  NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band
absolute magnitude of -18 and a prominent, massive, intermediate-age nucleus at
a distance from Earth of 3.8 megaparsecs. It is wreathed in an extraordinary
neutral hydrogen (H I) complex, which includes rings, shells and a
counter-rotating core, spanning 90 kiloparsecs. NGC 4449 is relatively
isolated, although an interaction with its nearest known companion-the galaxy
DDO 125, some 40 kpc to the south-has been proposed as being responsible for
the complexity of its HI structure. Here we report the presence of a dwarf
galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band
absolute magnitude of -13.4 and a half-light radius of 2.7 kpc, with a full
extent of around 8 kpc. It is in a transient stage of tidal disruption, similar
to that of the Sagittarius dwarf near the Milky Way. NGC 4449B exhibits a
striking S-shaped morphology that has been predicted for disrupting galaxies
but has hitherto been seen only in a dissolving globular cluster. We also
detect an additional arc or disk ripple embedded in a two-component stellar
halo, including a component extending twice as far as previously known, to
about 20 kpc from the galaxy's centre.
