Deep Extragalactic VIsible Legacy Survey (DEVILS): New robust merger rates at intermediate redshifts

Abstract

Mergers are fundamental to our understanding of the processes driving the evolution of the structure and morphology of galaxies, star formation, AGN activity, and the redistribution of stellar mass in the Universe. Determining the fraction and properties of mergers across cosmic time is critical to understanding the formation of the Universe we observe today. This fraction and its evolution also provide inputs and constraints for cosmological simulations, crucial for theoretical models of galaxy evolution. We present robust estimates of major close-pair fractions and merger rates at $0.2 < z < 0.9$ in the Deep Extragalactic VIsible Legacy Survey (DEVILS). We identify major mergers by selecting close-pairs with a projected spatial separation $r_{\mathrm{sep}} < 20$ h$^{-1}$ kpc and a radial velocity separation $v_{\mathrm{sep}} < 500$ km s$^{-1}$. For galaxies with stellar masses of log$_{10}$($M_\star$/$M_\odot$) = 10.66 $\pm$ 0.25 dex, we find a major close-pair fraction of $\approx 0.021$ at $0.2 < z < 0.34$ using a highly complete, unbiased spectroscopic sample. We extend these estimates to $0.2 < z < 0.9$ by combining the full probability distribution of redshifts for galaxies with high-quality spectroscopic, photometric, or grism measurements. Fitting a power-law $\gamma_{m} = A(1 + z)^m$, we find $A = 0.024 \pm 0.001$ and $m = 0.55 \pm 0.22$. Consistent with previous results, the shallow slope suggests weak redshift evolution in the merger fraction. When comparing with large hydrodynamical simulations, we also find consistent results. We convert close-pair fractions to merger rates using several literature prescriptions for merger timescales and provide all measurements for future studies.