The narrow-gap semiconductor ${\mathrm{FeGa}}_3$ has attracted considerable attention due to its colossal thermoelectric Seebeck effect, intriguing magnetic anomaly, and quantum criticality. Although the quasiparticle excitations in the band gap have been probed and proposed to be directly associated with exotic characters of ${\mathrm{FeGa}}_3$, the physical origin of these in-gap states and their direct connection to either of these puzzling properties are under debate. Here, we report the discovery of the dispersive in-gap states in semiconducting ${\mathrm{FeGa}}_3$ through high-resolution angle-resolved photoemission spectroscopy. Combined with first-principles calculations and comprehensive analysis, the observed coherent in-gap states are tentatively identified as a manifestation of slightly off-stoichiometry induced ordered secondary phase, whose formula and exact lattice structure require further clarification. Notably, these dispersive in-gap states provide a consistent explanation for the exotic electronic and magnetic properties of ${\mathrm{FeGa}}_3$, establishing their origin in the previously overlooked secondary phase rather than in strong correlation effects or the trivial disorders.

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• Crystal defects
• Electronic structure
• Intermetallic semiconductors
• Narrow band gap systems
• Angle-resolved photoemission spectroscopy