A Possible Distance Bias for Type Ia Supernovae with Different Ejecta Velocities

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Abstract

After correcting for their light-curve shape and color, Type Ia supernovae (SNe Ia) are precise cosmological distance indicators. However, there remains a non-zero intrinsic scatter in the differences between measured distance and that inferred from a cosmological model (i.e., Hubble residuals or HRs), indicating that SN Ia distances can potentially be further improved. We use the open-source relational database kaepora to generate composite spectra with desired average properties of phase, light-curve shape, and HR. At many phases, the composite spectra from two subsamples with positive and negative average HRs are significantly different. In particular, in all spectra from 9 days before to 15 days after peak brightness, we find that SNe with negative HRs have, on average, higher ejecta velocities (as seen in nearly every optical spectral feature) than SNe with positive HRs. At +4 days relative to $B$-band maximum, using a sample of 62 SNe Ia, we measure a $0.091\pm0.035$ mag ($2.7\sigma$) HR step between SNe with Si II $\lambda$6355 line velocities ($v_{Si {II}}$) higher/lower than $-11{,}000$ km s$^{-1}$ (the median velocity). After light-curve shape and color correction, SNe with higher velocities tend to have underestimated distance moduli relative to a cosmological model. The intrinsic scatter in our sample reduces from 0.094 mag to 0.082 mag after making this correction. Using the Si II $\lambda$6355 velocity evolution of 115 SNe Ia, we estimate that a velocity difference $>$500 km s$^{-1}$ exists at each epoch between the positive-HR and negative-HR samples with 99.4% confidence. Finally at epochs later than +37 days, we observe that negative-HR composite spectra tend to have weaker spectral features in comparison to positive-HR composite spectra.