Dwarf galaxies of the Local Group

1. Tucana
   1. From the magnitude of the RGB tip, the first accurate distance modulus of this galaxy has been determined ( ). This has confirmed that Tucana is an isolated dwarf spheroidal located almost at the border of the Local Group (870 kpc from our Galaxy).
   2. From the color of the red giant branch tip and by direct comparison with the giant branches of galactic globular clusters, a metallicity [Fe/H] dex has been estimated, and no clear indication for a metallicity spread has been found.
   3. It has been shown that the color-magnitude diagram indicates that Tucana has had a single star formation burst at the epoch of the Galactic globular cluster star formation.
   4. The luminosity profile, and the surface density profile of resolved stars, have been derived. Consequently, the structural parameters of Tucana have been found, from which it has been confirmed that Tucana participates to the general metallicity-surface brightness-absolute magnitude relations defined by the Galaxy and M31 dwarf spheroidal and dwarf elliptical companions.
   5. It has thus been shown that Tucana is a clear exception to the scenario suggested by van den Bergh (1994), suggesting that young or intermediate-age populations are preferentially found in dSph's far from the Milky Way. In the case of Tucana, it was speculated that galactic winds expelled the interstellar medium at early epochs and halted star formation, a process apparently not dependent on environment.
2. Phoenix
   1. The extended HB of Phoenix has been for the first time revealed at . This demonstrates the presence of a minority yet significant population older than 10 Gyr. It has also allowed for the first time an independent estimate of the distance to Phoenix, . The spatial distribution of blue HB stars is significantly more extended than that of red giant stars. It therefore appears that the early star formation episode occurred in Phoenix on a larger spatial scale that subsequent bursts.
   2. A mean metal abundance [Fe/H] has been obtained with a direct comparison of the upper part of the RGB with the fiducial sequences of template Galactic globular clusters. A careful analysis based on extensive artificial star tests has given a metallicity dispersion .
   3. Thanks to the wide photometric baseline, new information has been provided on the young stellar population in Phoenix. It has been found that the recent star formation episode started at least 0.6 Gyr ago. The blue stars which trace the most recent burst (  yr ago) are concentrated in clumps or ``associations'' near the galaxy center, with a spatial distribution elongated in a direction perpendicular to the major axis defined by the diffuse galaxy light, and slightly offset towards the HI cloud observed by Young & Lo (1997, ApJ 490, 710). It was suggested that the neutral gas could have been blown out by the recent burst, a possibility that should be further investigated when the hypothesis of a physical link receives support by measurements of the galaxy radial velocity.
3. Fornax
   1. Broadband , , CCD photometry of about 40 000 stars in four regions, down to , has been obtained, which represents the largest three-color data set available for this galaxy.
   2. 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, and many age-tracers have been investigated: the conspicuous young main sequence, the helium-burning phase of the young population, the extended upper AGB tail, the prominent red HB clump pertaining to the dominant intermediate-age population, and the extended HB of an old population.
   3. New CMD features has been detected: we show that blue HB stars may be present in the outer regions, and measure the luminosity of the AGB bump. These are examples 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.
   4. Based on precise detection of the tip of the RGB in a selected RGB sample, a corrected distance modulus has been measured. An independent estimate of the distance to Fornax is also obtained from the mean magnitude of old horizontal branch stars, yielding a distance modulus , in good agreement with the distance estimated from the red giant branch tip.
   5. It has been found that the color distribution of the red giant stars can be approximately described as the superposition of two populations. The dominant component, comprising of the red giant stars, consists of relatively metal-enriched intermediate-age stars ([Fe/H] ). Once the younger mean age of Fornax is taken into account, the best estimate for the mean abundance becomes [Fe/H] . The dominant intermediate-age component has been shown to have an intrinsic color dispersion corresponding to a relatively low abundance dispersion,  dex. In addition, 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, it has been shown 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.
   6. The above result has clarified 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 ( ). 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, then a major SF episode occurred (probably with a discontinuous rate) after several Gyr.
4. Leo I
   1. A wide-area search with the ESO New Technology Telescope lead us to the discovery of a significant old population. The color-magnitude diagram now reveals an extended horizontal branch, unambiguously indicating the presence of an old, metal-poor population in the outer regions of this galaxy.
   2. The presence of an old stellar population is now demonstrated in all of the Local Group dwarf spheroidals.
   3. We have found little evidence for a stellar population gradient, at least outside ( kpc), since the old horizontal branch stars are radially distributed as their more numerous intermediate-age helium-burning counterparts.
   4. The discovery of a definitely old population in the predominantly young dwarf spheroidal galaxy LeoI points to a sharply defined first epoch of star formation common to all of the Local Group dSph's as well as to the halo of the Milky Way.