Program: GS-2006B-C-3

Although warps in the disks of galaxies have been known for decades, their origin and evolutionary role are unclear: are they disk material perturbed by tidal effects, or material of extragalactic origin? A prominent hypothesis is that they are signatures of accretion of cold gas from the halo. In that case, they would play a critical role in the lives of galaxies and be important to understanding how galaxies acquire their gas. This hypothesis can be tested by measuring the kinematics of warps to determine whether they are corotating at the rotational velocity of the galaxy (and thus likely simply perturbed disk material) or lagging behind (as would be expected of low-angular-momentum material in the process of accretion). This test requires kinematic measurements of gas in the outer disk with good spatial resolution. 21-cm radio observations, though a possible way, require enormous efforts to simultaneously achieve good kinematic and spatial resolution with sufficient sensitivity. Here, we propose an alternative route: Gas at sufficiently large scale heights in the warped outer disk is irradiated and partially ionized by the UV flux from star formation in the inner disk. It therefore emits H-alpha at a surface brightness ~10^-18 erg s^-1 cm^-2 arcsec^-2. This is well within the reach of optical spectroscopy with GMOS-S. We propose to target 6-12 edge-on spiral galaxies with a long-slit _offset_ from the major axis to intercept the outer-disk warps and measure their velocity within ~10%. Comparison to the rotational velocity of the galaxy will then show if the warp material is kinematically a part of the disk or not. Kinematic measurements of accreting gas may also potentially discriminate between traditional hot-mode accretion and the novel cold-mode accretion model.