The Distance to NGC 4993: The Host Galaxy of the Gravitational-wave Event GW170817

The historic detection of gravitational waves from a binary neutron star merger (GW170817) and its electromagnetic counterpart led to the first accurate (sub-arcsecond) localization of a gravitational-wave event. The transient was found to be ∼10″ from the nucleus of the S0 galaxy NGC 4993. We report here the luminosity distance to this galaxy using two independent methods. (1) Based on our MUSE/VLT measurement of the heliocentric redshift (z _helio =0.009783 ±0.000023), we infer the systemic recession velocity of the NGC 4993 group of galaxies in the cosmic microwave background (CMB) frame to be v _CMB =3231 ±53 km s^-1. Using constrained cosmological simulations we estimate the line-of-sight peculiar velocity to be v _pec =307 ±230 km s^-1, resulting in a cosmic velocity of v _cosmic =2924 ±236 km s^-1 (z _cosmic =0.00980 ±0.00079) and a distance of D _z =40.4 ±3.4 Mpc assuming a local Hubble constant of H ₀ =73.24 ±1.74 km s^-1 Mpc^-1. (2) Using Hubble Space Telescope measurements of the effective radius (15.″5 ±1.″5) and contained intensity and MUSE/VLT measurements of the velocity dispersion, we place NGC 4993 on the Fundamental Plane (FP) of E and S0 galaxies. Comparing to a frame of 10 clusters containing 226 galaxies, this yields a distance estimate of D _FP =44.0 ±7.5 Mpc. The combined redshift and FP distance is D _{NGC 4993} =41.0 ±3.1 Mpc. This "electromagnetic" distance estimate is consistent with the independent measurement of the distance to GW170817 as obtained from the gravitational-wave signal ( Mpc) and confirms that GW170817 occurred in NGC 4993.