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 similar to 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 ourMUSE/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 H0 = 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 (D-GW 43.8(-6.9) (+2.9) Mpc) and confirms that GW170817 occurred in NGC 4993.