2.5. Surface Photometry and Galaxy Colors
Since the colors of stars are sensitive to both age and metallicity (as pointed out in Section 2.3 above) it is readily concluded that the colors of galaxies similarly depend on age and metallicity. The situation is a bit more complicated because in galaxies the colors represent composite stellar populations. A further complication is the ubiquitous presence of dust, which has a clumpy distribution mixed with the stars rather than a uniform screen. Nevertheless, the analysis of galaxy colors could provide a useful means of studying averaged ages and metallicities for stellar populations in very large samples of galaxies covering a wide range of redshifts, particularly galaxies that are too faint for spectroscopy.
As with stars, of course, a difficulty with using colors is that variations in age and metallicity cause similar variations in galaxy colors. This is especially true for optical colors, which have been known for some time to be almost completely degenerate with regard to variations in age and metallicity (see Figure 5). This degeneracy can be broken to a large extent by including IR photometry, particularly K-band surface photometry, as illustrated in Figure 6 from Bell & de Jong (2000). Bell & de Jong have exploited this property to derive luminosity-weighted mean ages and metallicities for a sample of low-inclination disk galaxies. Note that the luminosity weighting means that the derived properties do not represent the present-day metallicities, as the emission-line measurements do.
Figure 5. Synthetic B-R vs. B-V colors for disk galaxies based on population synthesis models. The colors are from Bruzual & Charlot models with an exponentially decaying star formation rate with timescale ranging from 0 Gyr to infinity and metallicity Z ranging from 0.0001 to 0.05; see Bell & de Jong (2000) for details of the models. Solid lines connect models of constant Z, while dashed lines connect models of constant . All galaxies start forming stars 12 Gyr ago. The effects of interstellar reddening by foreground screen with AV = 0.3 magnitude is shown by the vector in the upper left corner. Diagram courtesy of Eric Bell.
Figure 6. Synthetic B-R vs. R-K colors for disk galaxies from Bell & de Jong (2000); see this paper for complete details. Here the four panels show results for galaxies in different ranges of K-band absolute magnitude. The labels on the model grid are the same as in Figure 5; note that the bottom right panel shows the mean age <A> rather than the star formation timescale . The solid broken lines in each plot represent observed surface colors for disk galaxies in the Bell & de Jong sample; the attached open or filled circles are the central colors for each galaxy. The symbols in the bottom right of each panel show the calibration and sky subtraction error bars for the inner and outer annuli of a galaxy. Diagram courtesy of Eric Bell.
Note that because this method depends on synthesis models for the colors of the stellar population, it suffers the same limitations. The model colors depend on the star formation and chemical enrichment history of a given galaxy. At present very simple star formation histories are assumed: either an instantaneous burst or exponentially decaying continuous star formation (which approximates a constant star formation rate for very long decay timescales). These approximations may break down in cases of galaxies which have undergone multiple starbursts separated by long periods, or galaxies which have truncated star formation histories, possibly punctuated by starbursts as well. Dwarf galaxies, in particular, may not be well-reproduced by the synthesis models. Note also that the sensitivity of the color-color diagram decreases rapidly for very metal-poor or very old stellar populations.
Note also that the mean ages and metallicities of a given position are very sensitive to the dust correction, although the slopes of age and metallicity gradients are not. Finally, the uncertainties in photometry and sky subtraction grow as the surface brightness decreases, so inferred ages and metallicities become increasingly more uncertain in the outer parts of disks and in low surface brightness galaxies. In contrast, metallicity gradients derived from H II regions are more stable, as the luminosity of an H II region is largely independent of its position within a galaxy.