Stellar populations across the NGC 4244 truncated galactic disk

We use the Hubble Space Telescope ACS to study the resolved stellar populations of the nearby, nearly edge-on galaxy NGC 4244 across its outer disk surface density break. The stellar photometry allows us to study the distribution of different stellar populations and reach very low equivalent surface brightnesses. We find that the break occurs at the same radius for young, intermediate-age, and old stars. The stellar density beyond the break drops sharply by a factor of at least 600 in 5 kpc. The break occurs at the same radius independent of height above the disk, but is sharpest in the midplane and nearly disappears at large heights. These results make it unlikely that truncations are caused by a star formation threshold alone: the threshold would have to keep the same radial position from less than 100 Myr to 10 Gyr ago, in spite of potential disturbances such as infall and redistribution of gas by internal processes. A dynamical interpretation of truncation formation is more likely, such as due to angular momentum redistribution by bars or density waves, or heating and stripping of stars caused by the bombardment of dark matter subhalos. The latter explanation is also in quantitative agreement with the small diffuse component we see around the galaxy.