Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Dávid Puskás, Sandro Tacchella, Charlotte Simmonds, Kevin Hainline, Francesco D’Eugenio
| Summary:
We present a comprehensive analysis of galaxy close-pair fractions and major
merger rates to evaluate the importance of mergers in the hierarchical growth
of galaxies over cosmic time. This study focuses on the previously poorly
understood redshift range of $z approx 3-9$ using JADES observations. Our
mass-complete sample includes primary galaxies with stellar masses of ${rm
log}(M_star/{rm M_odot}) = [8, 10]$, having major companions (mass ratio
$geq 1/4$) selected by $5-30$ pkpc projected separation and redshift proximity
criteria. Pair fractions are measured using a statistically robust method
incorporating photometric redshift posteriors and available spectroscopic data.
The pair fraction evolves steeply with redshift, peaking at $z sim 5-6$,
followed by a turnover, and shows dependence on the stellar mass: the pair
fraction peaks at later cosmic times (lower redshifts) for more massive
galaxies. Similarly, the derived galaxy major merger rate increases and
flattens beyond $z sim 6$ to $2-10~{rm Gyr^{-1}}$ per galaxy, showing a weak
scaling with stellar mass, driven by the evolution of the galaxy stellar mass
function. A comparison between the cumulative mass accretion from major mergers
and the mass assembled through star formation indicates that major mergers
contribute approximately $5-14%$ to the total mass growth over the studied
redshift range, which is in agreement with the ex-situ mass fraction estimated
from our simple numerical model. These results highlight that major mergers
contribute little to the direct stellar mass growth compared to in-situ star
formation but could still play an indirect role by driving star formation
itself.
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