HST resolves stars in a tiny body falling on the dwarf galaxy DDO 68
  We present new Hubble Space Telescope (HST) imaging of a stream-like system
associated with the dwarf galaxy DDO 68, located in the Lynx-Cancer Void at a
distance of D$\sim$12.65 Mpc from us. The stream, previously identified in deep
Large Binocular Telescope images as a diffuse low surface brightness structure,
is resolved into individual stars in the F606W (broad V) and F814W ($\sim$I)
images acquired with the Wide Field Camera 3. The resulting V, I
color-magnitude diagram (CMD) of the resolved stars is dominated by old
(age$\gtrsim$1-2 Gyr) red giant branch (RGB) stars. From the observed RGB tip,
we conclude that the stream is at the same distance as DDO 68, confirming the
physical association with it. A synthetic CMD analysis indicates that the large
majority of the star formation activity in the stream occurred at epochs
earlier than $\sim$1 Gyr ago, and that the star formation at epochs more recent
than $\sim$500 Myr ago is compatible with zero. The total stellar mass of the
stream is $\sim10^{6} M_{\odot}$, about 1/100 of that of DDO~68. This is a
striking example of hierarchical merging in action at the dwarf galaxy scales.
