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

Abstract

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$\mathrm{\mathring{A}}$ line with flux greater than $\sim 10^{-18} \, {\rm erg\, s^{-1}\, cm^{-2}}$, 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 $\sim 50 \, {\rm Gyr^{-1}}$. The latter type of young clumps is formed from gas debris in the tidal tails of major mergers with baryonic mass ratios of $1 \leq q < 4$. 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 $\sim 10^{-5}\, {\rm cMpc^{-3}}$, which is large enough to be detected in recent JWST surveys.