2021MNRAS.504.2675H

We present the results of our multifrequency observations for 10 ultraluminous infrared galaxies (ULIRGs) made by the Karl G. Jansky Very Large Array at 1.4, 5.5, 9.0, and 14.0 GHz. Our sample is selected from ULIRGs whose active galactic nuclei (AGNs) are not found at optical wavelengths (∼70 per cent of the total ULIRGs), but whose presence is suggested by mid-infrared or submillimetre observations (>50 per cent of the non-AGN ULIRGs at optical wavelengths). The statistical properties of the targets are similar to those of the entire ULIRG sample, which implies that ULIRGs have common radiative processes regardless of the presence of optical AGNs, and thus AGNs might equally contribute to the radio emission of every ULIRG. Although their spectra are mainly explained by starbursts and/or merger activity, some individual sources suggest contributions from AGNs. IRAS 00188-0856, whose optical morphology is not disturbed, shows a large non-thermal fraction and a spectral break at high frequency, which can be explained by synchrotron ageing of non-thermal plasma emitted from AGNs. In addition, we find 100-kpc-scale extended emission associated with IRAS 01004-2237. The two-sided morphology and absence of extended X-ray emission suggest that this system is not induced by a merger in a cluster but originates from AGN activity.