Author: Dijkstra et. al.
Link to the paper: http://arxiv.org/abs/0810.0014
The paper discusses the variation of the LW background required to dissociate the hydrogen molecules and prevent fragmentation of gas clouds into stars-thereby producing SMBH candidates.
They take into account the (i) the clustering of DM halos, (ii) Poisson fluctuations in the number of corresponding star forming galaxies, and (iii) scatter in the LW luminosity produced by halos. Although most of the haloes would be exposed to global mean value of the LW background, some haloes could be exposed to the high critical LW flux (Jcrit) that can prevent fragmentation.
This fraction, although low ~ 10e-8 to 10e-6 of all the haloes with Tvir>10e4 K, has an exponential dependance on the value of the flux; i.e. any small change in the value of J(crit) will exponentially affect the number of haloes that are exposed to this critical flux.