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Water and hydroxyl in luminous infrared galaxies: Spectroscopic observations and modelling

Many luminous infrared galaxies (LIRGs) contain compact obscured nuclei (CONs) where luminosities in excess of 10^9 Lsun emerge from inside dusty regions smaller than 100 pc in diameter. Due to the obscuring material in these regions, the nature of the nuclear power source cannot be determined using direct observations in the optical or IR. In addition, extreme column densities towards the central regions might render these objects heavily Compton-thick, blocking even X-rays originating in the nucleus. It is nevertheless important to reveal and understand the nature of the source behind the high luminosity as this may aid our understanding of the evolution of, and connection between, starbursts and active galactic nuclei (AGN). 

This thesis describes the ongoing observational and theoretical work to use far-infrared and submillimeter molecular lines to study the central regions of CONs. From excitation analysis, it is possible to construct models of the molecular lines that probe these regions. These models can then constrain the nature of the buried power source. 

Observations of the LIRG Zw 049.057, obtained with the Herschel Space Observatory, are presented. These observations reveal a rich spectrum of highly excited water (H2O) and hydroxyl (OH), species that couple very well to the intense far-infrared radiation field in the centre of this galaxy. The results of our radiative transfer modelling indicate that Zw 049.057 hosts an optically thick, dusty core with a surface brightness of 10^13-10^14 Lsun kpc^-2 and a column density of molecular hydrogen (H2) approaching 10^25 cm^-2. A surface brightness this high in the central parsecs of a galaxy is expected to arise from a compact starburst or an active galactic nucleus, neither of which can be ruled out based on these results.