We present the first results of an ongoing project to investigate thepresent-day chemical abundances of the extreme outer parts of galactic disks,as probed by the emission line spectra of a new sample of HII regions. Thegalaxies studied here, NGC628, NGC1058 and NGC6946, are all late-type spiralgalaxies, characterized by larger than average HI-to-optical sizes. Our deepHalpha images have revealed the existence of recent massive star formation,traced by HII regions, out to, and beyond, two optical radii in these galaxies(defined by the B-band 25th magnitude isophote). Optical spectra of thesenewly-discovered HII regions are used to investigate their densities,ionization parameters, extinctions and in particular their oxygen and nitrogenabundances. Our measurements reveal gas-phase abundances of O/H~10-15% of thesolar value, and N/O~20-25% of the solar value, at radii of 1.5-2 R25. Clearevidence also exists for diminished dust extinction (Av~0-0.2) at large radii.The combination of our measurements of outer disk HII region abundances withthose for inner disk HII regions published in the literature is a powerfulprobe of the shape of abundance gradients over unprecedented radial baselines.Within the limits of the current dataset, the radial abundance variations areconsistent with single log-linear relationships, although the derived slopescan often differ considerably from those found if only inner disk HII regionsare used to define the fit. Interestingly, both the mean level of enrichmentand the ratio of N/O measured in extreme outer galactic disks are similar tothose values measured in some high redshift damped Lyman-alpha absorbers,suggesting that outer disks at the present epoch are relatively unevolved.(abridged)
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