The Extinction Law in Type Ia Supernovae Hosts from Background Galaxy Measurements: Toward a 1% Determination of H 0

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Abstract In the most precise distance ladder determination of H 0 , the observed near-infrared fluxes of Cepheids are corrected for dust, assuming that the extinction law in large, star-forming spiral hosts of Type Ia supernovae (SNe Ia) is similar to the Milky Way’s average R V ≈ 3.1. Intriguingly, studies of SNe Ia often point to lower values for their hosts ( R V ≲ 2). To better resolve extragalactic extinction laws, we directly measure the wavelength-dependent absorption of background galaxies seen in the Hubble Space Telescope and JWST images (0.5–2.7 μ m). We take the following steps: (i) subtract foreground stars to measure accurate photometry of background galaxies with a tool, SPHOT ; (ii) measure their redshifts and spectroscopic features with Keck/DEIMOS; (iii) determine their intrinsic spectral energy distributions (SEDs) from empirical templates which match the observed absorption lines and breaks, and (iv) measure R V by fitting the extinction model to the template-observed difference in the SEDs. The above steps are tested with artificial datasets to ensure they accurately recover the input R V . We apply these steps to a first case, NGC 5584, an SN Ia host and a calibrator of H 0 . The estimated value, R V , N5584 = 3.4 9 − 0.47 + 0.65 ( stat ) ± 0.19 ( syst ) , is consistent with the Milky Way–like extinction law and is ≳4 σ away from R V = 2. This result shows no evidence for systematics in H 0 from extinction laws and indicates that dust-related systematics in SNe Ia are smaller than previously assumed, pushing the H 0 measurements toward ∼1% precision. We are now undertaking a statistical study of 5–10 SH0ES hosts to determine the distribution of host extinction laws.

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