Consistent extinction model for type Ia supernovae in Cepheid-based calibration galaxies and its impact on <i>H</i>0

ABSTRACT 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 ($z\gt 0.03$) 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 ($M_{\star }\gt 10^{10}\, \mathrm{M}_{\odot }$) calibration galaxies. This can be traced back to the fact that for these galaxies, a low total-to-selective extinction coefficient ($R_{\rm B}\sim 3$) is assumed, while for the low stellar-mass analogues a more standard $R_{\rm B}\sim 4$ 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 $R_{\rm B}$ (with mean $R_{\rm B}$ 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 ($\Delta \mathrm{ BIC}=-11$) to the calibration data and results in a lower value of $H_{0}$. Our result is $H_{0}=70.5\pm 1$ km s$^{-1}$ Mpc$^{-1}$ implying a reduction of the Hubble constant tension from $5.2\sigma$ to $2.8\sigma$.