The MOSDEF Survey: No Significant Enhancement in Star Formation or Deficit in Metallicity in Merging Galaxy Pairs at 1.5<=z<=3.5

TL;DR

This study investigates 30 galaxy pairs at redshifts 1.4 to 3.8 to determine if early-stage mergers enhance star formation or reduce metallicity, finding no significant differences compared to isolated galaxies, contrary to low-redshift observations.

Contribution

First high-redshift spectroscopic analysis of galaxy pairs showing no significant star formation or metallicity differences from isolated galaxies.

Findings

No measurable star formation rate enhancement in galaxy pairs.

No metallicity deficit observed in pairs compared to controls.

Results suggest reduced impact of pre-coalescence mergers at high redshift.

Abstract

We study the properties of 30 spectroscopically-identified pairs of galaxies observed during the peak epoch of star formation in the universe. These systems are drawn from the MOSFIRE Deep Evolution Field (MOSDEF) Survey at $1.4 \leq z \leq 3.8$, and are interpreted as early-stage galaxy mergers. Galaxy pairs in our sample are identified as two objects whose spectra were collected on the same Keck/MOSFIRE spectroscopic slit. Accordingly, all pairs in the sample have projected separations $R_{{\rm proj}}\leq 60$ kpc. The velocity separation for pairs was required to be $\Delta v \leq 500 \mbox{ km s}^{-1}$, which is a standard threshold for defining interacting galaxy pairs at low redshift. Stellar mass ratios in our sample range from 1.1 to 550, with 12 ratios closer than or equal to 3:1, the common definition of a "major merger." Studies of merging pairs in the local universe indicate anenhancement in star-formation activity and deficit in gas-phase oxygen abundance relative to isolated galaxies of the same mass. We compare the MOSDEF pairs sample to a control sample of isolated galaxies at the same redshift, finding no measurable SFR enhancement or metallicity deficit at fixed stellar mass for the pairs sample. The lack of significant difference between the average properties of pairs and control samples appears in contrast to results from low-redshift studies, although the small sample size and lower signal-to-noise of the high-redshift data limit definitive conclusions on redshift evolution. These results are consistent with some theoretical works suggesting a reduced differential effect of pre-coalescence mergers on galaxy properties at high redshift -- specifically that pre-coalescence mergers do not drive strong starbursts.