Colour pairs for constraining the age and metallicity of stellar populations

Abstract

Using a widely used stellar‐population synthesis model, we study the possibility of using pairs of AB system colours to break the well‐known stellar age–metallicity degeneracy and to give constraints on two luminosity‐weighted stellar‐population parameters (age and metallicity). We present the relative age and metallicity sensitivities of the AB system colours that relate to the u, B, g, V, r, R, i, I, z, J, H and K bands, and we quantify the ability of various colour pairs to break the age–metallicity degeneracy. Our results suggest that a few pairs of colours can be used to constrain the above two stellar‐population parameters. This will be very useful for exploring the stellar populations of distant galaxies. In detail, colour pairs [(r–K), (u–R)] and [(r–K), (u–r)] are shown to be the best pairs for estimating the luminosity‐weighted stellar ages and metallicities of galaxies. They can constrain two stellar‐population parameters on average with age uncertainties less than 3.89 Gyr and metallicity uncertainties less than 0.34 dex for typical colour uncertainties. The typical age uncertainties for young populations (age < 4.6 Gyr) and metal‐rich populations (Z≥ 0.001) are small (about 2.26 Gyr) while those for old populations (age ≥ 4.6 Gyr) and metal‐poor populations (Z < 0.001) are much larger (about 6.88 Gyr). However, the metallicity uncertainties for metal‐poor populations (about 0.0024) are much smaller than for other populations (about 0.015). Some other colour pairs can also possibly be used for constraining the two parameters. On the whole, the estimation of stellar‐population parameters is likely to be reliable only for early‐type galaxies with small colour errors and globular clusters, because such objects contain less dust. In fact, no galaxy is totally dust‐free and early‐type galaxies are also likely have some dust [e.g. E(B–V) ∼ 0.05], which can change the stellar ages by about 2.5 Gyr and metallicities (Z) by about 0.015. When we compare the photometric estimates with previous spectroscopic estimates, we find some differences, especially when comparing the stellar ages determined by two methods. The differences mainly result from the young populations of galaxies. Therefore, it is difficult to obtain the absolute values of stellar ages and metallicities, but the results are useful for obtaining some relative values. In addition, our results suggest that colours relating to both UBVRIJHK and ugriz magnitudes are much better than either UBVRIJHK 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 Survey and the Two‐Micron All‐Sky Survey can be estimated via photometry data.

Extending the baseline:Spitzermid-infrared photometry of globular cluster systems in the Centaurus A and Sombrero Galaxies

“…They concluded that the colour pairs, R − K versus u − R and r − K versus u − r , are most capable of partially breaking the age–metallicity degeneracy. The closest colour combination in the NGC 5128 GC catalogue to the preferred pair found in Li & Han (2008) is R ‐[3.6] versus U − R . Analysis in of Fig.…”

“…Li & Han (2008) investigated the relative sensitivities of all possible colour–colour pairs among the standard BVRIJHK ugriz bandpasses using the SSP of Bruzual & Charlot (2003). They concluded that the colour pairs, R − K versus u − R and r − K versus u − r , are most capable of partially breaking the age–metallicity degeneracy.…”

An isochrone data base and a rapid model for stellar population synthesis

“…V − K and I − K ) for bs‐SSPs. Note that ( B − V ) is sensitive to stellar age while ( V − K ) and ( I − K ) to metallicity (see Li & Han 2008).…”

Colors, Ages, and Metallicities of Galaxies in Six Nearby Galaxy Clusters

“…Recently Li & Han (2008) studied the ability of breaking the age-metallicity degeneracy using the stellar population synthesis model of Bruzual & Charlot (2003) (called BC03 hereafter) for various combinations of the Johnson-Cousins-Glass BV RIJHK system and the AB ugriz system. In their results all the best color pairs are combinations of the two systems, and the (B − V ) -(I − K) is the only pair composed of the Johnson-Cousins-Glass system.…”