We present deep CCD photometry in the VI passbands using the WIYN 3.5m telescope of a field located approximately 20 ’ southeast of the center of M33 ; this field includes the region studied by Mould & Kristian in their 1986 paper .
The color-magnitude diagram ( CMD ) extends to I \sim 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 \pm 0.06 , which combined with an adopted reddening of E ( V - I ) = 0.06 \pm 0.02 yields an absolute modulus of ( m - M ) _ { 0 } = 24.69 \pm 0.07 ( 867 \pm 28 kpc ) for M33 .
Over the range of deprojected radii covered by our field ( \sim 8.5 to \sim 12.5 kpc ) , we find a significant age gradient with an upper limit of \sim 1 Gyr ( \sim 0.25 Gyr/kpc ) .
Comparison of the RGB photometry to empirical giant branch sequences for Galactic globulars 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 ] \sim –1.0 with a tail to lower abundances .
The peak does show radial variation with a slope of \Delta [ Fe/H ] / \Delta R _ { deproj } = –0.06 \pm 0.01 dex/kpc .
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 .