Confirmation of a star formation bias in type IA supernova distances and its effect on the measurement of the Hubble constant

Abstract

Previously we used the Nearby Supernova Factory sample to show that Type Ia supernovae (SNe Ia) having locally star-forming environments are dimmer than SNe Ia having locally passive environments. Here we use the Constitution sample together with host galaxy data from GALEX to independently confirm that result. The effect is seen using both the SALT2 and MLCS2k2 lightcurve fitting and standardization methods, with brightness differences of 0.094 ± 0.037 mag for SALT2 and 0.155 ± 0.041 mag for MLCS2k2 with RV = 2.5. When combined with our previous measurement the effect is 0.094 ± 0.025 mag for SALT2. If the ratio of these local SN Ia environments changes with redshift or sample selection, this can lead to a bias in cosmological measurements. We explore this issue further, using as an example the direct measurement of H0. GALEX observations show that the SNe Ia having standardized absolute magnitudes calibrated via the Cepheid period-luminosity relation using the Hubble Space Telescope originate in predominately star-forming environments, whereas only 50% of the Hubble-flow comparison sample have locally star-forming environments. As a consequence, the H0 measurement using SNe Ia is currently overestimated. Correcting for this bias, we find a value of 70.6 ± 2.6 km s-1 Mpc-1 when using the LMC distance, Milky Way parallaxes, and the NGC 4258 megamaser as the Cepheid zero point, and 68.8 ± 3.3 km s-1 Mpc-1 when only using NGC 4258. Our correction brings the direct measurement of H0 within 1 σ of recent indirect measurements based on the cosmic microwave background power spectrum.