Discovering how gas in the diffuse IGM transfers into galaxy disks has become one of the main observational challenges of the LCDM paradigm. Observations have yet to conclusively reveal this gas accretion and so cannot explain the high star-formation rates found in the local Universe. Modeling suggests that infalling gas, which is often misaligned with the angular momentum of the galaxy disk, will form a warped outer gaseous disk. However, such accretion events are not the only possible explanation of these warps. They may also form through gravitational torques, which could, for example, arise through tidal interactions or misalignments between the disk and dark-matter halo. Fortunately, resolved stellar populations in the gas warps of nearby galaxies can discriminate between the different mechanisms for forming warps. Specifically, the models differ in their distribution and metallicity of old stars. If we do observe the predicted stellar populations due to accretion, then this will directly reveal the ongoing gas fueling of galaxy disks. We thus propose to use ACS and WFC3 in parallel to study the resolved stellar populations in all nearby warped galaxies as selected from the largest sample of such edge-on systems. Using our previous experience with deep color-magnitude diagrams from HST, we will comprehensively explore age and metallicity distributions of stars both in and outside the warp. We will compare these distributions with simulations covering a similar mass range in order to study the underlying formation mechanisms. This unique method directly addresses the issue of sufficiently fuelling the current star formation rate observed in the local Universe. We propose to determine the stellar content of the warps of three galaxies. We will use ACS and WFC3 coordinated parallel observations in F606W and F814W to resolve stellar populations. Although F475W offers better color separation than F606W in order to distinguish ages, the sensitivity of the filter to red giant branch stars is over 0.6 mag worse (equivalent to three times the required exposure length). As our key observation is to see if the older stars follow the warp, we require F606W for its greater sensitivity in finding these redder stars. In order to measure metallicities from RGB colors, we need to reach an optimal S/N~8 at 1 mag below the tip of the RGB, which occurs at I=-4 mag. We have used the ETC and our GHOSTS experience to calculate the required exposure times for optimal PSF fit photometry based on the best available distances. Per orbit we will typically expose 1000s and 1400s in F606W and F814W with WFC3, and 1000s and 1200s with ACS, using a dither in each exposure for removal of cosmic rays and bad pixels. However, we specify Low Sky for NGC 3510, which even with the required Increased Scheduling Flexibility option, increases the S/N despite the shorter exposure allowing us to sufficiently sample the RGB. The ACS observations will reach ~0.2 mag deeper than the WFC3 indicated limit. By using ACS and WFC3 in parallel we double the area covered, and in all systems we are able to observe the warp on both sides of the galaxy simultaneously. POS-TARGs are used for dithering and are placed outside the aperture between the ACS and WFC3 apertures. This places the center of rotation of the primary and parallel fields between the cameras so that larger orient ranges can be used for scheduling. COSMOLOGY 12 false false 0.0 0.0 false false false SUPPORTED GALAXY DISK HALO SPIRAL This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 GALAXY DISK HALO SPIRAL This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 GALAXY DISK HALO SPIRAL This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. 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