HII Region Metallicity Distribution in the Milky Way Disk
  The distribution of metals in the Galaxy provides important information about
galaxy formation and evolution. HII regions are the most luminous objects in
the Milky Way at mid-infrared to radio wavelengths and can be seen across the
entire Galactic disk. We used the NRAO Green Bank Telescope (GBT) to measure
radio recombination line and continuum emission in 81 Galactic HII regions. We
calculated LTE electron temperatures using these data. In thermal equilibrium
metal abundances are expected to set the nebular electron temperature with high
abundances producing low temperatures. Our HII region distribution covers a
large range of Galactocentric radius (5 to 22 kpc) and samples the Galactic
azimuth range 330 degree to 60 degree. Using our highest quality data (72
objects) we derived an O/H Galactocentric radial gradient of -0.0383 +/- 0.0074
dex/kpc. Combining these data with a similar survey made with the NRAO 140 Foot
telescope we get a radial gradient of -0.0446 +/- 0.0049 dex/kpc for this
larger sample of 133 nebulae. The data are well fit by a linear model and no
discontinuities are detected. Dividing our sample into three Galactic azimuth
regions produced significantly different radial gradients that range from -0.03
to -0.07 dex/kpc. These inhomogeneities suggest that metals are not well mixed
at a given radius. We stress the importance of homogeneous samples to reduce
the confusion of comparing data sets with different systematics. Galactic
chemical evolution models typically derive chemical evolution along only the
radial dimension with time. Future models should consider azimuthal evolution
as well.
