H I mapping of outer disks of galaxies - M33 and NGC 3344

We have mapped the neutral-hydrogen emission of two late-type galaxies, M33 and NGC 3344, using the Arecibo telescope. To study the detailed structure of the outer disks requires both good resolution and high sensitivity; the H I distribution in these galaxies is sufficiently extended relative to our beam size that we can meet both requirements. For M33, we present 166 spectra sampling the entire H I disk, and along one side of the major axis we examine the H I falloff down to column densities ~2 X 10^18^ cm^-2^. The H I disk extends out to twice the optical radius, and its kinematics are more complex than indicated by previous models of warping beyond the optical disk. In the outermost regions, the column density exhibits a secondary falloff, which is much steeper than the gradual decline seen just beyond the optical disk. The H I distribution also shows asymmetric gaps, and some evidence for a rising rotation curve. We have nearly complete coverage of the H I emission in NGC 3344 down to column densities ~2 X 10^19^ cm^-2^. The H I distribution in the isolated galaxy NGC 3344 shows a distinct outer ring rather than a continuous warp. A detailed analysis of the Arecibo flat feed is also given in the Appendix.