| Authors | Soria, Roberto; Mould, Jeremy R.; Watson, Alan M.; Gallagher, John S., III; Ballester, Gilda E.; Burrows, Christopher J.; Casertano, Stefano; Clarke, John T.; Crisp, David; Griffiths, Richard E.; Hester, J. Jeff; Hoessel, John G.; Holtzman, Jon A.; Scowen, Paul A.; Stapelfeldt, Karl R.; Trauger, John T.; Westphal, James A. |
| Abstract | We present a color-magnitude diagram of more than 10,000 stars in the halo of the galaxy NGC 5128 (Centaurus A), based on WFPC2 images through V-band and I-band filters. The position of the red giant branch (RGB) stars is compared with the loci of the red giant branch in six well- studied globular clusters and in the dwarf elliptical galaxy NGC 185; the tip of the RGB is signalled by an observed turn-up in the luminosity function at I ~ 24.1 +/- 0.1 mag; this yields a distance modulus (m - M)_0_ = 27.8 +/- 0.2 for NGC 5128 (i.e., a distance of 3.6 +/- 0.2 Mpc), in agreement with previous determinations based on the planetary nebulae luminosity function and on the surface brightness fluctuations technique. The presence of an intermediate-age stellar population (5 Gyr) is suggested by the luminosity function of the asymptotic giant branch stars, extending up to I = 22.6 mag (for V - I > 2) and M_bol_~-5 mag; however, the number of these stars constrains the intermediate-age stellar population in the halo of NGC 5128 to be less than ~10% of the total. The color distribution at constant I magnitude, albeit affected by the completeness level of our sample, strongly suggests a mean value of [Fe/H] > -0.9 dex, possibly similar to the value found in M31 and higher than that observed in NGC 185. Like the M31 halo, the halo of NGC 5128 exhibits a broad range of levels of chemical enrichment. |
