Why are only some supermassive black holes actively feeding as AGN?

At the heart of most, if not all, massive galaxies lies a supermassive black hole (SMBH; Ferrarese & Ford, 2005, Space Sci. Rev.116, 523). SMBHs grow through the self-regulated accretion of material, during which time they present as active galactic nuclei (AGN) and release enormous amounts of radiative and mechanical energy. This energy impacts both their host galaxy, by driving gaseous outflows, and their larger environment through heating the surrounding intergalactic or intracluster medium. The ubiquity of SMBHs, however, contrasts with the paucity of AGN. Less than 10% of massive galaxies are AGN (Martini et al., 2013, ApJ, 768, 1), meaning that SMBHs lie in a dormant phase for most of their lifetime. This presents a conundrum as most massive galaxies have a ready supply of hot or cold gas which could feed the SMBH. Given the link between SMBH and galaxy growth (Ferrarese & Ford, 2005), as well as the devastating impact of AGN feedback on galaxy evolution (Bower et al. 2012, MNRAS, 422, 2816), our lack of understanding of what triggers AGN is a key unresolved issue in galactic astrophysics.
The environment of AGN provides a fundamental test for a variety of AGN triggers (Haines et al., 2012; ApJ, 754,97). For example, if cold gas or galaxy mergers are required for nuclear activity then AGN should be distributed primarily in low mass groups and the filaments that feed into clusters. On the other hand, if nuclear activity is fuelled by hot gas we would expect AGN to be most prevalent in the core regions of groups and clusters. If AGN activity is a stochastic phenomenon, unrelated to environment, then we would expect the distribution of AGN to mirror that of star forming galaxies (their most common host galaxy). So far, the environment of AGN has been investigated in limited scope due to the expense of obtaining spectra and identifying AGN over wide areas. To solve this lack of data, the 1000-fibre-fed multi-object spectrograph (WEAVE) will be placed on the 4.2m William Herschel Telescope in La Palma in 2020. The forthcoming UK-led WEAVE-LOFAR (W-L) survey will use this instrument to gather more than a million spectra of active galaxies, including AGN. In this project you will use W-L to identify the precise environmental history of different types of AGN.