On the environmental dependence of halo formation

A generic prediction of hierarchical gravitational clustering models is that the distribution of halo formation times should depend relatively strongly on halo mass, massive haloes forming more recently, and depend only weakly, if at all, on the large-scale environment of the haloes. We present a novel test of this assumption, which uses the statistics of weighted or `marked' correlations, which prove to be particularly well-suited to detecting and quantifying weak correlations with environment. We find that close pairs of haloes form at slightly higher redshifts than more widely separated halo pairs, suggesting that haloes in dense regions form at slightly earlier times than haloes of the same mass in less dense regions. The environmental trends we find are useful for models that relate the properties of galaxies to the formation histories of the haloes that surround them.