The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale
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- Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
- Northwestern Univ., Evanston, IL (United States)
- Univ. of Chicago, Chicago, IL (United States)
- Ohio Univ., Athens, OH (United States)
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Univ. of Pennsylvania, Philadelphia, PA (United States)
- Syracuse Univ., Syracuse, NY (United States)
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); The Univ. of Chicago, Chicago, IL (United States)
- Columbia Univ., New York, NY (United States)
- Space Telescope Science Institute, Baltimore, MD (United States); The Johns Hopkins Univ., Baltimore, MD (United States)
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Brandeis Univ., Waltham, MA (United States)
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 $$\gtrsim 10$$ 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 $$11.2^{+0.7}_{-1.4}$$ Gyr, with a 90% confidence range of $6.8-13.6$ 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 (0.64$$r_{e}$$) 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 $$\sim 200$$ 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.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States); Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA), Office of Defense Nuclear Nonproliferation; USDOE Office of Science (SC), High Energy Physics (HEP)
- Grant/Contract Number:
- AC52-06NA25396; AC02-07CH11359
- OSTI ID:
- 1417825
- Alternate ID(s):
- OSTI ID: 1409349
- Report Number(s):
- LA-UR-17-28901; arXiv:1710.05458; FERMILAB-PUB-17-473-A-AE-CD
- Journal Information:
- The Astrophysical Journal. Letters (Online), Vol. 848, Issue 2; ISSN 2041-8213
- Publisher:
- Institute of Physics (IOP)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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