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.
I have already investigated the IR continuum emitted by dust surrounding SBs for such diagnostics. It was found that the IR continuum provides a handle on the total energy budget of such a system, but no specifics on the physical conditions in the core. One of the problems of using IR for diagnostics is that the dust and gas surrounding the core has a high optical depth and therefore the mean free path of the IR photons is short. The radiation will therefore be reprocessed several times before escaping. This problem can be reduced by moving to longer wave lengths.
We are now using mm line emission from molecular gas for diagnostics. This has two advantages. First of all, due to it longer wave length, this emission will 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. Also, models show that ratios of line strengths of different species are very sensitive to the (column) density of the gas and the harshness of the radiation field. These models also clearly show different behavior between gas radiated by a UV field (PDR, which in our case would correspond to SBs) and gas dominated by X-rays (XDR, in our case AGNs).
We are combining the existing models to construct a model which will not only include both star formation and an AGN contribution, but also the reprocessing of the radiation coming from the core by dust and gas. With this model we will study the IR and mm properties of a large sample of active galaxies. Using the emission properties of this sample we want to determine the state of the nuclear ISM of these galaxies and investigate the differences and possible (evolutionary) connections between SB and AGN driven systems.
Publications
Loenen, A.F., Baan, W.A., & Spaans, M.
Diagnostics of active galaxies: Modeling IR properties of dusty cores of starburst
A&A 458, 89-100 (2006)
Loenen, A.F.,Baan, W.A., & Spaans, M.
Molecular properties of (U)LIRGs: CO, HCN, HNC and HCO+
IAU Symposium 242 (astro-ph:0709.3424)
Baan, W.A., Henkel, C., Loenen, A.F., Baudry, A. & Wiklind, T.
Dense gas in Ultra-Luminous FIR galaxies
A&A in press (astro-ph:0710.0141)
Talks
"Molecuar properties of active galaxies", contributed talk at IAU symposium 242 on astrophysical masers and their environments.
Posters
"Extragalactic Chemistry", presented at the FIR Workshop, 5-7 November 2007 at the Physikzentrum, Bad Honnef Germany.
