HGAGN · Host galaxy effects on the observational properties and evolution of active galactic nuclei

Host galaxy effects on the observational properties and evolution of active galactic nuclei

The purpose of the program is the investigation of the observational signatures of AGN on the properties of their host galaxies, and the effect AGN have on the evolution of the host. The proposed technique to detect the signature of the AGN on the host is SED decomposition. The SED (spectral energy distribution) is the energy emitted in different bands of the electromagnetic spectrum and different physical processes have different SEDs. The processes we are interested in in this case are stellar processes which emit optical light usually obscured by dust, the AGN which emit optical light form the accretion disc and infrared light from the heated dust, and star formation which emits infrared light by heated dust. The infrared light from the AGN and the star-formation processes have different characteristic temperatures and their picks of emission happen at different wavelengths. After we identify the different processes forming the SEDs of AGN-host systems we can then investigate how they evolve with respect to each other. In order to do this we need large unbiased samples of AGN. It often happens that AGN are obscured in the bands that we usually detect them (X-rays), and in such cases the SED decomposition of large samples of extragalactic sources is a useful alternative method. The objective of the program is to both provide a tool to perform SED fitting and de-composition of AGN-host systems and apply it to a large number of extragalactic sources to discover hidden AGN, construct a large unbiased AGN sample with the host and AGN part de-composed, and use it to explore the possible co-evolution of the AGN and its host. * Most of the time was spent in creating the SED de-composition software. The java programming language was used for this purpose mainly because it is system-independent so that anyone can easily use the software. The software utilises SED templates provided by the user and performs a maximum-likelihood fit to the input photometry (again provided by the user). There is also the possibility to use prior information before calculating the likelihoods, such as an energy equilibrium between the flux absorbed by the dust from the stellar component and the flux emitted in the infrared, a prior knowledge of the fraction of light emitted by the AGN in the optical bands, and a prior knowledge of the star-formation rate. The user has control over much of the functionality of the program, like the types of output, the input SED templates, and the cosmological parameters used. The user interface is graphical to make it easier to use. The program is publicly available at: http://astro.dur.ac.uk/~erovilos/SEABASs. This is a website hosted by the Durham university, dedicated to the maintenance and distribution of the software. The documentation describing the methodology and the functionality of the software is also included, as well as examples and a description of the outputs. The software is already used by at least three groups other than the Astronomy group of Durham in three different European countries, with results to be published soon. * The software described above was used on a large number of extragalactic sources observed in the X-rays by XMM-Newton, in the optical by SDSS and in the mid-infrared by WISE, in order to identify possible heavily obscured AGN. A sample of 31 candidates was constructed based on their SED decomposition results and indeed most of them proved to be heavily obscured. The research paper describing this study is published in MNRAS. * The deepest X-ray and far-infrared surveys made with ESA's XMM-Newton and Herschel respectively were used to investigate the evolutionary connection between the AGN and their hosts. We have used a sample of a few hundred AGN and wealth of photometry information from the optical to the far-infrared bands and we used the SED de-composition software to differentiate between the different physical processes. We have found evidence of possible co-evolution of the host and the AGN at higher redshifts (z>1) and very weak evidence locally. The results of this study are published in Astronomy & Astrophysics and have been cited 25 times already. * The compilation of a large database based on SDSS spectroscopically identified galaxies observed with the WISE infrared telescope (~1.8 mil. sources) is built and and their optical-infrared SED are being fit with the software described above. When this will work will be completed it will provide the largest extragalactic mid-infrared SED resource and it will serve as the base for numerous projects.

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