Investigating the role of mergers in galaxy assembly in the early Universe (z > 5)

Abstract

Galaxy mergers play a crucial role in shaping the morphology, the star formation, and the mass growth of galaxies across cosmic time. While mergers have been extensively investigated in the local Universe, the evolution of their frequency and physical properties in the early Universe has yet to be fully understood. We investigate the role of mergers in a large spectroscopic sample of 1233 galaxies in the range 5<z<14 with good detection (S/N-pixel > 3) in JWST imaging, covering six different extragalactic fields. We identify mergers from rest-frame optical disturbances in F444W, using a combination of Gini, M-20, and Asymmetry parameters. We find a morphological merger fraction f_m that does not strongly evolve with redshift from z=0 to z ~ 8. The average f_m of our primary major merger condition (Gini+0.14xM-20 > 0.33, A>0.35) is ~ 5 %, which increases to ~13 % for major+minor merger tracers. Accounting for the evolving observability timescale of each tracer, we find that the merger rate is strongly increasing from z=1 to 7 by more than 1 dex, averaging ~ 2 merger/galaxy/Gyr at 5<z<10 for major mergers (in agreement with photometric pair studies), and a factor of 3 higher for minor+major mergers. We also perform SED modeling using available HST+JWST photometry to infer stellar masses and SFRs, using a non parametric star-formation history. We find that mergers at z > 5 have a significant impact, although significantly lower than at z<1, on the SFR of galaxies. When averaged over 10 Myr (comparable to the observability timescale of morphological disturbances), their SFRs are a factor of 1.7 higher than a mass and redshift matched sample of non-mergers, suggesting that mergers trigger new star-formation through short-lived powerful bursty episodes. Despite this, mergers contribute only by 5% - 10% to the mass build-up of galaxies in the redshift range explored.

Figures (12)

• Figure 1: Top: Histogram of the redshift distribution of the original spectroscopic sample selected from DJA with $z_{spec} > 3$ (gray line) and of the final selected sample after applying the S/N$_{\rm pixel}$ cut (red line). Bottom: Scatter plot highlighting the M$_{\rm UV}$ distribution as a function of redshift for the original spectroscopic sample and the final sample (gray and red circles, respectively). The median values of M$_{\rm UV}$ in increasing redshift bins are highlighted with black squares. The histogram of M$_{\rm UV}$ for the selected sample is also shown on the y right axis, with the median M$_{\rm UV}$ indicated by a horizontal dashed line.
• Figure 2: Histogram of the stellar mass distribution of the final selected, spectroscopic sample. The median stellar mass of the sample is highlighted with a vertical black shaded line.
• Figure 3: Diagram of the Asymmetry vs f(Gini,M$_{20}$) for our selected sample, where f(Gini,M$_{20}$) $=$ Gini $+ 0.14 \times$ M$_{20}$. The two merger criteria defined in Equation \ref{['eq:merger_criterion']} are shown with a vertical and horizontal green line. Non merger galaxies are highlighted with simple gray circles. On top of them, we show a black cross for Gini-M$_{20}$ mergers and a blue empty square for asymmetry mergers.
• Figure 4: Merger observability timescales $\tau_m$ in the redshift range $0$-$10$ that we assume in this work for the close pair method (from conselice22, shaded dark blue region) and for the golden merger sample (light blue region). The shaded region encompasses the $\tau_{obs}$ expected for mass ratios $\mu$ between $1$ and $4$.
• Figure 5: Merger fraction f$_m$ as a function of redshift for different merger identification methods : panel A) gold merger sample; panel B) asymmetry mergers ($A\geq 0.35$); panel C) mergers identified through the photometric pairs; panel D) Gini-M$_{20}$ mergers. In each panel, our data is compared to several works in the literature at lower and similar redshifts adopting a similar merger identification technique.