Evidence for merger-driven activity in the clustering of high-redshift quasars

Recently, a very large clustering length has been measured for quasars at a redshift of z similar to 4. In combination with the observed quasar luminosity function, we assess the implications of this clustering for the relationship between quasar luminosity and dark matter halo mass. Our analysis allows for non-linearity and finite scatter in the relation between quasar luminosity and halo mass, as well as a luminosity dependent quasar lifetime. The additional novel ingredient in our modelling is the allowance for an excess in the observed bias over the underlying halo bias owing to the merger driven nature of quasar activity. We find that the observations of clustering and luminosity function can be explained only if both of the following conditions hold: (i) the luminosity to halo mass ratio increases with halo mass; (ii) the observed clustering amplitude is in excess of that expected solely from halo bias. The latter result is statistically significant at the 99 per cent level. Taken together, the observations provide compelling evidence for merger driven quasar activity, with a black-hole growth that is limited by feedback. In difference from previous analyses, we show that there could be scatter in the luminosity-halo mass relation of up to 1 dex, and that quasar clustering cannot be used to estimate the quasar lifetime.