tokens: geometry, but, e.g., if we assume a 3D isotropic velocity dispersion it would reduce the dynamical time by ). So far, we have no measurement of the shell’s velocity dispersion, but estimates from the literature suggest for similar shells in other galaxies (Quinn 1984 ). This would give a dynamical timescale of t dyn ≈ 192 Myr. Detailed simulations of shells in other galaxies suggest that survival time could be even larger that 1 Gyr, depending on the assumed scenario (Pop et al. 2017 ). The survival time of the shell could be used as an upper limit for the time the minor merger took place, i.e., t dyn ≥ t merg , so we estimate t merg ≲ 200 Myr. If the BNS was formed as such in a shell, then we would have expected to see evidence for recent star formation, but we find no indication of this. In the absence of star formation, it is plausible that the BNS coalescence was triggered by a dynamical process, e.g., NS–NS capture or the destabilization of a pre-existing wide-separation binary. These processes will be quite sensitive to the stellar density, which, given the Sérsic index and the luminosity from the residual image found in Section 3.2 , is high in the center of NGC 4993 and around the transient position. If this dynamical hypothesis is true, then the delay time Δ t NSM between the BNS formation and coalescence is ≲200 Myr. On the other hand, Blanchard et al. ( 2017 ) find a median delay time of under the assumption that the binary was formed through secular SF. 5.2. Galaxy Environment If the binary formation is related to dynamical processes in galaxy merging, as we are investigating here, then this is most likely to happen in galaxy groups and low-mass clusters. According to the 2MASS catalog (Tully 2015 ), NGC 4993 resides in a group of which we analyze the remaining seven galaxies. A spectral analysis shows that NGC 4993 is not the only galaxy showing AGN activity (see Figure 3 ), but it is peculiar in terms of age, metallicity, and mass-to-light ratio. It shows an older stellar population (the mean age of the other 13 galaxies is Log (Age) = 9.56 ± 0.17), lower metallicity (mean: M / H = −0.31 ± 0.11), and higher M / L r (mean: 2.41 ± 0.45) than the average. The group has a projected virial radius of R vir = 0.36 Mpc and a line-of-sight velocity dispersion (Tully 2015 ). The crossing time is therefore . If galaxy mergers are correlated to BNS coalescence, future GW studies could possibly concentrate on galaxy groups (but note that these are crowded regions and therefore matching candidates to a host could be difficult). In order to have precise measurements of H 0 , one needs to identify the host galaxy redshift clearly. When the match is clear, the properties of the type of host galaxy found could help future studies to select the right host galaxy or create galaxy catalogs of likely hosts for GW–EM follow-up and untriggered kilonova searches (Doctor et al. 2017 ). In fact, large photometric surveys such as DES, LSST, or WFIRST are
ner_tags: O O O O O O O B-Model I-Model I-Model I-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation O O O O O O O O B-Formula I-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula I-Formula I-Formula O O O O B-Formula I-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Dataset I-Dataset B-Citation I-Citation O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula I-Formula I-Formula I-Formula O O O B-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula I-Formula I-Formula O O O O O B-Citation I-Citation O O O O O O O O O O O O O O O O B-EntityOfFutureInterest O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-EntityOfFutureInterest I-EntityOfFutureInterest O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O B-Survey B-Survey O B-Survey O