RESOLVING THE DISCREPANCY OF GALAXY MERGER FRACTION MEASUREMENTS AT z ∼ 0–3

Abstract

We measure the merger fraction of massive galaxies using the UltraVISTA/COSMOS Ks-band selected catalog, complemented with the deeper, higher resolution 3DHST+CANDELS catalog selected in the HST /WFC3 H -band, presenting the largest mass-complete photometric merger sample up to z ∼ 3. We find that selecting mergers using the H 160 -band flux ratio leads to an increasing merger fraction with redshift, while selecting mergers using the stellar mass ratio causes a diminishing redshift dependence. Defining major and minor mergers as having stellar mass ratios of 1:1 -4:1 and 4:1 -10:1 respectively, the results imply ∼1 major and 1 minor merger for an average massive (log(M ⋆ /M ⊙ ) 10.8) galaxy during z = 0.1 − 2.5. There may be an additional ∼ 0.5(0.3) major (minor) merger if we use the H-band flux ratio selection. The observed amount of major merging alone is sufficient to explain the observed number density evolution for the very massive (log(M ⋆ /M ⊙ ) 11.1) galaxies. We argue that these very massive galaxies can put on a maximum of 6% of stellar mass in addition to major and minor merging, so that their number density evolution remains consistent with observations. The observed number of major and minor mergers can increase the size of a massive quiescent galaxy by a factor of two at most. This amount of merging is enough to bring the compact quiescent galaxies formed at z > 2 to lie at 1σ below the mean of the stellar mass-size relation as measured in some works (e.g. Newman et al. 2012), but additional mechanisms are needed to fully explain the evolution, and to be consistent with works suggesting stronger evolution (e.g. van der Wel et al. 2014).

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

“…In Figure 5, we compare our lightcone-based Illustris merger fraction estimates to observational measurements by Man et al (2016). Here we use the same stellar mass (M * > 10 10.8 M ) and separation (10 < d/(kpc/h) < 30) criteria for the simulated points as Man et al (2016) used for the observed ones.…”

“…At z 1, we find severe cosmic variance owing to the small volume surveyed, and so we cannot make strong conclusions about how these lightconebased pair fractions evolve to z = 0. With larger simulations, we would expect to recover the expected decrease in the merger fraction from z = 1 to z = 0 (e.g., Lotz et al 2011;Man et al 2016). Figure 1 which merge by z = 0.…”

“…Here we use the same stellar mass (M * > 10 10.8 M ) and separation (10 < d/(kpc/h) < 30) criteria for the simulated points as Man et al (2016) used for the observed ones. We combine the observational data for both major and minor pairs, and show the same stellar mass ratio selection (Msec/Mpri > 0.1) for Illustris.…”

“…Observationally, Man et al (2016) used photometric redshifts, which have considerable uncertainties. While we used an identical redshift difference selection for our pairs (∆z < 0.2(1 + z)) at most redshifts, for simplicity we did not include redshift uncertainties in our lightcone catalogs.…”

“…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).…”