tokens: in such a faint galaxy during our survey (Section 3 ) and we argue that no host galaxy exists at the SN location even if we take this redshift possibility. The nearest galaxy on the sky (galaxy A in Figure 1 ) could be at z ∼ 0.25 (Figure 7 ). With the 3.86 kpc arcsec −1 at z = 0.2455, the 07 separation corresponds only to the distance of 2.7 kpc from the center of galaxy A. An interesting coincidence is that the extended galaxy at the north of HSC16aayt (galaxy B in Figure 1 ) is at z =0.25053 ± 0.00005, which is spectroscopically measured by the Sloan Digital Sky Survey 20 20 https://www.sdss.org/ (SDSS). Therefore, this galaxy B can also be the host galaxy of HSC16aayt if it is at z = 0.2455, although the galaxy center at (R.A., decl.)=(10:02:05.46, +02:58:10.62) has a separation of 124 from HSC16aayt. The angular scale at z = 0.2505 is 3.91 kpc arcsec −1 and the physical distance between the galaxy center to the SN location is 48.5 kpc at z = 0.2505. Again, assuming a typical massive star lifetime of 10 Myr, the progenitor needs to have the tangential velocity of to travel 48.5 kpc before the explosion. Another intriguing fact is the redshift difference between the host galaxy ( z = 0.2505) and HSC16aayt ( z = 0.2455). Assuming that HSC16aayt is actually at z = 0.2505, the redshift difference indicates that HSC16aayt is moving away from the host toward us with the velocity of . Combined with the tangential velocity of , the progenitor of HSC16aayt was moving away from the host galaxy with a velocity of if galaxy B is the host galaxy. The SDSS spectrum of galaxy B shows that it is an active galactic nucleus. Thus, we can estimate its central BH mass from its spectrum in this case. By using the H α luminosity (3.6 × 10 41 erg s −1 ) and the FWHM of the H α emission from the host galaxy (46.8 Å in the rest frame) obtained by the SDSS spectrum, the central BH mass is estimated to be (Greene Ho 2007 ). The velocity that can be gained by the BH interaction through the Hills mechanism iswhere a is the binary separation, M total is the total mass of the binary system, M BH is the BH mass, and f R is a factor of unity (Brown 2015 ). The extreme velocities of the order of estimated above can be achieved by the BH at the center of galaxy B. 6.2.2. <italic>z</italic> = 1.92 This redshift possibility appears if we assume the strongest line is Mg ii at 2800 Å in the rest frame. It is one of the strongest lines in ultraviolet in SNe IIn (e.g., Fransson et al. 2002 , 2014 ). However, no other possible emission lines can be identified in this redshift. If HSC16aayt is at z = 1.92, the peak SN magnitudes in the rest-frame ultraviolet are around −22 mag and HSC16aayt is a long-lasting SLSN IIn like SN 2006gy (Smith et al. 2010 ). If we assume z = 1.92, a host galaxy fainter than −17.7 mag can exist at the SN location and we cannot exclude the possibility that there is a non-detected host galaxy at the SN location. Galaxy A is, again, not likely to be a host
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