Radio properties of ten nearby ultraluminous infrared galaxies with signatures of luminous buried active galactic nuclei

TL;DR

This study uses multifrequency radio observations to analyze the properties of 10 ULIRGs with buried AGN signatures, revealing common radiative processes and evidence of AGN contributions in some sources.

Contribution

First comprehensive radio analysis of ULIRGs with hidden AGNs, showing their similar radiative properties and identifying AGN-related features in specific cases.

Findings

ULIRGs have similar radiative processes regardless of optical AGN detection.

Some ULIRGs show signs of AGN activity through spectral features and extended emission.

Evidence of synchrotron ageing and large-scale AGN-related structures was found.

Abstract

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 ($\sim$70% of the entire ULIRGs), but whose presence is suggested by mid-infrared or submillimetre observations ($>50$% of the non-AGN ULIRGs at optical wavelengths). The statistical properties of the targets are similar to those of the entire ULIRG sample, which implies that ULIRGs have common radiative processes regardless of the presence of optical AGNs, and thus AGNs might equally contribute to the radio emission of every ULIRG. Although their spectra are mainly explained by starbursts and/or merger activity, some individual sources suggest contributions from AGNs. IRAS 00188$-$0856, whose optical morphology is not disturbed, shows a large nonthermal fraction and a spectral break at high frequency, which can be explained by synchrotron ageing of nonthermal plasma emitted from AGNs. In addition, we find 100-kpc scale extended emission associated with IRAS 01004$-$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.