Less than 10 percent of star formation in z=0.6 massive galaxies is triggered by major interactions

TL;DR

This study shows that major galaxy interactions at redshift 0.4-0.8 only modestly increase star formation rates and contribute less than 10% to the overall star formation, indicating other processes dominate galaxy growth.

Contribution

The paper quantifies the limited role of major mergers in triggering star formation at intermediate redshifts using combined observational data and correlation function analysis.

Findings

Major interactions increase SFR by about 1.8 times.

Less than 10% of star formation is directly triggered by major mergers.

Major mergers are not the primary driver of stellar mass build-up since z=1.

Abstract

Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of starformation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses and UV SFRs from COMBO-17, 24 micron SFRs from Spitzer and morphologies from two deep HST cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically-selected very close pairs (separation <2 arcsec) and merger remnants from the HST imaging. Our analysis confirms that the most intensely star-forming systems are indeed interacting or merging. Yet, for massive (M* > 10^10 Msun) star-forming galaxies at 0.4<z<0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios <1:4 and separations < 40 kpc) are only 1.80 +/- 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. We demonstrate that these results imply that <10% of star formation at 0.4 < z < 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z=1.