THE MAJOR AND MINOR GALAXY MERGER RATES ATz< 1.5

Abstract

Calculating the galaxy merger rate requires both a census of galaxies identified as merger candidates, and a cosmologically-averaged 'observability' timescale T obs (z) for identifying galaxy mergers. While many have counted galaxy mergers using a variety of techniques, T obs (z) for these techniques have been poorly constrained. We address this problem by calibrating three merger rate estimators with a suite of hydrodynamic merger simulations and three galaxy formation models. We estimate T obs (z) for (1) close galaxy pairs with a range of projected separations, (2) the morphology indicator G − M 20 , and (3) the morphology indicator asymmetry A. Then we apply these timescales to the observed merger fractions at z < 1.5 from the recent literature. When our physically-motivated timescales are adopted, the observed galaxy merger rates become largely consistent. The remaining differences between the galaxy merger rates are explained by the differences in the range of mass-ratio measured by different techniques and differing parent galaxy selection. The major merger rate per unit comoving volume for samples selected with constant number density evolves much more strongly with redshift (∝ (1 + z) +3.0±1.1 ) than samples selected with constant stellar mass or passively evolving luminosity (∝ (1 + z) +0.1±0.4 ). We calculate the minor merger rate (1:4 < M sat /M primary 1:10) by subtracting the major merger rate from close pairs from the 'total' merger rate determined by G − M 20 . The implied minor merger rate is ∼ 3 times the major merger rate at z ∼ 0.7, and shows little evolution with redshift.

The rise and fall of stellar across the peak of cosmic star formation history: effects of mergers versus diffuse stellar mass acquisition

“…The sample is affected by mergers at an average rate around 1 − 2 × 10 −3 Gyr −1 h 3 Mpc −3 and 3 − 5 × 10 −4 Gyr −1 h 3 Mpc −3 for all mergers and major mergers respectively. Note that these values are completely consistent with observations by Lotz et al (2011) and in good agreement with the cumulative merger rates per galaxy derived from the Illustris simulation (Rodriguez-Gomez et al 2015) .…”

Massive close pairs measure rapid galaxy assembly in mergers at high redshift

“…Assuming a roughly constant observability timescale, i.e., the duration a merging pair is identified in a catalog, these observations of galaxy pairs matched well with expectations (e.g., Hopkins et al 2010;Lotz et al 2011;Robotham et al 2014). Moreover, different windows on the merger process, such as asymmetric morphologies (Conselice et al 2003) and post-starburst signatures (Snyder et al 2011) all generally agree with this picture, so long as we adopt the proper observability timescales.…”

“…While these results were originally somewhat uncertain, multiple groups have now reached agreement about this conclusion (e.g., Mantha et al 2016, Mantha et al in prep.). Other selections, such as late-stage morphological disturbance (e.g., Bluck et al 2011) and major flux-ratio pairs (e.g., Man et al 2016), show the expectedly increasing merger fraction at z 2, but these selections likely include a large number of minor mergers with stellar mass ratio less than 1 to 4 (e.g., Lotz et al 2011).…”

THE SFR–M* RELATION AND EMPIRICAL STAR FORMATION HISTORIES FROM ZFOURGE AT 0.5 < z < 4*

“…It is important to note that galaxy merging is not accounted for in the inferred SMFs, thus, at least part of this offset must be caused by this effect. However this would require between 25% and 65% of these galaxies to merge with a more massive galaxy per Gyr, which substantially exceeds current estimates of galaxy merger rates (e.g., Lotz et al 2011;Williams et al 2011;Leja et al 2015).…”