The Discovery of Cepheids and a Distance to NGC 5128^*

We discuss a new distance to NGC 5128 (Centaurus A) based on Cepheid variables observed with the Hubble Space Telescope. Twelve F555W (V) and six F814W (I) epochs of cosmic-ray-split WFPC2 observations were obtained. A total of 56 bona fide Cepheids were discovered, with periods ranging from 5 to ~50 days; five of these are likely Population II Cepheids of the W Virginis class, associated with the bulge or halo of NGC 5128. Based on the period and V- and I-band luminosities of a subsample of 42 classical (Population I) Cepheids, and adopting an LMC distance modulus and extinction of 18.50 ± 0.10 mag and E(B - V) = 0.10 mag, respectively, the true reddening-corrected distance modulus to NGC 5128 is μ₀ = 27.67 ± 0.12 (random) ± 0.16 (systematic) mag, corresponding to a distance of 3.42 ± 0.18 (random) ± 0.25 (systematic) Mpc. The random uncertainty in the distance is dominated by the error on the assumed value for the ratio of total to selective absorption, R_V, in NGC 5128, and by the possible metallicity dependence of the Cepheid period-luminosity relation at V and I. This represents the first determination of a Cepheid distance to an early-type galaxy.