WiFeS observations of nearby southern Type Ia supernova host galaxies

Abstract

We present high-resolution observations of nearby galaxies that have hosted Type Ia supernovae to measure systemic spectroscopic redshifts using the wide field spectrograph (WiFeS) instrument on the Australian National University 2.3 m telescope at Siding Spring Observatory. While most of the galaxies targeted have previous spectroscopic redshifts, we provide demonstrably more accurate and precise redshifts with competitive uncertainties, motivated by potential systematic errors that could bias estimates of the Hubble constant . The WiFeS instrument is remarkably stable; after calibration, the wavelength solution varies by 0.5 Å in red and blue with no evidence of a trend over the course of several years. By virtue of the arcsec field of view, we are always able to measure the redshift of the galactic core, or the entire galaxy in the cases where its angular extent is smaller than the field of view, reducing any errors due to galaxy rotation. We observed 185 southern SN Ia host galaxies and measured the redshift of each via at least one spatial region of (a) the core and (b) the average over the full-field/entire galaxy. Overall, we find stochastic differences between historical redshifts and our measured redshifts on the order of with a mean offset of 4.3 and normalised median absolute deviation of 1.2. We show that a systematic redshift offset at this level is not enough to bias cosmology, as shifts by km s Mpc when we replace Pantheon+ redshifts with our own, but the occasional large differences are interesting to note. Show more