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 _ { \Sun } 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 _ { \Sun } , 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 .