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

Jens Hjorth, Andrew J. Levan, Nial R. Tanvir, Joe D. Lyman, Radosław Wojtak, Sophie L. Schroder, Ilya Mandel, Christa Gall, Sofie H. Bruun

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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.

Original languageEnglish
Article numberL31
Pages (from-to)1-6
Number of pages6
JournalThe Astrophysical Journal Letters
Issue number2
Publication statusPublished - 20 Oct 2017
Externally publishedYes


  • galaxies: distances and redshifts
  • galaxies: fundamental parameters
  • galaxies: individual (NGC 4993)

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