The Stellar Populations in the Outer Regions of M33. I. Metallicity Distribution Function

We present deep CCD photometry in the VI passbands using the WIYN 3.5 m telescope of a field located approximately 20′ southeast of the center of M33; this field includes the region studied by Mould & Kristian in 1986. The color-magnitude diagram (CMD) extends to I ∼ 25 and shows a prominent red giant branch (RGB), along with significant numbers of asymptotic giant branch and young main-sequence stars. The red clump of core helium burning stars is also discernable near the limit of our CMD. The I-band apparent magnitude of the red giant branch tip implies a distance modulus of (m - M)I = 24.77 ± 0.06, which combined with an adopted reddening of E(V-I) = 0.06 ± 0.02 yields an absolute modulus of (m - M)0 = 24.69 ± 0.07 (867 ± 28 kpc) for M33. Over the range of deprojected radii covered by our field (from about 8.5 to about 12.5 kpc), we find a significant age gradient with an upper limit of ∼1 Gyr (∼0.25 Gyr kpc-1). Comparison of the RGB photometry with empirical giant branch sequences for Galactic globular clusters allows us to use the dereddened color of these stars to construct a metallicity distribution function (MDF). The primary peak in the MDF is at a metallicity of [Fe/H] ∼ -1.0 with a tail to lower abundances. The peak does show radial variation with a slope of Δ[Fe/H]/ΔRdeproj = -0.06 ± 0.01 dex kpc-1. This gradient is consistent with the variation seen in the inner disk regions of M33. As such, we conclude that the vast majority of stars in this field belong to the disk of M33, not the halo as previously thought.