The stellar populations of the Fornax dwarf spheroidal galaxy

We present B, V: I CCD photometry of about 40000 stars in four regions of t he Fornax dwarf spheroidal galaxy down to V similar to 23.5, the largest th ree-color data set obtained for this galaxy until now. The resultant color- magnitude diagrams, based on a wide color baseline, show a variety of featu res tracing the history of star formation of this dwarf galaxy. One of the most distinctive features in our diagrams is the conspicuous young main seq uence, indicating recent star formation until approximately 2 x 10(8) yr ag o. A plume of stars brighter than the red HE clump, with (B - I) similar to 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]similar to -0.7) with age 300-400 Myr. This represents an important constraint for understanding the chemical enrichment history of Fornax. An extended upper AGE tail and a prominent r ed HE clump sign the presence of the well-known dominant intermediate-age p opulation with an age range 2-10 Gyr, for which we have estimated a mean ag e 5.4 +/- 1.7. About 0.2 mag below the red clump, an extended HE is indicat ive of an old population. We show that blue HE 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 t hat in the Fornax globular clusters. We have identified the AGE bump, a clu stering of stars that occurs at the beginning of helium shell-burning evolu tion, at a luminosity M-V similar or equal to -0.4. This is an example of t he short-lived evolutionary phases that can be revealed in stellar populati ons using adequately large star data samples, whose measurements provide po werful 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 ind ependent 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 t he 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 anal yze in detail the color distribution of the red giant strars. We rind that it can be approximately described as the superposition of two populations. The dominant component, comprising similar to 70% of the red giant stars, c onsists of relatively metal-enriched intermediate-age stars. Its mean metal licity is [Fe/H]=-1.39 +/- 0.15, based on a compar- ison of the fiducial lo cus of the bulk or the Fornax red giants with the homogeneous Galactic glob ular cluster set of Da Costa & Armandroff ( 1990). Once the younger mean ag e of Fornax is taken into account, our best estimate for the mean abundance of the bulk of the galaxy is [Fe/H]approximate to -1.0+/-0.15. The dominan t intermediate-age component has an intrinsic color dispersion sigma(0)(B - I) = 0.00 +/- 0.01 mag, corresponding to a relatively low abundance disper sion, 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 co uld be either old or young red giants, we show that their spatial distribut ion is consistent with the radial gradient of old horizontal branch stars, and completely different from that of the younger population. This unambigu ously qualifies s them as old and metal-poor This result clarifies the natu re 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 abun dance range (-2.0 < [Fe/H] < -0.7). This evidence suggests a scenario in wh ich the Fornax dSph started forming a stellar halo and its surrounding clus ters together about 10-13 Gyr ago, Followed by a major star formation epoch (probably with a discontinuous rate) after several Gyr.