| Abstract | We present B,V,I CCD photometry of about 40000 stars in four regions of the Fornax dwarf spheroidal galaxy down to V ~ 23.5, the largest three-color data set obtained for this galaxy until now. The resultant color-magnitude diagrams, based on a wide color baseline, show a variety of features tracing the history of star formation of this dwarf galaxy. One of the most distinctive features in our diagrams is the conspicuous young main sequence, indicating recent star formation until approximately 2*E8 yr ago. A plume of stars brighter than the red HB clump, with (B- I) ~ 0.5, trace the helium-burning phase of the young population. A comparison of the color and extension of this feature with model isochrones suggests a relatively metal-rich population ([Fe/H] ~ -0.7) with age 300-400 Myr. This represents an important constraint for understanding the chemical enrichment history of Fornax. An extended upper AGB tail and a prominent red HB clump sign the presence of the well-known dominant intermediate-age population with an age range 2-10 Gyr, for which we have estimated a mean age 5.4+/-1.7. About 0.2 mag below the red clump, an extended HB is indicative of an old population. We show that blue HB stars may be present in the outer regions. Together with previous detection of RR Lyrae, this provides evidence for a minority field population that is as old and metal-poor as that in the Fornax globular clusters. We have identified the AGB bump, a clustering of stars that occurs at the beginning of helium shell-burning evolution, at a luminosity M_V =~ -0.4. This is an example of the short-lived evolutionary phases that can be revealed in stellar populations using adequately large star data samples, whose measurements provide powerful tests of theoretical models. Based on precise detection of the tip of the RGB in a selected RGB sample, we measure a corrected distance modulus (m-M)_0 = 20.70 +/- 0.12. An independent estimate of the distance to Fornax was also obtained from the mean magnitude of old horizontal branch stars, yielding a distance modulus (m-M)_0=20.76+/-0.04, in good agreement with the distance estimated from the red giant branch tip and previous results. The large baseline of the (B-I) colors together with the size of the stellar sample allowed us to analyze in detail the color distribution of the red giant stars. We find that it can be approximately described as the superposition of two populations. The dominant component, comprising ~ 70% of the red giant stars, consists of relatively metal-enriched intermediate-age stars. Its mean metallicity is [Fe/H]=-1.39 +/- 0.15, based on a comparison of the fiducial locus of the bulk of the Fornax red giants with the homogeneous Galactic globular cluster set of Da Costa & Armandroff (\cite{daco+arma90}). Once the younger mean age of Fornax is taken into account, our best estimate for the mean abundance of the bulk of the galaxy is [Fe/H]~ -1.0 +/- 0.15. The dominant intermediate-age component has an intrinsic color dispersion sigma_0 (B-I) = 0.06 +/- 0.01 mag, corresponding to a relatively low abundance dispersion, sigma_ [Fe/H] = 0.12 +/- 0.02 dex. Further, there is a distinct small population of red giants on the blue side of the RGB. While these stars could be either old or young red giants, we show that their spatial distribution is consistent with the radial gradient of old horizontal branch stars, and completely different from that of the younger population. This unambiguously qualifies them as old and metal-poor. This result clarifies the nature of the red giant branch of Fornax, suggesting that its exceptional color width is due to the presence of two main populations yielding a large abundance range (-2.0 < [Fe/H] < -0.7). This evidence suggests a scenario in which the Fornax dSph started forming a stellar halo and its surrounding clusters together about 10-13 Gyr ago, followed by a major star formation epoch (probably with a discontinuous rate) after several Gyr. Based on data collected at the European Southern Observatory, La Silla, Chile, Proposal N. 56.A-0538 |
