2.6. Distance, Extinction & Metallicity
The accurate interpretation of the indicators described above depends critically upon having reliable estimates for the distance, the extinction and the metallicity of a galaxy. Ideally we would also like to know if the extinction is patchy and on what scale, and what has been the evolution of the metallicity of the stellar population with time. These three basic parameters, in conjunction with observational errors and incompleteness make the most significant impact on the properties of the CMD and hence the final SFH model (Tolstoy et al. 1998). There are a number of difficulties in accurately determining these basic properties but they can be resolved with careful observation and analysis techniques.
The Distance is the most crucial parameter for accurate analysis of a CMD, partly because it can easily be wrong by many orders of magnitude (for example the young, red supergiants can be mistaken for the RGB, if the observed CMD isn't deep enough to confirm the identification, i.e. by detecting a RC or HB). If the distance to a galaxy is incorrect this will result in the assumed ages of the different populations being completely wrong. To be sure of the distance to small faint galaxies it is necessary to have a CMD which goes deep enough to extend below the unambiguous RC/HB strucutre (Tolstoy et al. 1998).
The Extinction, both internal to a galaxy and between us and a galaxy can affect the accurate analysis of a CMD. If the extinction is incorrectly determined it will have the same effect as a distance error, and hence effect the reliability of the SFH models (e.g. Gallagher et al. 1998).
Metallicity: When a galaxy makes stars, then the detritus of this process (e.g., from SN explosions and stellar winds) make it unlikely that the galaxy can avoid metallicity evolution altogether. However, there is no concrete observational evidence that this is true, although abundance ratios of different elements do give us model dependent suggestions (Pagel 1994). Determining the effect of metallicity evolution in a CMD is difficult. It is impossible to determine a unique model based solely on the RGB because of age-metallicity degeneracy, and if metallicity evolution is neglected then the best model for that galaxy will typically be younger than if metallicity evolution were included (e.g. Tolstoy et al. 1998).