubble tension. However, in order to break the tension we must first develop a robust understanding of the errors and uncertainties inherent to such measurements. The error on H₀ measurements using gravitational wave events is comprised of two components: (i) rigorously-modeled statistical and systematic errors in the standard siren distance and (ii) error in the Hubble velocity measurement. Because the Hubble velocity cannot be directly observed, it must instead be estimated by indirect means. If this estimation is not done carefully, it is easy to introduce unaccounted systematic biases in the Hubble velocity and therefore H₀. One possible source of such error is the incorrect association of galaxies with a host group, which could severely impact peculiar velocity corrections to the Hubble velocity measurement. By applying the grouping algorithm used in the 2MASS Redshift Survey (2MRS) group catalog to cosmological simulations, we investigate the possibility that NGC4993, the host galaxy of GW170817, is incorrectly associated with its group in the 2MRS group catalog. For groups of similar mass to the NGC4993 group, we find that the probability for incorrect association of group members is significant. We present our methods in detail and discuss techniques to obtain realistic Hubble velocities for future H₀ measurements.