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We present sensitive aperture synthesis observations of the nearby, late-typespiral galaxy NGC 6503, and produce HI maps of considerably higher quality thanprevious observations by van Moorsel & Wells (1985). We find that the velocityfield, while remarkably regular, contains clear evidence for irregularities.The HI is distributed over an area much larger than the optical image of thegalaxy, with spiral features in the outer parts and localized holes within theHI distribution. The absence of absorption towards the nearby quasar 1748+700yields an upper limit of 5 10^{17} cm^{-2} for the column density of cold HIgas along a line of sight which should intersect the disk at a radius of 29kpc. This suggests that the radial extent of the HI disk is not much largerthan that which we trace in HI emission (23 kpc). The observed HI distributionis inconsistent with models of a single thin or thick disk. Instead, the datarequire a model containing a thin disk plus a thicker low column-density HIlayer that rotates more slowly than the thin disk and that extends only toapproximately the optical radius. This suggests that the presence ofextra-planar gas in this galaxy is largely the result of star formation in thedisk rather than cold gas accretion. Improved techniques for interferometricimaging including multi-scale Clean that were used in this work are alsodescribed.

Aperture Synthesis Observations of the Nearby Spiral NGC 6503: Modeling the Thin and Thick HI Disks