SN 2017fgc: A Fast-expanding Type Ia Supernova Exploded in Massive Shell Galaxy NGC 474

Abstract We present extensive optical photometric and spectroscopic observations of the high-velocity (HV) Type Ia supernova (SN Ia) 2017fgc, covering the phase from ∼12 days before to ∼389 days after maximum brightness. SN 2017fgc is similar to normal SNe Ia, with an absolute peak magnitude of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>max</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>≈</mml:mo> </mml:math> −19.32 ± 0.13 mag and a post-peak decline of Δ m 15 ( B ) = 1.05 ± 0.07 mag. Its peak bolometric luminosity is derived as (1.32 ± 0.13) × 10 43 erg s −1 , corresponding to a 56 Ni mass of 0.51 ± 0.03 M ⊙ . The light curves of SN 2017fgc are found to exhibit excess emission in the UBV bands in the early nebular phase and pronounced secondary shoulder/maximum features in the RrIi bands. Its spectral evolution is similar to that of HV SNe Ia, with a maximum-light Si ii velocity of 15,000 ± 150 km s −1 and a post-peak velocity gradient of ∼120 ± 10 km s −1 day −1 . The Fe ii and Mg ii lines blended near 4300 Å and the Fe ii , Si ii , and Fe iii lines blended near 4800 Å are obviously stronger than those of normal SNe Ia. Inspecting a large sample reveals that the strength of the two blends in the spectra, and the secondary peak in the i / r -band light curves, are found to be positively correlated with the maximum-light Si ii velocity. Such correlations indicate that HV SNe Ia may experience more complete burning in the ejecta and/or that their progenitors have higher metallicity. Examining the birthplace environment of SN 2017fgc suggests that it likely arose from a stellar environment with young and high-metallicity populations.