Merger and ring galaxy formation rates at z≤ 2

We compare the observed merger rate of galaxies over cosmic time and the frequency of collisional ring galaxies (CRGs), with analytic models and halo merger and collision rates from a large cosmological simulation. In the Λ cold dark matter (ΛCDM) model, we find that the cosmic merger fraction does not evolve strongly between 0.2 ≤z≤ 2, implying that the observed decrease in the cosmic star formation rate since z∼ 1 might not be tied to a disappearing population of major mergers. Haloes hosting massive galaxies undergo on average ∼2 mergers from z≤ 2 up to present day, reflecting the late assembly time for the massive systems and the related down-sizing problem. The cosmic merger rate declines with redshift: at the present time, it is of a factor of 10 lower than at z∼ 2 in reasonable agreement with the current available data. The rate of CRG formation derived from the interactions between halo progenitors up to z= 2 is found to be a good tracer of the cosmic merger rate. In the ΛCDM model, the rate of CRGs as well as the merger rate do not scale as (1 +z)^m as suggested by previous models. Our predictions of cosmic merger and CRG rates may be applied to forthcoming surveys such as GOODS and zCOSMOS.