The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. VII. properties of the host galaxy and constraints on the merger timescale

Research output: Contribution to journalArticlepeer-review


  • P. K. Blanchard
  • E. Berger
  • W. Fong
  • J. Leja
  • C. Conroy
  • K. D. Alexander
  • R. Margutti
  • P. K. G. Williams
  • Z. Doctor
  • R. Chornock
  • V. A. Villar
  • P. S. Cowperthwaite
  • J. Annis
  • D. Brout
  • D. A. Brown
  • H. -Y. Chen
  • T. Eftekhari
  • J. A. Frieman
  • D. E. Holz
  • B. D. Metzger
  • A. Rest
  • M. Sako
  • M. Soares-Santos

Colleges, School and Institutes


We present the properties of NGC 4993, the host galaxy of GW170817, the first gravitational-wave (GW) event from the merger of a binary neutron star (BNS) system and the first with an electromagnetic (EM) counterpart. We use both archival photometry and new optical/near-IR imaging and spectroscopy, together with stellar population synthesis models to infer the global properties of the host galaxy. We infer a star formation history peaked at ≳ 10 {Gyr} ago, with subsequent exponential decline leading to a low current star formation rate of 0.01 {M} yr-1, which we convert into a binary merger timescale probability distribution. We find a median merger timescale of {11.2}-1.4+0.7 Gyr, with a 90% confidence range of 6.8{--}13.6 {Gyr}. This in turn indicates an initial binary separation of ≈ 4.5 {R}, comparable to the inferred values for Galactic BNS systems. We also use new and archival Hubble Space Telescope images to measure a projected offset of the optical counterpart of 2.1 kpc (0.64r e ) from the center of NGC 4993 and to place a limit of {M}r≳ -7.2 mag on any pre-existing emission, which rules out the brighter half of the globular cluster luminosity function. Finally, the age and offset of the system indicates it experienced a modest natal kick with an upper limit of ̃200 km s-1. Future GW-EM observations of BNS mergers will enable measurement of their population delay time distribution, which will directly inform their viability as the dominant source of r-process enrichment in the universe....


Original languageEnglish
Article numberL22
JournalAstrophysical Journal Letters
Issue number2
Publication statusPublished - 16 Oct 2017


  • galaxies: individual (NGC 4993), gravitational waves, stars: neutron