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.
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).
Acknowledgements
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.