Involved people at OAB: Bellazzini, Ferraro, Fusi Pecci.
Dwarf spheroidal galaxies (dSph) are thought to be the ``galactic remnants'' of the process of formation of major galaxies. They are the smallest known galactic systems and display the highest mass to light ratios known. Thus, the study of local dSph is of paramount importance to recover the past history of the Milky Way galaxy and for a better understanding of the whole process of evolution and organization of matter in the Universe.
The recently discovered Sgr dSph [Ibata, Gilmore & Irwin (1994)], in particular promises to be a keystone in the study of Galaxy formation, since (a) it is the nearest galactic satellite, (b) it is the most massive dSph known, (c) it has its own globular cluster system and (d) the process of disruption and final merger with the Milky Way is currently going on, in an advanced stage.
The study of the stellar content and of the globular cluster system of the Sgr
dSph has promptly started at the OAB [Montegriffo et al. (1998, MNRAS, 294,
315)] and achieved a major result with the completion of the Sagittarius Dwarf
Galaxy Survey [SGDS, Bellazzini, Ferraro & Buonanno (1999a,b)], the widest
photometric survey of the Sgr dSph. Three wide (
)
fields have been observed with the ESO-NTT, at 
, 
, 
from the
center of the galaxy along the major axis and the V,I photometry of 90000 stars
has been obtained, to 
. The resulting Color Magnitude Diagrams has
been statistically decontaminated by the strong Galactic foreground using the
photometry of an additional control field and, finally, important
constraint on the stellar
content, metallicity distribution and star formation history of the Sgr dSph
have been derived [see Bellazzini, Ferraro & Buonanno (1999b)].
In the dSph satellite system of the Milky Way there is just another galaxy that
can be compared with the Sgr dSph, i.e. the Fornax dSph that has a similar total
luminosity and is the only other dSph having a globular cluster system.
At odds with Sgr dSph, Fornax has evolved undisturbed, very far (
Kpc)
from the center of the Milky Way.
OAB researchers has been involved in the study of this galaxy, and in particular
of its globular clusters, since its pioneering phase [Buonanno et al. (1985)].
In the last two years has been completed the photometric study of the globulars
and the field of Fornax performed with HST, reaching for the first time the Turn
Off point of very old populations in this distant galaxy [see Buonanno et al.
(1998, ApJ, 501, 33) and Buonanno et al. (1999)]. The age of all
the Fornax globulars was accurately measured for the first time, and the star
formation history of the galaxy has been derived.
The most striking result from the above described studies on dSphs is perhaps that, despite the huge differences in size, environment, morphology and star formation history of the parent galaxies, the oldest globular clusters in the Sgr dSph, Fornax dSph, LMC [Testa, Ferraro, Brocato & Castellani (1995, MNRAS, 275, 454] and in the Milky Way all have the same age and similar heavy element abundance. This occurrence indicates that some fundamental step in the formation of galaxies has been simultaneous, at least in the local Universe.