The Perfect Host: JWST Cepheid Observations in a Background-free Type Ia Supernova Host Confirm No Bias in Hubble-constant Measurements

Abstract Cycle 1 JWST observations of Cepheids in Type Ia supernova (SN Ia) hosts resolved their red-giant-dominated near-infrared backgrounds, sharply reducing crowding and showing that photometric bias in lower-resolution Hubble Space Telescope (HST) data does not account for the Hubble tension. We present cycle 2 JWST observations of >100 Cepheids in NGC 3447, a unique system that pushes this test to the limit by transitioning from low to no background contamination. NGC 3447, an SN Ia host at D ≈ 25 Mpc, is an interacting pair comprising (i) a spiral with mixed stellar populations, typical of H 0 calibrators, and (ii) a young, star-forming companion (NGC 3447A) devoid of old stars and hence stellar crowding—a rare “perfect host” for testing photometric bias. We detect ∼60 long-period Cepheids in each, enabling a “three-way comparison” across HST, JWST, and background-free conditions. We find no component-to-component offset ( σ < 0.03 mag; a calibration-independent test) and a 50% reduction in scatter to ∼0.12 mag in the background-free case, the tightest seen for any SN Ia host. Across cycles 1–2, we also measure Cepheids in all SH0ES hosts observed by JWST (19 hosts of 24 SNe Ia; >50% of the sample) and find no evidence of bias relative to HST photometry, including for the most crowded, distant hosts. These observations constitute the most rigorous test yet of Cepheid distances and provide strong evidence for their reliability. Combining JWST Cepheid measurements in 19 hosts (24 SNe Ia) with HST data (37 hosts, 42 SNe Ia) yields H 0 = 73.49 ± 0.93 km s −1 Mpc −1 . Including 35 TRGB-based calibrations (from HST and JWST) totals 55 SNe Ia and gives H 0 = 73.18 ± 0.88 km s −1 Mpc −1 –∼6 σ above the ΛCDM+cosmic microwave background expectation.