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

TL;DR

This paper proposes that giant clumps in high-redshift galaxies are formed from smaller substructures called clump clusters, explaining their observed properties through high-resolution simulations without needing additional energy sources.

Contribution

It introduces a bottom-up formation scenario for giant clumps, supported by hydrodynamical simulations that replicate observed properties and internal motions of these structures.

Findings

Clump clusters appear as single entities with sizes 0.9-1.4 kpc and masses 1.5-3 billion solar masses.

Internal irregular motions explain high velocity dispersions without stellar feedback.

Simulated velocity gradients match observed prograde and retrograde rotations.

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.