  We report a new active asteroid in the main belt of asteroids between Mars
and Jupiter. Object (62412) 2000 SY178 exhibited a tail in images collected
during our survey for objects beyond the Kuiper Belt using the Dark Energy
Camera (DECam) on the CTIO 4 meter telescope. We obtained broad-band colors of
62412 at the Magellan telescope, which along with 62412's low albedo suggest it
is a C-type asteroid. 62412's orbital dynamics and color strongly correlate
with the Hygiea family in the outer main belt, making it the first active
asteroid known in this heavily populated family. We also find 62412 to have a
very short rotation period of 3.33+-0.01 hours from a double-peaked light curve
with a maximum peak-to-peak amplitude of 0.45+-0.01 magnitudes. We identify
62412 as the fastest known rotator of the Hygiea family and the nearby Themis
family of similar composition, which contains several known main belt comets.
The activity on 62412 was seen over 1 year after perihelion passage in its 5.6
year orbit. 62412 has the highest perihelion and one of the most circular
orbits known for any active asteroid. The observed activity is probably linked
to 62412's rapid rotation, which is near the critical period for break-up. The
fast spin rate may also change the shape and shift material around 62412's
surface, possibly exposing buried ice. Assuming 62412 is a strengthless rubble
pile, we find the density of 62412 to be around 1500 kg/m3.
