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We compare mass-selected close pairs at z > 1 with the intrinsic galaxymerger rate in the Illustris Simulations. To do so, we construct three 140arcmin^2 lightcone catalogs and measure pair fractions, finding that theychange little or decrease with increasing redshift at z > 1. Consistent withcurrent surveys, this trend requires a decrease in the merger-pairobservability time, roughly as (1 + z)^-2, in order to measure the merger ratesof the same galaxies. This implies that major mergers are more common at highredshift than implied by the simplest arguments assuming a constantobservability time. Several effects contribute to this trend: (1) The fractionof massive, major (4:1) pairs which merge by today increases weakly from ~0.5at z=1 to ~0.8 at z=3. (2) The median time elapsed between an observed pair andfinal remnant decreases by a factor of two from z~1 to z~3. (3) An increasingspecific star formation rate (sSFR) decreases the time during which commonstellar-mass based pair selection criteria could identify the mergers. Theaverage orbit of the pairs at observation time varies only weakly, suggestingthat the dynamical time is not varying enough to account by itself for the pairfraction trends. Merging pairs reside in dense regions, having overdensity ~10to ~100 times greater than the average massive galaxy. We forward model thepairs to reconstruct the merger remnant production rate, showing that it isconsistent with a rapid increase in galaxy merger rates at z > 1.

Massive Close Pairs Measure Rapid Galaxy Assembly in Mergers at High Redshift