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Palmese, A. and Hartley, W. and Tarsitano, F. and Conselice, Christopher J. and Lahav, O. and Allam, S. and Annis, J. and Lin, H. and Soares-Santos, M. and Tucker, D. and Brout, D. and Banerji, M. and Bechtol, K. and Diehl, H.T. and Fruchter, A. and García-Bellido, J. and Herner, K. and Levan, A.J. and Li, T.S. and Lidman, C. and Misra, K. and Sako, M. and Scolnic, D. and Smith, M. and Abbott, T.M.C. and Abdalla, F.B. and Benoit-Lévy, A. and Bertin, E. and Brooks, D. and Buckley-Geer, E. and Rosell, A. Carnero and Kind, M. Carrasco and Carretero, J. and Castander, F.J. and Cunha, C.E. and D’Andrea, C.B. and da Costa, L.N. and Davis, C. and DePoy, D.L. and Desai, S. and Dietrich, J.P. and Doel, P. and Drlica-Wagner, A. and Eifler, T.F. and Evrard, A.E. and Flaugher, B. and Fosalba, P. and Frieman, J. and Gaztanaga, E. and Gerdes, D.W. and Giannantonio, T. and Gruen, D. and Gruendl, R.A. and Gschwend, J. and Gutierrez, G. and Honscheid, K. and Jain, B. and James, D.J. and Jeltema, T. and Johnson, M.W.G. and Johnson, M.D. and Krause, E. and Kron, R. and Kuehn, K. and Kuhlmann, S. and Kuropatkin, N. and Lima, M. and Maia, M.A.G. and March, M. and Marshall, J.L. and McMahon, R.G. and Menanteau, F. and Miller, C.J. and Miquel, R. and Neilsen, E. and Ogando, R.L.C. and Plazas, A.A. and Reil, K. and Romer, A.K. and Sanchez, E. and Schindler, R. and Smith, R.C. and Sobreira, F. and Suchyta, E. and Swanson, M.E.C. and Tarle, G. and Thomas, D. and Thomas, R.C. and Walker, A.R. and Weller, J. and Zhang, Y. and Zuntz, J. (2017) Evidence for dynamically driven formation of the GW170817 neutron star binary in NGC 4993. The Astrophysical Journal, 849 (2). L34. ISSN 2041-8213

Abstract

We present a study of NGC 4993, the host galaxy of the GW170817 gravitational-wave event, the GRB 170817A short gamma-ray burst (sGRB), and the AT 2017gfo 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. NGC 4993 is a nearby early-type galaxy, with an i-band Sérsic index n = 4.0 and low asymmetry (A = 0.04 ± 0.01). These properties are unusual for sGRB hosts. However, NGC 4993 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 ongoing 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 ${R}_{\mathrm{NSM}}^{\mathrm{gal}}={5.7}_{-3.3}^{+0.57}\times {10}^{-6}{\mathrm{yr}}^{-1}$. 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 ~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.

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