  We use HST/ACS to study the resolved stellar populations of the nearby,
nearly edge-on galaxy NGC4244 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 sub-halos. The latter
explanation is also in quantitative agreement with the small diffuse component
we see around the galaxy.
