Several authors claim having detected a metallicity gradient in the stellar population of Sgr, the centre being more
metal-rich than the outer regions. This finding is
based on the redder morphology of the Red Giant Branch and the Horizontal Branch at the centre of Sgr relative to the outer regions
(Marconi et al. 1998; Bellazzini et al. 1999b; Alard 2001). If confirmed, this feature would
be in concordance with observations of other dwarf galaxies (e.g. Saviane et al. 2001 and references therein).We now question whether such a gradient is apparent in the RR Lyrae population.
Any metallicity gradient would translate into a gradient in the mean period of RRab stars.
Figure 9a presents the mean period of RRab stars in Sgr (solid line) as a function of distance from M54 projected onto the
main axis. Each bin size has been adapted in order to contain the
same number of RRab stars. A linear least square fit through
these points yields a slope of
.
From Eq. (2),
it results in a metallicity difference <0.1 dex between
the two extremities of the field. Clearly, there is no significant metallicity gradient in the RR Lyrae
population of Sgr. For comparison, we present in Fig. 9a the average period of Galactic RRab as a function of
latitude (dotted line), where the period dependence as a function of position is evident.
To test the homogeneity further, we divided the RRab catalogue into two subsamples around the median distance from M 54
and compared the period distributions (Fig. 9b). A KS test shows that the two histograms are drawn from the same
parent distribution with a probability of 82,
suggesting a similar horizontal branch morphology in the two subsamples.
Finally, in Fig. 9c we compare the spatial distributions of the 250
longest and shortest period RRab variables. The KS test yields a probability of 88
for the two histograms to be issued from the
same parent distribution. This definitely excludes any modification of the period distribution with position. Thus, unless a metallicity gradient
and the second parameter effect conspire to keep the
horizontal branch morphology constant over the field, the RR Lyrae population should be homogeneous in the main body of Sgr. Furthermore, if
the metallicity gradient was confirmed, this
may indicate that the RR Lyrae population is not associated to the prominent red horizontal branch which is apparent in the CMDs of Sgr.
Copyright ESO 2001