Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch
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abstract
Using the HST Wide Field Camera of the Advanced Camera for Surveys (ACS), we
have obtained deep (V,I) photometry of an outer-halo field in NGC 5128, to a
limiting magnitude of I~29. Our photometry directly reveals the core
helium-burning stellar population (the ``red clump'' or horizontal branch) in a
giant E/S0 galaxy for the first time.
The color-magnitude diagram displays a very wide red giant branch (RGB), an
asymptotic giant branch (AGB) bump, and the red clump; no noticeable population
of blue HB stars is present, confirming previous suggestions that old, very
metal-poor population is not ubiquitous in the halo of this galaxy. From the
upper RGB we derive the metallicity distribution, which we find to be very
broad and moderately metal-rich, with average [M/H]=-0.64 and dispersion 0.49
dex. The MDF is virtually identical to that found in other halo fields observed
previously with the HST, but with an enhanced metal-rich population which was
partially missed in the previous surveys due to V-band incompleteness for these
very red stars. Combining the metallicity sensitive colors of the RGB stars
with the metallicity and age sensitive features of the AGB bump and the red
clump, we infer the average age of the halo stars to be 8^{+3}_{-3.5} Gy.
As part of our study, we present an empirical calibration of the ACS F606W
and F814W filters to the standard V and I bands, achieved with ground-based
observations of the same field made from the EMMI camera of the New Technology
Telescope of the ESO La Silla Observatory.
