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Title: The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale

Here, 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 $$\gtrsim 10\,\mathrm{Gyr}$$ ago, with subsequent exponential decline leading to a low current star formation rate of 0.01 $${M}_{\odot }$$ 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 $$6.8\mbox{–}13.6\,\mathrm{Gyr}$$. This in turn indicates an initial binary separation of $$\approx 4.5$$ $${R}_{\odot }$$, 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 $${M}_{r}\gtrsim -7.2$$ 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.
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  1. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  2. Northwestern Univ., Evanston, IL (United States)
  3. Univ. of Chicago, Chicago, IL (United States)
  4. Ohio Univ., Athens, OH (United States)
  5. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
  6. Univ. of Pennsylvania, Philadelphia, PA (United States)
  7. Syracuse Univ., Syracuse, NY (United States)
  8. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); The Univ. of Chicago, Chicago, IL (United States)
  9. Columbia Univ., New York, NY (United States)
  10. Space Telescope Science Institute, Baltimore, MD (United States); The Johns Hopkins Univ., Baltimore, MD (United States)
  11. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Brandeis Univ., Waltham, MA (United States)
Publication Date:
Report Number(s):
Journal ID: ISSN 2041-8213
Grant/Contract Number:
Accepted Manuscript
Journal Name:
The Astrophysical Journal. Letters
Additional Journal Information:
Journal Volume: 848; Journal Issue: 2; Journal ID: ISSN 2041-8213
Institute of Physics (IOP)
Research Org:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org:
USDOE National Nuclear Security Administration (NNSA), Office of Defense Nuclear Nonproliferation (NA-20)
Country of Publication:
United States
79 ASTRONOMY AND ASTROPHYSICS; galaxies: individual (NGC 4993); gravitational waves; stars: neutron
OSTI Identifier: