Clusters of Small Clumps Can Explain The Peculiar Properties of Giant Clumps in High-Redshift Galaxies

Abstract

Giant clumps are a characteristic feature of observed high-redshift disk galaxies. We propose that these kpc-sized clumps have a complex substructure and are the result of many smaller clumps self-organizing themselves into clump clusters (CC). This bottom-up scenario is in contrast to the common top-down view that these giant clumps form first and then sub fragment. Using a high resolution hydrodynamical simulation of an isolated, fragmented massive gas disk and mimicking the observations from Genzel et al. (2011) at $z \sim 2$, we find remarkable agreement in many details. The CCs appear as single entities of sizes $R_{\mathrm{HWHM}} \simeq 0.9-1.4$ kpc and masses $\sim 1.5-3 \times 10^9 \ \mathrm{M_{\odot}}$ representative of high-z observations. They are organized in a ring around the center of the galaxy. The origin of the observed clumps' high intrinsic velocity dispersion $\sigma_{\mathrm{intrinsic}} \simeq 50 - 100 \ \mathrm{km \ s^{-1}}$ is fully explained by the internal irregular motions of their substructure in our simulation. No additional energy input, e.g. via stellar feedback, is necessary. Furthermore, in agreement with observations, we find a small velocity gradient $V_{\mathrm{grad}} \simeq 8 - 27 \ \mathrm{km \ s^{-1} \ kpc^{-1}}$ along the CCs in the beam smeared velocity residual maps which corresponds to net prograde and retrograde rotation with respect to the rotation of the galactic disk. The CC scenario could have strong implications for the internal evolution, lifetimes and the migration timescales of the observed giant clumps, bulge growth and AGN activity, stellar feedback and the chemical enrichment history of galactic disks.