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

Dwarfs Gobbling Dwarfs: A Stellar Tidal Stream Around NGC 4449 and Hierarchical Galaxy Formation on Small Scales