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Observational Cosmology

— our view of the universe through telescopes.

On the relationship between Type Ia supernova luminosity and host-galaxy properties
Observational Cosmology - Supernovae - Statistical Analysis
Murakami, Stahl, Zhang, et al. submitted to MNRAS. arxiv: 2012.06217
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"A string of recent studies has debated the possible presence of an evolutionary trend between the peak luminosity of Type Ia supernovae (SNe Ia) and the properties of the galaxies that host them. We shed new light on the discussion by presenting an analysis of ∼200 low-redshift SNe Ia in which we measure the separation of Hubble residuals (HR; as probes of luminosity) between two host-galaxy morphological types. We show that this separation can test the predictions made by recently proposed models, using an independently and empirically determined distribution of each morphological type in host-property space. Our results are consistent with the previously known HR–mass step (or slope), but inconsistent with newly proposed HR–age slopes, which we find to significantly overstate what amounts only to a slight trend. In addition, we show that these two trends— HR–mass and HR–age correlation — need to be consistent with each other, given the significant correlation that we identify between age and mass in a sample of galaxies. While our result clearly rejects the recently proposed large HR–age slope, the correlations between mass, age, morphology, and HR values are evident, keeping the HR–age slope relevant as an interesting topic for discussion and further studies."

Statistical methods on Type Ia supernova luminosity evolution
Observational Cosmology - Supernovae - Statistical Analysis
Zhang, Murakami, Stahl, et al. submitted to MNRAS. arxiv: 2012.06215

"Much of the cosmological utility thus far extracted from Type Ia supernovae (SNe Ia) relies on the assumption that SN Ia peak luminosities do not evolve significantly with the age (local or global) of their stellar environments. Two recent studies have provided conflicting results in evaluating the validity of this assumption, with one finding no correlation between Hubble residuals (HR) and stellar environment age, while the other claims a significant correlation. In this Letter we perform an independent reanalysis that rectifies issues with the statistical methods employed by both of the aforementioned studies. Our analysis follows a principled approach that properly accounts for regression dilution and critically (and unlike both prior studies) utilizes the Bayesian-model-produced SN environment age estimates (posterior samples) instead of point estimates. Moreover, the posterior is used as an informative prior in the regression. We find the Pearson correlation between the HR and local (global) age to be in excess of 4σ (3σ). Assuming there exists a linear relationship between HR and local (global) age, we find a corresponding slope of −0.035±0.007mag/Gyr (−0.036±0.007mag/Gyr). We encourage further usage of our approach to examine possible cosmological implications of the HR and age correlation."