H II REGION METALLICITY DISTRIBUTION IN THE MILKY WAY DISK

Abstract

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