The metallicity distribution in the outer halo of M33

We present the results of deep I- and V-band photometry of the halo stars near the southeast minor axis of the Local Group spiral galaxy M33. An (I, V - I) color-magnitude diagram distinctly reveals the red giant branch stars at the bright end of the color-magnitude diagram. A luminosity function in the I band that uses a background field to remove the contaminating effects of Galactic foreground stars and distant, unresolved galaxies reveals the presence of the tip of the red giant branch at (m - M)(I) = 20.70 +/- 0.10. Assuming an absolute magnitude of the tip of the red giant branch of M-I,M-TRGB = -4.1 +/- 0.1 and a foreground reddening of E(V-I) = 0.054 +/- 0.020, the distance modulus of M33 is determined to be (m - M)(0) = 24.72 +/- 0.14 (880 +/- 60 kpc). The metallicity distribution function derived by interpolating between evolutionary tracks of red giant branch models is dominated by a relatively metal-poor stellar population, with a mean metallicity of [M/H] = - 0.94 +/- 0.04 or [Fe/H] = -1.24. We fit a leaky-box chemical enrichment model to the halo data, which shows that the halo is well represented by a single-component model with an effective yield of y(eff) = 0.0024. The mean metallicity of a sample of globular clusters in the M33 halo ([Fe/H] = - 1.3 +/- 0.1) matches very closely the metallicity of the halo, but the cluster sample is too small to investigate other aspects of the metallicity distribution function of the clusters.