Reconciliation of the Surface Brightness Fluctuation and Type Ia Supernova Distance Scales

Abstract

We present Hubble Space Telescope measurements of surface brightness fluctuation (SBF) distances to early-type galaxies that have hosted Type Ia supernovae (SNe Ia). The agreement in the relative SBF and SN Ia multicolor light-curve shape and delta-m15 distances is excellent. There is no systematic scale error with distance, and previous work has shown that SBFs and SNe Ia give consistent ties to the Hubble flow. However, we confirm a systematic offset of ~0.25 mag in the distance zero points of the two methods, and we trace this offset to their respective Cepheid calibrations. SBFs have in the past been calibrated with Cepheid distances from the H0 Key Project team, while SNe Ia have been calibrated with Cepheid distances from the team composed of Sandage, Saha, and collaborators. When the two methods are calibrated in a consistent way, their distances are in superb agreement. Until the conflict over the long and short extragalactic Cepheid distances among many galaxies is resolved, we cannot definitively constrain the Hubble constant to better than ~10%, even leaving aside the additional uncertainty in the distance to the Large Magellanic Cloud, common to both Cepheid scales. However, recent theoretical SBF predictions from stellar population models favor the Key Project Cepheid scale, while the theoretical SN Ia calibration lies between the long and short scales. In addition, while the current SBF distance to M31/M32 is in good agreement with the RR Lyrae and red giant branch distances, calibrating SBFs with the longer Cepheid scale would introduce a 0.3 mag offset with respect to the RR Lyrae scale.