Evidence for Dynamically Driven Formation of the GW170817 Neutron Star Binary in NGC 4993

Abstract

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 (40 Mpc) 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 $R_{NSM}^{gal}=5.7_{-3.3}^{+0.57}\times 10^{-6}{\mathrm{yr}}^{-1}$. 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 $0.038_{-0.022}^{+0.004}$, as opposed to $\sim 0.5$ from all galaxy types. Hypothesizing that the binary system 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 $t_{\mathrm{mer}}\lesssim 200\mathrm{Myr}$ prior to the BNS coalescence.

9 pages, 4 figures, 1 table. Edited to match the ApJL version

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• OzDES multi-object fibre spectroscopy for the Dark Energy Survey: Results and second data release
• Measuring the eccentricity of GW170817 and GW190425
• Swift/UVOT follow-up of Gravitational Wave Alerts in the O3 era
• Examining transitional galaxies to understand the role of clusters and their dynamical status in galaxy quenching
• Gravitational wave cosmology and astrophysics with large spectroscopic galaxy surveys
• Binary Neutron Star (BNS) merger: What we learned from relativistic ejecta of GW/GRB~170817A
• Host galaxies and electromagnetic counterparts to binary neutron star mergers across the cosmic time: Detectability of GW170817-like events