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Hydrogen-poor superluminous supernovae (SLSNe-I) have been predominantlyfound in low-metallicity, star-forming dwarf galaxies. Here we identifyGaia17biu/SN 2017egm as an SLSN-I occurring in a "normal" spiral galaxy (NGC3191) in terms of stellar mass (several times 10^10 M_sun) and metallicity(roughly Solar). At redshift z=0.031, Gaia17biu is also the lowest redshiftSLSN-I to date, and the absence of a larger population of SLSNe-I in dwarfgalaxies of similar redshift suggests that metallicity is likely less importantto the production of SLSNe-I than previously believed. With the smallestdistance and highest apparent brightness for an SLSN-I, we are able to studyGaia17biu in unprecedented detail. Its pre-peak near-ultraviolet to opticalcolor is similar to that of Gaia16apd and among the bluest observed for anSLSN-I while its peak luminosity (M_g = -21 mag) is substantially lower thanGaia16apd. Thanks to the high signal-to-noise ratios of our spectra, weidentify several new spectroscopic features that may help to probe theproperties of these enigmatic explosions. We detect polarization at the ~0.5%level that is not strongly dependent on wavelength, suggesting a modest, globaldeparture from spherical symmetry. In addition, we put the tightest upper limityet on the radio luminosity of an SLSN-I with <5.4x10^26 erg/s/Hz (at 10 GHz),which is almost a factor of 40 better than previous upper limits and one of thefew measured at an early stage in the evolution of an SLSN-I. This limitlargely rules out an association of this SLSNe-I with known populations ofgamma-ray burst (GRB) like central engines.

Gaia17biu/SN 2017egm in NGC 3191: The closest hydrogen-poor superluminous supernova to date is in a "normal", massive, metal-rich spiral galaxy