  We present a study of spatial variations in the metallicity of old red giant
branch stars in the Andromeda galaxy. Photometric metallicity estimates are
derived by interpolating isochrones for over seven million stars in the
Panchromatic Hubble Andromeda Treasury (PHAT) survey. This is the first
systematic study of stellar metallicities over the inner 20 kpc of Andromeda's
galactic disk. We see a clear metallicity gradient of $-0.020\pm0.004$ dex/kpc
from $\sim4-20$ kpc assuming a constant RGB age. This metallicity gradient is
derived after correcting for the effects of photometric bias and completeness
and dust extinction and is quite insensitive to these effects. The unknown age
gradient in M31's disk creates the dominant systematic uncertainty in our
derived metallicity gradient. However, spectroscopic analyses of galaxies
similar to M31 show that they typically have small age gradients that make this
systematic error comparable to the 1$\sigma$ error on our metallicity gradient
measurement. In addition to the metallicity gradient, we observe an asymmetric
local enhancement in metallicity at radii of 3-6 kpc that appears to be
associated with Andromeda's elongated bar. This same region also appears to
have an enhanced stellar density and velocity dispersion.

Astrophysics of Galaxies