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.