UC Santa Cruz

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 distances inferred from a cosmological model (i.e., Hubble residuals). We have found that Hubble residuals are correlated with different properties of SN Ia host galaxies (e.g., mass, star-formation rate, and metallicity). Cosmological analyses attempt to correct for these effects empirically without any physical knowledge of the progenitor system, potentially introducing a bias. In this thesis, I detail several projects aimed at uncovering the dominant physical sources of intrinsic scatter.

First, I will present Kaepora, a relational database for SN Ia observations containing 4975 public spectra of 777 SNe Ia. We have significantly improved this data set by inspecting these spectra for quality, removing galactic emission lines and cosmic rays, generating variance spectra, and correcting for the reddening caused by both MW and host-galaxy dust. Using this large homogenized sample, we produce composite spectra that have been precisely controlled for phase and light-curve shape. These composite spectra reproduce known trends with other optical properties of SNe Ia, and by controlling for these effects, they can be used to investigate a wide parameter space. Critically, with these methods I will show that SN Ia distances can potentially be improved with the knowledge of their ejecta velocities.

I will then describe late time observations of the UV-flashing high-velocity SN Ia 2019yvq. We identify strong Calcium emission in the nebular spectrum of this SN and argue that this indicates SN 2019yvq was likely the result of a sub-Chandrasekhar mass double-detonation explosion. It is possible that 2019yvq-like SNe contribute to our cosmological samples, and the presence of multiple progenitor channels could be a source of Hubble residual intrinsic scatter. A better understanding of SN Ia progenitor environments (which may correlate with progenitor channel) could help us discern the relative contributions of multiple progenitor channels in cosmological samples.

Finally, I will present preliminary results from the Foundation/Swope host galaxy survey (a project which has spanned my entire graduate career). We have obtained optical spectroscopy for 372 out of 517 total SN host galaxies in the Foundation and Swope combined cosmological SN sample (the largest low-z sample). The mass-metallicity relationship of our preliminary sample is consistent with similar samples and contains some of the lowest mass galaxies studied in a cosmological sample. Our sample also shows evidence for a "metallicity-step" using measurements from both the galaxy nucleus and the supernova location. With the full sample, we hope to constrain the functional form of this relationship and better understand the underlying physical cause of the empirical host-galaxy correction.