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We examine the evolution of late-time, optical nebular features of Type Iasupernovae (SNe Ia) using a sample consisting of 160 spectra of 27 normal SNeIa taken from the literature as well as unpublished spectra of SN 2008Q andASASSN-14lp. Particular attention was given to nebular features between4000-6000 A in terms of temporal changes in width and central wavelength.Analysis of the prominent late-time 4700 A feature shows a progressive centralwavelength shift from ~4600 A to longer wavelengths out to at least day +300for our entire sample. We find no evidence for the feature's red-ward shiftslowing or halting at an [Fe III] blend centroid of 4701 A as has beenproposed. The width of the feature also steadily increases with a FWHM ~170 Aat day +100 growing to 200 A or more by day +350. Two weaker adjacent featuresaround 4850 and 5000 A exhibit very similar red shifts to that of the 4700 Afeature but show no change in width until very late times. We discuss possiblecauses for the observed red-ward shifting of these late-time optical featuresincluding contribution from [Co II] emission at early nebular epochs and theemergence of additional features at later times. We conclude that theubiquitous red shift of these common late-time, nebular SN Ia spectral featuresis not mainly due to a decrease in a blueshift of forbidden Fe lines but theresult, in part, of decreasing velocity and/or optical depth of permitted Felines.

Progressive Red Shifts in the Late-Time Spectra of Type Ia Supernovae