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

Abstract

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 τ ∝ (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 stellarmass 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 δ ∼ 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.

The MUSEHubbleUltra Deep Field Survey

“…The fact that the fraction of major mergers remains constant or turns over beyond z ∼ 1 is in agreement with the prediction of recent cosmological simulations, such as Horizon-AGN (Kaviraj et al 2015), EAGLE (Qu et al 2017) and Illustris (Snyder et al 2017). It remains also an intriguing question, down to which galaxy masses mergers will play an important role.…”

“…For these predictions a combined power-law and exponential fitting function, f MM ∼ a(1 + z) b e −c(1+z) , was used (see Qu et al 2017 for details). Finally, the blue diamonds correspond to the major pair fraction for massive galaxies in the ILLUSTRIS simulation (Snyder et al 2017). …”

“…1). Estimates of the major merger fraction evolution with redshift are available from the HORIZON-AGN (Kaviraj et al 2015), EAGLE (Qu et al 2017), and Illustris (Snyder et al 2017) simulations. Table 1: Major merger fractions up to z ≈ 6 from the HDF-S, udf-10 and UDF-Mosaic combined analysis.…”

“…From the Illustris simulation, Snyder et al (2017) have created three synthetic light cone catalogues and measured pair fractions using a velocity criterion inspired by photometric redshift precision in deep surveys, i.e. ∆v max = 18000 km s −1 at z = 2.…”

“…which is so far unconstrained for very high-redshift and/or low-mass galaxies (see e.g. Snyder et al 2017). …”