University of Leicester
Hjorth_2017_ApJL_848_L31.pdf (2.12 MB)

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

Download (2.12 MB)
journal contribution
posted on 2018-05-01, 11:04 authored by Jens Hjorth, Andrew J. Levan, Nial R. Tanvir, Joe D. Lyman, Radosław Wojtak, Sophie L. Schrøder, Ilya Mandel, Christa Gall, Sofie H. Bruun
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.


R.W. thanks Vinicius Araújo Barbosa de Lima for his help in handling the simulation data. We thank the LIGO and Virgo Scientific Collaborations for sharing information prior to publication and for helpful comments on the manuscript. We thank the anonymous referee for a swift and insightful review. This work was supported by a VILLUM FONDEN Investigator grant to J.H. (project number 16599). A.J.L. is supported by STFC and the ERC (grant #725246). J.D.L. gratefully acknowledges support from STFC (ST/P000495/1). I.M. acknowledges partial support from STFC. C.G. acknowledges support from the Carlsberg Foundation. This work is based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 099.D-0668. Facilities: Hubble Space Telescope - , Very Large Telescope - . Software: astropy (Astropy Collaboration 2013).



Astrophysical Journal Letters, 2017, 848 (2)

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


  • VoR (Version of Record)

Published in

Astrophysical Journal Letters


American Astronomical Society, IOP Publishing





Acceptance date


Copyright date


Available date


Publisher version



Usage metrics

    University of Leicester Publications