Aperture Synthesis Observations of the Nearby Spiral NGC 6503: Modeling
  the Thin and Thick HI Disks
  We present sensitive aperture synthesis observations of the nearby, late-type
spiral galaxy NGC 6503, and produce HI maps of considerably higher quality than
previous observations by van Moorsel & Wells (1985). We find that the velocity
field, while remarkably regular, contains clear evidence for irregularities.
The HI is distributed over an area much larger than the optical image of the
galaxy, with spiral features in the outer parts and localized holes within the
HI distribution. The absence of absorption towards the nearby quasar 1748+700
yields an upper limit of 5 10^{17} cm^{-2} for the column density of cold HI
gas along a line of sight which should intersect the disk at a radius of 29
kpc. This suggests that the radial extent of the HI disk is not much larger
than that which we trace in HI emission (23 kpc). The observed HI distribution
is inconsistent with models of a single thin or thick disk. Instead, the data
require a model containing a thin disk plus a thicker low column-density HI
layer that rotates more slowly than the thin disk and that extends only to
approximately the optical radius. This suggests that the presence of
extra-planar gas in this galaxy is largely the result of star formation in the
disk rather than cold gas accretion. Improved techniques for interferometric
imaging including multi-scale Clean that were used in this work are also
described.
