tokens: decrease by more than 25% with respect to an older adjacent bin. The choice of just five coarse age bins is motivated by the low number of stars in the CMD, the large photometric errors, and the reached magnitude limit of just ∼1.5 mag below the RGB tip. Indeed, from the existence of an RGB we know that stars older than 1–2 Gyr are present in S1, but we are not able to infer in great detail the SFH from the age of the universe to ∼2 Gyr ago. The best-fit solution from SFERA is presented in Figure 9 ; in the top panels, we show the empirical and the model CMDs in the form of Hess diagrams, i.e., density plots, and the residuals in units of Poisson uncertainties ((data−model)/). As visible from the residuals, the model reproduces quite well the observed CMD above F814W ∼ 28. The TP-AGB star region at F814W ∼ 26, F606W−F814W ≳ 1.2 is better populated than in the previous SSP-fit case, although a shortage of counts is still present in the synthetic CMD. However, we do not consider this partial discrepancy as a problem here, as it is known that this evolutionary phase is very difficult to model and still lacks proper calibration over the whole range of metallicity regimes. Figure 9. Results of the SFERA procedure for the SFH recovery in S1. Top panels: Hess diagrams of the observed CMD, of the best-fit model, and of the residuals. Middle panel: derived metallicity behavior as a function of look-back time. Bottom panel: derived star formation rate as a function of look-back time. The total formed stellar mass is indicated. The middle and bottom panels of Figure 9 show the resulting metallicity and star formation behaviors as a function of look-back time. The solution recovered by SFERA implies that the vast majority of the SF activity occurred at epochs earlier than ∼500 Myr ago; the peak of the activity is predicted to have occurred in the 2–6 Gyr bin, while the adjacent 500 Myr–2 Gyr and 6–13.7 Gyr intervals were characterized by a lower SF. As already expected from the CMD appearance in Figure 7 , the SF rate in S1 was compatible with zero at epochs more recent than ∼0.5 Gyr ago. At earlier epochs we are not able to infer the details of the SFH with the data in our hands; however, the total formed stellar mass of (1.27 ± 0.41) × 10 6 M ⊙ can be considered a quite robust result and is in reasonable agreement with the estimates from both the SSP-fitting and the integrated light in Annibali et al. ( 2016 ). Concerning metallicity, the best-fit solution implies a continuous increase in Z starting from ∼0.0006 at the oldest look-back times and reaching up to Z ∼ 0.006 (∼40% solar) at recent epochs. This value is significantly higher than the Z ∼ 0.0004 (∼2%) solar metallicity measured in DDO 68's H ii regions, and also much higher than expected for a ∼10 6 M ⊙ stellar mass galaxy. Admittedly, given the strong age–metallicity degeneracy in the RGB phase combined with the relatively large photometric errors of our data, we can not claim a robust
ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O B-Software O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model I-Model O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula O O O O O B-Formula I-Formula I-Formula B-Formula I-Formula I-Formula O O O O O O O B-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Software O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Software O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula O O O O O O O O O O O O B-Formula I-Formula I-Formula O B-EntityOfFutureInterest I-EntityOfFutureInterest O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O