Nebular-phase spectra of superluminous supernovae: physical insights from observational and statistical properties

We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN 2017egm and analyze these with a larger sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 e-folding times after light-curve peak, with the rate of spectroscopic evolution correlated to the light-curve timescale. Emission lines are identified with well-known transitions of oxygen, calcium, magnesium, sodium, and iron. SLSNe are differentiated from other SNe Ic by a prominent O I λ7774 line and higher ionization states of oxygen. The iron-dominated region around 5000 Å is more similar to broad-lined SNe Ic than to normal SNe Ic. Principal component analysis shows that five “eigenspectra” capture ≳70% of the variance, while a clustering analysis shows no clear evidence for multiple SLSN subclasses. Line velocities are 5000-8000 km s^-1 and show stratification of the ejecta. O I λ7774 likely arises in a dense inner region that also produces calcium emission, while [O I] λ6300 comes from farther out until 300-400 days. The luminosities of O I λ7774 and Ca II suggest significant clumping, in agreement with previous studies. Ratios of [Ca II] λ7300/[O I] λ6300 favor progenitors with relatively massive helium cores, likely ≳6 {M}_☉ , though more modeling is required here. SLSNe with broad light curves show the strongest [O I] λ6300, suggesting larger ejecta masses. We show how the inferred velocity, density, and ionization structure point to a central power source....