Red Giant Stars in NGC 5128

Abstract

I present a selection of results obtained from VLT FORS1 and ISAAC photometric monitoring of late-type giants in NGC 5128 (=Centaurus A). The combination of optical and near-IR photometry allows the full metallicity range of the stars on the upper red giant branch to be probed, owing to the combined low-metallicity sensitivity of the optical and high-metallicity sensitivity of the near-IR bands. The metallicity distribution covers a wide range with the mean value around [M/H] ~ 0.45 dex. The near-IR monitoring of the variable AGB stars allows insight to be gained into the age distribution. The period distribution of these long period variables indicates only about 10% contribution of the intermediate-age component (age ≲ 5 Gyr) to the predominantly old stellar halo. Among the brightest, large amplitude and long period variables, only very few have near-IR and optical colors consistent with carbon-rich giants.