Molecular observations of obscured galaxy nuclei
It is becoming evident that some (ultra)luminous infrared galaxies ((U)LIRGs) contain compact obscured nuclei (CONs) where luminosities in excess of 10^9 L☉ emerge from inside dusty regions smaller than 100 pc in diameter. Due to the obscured nature of these objects, their inner regions are hidden from examination using optical or infrared lines. In addition, extreme column densities (≳10^24 cm^-2) towards the central regions might render them heavily Compton-thick, blocking even X-rays originating in the nuclei. It is nevertheless important to reveal and understand the nature of the sources behind the high luminosities, as well as their connection to the host galaxies, as this may aid our understanding of galaxy evolution and the connection between starbursts and active galactic nuclei (AGN). This thesis describes observational efforts at far-infrared, (sub)millimeter, and radio wavelengths, with a focus on molecular line emission and absorption, combined with theoretical work, to investigate the central regions of CONs. Far-infrared and submillimeter observations of the LIRGs Zw 049.057 and Arp 299A, and the ULIRG IRAS 13120-5453 obtained with the Herschel Space Observatory as well as interferometric (sub)millimeter and radio observations of Zw 049.057 obtained with current state of the art interferometers are presented. Based on the Herschel observations, which targeted spectral lines of water (H2O) and hydroxyl (OH), we construct radiative transfer models in order to constrain the physical conditions in the three galaxies. In this way we find that both Zw~049.057 and Arp~299A host compact obscured nuclei, while IRAS 13120-5453 does not. It is however still difficult to determine the nature of the power sources in the two CONs, and high angular resolution observations of tracers that are able to probe highly enshrouded and compact regions are definitely required in order to do this. The Herschel observations also included signatures of inflowing gas in both Zw 049.057 and Arp 299A as well as a tentative signature of outflowing gas in Zw 049.057. The interferometric follow-up observations of Zw 049.057 further revealed a nuclear molecular outflow detected both in radio absorption lines of OH and (sub)millimeter emission lines of CO. We interpret this combination of in- and outflowing gas motions close to the nucleus as evidence that these objects are in a short transition phase, close to the onset of strong feedback from the nucleus. The study of CONs might thus be essential for our understanding of the global evolution of galaxies.