We present a statistically decontaminated Color Magnitude Diagram of a 1 \degr \times 1 \degr field in the core of the Sagittarius dSph galaxy .
Coupling this CMD with the most recent metallicity distributions obtained from high resolution spectroscopy we derive robust constraints on the mean age of the stellar population that dominates the galaxy ( Pop A ) .
Using three different sets of theoretical isochrones in the metallicity range -0.4 \leq [ M / H ] \leq - 0.7 and taking into consideration distance moduli in the range 16.90 \leq ( m - M ) _ { 0 } \leq 17.20 we find that the mean age of Pop A is larger than 5 Gyr , and the best-fit value is age = 8.0 \pm 1.5 Gyr .
Since Pop A provides the vast majority of the M giants that traces the tidal stream of Sgr dSph all over the sky , our estimate resolves the so called “ M giant conundrum ” ( Majewski et al .
2003 ) .
The time needed by the M giants that currently populates the stream to diffuse within the main body of Sgr and to reach the extremes of the tidal tails once torn apart from the parent galaxy ( \simeq 3 - 4 Gyr ) can be easily accommodated into the time lapsed since their birth ( \simeq 5.5 - 9.5 Gyr ) .