Using resolved galaxies in Hubble Space Telescope images to measure absolute proper motions

We explore the possibility of using resolved background galaxies to measure accurate absolute proper motions with the Hubble Space Telescope. Because galaxies are not point sources, we cannot use stellar point-spread functions (PSFs) to measure their positions. Rather, we must develop a template for each galaxy that can be used to measure a consistent position for it in each exposure at each epoch. We find that there are enough measurable galaxies in a 1200-s exposure in F606W and F775W to define an astrometric reference frame to better than 0.02 WFC pixel (1 mas). There are, however, some limitations. For example, observations taken at different orientations may suffer from the fact that the PSF is asymmetric, which can induce apparent shifts in the positions of stars and galaxies when the field is rotated relative to the PSF. This is present at the 0.03-pixel level. Nonetheless, being able to measure absolute proper motions relative to background galaxies will open up some interesting possibilities for targeted observations and pencil-beam surveys. This investigation uses the Ultra Deep Field, but the concept could work for any deep field that has extragalactic sources and dithered observations.