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Register a new userThe Dependence of the Type Ia Supernova Host Bias on Observation or Fitting Technique
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Brighter type Ia supernovae (SNe Ia) prefer less massive hosts with higher star formation. This bias is over-corrected for SNe Ia standardized using the standard Tripp relation, resulting in a step-like dependence of standardized distance on host properties. Using the PISCO supernova host sample and SDSS, GALEX, and 2MASS photometry, we compare host galaxy stellar mass and star formation rate (SFR) estimates from different observation and fitting techniques and their impact on the mass step and sSFR step biases. The step size for FAST++ mass estimates was $-0.04pm0.02$ mag for FAST++ and STARLIGHT, increasing by 0.02 mag for ZPEG. UV information had no effect on measured mass step size or location. Our small sample sizes resulted in all mass step size uncertainties being within 2$sigma$ significance of a zero step due. Regardless, mass step sizes were all consistently within 1$sigma$ of each other. Specific SFR (sSFR) step sizes are $0.05pm0.03$ mag (H$alpha$) and $0.06pm0.03$ mag (UV) for a reduced 51 host sample with SDSS and GALEX coverage, with 50% increase in step size uncertainties. Step location was determined by mass sample used to normalize sSFR. The step size reduces by 0.04 mag with an unconstrained location using all available 73 hosts with H$alpha$ measurements. Despite reduced sample sizes, we find no evidence that observation or fitting technique choice drives mass step measurement, but cannot conclude the same for the sSFR step. Further work will focus on differing star formation epochs and dust attenuation corrections effects on the sSFR bias.
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We analyse spectroscopic measurements of 122 type Ia supernovae (SNe Ia) with z<0.09 discovered by the Palomar Transient Factory, focusing on the properties of the Si II 6355 and Ca II `near-infrared triplet absorptions. We examine the velocities of the photospheric Si II 6355, and the velocities and strengths of the photospheric and high-velocity Ca II, in the context of the stellar mass (Mstellar) and star-formation rate (SFR) of the SN host galaxies, as well as the position of the SN within its host. We find that SNe Ia with faster Si II 6355 tend to explode in more massive galaxies, with the highest velocity events only occuring in galaxies with Mstellar > 3*10^9 solar mass. We also find some evidence that these highest velocity SNe Ia explode in the inner regions of their host galaxies, similar to the study of Wang et al. (2013), although the trend is not as significant in our data. We show that these trends are consistent with some SN Ia spectral models, if the host galaxy stellar mass is interpreted as a proxy for host galaxy metallicity. We study the strength of the high-velocity component of the Ca II near-IR absorption, and show that SNe Ia with stronger high-velocity components relative to photospheric components are hosted by galaxies with low stellar mass, blue colour, and a high sSFR. Such SNe are therefore likely to arise from the youngest progenitor systems. This argues against a pure orientation effect being responsible for high-velocity features in SN Ia spectra and, when combined with other studies, is consistent with a scenario where high-velocity features are related to an interaction between the SN ejecta and circumstellar medium (CSM) local to the SN.
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