We have obtained a robust detection of the I magnitude of the TRGB in a field located at ~ 15' from the center of M 33, near the galaxy minor axis. Adopting the median metallicity we derived from the same data and the calibration of MTRGBI as a function of the global metallicity ([M / H]) provided by Bellazzini et al. (2004), we have obtained a new estimate of the distance modulus of M 33, (m - M)0 = 24.64 ± 0.15. All the sources of uncertainty have been taken into account in the reported error bar.
In Fig. 10, our distance modulus is compared with previous estimates available in the literature. The large majority of the reported values are compatible, within the formal 1- errors, with our estimate. The only exceptions are provided by the oldest analysis of Cepheid variables, based on photographic plates (Madore et al., 1985; Christian & Schommer, 1987). If we exclude these two estimates as well as those by Argon et al. (1998) and Greenhill et al. (1993), that are affected by very large uncertainties, we obtain, from 19 independent estimates including ours, an average distance modulus (m - M)0 = 24.69 ± 0.15 (average ± standard deviation) in excellent agreement with our result. It is interesting to note that the standard error on the above average is just = / 19 = 0.03 mag.
Figure 10. Previous estimates of the distance modulus from the literature (open triangles) are compared with the estimate obtained in the present study (continuous vertical line). The dashed line encloses the uncertainty range of our estimate. The method adopted in the different estimate is indicated on the left part of the plot, the sources on the right part. HB = Horizontal Branch; PNLF = Planetary nebulae Luminosity Function; LPV = Long Period Variables.
The photometric metallicity distributions described in Sect. 3.3 indicate that (a) the observed RGB population in the outskirts of M 33 has a typical metallicity ([Fe / H]ZW - 1.0) that is intermediate between that of the halo of the Milky Way ([Fe / H]ZW - 1.5) and that of M 31 ([Fe / H]ZW - 0.6) (see Bellazzini et al., 2003, for discussion and references); (b) the MDs are quite similar everywhere, within the sampled regions, similar to the case of M 31 (Bellazzini et al., 2003).
In the radial range 10' R 20', the AGB and RGB stars have a similar radial distribution, much more extended than that of young MS stars which appear to decrease abruptly around R = 18'. The distribution of RGB stars is equally well fitted by an exponential law (but not the same that fits the distribution of MS stars) or by the same R1/4 law that fits the central bulge (Boulesteix et al., 1980). The density of RGB stars is observed to fall far below that predicted by both the adopted best-fit models at R ~ 27', but the actual break may occur anywhere between R = 20' and R = 27' (e.g. between 4 and ~ 5 exponential scalelenghts, in good agreement with the complete profile obtained by Ferguson et al. (2004) from the huge database presented in McConnachie et al. (2004); A. Ferguson, private communication). The above results may suggest that a weak old-halo component is indeed present also in M 33, but probably the point could be established only with an analysis of the stellar kinematics in the considered radial ranges. The possibility to interpret the observations of the stellar content at large distance from the center of the galaxy in terms of an unexpectedly metal rich halo or of a very extended old disc is (again) reminescent of the case of M 31 (see Bellazzini et al., 2003; Ferguson & Johnson, 2001, for discussions and references).
This research is partially supported by the Italian Ministero dell'Universitá e della Ricerca Scientifica (MURST) through the COFIN grant p. 2002028935-001, assigned to the project Distance and stellar populations in the galaxies of the Local Group. This work was supported by a fellowship (S.G.) from the Consorzio Nazionale Astronomia ed Astrofisica-CNAA and contributions from MIUR-COFIN. Part of the data analysis has been performed using software developed by P. Montegriffo at the INAF - Osservatorio Astronomico di Bologna. This research has made use of NASA's Astrophysics Data System Abstract Service. The kind assistance of the TNG staff during the observing run is also acknowledged.