Supernova brightness shows correlation with host galaxy mass
30 October 2013
More than a decade after the discovery of the accelerating expansion of the universe, Type Ia supernovae (SNe Ia) remain the best standard candles for cosmology. However, recent studies have indicated that the standardised brightness of SNe Ia may have a residual dependence on their host galaxy environment. In a pair of papers from the Nearby Supernova Factory (SNfactory) led by ANU-based CAASTRO researcher Michael Childress, the authors derive precision estimates of SN Ia host galaxy properties and show that SNe Ia show a sharp transition in their standardised brightness when going from low mass to high mass host galaxies.
SNe Ia are used to measure distances by comparing their observed brightness to the brightness predicted for their observed light curve shape and colour. Childress and the SNfactory team computed the light curve properties for a sample of 115 new SNe Ia from SNfactory, and derived the mass, metallicity, and star-formation rates for their host galaxies. Combining these data with three other major surveys to form a sample of over 600 SNe Ia, the team found that SNe Ia in low mass galaxies were too faint while those in high mass galaxies were too bright. Such a large sample of SNe allowed the team to inspect the structure of the luminosity trend with host galaxy mass and to find a difference of 0.10 magnitudes that undergoes a sharp transition at a galaxy mass scale of about 1010 solar masses.
While previous work interpreted this SN Ia "host bias" as likely arising from metallicity effects, the SNfactory analysis inspected all physical properties which evolve along the galaxy mass sequence. Age, metallicity, and dust were all compared to the luminosity-mass trend, and the authors found SN Ia age showed the most consistency with the data. The authors also uncovered a tentative trend of SN Ia colour with host galaxy metallicity. These results illustrate the importance of studying SN Ia environments to fully quantify the environmental impact on SN Ia properties.
by M. Childress
Deviations from standardised brightness ("Hubble Residuals") for over 600 SNe Ia (binned as black points) as a function of host galaxy mass, showing the rapid "transition" region (framed by vertical dashed black lines). The data are compared to several physical or analytic models: age-dependent luminosity (thick solid blue) metallicity-dependent SN Ia colour (dashed green), host-mass-dependent dust extinction (dotted magenta), a linear fit (thick dashed orange), a sigmoid-function fit (thin solid red).