An infrared/submillimetre perspective on Active Galactic Nuclei

This thesis presents a Far Infrared / submillimeter view on local Active Galactic Nuclei (AGN). Two aspects are discussed: the potential to detect the molecular torus through high rotational transitions in CO, and the driving of molecular outflows. High-J CO lines arise from dense and warm regions, and could therefore trace the gas around AGN that is hold responsible for absorbing the X-rays from the AGN. Whether this is possible with lines as high as CO(40-39), was tested on the nearby Compton-thick AGN NGC 1068. A deep observation with the Photodetector Array Camera and Spectrometer (PACS) onboard the Herschel Space Observatory resulted in a non-detection of CO(40-39). A radiative transfer model of X-ray dominated regions clarifies the non-detection: with an X-ray input spectrum typical of AGN, the CO(40-39) line only reaches the detection limit at a column density N_H = 10^25 cm^-2. If any dust is present in the gas, it will absorb the CO(40-39) emission over such a large column density. Very high CO rotational lines are therefore not ideal tracers of AGN tori. The second topic concerns galaxy-scale molecular outflows in local Ultra Luminous Infrared Galaxies (ULIRGs). The 22 ULIRGs in our sample are gas-rich mergers with active star formation and AGN activity. Most of these sources have fast (> 1000 km/s) outflows as detected in OH 119 micrometer, thought to be AGN-driven based on their high velocity and their relation between outflow velocity and AGN luminosity. Here it is investigated whether these outflows are traced well through broad wings in the [CII] fine structure line at 158 micrometer. Fourteen out of 22 objects show broad wings in [CII]. Taking only those objects with an outflow detected in both [CII] and OH119, we find a trend between two outflow velocities, as well as between the outflow masses derived from both tracers. We conclude that [CII] is a good tracer of molecular outflows. This is particularly useful at high redshift, where other tracers like OH119 and CO(1-0) are harder to observe. IRAS F08572+3915 is one of the ULIRGs which has a particularly fast and massive molecular outflow. It is studied here in more detail, using high spatial resolution observations of CO(1-0) which were taken with NOEMA (formerly known as Plateau de Bure Interferometer). We find that the source hosts a biconical outflow, aligned with the kinematic major axis of the disk. Possibly the outflow is coplanar, stripping gas off the disk. Spatially resolved observations of Na I D, H alpha and warm H2 show that the atomic and ionized gas overlap with the molecular outflow. The CO observations also reveal a second outflow, at about 6 kpc from the galaxy. If this is an AGN-driven outflow as well, it would imply AGN variability over time-scales of a few megayears. The outflow is slowing down at a rate of ~300 km/s/kpc, perhaps as result of gravitational pull and the absence of a driving mechanism.