Abstract
The historic detection of gravitational waves from a binary neutron star merger (GW170817) and its electromagnetic counterpart led to the first accurate (sub-arcsecond) localization of a gravitational-wave event. The transient was found to be $\sim$10" from the nucleus of the S0 galaxy NGC 4993. We report here the luminosity distance to this galaxy using two independent methods. (1) Based on our MUSE/VLT measurement of the heliocentric redshift ($z_{\rm helio}=0.009783\pm0.000023$) we infer the systemic recession velocity of the NGC 4993 group of galaxies in the cosmic microwave background (CMB) frame to be $v_{\rm CMB}=3231 \pm 53$ km s$^{-1}$. Using constrained cosmological simulations we estimate the line-of-sight peculiar velocity to be $v_{\rm pec}=307 \pm 230$ km s$^{-1}$, resulting in a cosmic velocity of $v_{\rm cosmic}=2924 \pm 236$ km s$^{-1}$ ($z_{\rm cosmic}=0.00980\pm 0.00079$) and a distance of $D_z=40.4\pm 3.4$ Mpc assuming a local Hubble constant of $H_0=73.24\pm 1.74$ km s$^{-1}$ Mpc$^{-1}$. (2) Using Hubble Space Telescope measurements of the effective radius (15.5" $\pm$ 1.5") and contained intensity and MUSE/VLT measurements of the velocity dispersion, we place NGC 4993 on the Fundamental Plane (FP) of E and S0 galaxies. Comparing to a frame of 10 clusters containing 226 galaxies, this yields a distance estimate of $D_{\rm FP}=44.0\pm 7.5$ Mpc. The combined redshift and FP distance is $D_{\rm NGC 4993}= 41.0\pm 3.1$ Mpc. This 'electromagnetic' distance estimate is consistent with the independent measurement of the distance to GW170817 as obtained from the gravitational-wave signal ($D_{\rm GW}= 43.8^{+2.9}_{-6.9}$ Mpc) and confirms that GW170817 occurred in NGC 4993.
Highlights
Mpc) and confirms that GW170817 occurred in NGC 4993
GW170817 was the first gravitational-wave event arising from a binary neutron star (NS) merger to have been detected by LIGO/Virgo (Abbott et al 2017a)
It provided the first realistic chance of detecting an electromagnetic counterpart, as outlined in Abbott et al (2017b): 2 s after the merger, Fermi and INTEGRAL detected a weak gamma-ray burst, and half a day after the event, an optical (Coulter et al 2017) and near-infrared (NIR; Tanvir et al 2017) counterpart was localized to sub-arcsecond precision, ∼10′′ from the nucleus of the S0 galaxy NGC 4993 (Abbott et al 2017b)
Summary
GW170817 was the first gravitational-wave event arising from a binary neutron star (NS) merger to have been detected by LIGO/Virgo (Abbott et al 2017a). The source was localized to a sky region of 28 deg purely using gravitational-wave data from the three interferometers As such, it provided the first realistic chance of detecting an electromagnetic counterpart, as outlined in Abbott et al (2017b): 2 s after the merger, Fermi and INTEGRAL detected a weak gamma-ray burst, and half a day after the event, an optical (Coulter et al 2017) and near-infrared (NIR; Tanvir et al 2017) counterpart was localized to sub-arcsecond precision, ∼10′′ from the nucleus of the S0 galaxy NGC 4993 (Abbott et al 2017b). The image was reduced via astrodrizzle, with the final scale set to 0 07
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