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Galaxy mergers induce the accretion of gas and dust toward the central regionof the galaxy, leading to subsequent activities of star formation and activegalactic nuclei (AGNs). These processes serve as energy sources for theinfrared emission in ultraluminous infrared galaxies (ULIRGs). Recently, aselected number of ULIRGs were reported to display extended radio continuumemission on a scale of $\sim100$\,kpc related to AGNs, whose properties androle in merging systems still need to be better understood. This paper presentsthe results of multifrequency observations of the nearby ULIRGIRAS\,01004$-$2237, exhibiting 100-kpc scale continuum emission at radiowavelengths, conducted by the Very Long Baseline Array at 2.3 and 8.4\,GHz. Theabsence of extended X-ray emission in IRAS\,0100$-$2237 suggested an AGN originfor the radio emission, but further evidence was needed. At 8.4\,GHz, we detectcompact radio continuum emission with a brightness temperature exceeding$10^{7.2}$\,K in the nuclear region of the object. Conversely, no significantemission is observed at 2.3\,GHz, indicating its flat or inverted spectrum.Both synchrotron self-absorption and free-free absorption can explain thespectral feature of the source. Given the suggestion of AGN presence at otherwavelengths, this emission is a plausible candidate for the AGN core poweringthe extended radio emission.

VLBI Detection of a Potential Active Radio Source Driving 100-kpc Scale Emission in the Ultraluminous Infrared Galaxy IRAS\,01004$-$2237