Colour Pairs for Constraining Stellar Populations

Using a widely used stellar population synthesis model, we study the ability of using pairs of AB system colours to break the well-known stellar age–metallicity degeneracy and give constraints on the two stellar-population parameters (age and metallicity). The relative age and metallicity sensitivities of AB system colours that relate to u, B, g, V , r, R, i, I, z, J , H , and K bands are presented, and the abilities of various colour pairs for breaking the age–metallicity degeneracy are quantified by a uncertainty parameter (UP ) method. Our results suggest that a few pairs of colours can be used to constrain the two above stellar-population parameters. This will be very useful for exploring the stellar populations of distant galaxies. In detail, colour pairs [(I − K), (u − R)] and [(I − K), (u − r)] are found to be the best two colour pairs for estimating stellar ages and metallicities. They can constrain the two stellar-population parameters with uncertainties smaller than 25% for typical uncertainty (6 0.03 mag) in colours. Some other colour pairs, such as [(g − J), (z − K)], [(B − K), (z − K)], [(R−K), (u−r)], [(R−K), (u−r)], [(u−K), (z−K)], and [(R−K), (u−R)], can possibly be used for constraining the two parameters, either. As a whole, our results suggest that colours relating to both UBV RIJHK and ugriz magnitudes are better than either UBVRIJHK colours or ugriz colours for breaking the well-known degeneracy. The results also show that the stellar ages and metallicities of galaxies observed by the Sloan Digital Sky Survery (SDSS) and the Two-Micron All-Sky Survey (2MASS) can be estimated via the photometry data.