A merger-driven scenario for clumpy galaxy formation in the epoch of reionization: Physical properties of clumps in the FirstLight simulation

Recent JWST observations with superb angular resolution have revealed the existence of clumpy galaxies at high redshift through the detection of rest-frame optical emission lines. We use the FirstLight simulation to study the properties of (sub-)galactic clumps that are bright in [OIII] 5007$\mathring{\mathrm{A}}$ line with flux greater than $\sim {10}^{-18}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}$, to be detected by JWST. For 62 simulated galaxies that have stellar masses of $(0.5-6)\times {10}^{10}{M}_{\odot }$ at $z=5$, we find clumps in 1828 snapshots in the redshift range $z=9.5-5.5$. The clumps are identified by the surface density of star formation rate. About one-tenth of the snapshots show the existence of clumpy systems with two or more components. Most of the clumps are formed by mergers and can be characterized by their ages; central clumps dominated by stellar populations older than 50 Myr, and off-centered clumps dominated by younger stellar populations with specific star formation rates of $\sim 50\mathrm{G}\mathrm{y}{\mathrm{r}}^{-1}$. The latter type of young clumps is formed from gas debris in the tidal tails of major mergers with baryonic mass ratios of $1\le q<4$. The merger-induced clumps are short-lived, and merge within a dynamical time of several tens million years. The number density of the clumpy systems is estimated to be $\sim {10}^{-5}\mathrm{c}\mathrm{M}\mathrm{p}{\mathrm{c}}^{-3}$, which is large enough to be detected in recent JWST surveys.