Observing the effect of galaxy environment on the evolution of active galactic nuclei in the era of multiplexed wide-field fibre-optic spectroscopy

Gordon, Yjan Arthur

February 2018

Thesis or dissertation

© 2018 Yjan Arthur Gordon. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

In this thesis, we use the environment of active galactic nuclei (AGN) to test AGN unification. Furthermore, we analyse how interaction between the environment and host galaxy can influence AGN evolution. This is done for a range of galaxy densities, from the field to the cores of galaxy clusters.

We utilise the high spectroscopic completeness of the Galaxy And Mass Assembly survey (GAMA) to perform a comparative analysis of the pair-wise environments of broad- and narrow-line AGN. Our observations show no difference in the frequency of broad- and narrow line AGN in all but the tightest galaxy pairs. Furthermore, our observations show no difference in the colour or star-forming properties of the neighbouring galaxies of AGN, contrary to previous works. These observations are consistent with AGN unification and indicate that close galaxy interactions may increase nuclear obscuration.

Optically selected AGN preferentially inhabit infall regions of massive clusters, and are rarely found in the cluster core. Using the depth and completeness of GAMA, we optically select 451 AGN from 695 groups in the halo mass range 11.5 . log10(M200/Mʘ) . 14.5 at z < 0.15. At log10(M200/Mʘ) > 13.5 our results are similar to previous works on clusters with a deficit of AGN observed in the group core at 3.6! confidence. At lower halo masses no preferred location for AGN is seen.

Weakly accreting radio AGN are found in dense galactic environments, and minor-mergers have been invoked as a plausible mechanism to ‘drip-feed’ these low-excitation radio galaxies (LERGs). We use deep optical imaging from the Dark Energy Camera Legacy Survey (DECaLS) to search for low-surface brightness tidal remnants in a sample of 189 LERGs. We observe such tidal features in 27+3.5 −3.0 per cent of LERGs compared to 31.5+2.5 −2.3 per cent of a control sample. Our results are thus inconsistent with the hypothesis that minor-mergers play a significant role in LERG fuelling.

School of Mathematics and Physics, The University of Hull
Pimbblet, Kevin A.; Owers, Matt S.
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