  A candidate diffuse stellar substructure was previously reported in the halo
of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and
analyze this feature using a unique combination of deep integrated-light images
from the Black Bird 0.5-meter telescope, and high-resolution wide-field images
from the 8-meter Subaru telescope, which resolve the nebulosity into a stream
of red giant branch stars, and confirm its physical association with NGC 4449.
The properties of the stream imply a massive dwarf spheroidal progenitor, which
after complete disruption will deposit an amount of stellar mass that is
comparable to the existing stellar halo of the main galaxy. The ratio between
luminosity or stellar-mass between the two galaxies is ~1:50, while the
indirectly measured dynamical mass-ratio, when including dark matter, may be
~1:10-1:5. This system may thus represent a "stealth" merger, where an
infalling satellite galaxy is nearly undetectable by conventional means, yet
has a substantial dynamical influence on its host galaxy. This singular
discovery also suggests that satellite accretion can play a significant role in
building up the stellar halos of low-mass galaxies, and possibly in triggering
their starbursts.
