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The Cepheids of Centaurus A (NGC 5128) and Implications for H0

Majaess D.

Acta Astronomica

2010

Vol. 60, No. 2

121--136

A VI Wesenheit and period-color analysis based on new OGLE observations reaffirms Ferrarese et al. discovery of 5 type II Cepheids in NGC 5128. The distance to that comparatively unreddened population is d=3.8±0.4(σx) Mpc. The classical Cepheids in NGC 5128 are the most obscured in the extragalactic sample (n=30) surveyed, whereas groups of Cepheids tied to several SNe host galaxies feature negative reddenings. Adopting an anomalous extinction law for Cepheids in NGC 5128 owing to observations of SN 1986G (RV≅2.4) is not favored, granted SNe Ia may follow smaller RV. The distances to classical Cepheids in NGC 5128 exhibit a dependence on color and CCD chip, which may arise in part from photometric contamination. Applying a color cut to mitigate contamination yields d≅3.5 Mpc (V-I ≤1.3 mag), while the entire sample's mean is d≅3.1 Mpc. The distance was established via the latest VI Galactic Wesenheit functions that include the 10 HST calibrators, and which imply a shorter distance scale than Sandage et al. (2004) by ≥10% at P≅25 d. HST monitored classical Cepheids in NGC 5128, and the SNe hosts NGC 3021 and NGC 1309, follow a shallower VI Wesenheit slope than ground-based calibrations of the Milky Way, LMC, NGC 6822, SMC, and IC 1613. The discrepancy is unrelated to metallicity since the latter group share a common slope over a sizeable abundance baseline (α=-3.34±0.08(2σ), Δ[Fe/H]≅1). A negligible distance offset between OGLE classical Cepheids and RR Lyr variables in the LMC, SMC, and IC 1613 bolsters assertions that VI-based Wesenheit functions are relatively insensitive to chemical abundance. In sum, a metallicity effect (VI) is not the chief source of uncertainty associated with the Cepheid distance to NGC 5128 or the establishment of the Hubble constant, but rather it may be the admittedly challenging task of obtaining precise, commonly standardized, multiepoch, multiband, comparatively uncontaminated extragalactic Cepheid photometry.