All Figures

	Figure 1: Random errors from numerical simulations in the measurement of the H indices defined by Vazdekis & Arimoto (1999), as a function of the mean signal-to-noise ratio per Å. The fit to the data points (full line) provides the error coefficients c(M) defined in Eq. (3). The dashed line indicates what should be the location of the data if the bandpasses defining those indices did not overlap.
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In the text

Figure 2: Panel a): H-Fe4668 diagram, with Bruzual & Charlot (2001) models over-plotted (dashed lines and dotted lines correspond to the predictions for constant age and metallicity, respectively; ages are labeled in Gyr, and metallicities are given as [Fe/H]). Dots correspond to 2000 simulations of a 12 Gyr old object with solar metallicity, assuming . Panel b): Errors in and for the same simulated data displayed in panel  a). Ellipses indicate the regions of 68.26, 95.44 and 99.73% probability, whereas the central error bars show the unbiased standard deviation in each axis. Panels c) and  d): Same than panels  a) and  b) after introducing the index instead of H, and using the same in the simulations. The index is plotted in an inverse scale to display the model predictions with roughly the same orientation in age and metallicity than in panel  a). Note also that the axis scales in panels  b) and  d) are different. See discussion in Sect. 4.2.

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In the text

	Figure 3: Three-dimensional representation of the values for all the intersecting points in the grid of models from Bruzual & Charlot (2001), for the H-Fe4668 diagram.
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In the text

Figure A.1: Relative errors in the D4000, , as a function of the signal-to-noise per Å, measured in the stellar library employed by Gorgas et al. (1999) to derive the empirical calibration of this spectral feature. See text in Appendix A for details.


In the text

Figure B.1: Random errors from numerical simulations in the measurement of three colors in the 131 stellar spectra from the library of Pickles (1998), as a function of the signal-to-noise ratio per Å. The full lines are the predictions of Eq. (B.16). See text for details.


In the text

Figure B.2: Comparison of the values as a function of the filter width, computed with Eqs. (B.13) and (B.20), respectively. The measurements have been performed using the response function of a set of 360 photometric bands collected from the literature (including those which are incorporated in the source code of the GISSEL models, and HST, KPNO and ESO filters, among others). The solid line is Eq. (B.21), a least square fit to the data, and the dashed line is the prediction of Eq. (B.19). The difference between both lines is due to the fact that filter response curves are not exactly box functions.


In the text

Figure C.1: Example of fit of local polynomials to describe the local behavior of line-strength indices as a function of physical parameters. Dashed and dotted lines are the predictions of Bruzual & Charlot (2001) models for single stellar populations of fixed age and metallicity, respectively (ages are given in Gyr, and metallicities as [Fe/H]). The thin solid lines indicates the resulting fit after using Eq. (C.1) with N=1, around the model predictions for a SSP of 12 Gyr and solar metallicity (the coefficients were determined using a least-squares fit to 5 points: the grid model point chosen as the origin of the local transformation, and the two closest points in both age and metallicity). It is clear that the fit can not be extrapolated very far from the central point. The thick solid lines show the result for N=2 (derived from the fit to the 9 points: the 5 points previously employed in the fit for N=1, plus the 4 additional corners of the region delineated by the thick line). In this case it is clear that the N=2 polynomial approximation provides a very reasonable representation of the geometric distortions when moving from the observational to the physical parameter space.


In the text