Progressive redshifts in the late-time spectra of Type Ia supernovae

Abstract

We examine the evolution of late-time, optical nebular features of Type Ia supernovae (SNe Ia) using a sample consisting of 160 spectra of 27 normal SNe Ia taken from the literature as well as unpublished spectra of SN 2008Q and ASASSN-14lp. Particular attention was given to nebular features between 4000 and 6000 Å in terms of temporal changes in width and central wavelength. Analysis of the prominent late-time 4700 Å feature shows a progressive central wavelength shift from ~4600 Å to longer wavelengths out to at least day +300 for our entire sample. We find no evidence for the feature's redward shift slowing or halting at an [Fe III] blend centroid ~4700 Å as has been proposed. The width of this feature also steadily increases with a FWHM ~170 Å at day +100 growing to 200 Å or more by day +350. Two weaker adjacent features at around 4850 and 5000 Å exhibit similar redshifts to that of the 4700 Å feature but show no change in width until very late times. We discuss possible causes for the observed redshifts of these late-time optical features including contribution from [Co II] emission at early nebular epochs and the emergence of additional features at later times. We conclude that the ubiquitous redshift of these common late-time SN Ia spectral features is not mainly due to a decrease in line velocities of forbidden Fe emissions, but the result of decreasing line velocities and opacity of permitted Fe absorption lines.