The Magnification of SN 1997ff, the Farthest Known Supernova

With a redshift of z ≈ 1.7, SN 1997ff is the most distant Type Ia supernova (SN Ia) discovered so far. This SN is close to several bright, z = 0.6-0.9 galaxies, and we consider the effects of lensing by those objects on the magnitude of SN 1997ff. We estimate their velocity dispersions using the Tully-Fisher and Faber-Jackson relations, corrected for evolution effects, and calculate, applying the multiple-plane lensing formalism, that SN 1997ff is magnified by 0.34 ± 0.12 mag. Due to the spatial configuration of the foreground galaxies, the shear from individual lenses partially cancels out, and the total distortion induced on the host galaxy is considerably smaller than that produced by a single lens having the same magnification. After correction for lensing, the revised distance to SN 1997ff is m-M = 45.49 ± 0.34 mag, which improves the agreement with the Ω_M = 0.35, Ω_Λ = 0.65 cosmology expected from lower redshift SNe Ia and which is inconsistent at the ~3 σ confidence level with a uniform gray dust model or a simple evolution model.