A Hydrogen-poor Superluminous Supernova with Enhanced Iron-group Absorption: A New Link between SLSNe and Broad-lined Type Ic SNe

We present optical observations of the Type I superluminous supernova (SLSN-I) SN 2017dwh at z ≈ 0.13, which reached M _i ≈ -21 mag at peak. Spectra taken a few days after peak show an unusual and strong absorption line centered near 3200 that we identify with Co ii, suggesting a high fraction of synthesized ⁵⁶ Ni in the ejecta. By ∼1 month after peak, SN 2017dwh became much redder than other SLSNe-I, instead strongly resembling broad-lined Type Ic supernovae (Ic-BL SNe) with clear suppression of the flux redward of ∼5000 , providing further evidence for a high mass of Fe-group elements. Late-time upper limits indicate a ⁵⁶ Ni mass of ≲0.6 M _o , leaving open the possibility that SN 2017dwh produced a ⁵⁶ Ni mass comparable to SN 1998bw (≈0.4 M _o ). Fitting the light curve with a combined magnetar and ⁵⁶ Ni model using MOSFiT, we find that the light curve can easily accommodate such masses without affecting the inferred magnetar parameters. We also find that SN 2017dwh occurred in the least-luminous detected host galaxy to date for an SLSN-I, with M _B = -13.5 mag and an implied metallicity of Z ∼ 0.08 Z _o . The spectral properties of SN 2017dwh provide new evidence linking SLSNe-I with Type Ic-BL SNe, and in particular, the high Fe-group abundance may be due to enhanced ⁵⁶ Ni production or mixing due to asphericity. Finally, we find that SN 2017dwh represents the most extreme end of a correlation between continuum shape and Co ii absorption strength in the near-peak spectra of SLSNe-I, indicating that Fe-group abundance likely accounts for some of the variation in their spectral shapes.