Tracing the Evolution of Ultraluminous Infrared Galaxies into Radio Galaxies with multifrequency Radio Observations

Ultraluminous infrared galaxies (ULIRGs) are characterised by extremely large infrared luminosities ($L_{IR}>$10$^{12}$L$\odot$), large dust masses and vigorous star formation. Studies show that they represent the end stages of mergers of gas-rich spiral galaxies. Their luminosity can be due to both starburst activity and active galactic nuclei (AGN). We study a sample of 13 ULIRGs that have optically identified AGN characteristics with 1.28~GHz Giant Metrewave Radio Telescope (GMRT) observations. Our aim is to resolve any core-jet structures or nuclear extensions and hence examine whether the ULIRGs are evolving into radio loud ellipticals. GMRT observations show marginal extension for only one source. However, the integrated radio spectra of most of the ULIRGs in this sample, show characteristics that are similar to that of GPS/CSS/CSO/young radio sources. The estimated spectral ages are 0.4 to 8 Myr and indicate that they are young radio sources and possible progenitors of radio galaxies. Few ULIRGs in our sample, show double peaked optical emission lines. The possible reason for these double peaks are dual AGN or outflow. We conclude that although most ULIRGs do not show kpc scale extended radio emission associated with nuclear activity, their radio spectral energy distributions do show signatures of young radio galaxies.