The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XIV. The Cepheids in NGC 1365

Abstract

We report the detection of Cepheid variable stars in the barred spiral galaxy NGC 1365, located in the Fornax cluster, using the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC2). Twelve V (F555W) and four I (F814W) epochs of observation were obtained. The two photometry packages ALLFRAME and DoPHOT were separately used to obtain profile-fitting photometry of all the stars in the HST field. The search for Cepheid variable stars resulted in a sample of 52 variables, with periods between 14 and 60 days, common to both data sets. ALLFRAME photometry and light curves of the Cepheids are presented. A subset of 34 Cepheids were selected on the basis of period, light curve shape, similar ALLFRAME and DoPHOT periods, color, and relative crowding, to fit the Cepheid period-luminosity relations in V and I for both ALLFRAME and DoPHOT. The measured distance modulus to NGC 1365 from the ALLFRAME photometry is 31.31 ± 0.20 (random) ± 0.18 (systematic) mag, corresponding to a distance of 18.3 ± 1.7 (random) + 1.6 (systematic) Mpc. The reddening is measured to be E(V - I) = 0.16 ± 0.08 mag. These values are in excellent agreement with those obtained using the DoPHOT photometry, namely a distance modulus of 31.26 ± 0.10 mag and a reddening of 0.15 ± 0.10 mag (internal errors only).