We present a study of NGC 4993 , the host galaxy of the GW170817 gravitational wave event , the GRB170817A short gamma–ray burst ( sGRB ) and the AT2017gfo kilonova .
We use Dark Energy Camera imaging , AAT spectra and publicly available data , relating our findings to binary neutron star ( BNS ) formation scenarios and merger delay timescales .
NGC4993 is a nearby early–type galaxy , with i -band Sérsic index n = 4.0 and low asymmetry ( A = 0.04 \pm 0.01 ) .
These properties are unusual for sGRB hosts .
However , NGC4993 presents shell–like structures and dust lanes indicative of a recent galaxy merger , with the optical transient located close to a shell .
We constrain the star formation history ( SFH ) of the galaxy assuming that the galaxy merger produced a star formation burst , but find little to no on–going star formation in either spatially–resolved broadband SED or spectral fitting .
We use the best–fit SFH to estimate the BNS merger rate in this type of galaxy , as R _ { NSM } ^ { gal } = 5.7 ^ { +0.57 } _ { -3.3 } \times 10 ^ { -6 } { yr } ^ { -1 } .
If star formation is the only considered BNS formation scenario , the expected number of BNS mergers from early–type galaxies detectable with LIGO during its first two observing seasons is 0.038 ^ { +0.004 } _ { -0.022 } , as opposed to \sim 0.5 from all galaxy types .
Hypothesizing that the binary formed due to dynamical interactions during the galaxy merger , the subsequent time elapsed can constrain the delay time of the BNS coalescence .
By using velocity dispersion estimates and the position of the shells , we find that the galaxy merger occurred t _ { mer } \lesssim 200 ~ { } { Myr } prior to the BNS coalescence .