Dynamical Properties of Molecular-forming Gas Clumps in Galaxies at the Epoch of Reionization

We study the properties of molecular-forming gas clumps (MGCs) at the epoch of reionization using cosmological zoom-in simulations. We identify MGCs in a $z\simeq 6$ prototypical galaxy ("Althæa") using an H₂ density-based clump finder. We compare their mass, size, velocity dispersion, gas surface density, and virial parameter (${\alpha }_{vir}$) to observations. In Althæa, the typical MGC mass and size are $M_{gas}\simeq {10}^{6.5}$ M _⊙ and $R\simeq 45\text{--}100$ pc, which are comparable to those found in nearby spirals and starburst galaxies. MGCs are highly supersonic and supported by turbulence, with rms velocity dispersions of ${\sigma }_{gas}\simeq$ 20–100 km s⁻¹ and pressure of $P/K_B\simeq {10}^{7.6}K$ ${cm}^{-3}$ (i.e., $>1000\times$ with respect to the Milky Way), similar to those found in nearby and z  ∼  2 gas-rich starburst galaxies. In addition, we perform stability analysis to understand the origin and dynamical properties of MGCs. We find that MGCs are globally stable in the main disk of Althæa. Densest regions where star formation is expected to take place in clouds and cores on even smaller scales instead have lower ${\alpha }_{vir}$ and Toomre Q values. Detailed studies of the star-forming gas dynamics at the epoch of reionization thus require a spatial resolution of ≲40 pc (≃$0\stackrel{\prime \prime }{.}01$), which is within reach with the Atacama Large (sub-)Millimeter Array and the Next Generation Very Large Array.