Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452

We study the red giant populations of two dE galaxies, AM 1339-445 and AM 1343-452, with the aim of investigating the number and luminosity of any upper asymptotic giant branch (AGB) stars present. The galaxies are members of the Centaurus A group ($D \approx 3.8$ Mpc) and are classified as outlying ($R \approx 350$ kpc) satellites of Cen A. The analysis is based on near-IR photometry for individual red giant stars, derived from images obtained with ISAAC on the VLT. The photometry, along with optical data derived from WFPC2 images retrieved from the HST science archive, enable us to investigate the stellar populations of the dEs in the vicinity of the red giant branch (RGB) tip. In both systems we find stars above the RGB tip, which we interpret as intermediate-age upper-AGB stars. The presence of such stars is indicative of extended star formation in these dEs similar to that seen in many, but not all, dEs in the Local Group. For AM 1339-445, the brightest of the upper-AGB stars have $M_{\rm bol} \approx-4.5$ while those in AM 1343-452 have $M_{\rm bol} \approx -4.8$ mag. These luminosities suggest ages of approximately $6.5 \pm 1$ and $4 \pm 1$ Gyr as estimates for the epoch of the last episode of significant star formation in these systems. In both cases the number of upper-AGB stars suggests that ~15% of the total stellar population is in the form of intermediate-age stars, considerably less than is the case for outlying dE satellites of the Milky Way such as Fornax and Leo I.