The distance of M 33 and the stellar population in its outskirts

Abstract

We present deep V, I photometry of two 9.4′ × 9.4′ field in the outer regions of the M 33 galaxy. We obtain a robust detection of the luminosity of the Red Giant Branch Tip (ITRGB = 20.72 ± 0.08) from which we derived a new estimate of the distance modulus of M 33, (m - M)0 = 24.64 ± 0.15, corresponding to a distance D = 847 ± 60 kpc. By comparison of the color and magnitude of the observed Red Giant Branch stars with ridge lines of template globular clusters we obtained the photometric metallicity distribution of the considered fields in three different metallicity scales. The derived metallicity distributions are very similar over a range of distances from the galactic center 10′ ≤ R ≤ 33′. and are characterized by a well defined peak at [M/H] ≃ -0.7 ([Fe/H] ≃ -1.0, in the Zinn & West scale) and a weak metal-poor tail reaching [M/H] ≲ -2.0. Our observations demonstrate that Red Giant Branch and Asymptotic Giant Branch stars have a radial distribution that is much more extended than the young MS stars associated with the star-forming disc.