Consistent extinction model for type Ia supernovae in Cepheid-based calibration galaxies and its impact on H0

The most recent SH0ES measurement of the Hubble constant employs corrections of type Ia supernova magnitudes due to extinction in their host galaxies. These corrections are estimated using a probabilistic model which is trained on Hubble flow (z> 0.03) supernovae and extrapolated to the calibration galaxies (those with observed Cepheids), despite the fact that the latter are selected based on criteria favouring discy and dust-rich systems. We show that this standard approach underestimates the brightness of reddened supernovae in the high stellar-mass (M> 10¹⁰\, M) calibration galaxies. This can be traced back to the fact that for these galaxies, a low total-to-selective extinction coefficient (RB~ 3) is assumed, while for the low stellar-mass analogues a more standard RB~ 4 is adopted. We propose a minimalistic modification of the extinction model in the calibration galaxies in order to alleviate this systematic effect. The modification is twofold and it involves: (i) the same, Milky Way-like distribution of RB (with mean RB of 4.3 – consistent with the extinction curve used for colour corrections of the Cepheids – and scatter 0.4) and (ii) a modified shape of the $E(B-V)$ reddening distribution while keeping the same effective slope of the supernova peak magnitude–colour relation and the same mean $E(B-V)$ reddening as measured for supernovae in the Hubble flow. We show that this new approach yields a significantly better fit (Δ BIC=-11) to the calibration data and results in a lower value of H₀. Our result is H₀=70.5± 1 km s⁻¹ Mpc⁻¹ implying a reduction of the Hubble constant tension from 5.2σ to 2.8σ.