Observational constraints on the merger history of galaxies since $z\approx 6$: Probabilistic galaxy pair counts in the CANDELS fields

Abstract

Galaxy mergers are expected to have a significant role in the mass assembly of galaxies in the early Universe, but there are very few observational constraints on the merger history of galaxies at $z>2$. We present the first study of galaxy major mergers (mass ratios $>$ 1:4) in mass-selected samples out to $z\approx 6$. Using all five fields of the HST/CANDELS survey and a probabilistic pair count methodology that incorporates the full photometric redshift posteriors and corrections for stellar mass completeness, we measure galaxy pair-counts for projected separations between 5 and 30 kpc in stellar mass selected samples at $9.7<{\log }_{10}({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })<10.3$ and ${\log }_{10}({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })>10.3$. We find that the major merger pair fraction rises with redshift to $z\approx 6$ proportional to $(1+z{)}^m$, with $m=0.8\pm 0.2$ ($m=1.8\pm 0.2$) for ${\log }_{10}({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })>10.3$ ($9.7<{\log }_{10}({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })<10.3$). Investigating the pair fraction as a function of mass ratio between 1:20 and 1:1, we find no evidence for a strong evolution in the relative numbers of minor to major mergers out to $z<3$. Using evolving merger timescales we find that the merger rate per galaxy ($\mathcal{R}$) rises rapidly from $0.07\pm 0.01$ Gyr${}^{-1}$ at $z<1$ to $7.6\pm 2.7$ Gyr${}^{-1}$ at $z=6$ for galaxies at ${\log }_{10}({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })>10.3$. The corresponding co-moving major merger rate density remains roughly constant during this time, with rates of $\Gamma \approx 10^{-4}$ Gyr${}^{-1}$ Mpc${}^{-3}$. Based on the observed merger rates per galaxy, we infer specific mass accretion rates from major mergers that are comparable to the specific star-formation rates for the same mass galaxies at $z>3$ - observational evidence that mergers are as important a mechanism for building up mass at high redshift as in-situ star-formation.

36 pages, 17 figures. Accepted for publication in ApJ

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