Engines of active galaxies
Despite decades of observations, a lot of the properties of the environment in cores of active galaxies is still a matter of debate. The two energy sources are active star formation (starburst; hereafter SB) and an accreting black hole (active galactic nucleus; hereafter AGN). These two can be distinguished by the radiation the emit: SBs emit most of their energy in the UV regime, whereas AGNs are strong in X-ray. However, the cores of active galaxies are usually highly obscured by dust and gas and most of the radiation from the central engine is reprocessed, so there is no direct way to determine the physical properties of the environment in the core of an active galaxy. Therefore, my research focuses on finding diagnostics in the reprocessed emission.
For this I use (sub-)mm line emission from the molecular gas. This has two advantages. First of all, due to the longer wave length, this emission suffers less from obscuration and will therefore most likely originate from a region closer to the core. Another advantage is that the molecular lines and their ratios are highly sensitive to the physical conditions of the gas. The relative abundances of different transitions of one molecule can tell a lot about the density and temperature of the gas. Similarly ratios of line strengths of different species are very sensitive to physical parameters like the (column) density of the gas and the harshness of the radiation field. Such line ratios also clearly show different behavior between gas radiated by a UV field (PDR, which in our case would correspond to SBs), gas dominated by X-rays (XDR, in our case AGNs) and gas that is heated by mechanical processes.
During my PhD I have collected a large amount of observations of molecular lines such as CO(1-0), HCN(1-0), HCO+(1-0), and HNC(1-0) towards nearby active galaxies (Baan et al., 2008). We have analysed these data using numerical models that calculate the thermal and chemical balance in PDRs and XDRs. It was found that about half of the galaxies in our sample actually could not be explained with either PDR or XRD models. In these systems the thermal balance and chemistry of the molecular ISM turns out to be dominated by mechanical heating (Loenen et al., 2008). In a following publication it is shown that these systems represent a later stage in the evolution of active galaxies when part of the molecular ISM is consumed by the ongoing star formation and feedback from supernovae heats the gas (Baan et al., 2010).
Currently, I am mostly working on data from the Herschel Space Observatory, that are part of the HerCULES and HEXGAL key programmes, which both study the ISM in nearby galaxies. In both programs the excitation of CO plays an important role (e.g., Loenen et al., 2010), but it was found that water and related species are also very prominent in these systems (e.g., van der Werf et al., 2010 and Gonzalez-Alfonso et al., 2010). This prompted us to look for water in a high redshift galaxy. We not only found it, but found that it has unusual excitation properties. Therefore, the paper (van der Werf et al., 2011) was accompanied with a press release.
Besides working on these two programmes, I continue with the research I started during my PhD, using observations of ground based telescopes, as well as theoretical modelling of the ISM (Meijerink et al., 2011).
