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

Understand

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 $\sim$10" from the nucleus of the S0 galaxy NGC 4993.
• We report here the luminosity distance to this galaxy using two independent methods.
• (1) Based on our MUSE/VLT measurement of the heliocentric redshift ($z_{\rm helio}=0.009783\pm0.000023$) we infer the systemic recession velocity of the NGC 4993 group of galaxies in the cosmic microwave background (CMB) frame to be $v_{\rm CMB}=3231 \pm 53$ km s$^{-1}$.