2.5. The Extended Asymptotic Giant Branch (EAGB)
The temperature and colour of the EAGB stars in a galaxy are determined by the age and metallicity of the population they represent. However there remain a number of uncertainties in the comparison between the models and the data (e.g. Gallart et al. 1994; Lynds et al. 1998). It is very important that more work is done to calibrate these very bright indicators of past star formation events. In Figure 3 theoretical EAGB isochrones (from Bertelli et al. 1992) are overlaid on the HST CMD of the post-starburst BCD galaxy, VII Zw403, and we can see that a large population of EAGB stars is a bright indicator of a past high sfr, and the luminosity spread depends upon metallicity and the age of the sfr.
Figure 3. EAGB isochrones for metallicities, Z = 0.001 and Z = 0.004, are shown superposed on the observed CMD of VII Zw403. For each metallicity the isochrones are for populations of ages 1.3, 2, 3, and 5 Gyrs, with the youngest isochrone being the brightest. This shows the potential discriminant between the age and metallicity of older populations, if the models could be calibrated.