Massive Close Pairs Measure Rapid Galaxy Assembly in Mergers at High
  Redshift
  We compare mass-selected close pairs at z > 1 with the intrinsic galaxy
merger rate in the Illustris Simulations. To do so, we construct three 140
arcmin^2 lightcone catalogs and measure pair fractions, finding that they
change little or decrease with increasing redshift at z > 1. Consistent with
current surveys, this trend requires a decrease in the merger-pair
observability time, roughly as (1 + z)^-2, in order to measure the merger rates
of the same galaxies. This implies that major mergers are more common at high
redshift than implied by the simplest arguments assuming a constant
observability time. Several effects contribute to this trend: (1) The fraction
of massive, major (4:1) pairs which merge by today increases weakly from ~0.5
at z=1 to ~0.8 at z=3. (2) The median time elapsed between an observed pair and
final remnant decreases by a factor of two from z~1 to z~3. (3) An increasing
specific star formation rate (sSFR) decreases the time during which common
stellar-mass based pair selection criteria could identify the mergers. The
average orbit of the pairs at observation time varies only weakly, suggesting
that the dynamical time is not varying enough to account by itself for the pair
fraction trends. Merging pairs reside in dense regions, having overdensity ~10
to ~100 times greater than the average massive galaxy. We forward model the
pairs to reconstruct the merger remnant production rate, showing that it is
consistent with a rapid increase in galaxy merger rates at z > 1.
