A First Look at Cepheids in a Type Ia Supernova Host with JWST

We report the first look at extragalactic Cepheid variables with the James Webb Space Telescope (JWST), obtained from an archival observation of NGC 1365, host of SNIa 2012fr, a calibration path used to measure the Hubble constant. As expected, the high-resolution observations with NIRCam through F200W show better source separation from line-of-sight companions than Hubble Space Telescope (HST) images at similar near-infrared wavelengths, the spectral region that has been used to mitigate the impact of host dust on distance measurements. Using the standard star P330E as a zero-point and point-spread function reference, we photometered 31 previously known Cepheids in the JWST field, spanning $1.15\lt \mathrm{log}P\lt 1.75$ including 24 Cepheids in the longer-period interval of $1.35\lt \mathrm{log}P\lt 1.75$. We compared the resultant period–luminosity (P-L) relations to that of 49 Cepheids in the full period range including 38 in the longer-period range observed with WFC3/IR on HST and transformed to the JWST photometric system (F200W, Vega). The P-L relations measured are in good agreement, with intercepts (at $\mathrm{log}P=1$) of 25.74 ± 0.04 and 25.72 ± 0.05 for HST and JWST, respectively. Our baseline result comes from the longer-period, higher signal-to-noise ratio Cepheids where we find 25.75 ± 0.05 and 25.75 ± 0.06 mag for HST and JWST, respectively. We find good consistency between this first JWST measurement and HST, and no evidence that HST Cepheid photometry is "biased bright" at the ∼0.2 mag level needed to mitigate the Hubble tension, though comparisons from more SN hosts are warranted and anticipated. We expect future optimized JWST observations to surpass these in quality.