tokens: contradicting the original hypothesis. A way to avoid this problem could be to look at the incidence of AGN activity as a function of environment, for example as was done by Arnold etal. ( 2009 ). With a sample of 10 groups and six clusters, they find that the fraction of AGN (with L 0.3-8keV 10 41 ergs 1 ) in groups is a factor of 2 higher than in clusters, although the result has marginal significance. Due to concerns about the differing morphological mix of galaxies in these different environments, they also looked at the AGN fraction in early-type galaxies, finding a similar result. Many other studies of the environment of X-ray selected AGN have used correlation analysis to assess the clustering bias on different scales as well as the typical halo mass. Recent examples include Gilli etal. ( 2009 ) who found a strong clustering signal for AGN with median z 0.98 and L 0.5-10keV 10 43.8 ergs 1 . The correlation length matched that of galaxies with stellar mass 10 10.5 M , and implied a typical halo mass of 10 12.4 M . Fanidakis etal. ( 2013 ) found that AGN (10 42 L 2-10keV [ergs 1 ] 10 44 ) typically reside in haloes of mass 10 13 M . DiPompeo etal. ( 2014 ) found similar halo masses for the infrared selected AGN they studied at z 1, with 10 13.3 M and 10 12.8 M depending on whether the AGN are obscured or not. These papers highlight that the luminosity, selection technique and redshift of an AGN population are also important factors in assessing environment. Indeed, Martini etal. ( 2013 ) show that at z 11.5 there may be a reversal in the incidence of AGN in clusters versus the field. More locally, using a large number of galaxies from the Sloan Digital Sky Survey (SDSS), with AGN identified via their [O iii ] line emission, Li etal. ( 2006 ) found little difference between the clustering of AGN and control galaxies on scales greater than 1Mpc, suggesting that the haloes of active and inactive galaxies have similar masses. Between 100kpc and 1Mpc AGN were more weakly clustered, a result these authors argued could be explained if AGN are preferentially located at the centres of their haloes. And at scales less than 70kpc, AGN appeared to be marginally more clustered. The importance of the local environment was also highlighted by Serber etal. ( 2006 ), who looked at quasars (identified by M i 22) in the SDSS, finding they are overdense on scales 100kpc, but have no difference compared to the general population at larger 1Mpc scales. This suggests that galaxygalaxy interactions might be important. However, from a snapshot survey of AGN (selected in the 0.33.5keV soft X-ray band, and identified as AGN using optical emission line widths and ratios), Schade, Boyle Letwasky ( 2000 ) emphasized the lack of evidence for strong interactions or merging activity. Indeed, Sabater, Best Heckman ( 2015 ) have argued that the availability of cold gas in the nuclear regions, rather than local galaxy density or galaxygalaxy interactions, is the key
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