A possible distance bias for type Ia supernovae with different ejecta velocities

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 ± 0.035 mag (2.7σ) HR step between SNe with Si ii λ6355 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 λ6355 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.

Near-IR Type Ia SN distances: host galaxy extinction and mass-step corrections revisited

“…In fact, there are indications that there are two SN populations, having different SN ejecta velocities and different intrinsic colors, which also trace the host galaxy stellar mass (see e.g. Polin et al 2019;Siebert et al 2020;Pan 2020). Childress et al (2013) and Gonzalez-Gaitan et al ( 2020) simulate the effect of having separate colorluminosity corrections for low-and high-mass galaxies.…”

The Zwicky Transient Facility Type Ia supernova survey: first data release and results

“…This is critical to remo v e contamination from core-collapse SNe and other non-SN Ia transients. Several studies in the literature have demonstrated that SNe Ia can be divided into subclasses based on spectral line velocities near maximum light, with potential implications for distance measurements (Wang et al 2009 ;F ole y & Kasen 2011 ;F olatelli et al 2013 ;Siebert et al 2020 ;Dettman et al 2021 ). Therefore, having spectra is important for improving cosmological inference with SNe Ia.…”

Understanding Type Ia Supernova Distance Biases by Simulating Spectral Variations

“…SN velocity is an active topic of investigation in the SN Ia community, with hints that high and low-velocity SN Ia vary in their intrinsic colors and intrinsic color scatter (e.g., Foley et al 2011;Foley 2012;Mandel et al 2014;Pan et al 2015;Siebert et al 2020). This correlation motivated the inclusion of a velocity perturber in our simulations.…”

“…Here, we simulate each SN with a dependency (SN velocity) that has no corresponding model component in SALT2. Ejecta velocities are simulated according to the observed population from Siebert et al (2020).…”

“…Today, it is generally accepted that the environment of a SN Ia correlates with the properties of its spectra and light curve, but it is unclear if current corrections (e.g., the mass step) account for these relationships with sufficient precision for much larger data sets that will be obtained over the next decade. Numerous studies have investigated the potential impact on SN Ia standardization of many variables beyond host mass including local specific star formation rate (sSFR, Rigault et al 2013;Kelly et al 2015), intrinsic luminosity evolution with redshift (Howell et al 2007), and metallicity (Childress et al 2013;Hayden et al 2013), as well as the ways in which SN properties such as ejecta velocity correlate with Type Ia standardization parameters and Hubble residuals (Foley et al 2011;Siebert et al 2020).…”