Merger and Ring Galaxy Formation Rates at z<=2

TL;DR

This study compares observed galaxy merger and collisional ring galaxy rates over cosmic time with models and simulations, revealing that merger fractions remain relatively constant while merger rates decline, providing insights into galaxy evolution.

Contribution

It introduces a detailed comparison of observed merger rates and CRG formation with LCDM model predictions, highlighting the non-evolution of merger fraction and the decline in merger rate since z~2.

Findings

Merger fraction remains roughly constant between z=0.2 and 2.

Merger rate declines by a factor of 10 from z=2 to present.

CRG formation rate closely traces the cosmic merger rate.

Abstract

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 Lambda cold dark matter (LCDM) model we find that the cosmic {\it merger fraction} does not evolve strongly between 0.2<z<2, implying that the observed decrease of the cosmic star formation rate since z~1 might not be tied to a disappearing population of major mergers. Halos 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 downsizing problem. The cosmic {\it merger rate} declines with redshift: at the present time it is 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 LCDM 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.