The first neutron star-neutron star ( NS-NS ) merger was discovered on August 17 , 2017 through gravitational waves ( GW170817 ) and followed with electromagnetic observations \citep GW170817 .
This merger was detected in an old elliptical galaxy with no recent star formation \citep Blanchard2017 , Troja2017 .
We perform a suite of numerical calculations to understand the formation mechanism of this merger .
We probe three leading formation mechanisms of double compact objects : classical isolated binary star evolution , dynamical evolution in globular clusters and nuclear cluster formation to test whether they are likely to produce NS-NS mergers in old host galaxies .
Our simulations with optimistic assumptions show current NS-NS merger rates at the level of 10 ^ { -2 } yr ^ { -1 } from binary stars , 5 \times 10 ^ { -5 } yr ^ { -1 } from globular clusters and 10 ^ { -5 } yr ^ { -1 } from nuclear clusters for all local elliptical galaxies ( within 100 Mpc ^ { 3 } ) .
These models are thus in tension with the detection of GW170817 with an observed rate 1.5 ^ { +3.2 } _ { -1.2 } yr ^ { -1 } ( per 100 Mpc ^ { 3 } ; LIGO/Virgo 90 \% credible limits ) .
Our results imply that either ( i ) the detection of GW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is a statistical coincidence ; or that ( ii ) physics in at least one of our three models is incomplete in the context of the evolution of stars that can form NS-NS mergers ; or that ( iii ) another very efficient ( unknown ) formation channel with a long delay time between star formation and merger is at play .