NIHAO XIII: Clumpy discs or clumpy light in high redshift galaxies?

TL;DR

This study uses high-resolution simulations and mock observations to analyze the nature, formation, and evolution of clumpy structures in high-redshift disc galaxies, revealing that observed clumps are mainly due to star formation rather than stellar mass concentrations.

Contribution

First quantitative comparison of observed and simulated clumpy galaxy fractions over redshift using mock HST observations, highlighting the role of dust and star formation in clump appearance.

Findings

Clumpy light distribution is due to star formation, not stellar mass.

Dust attenuation affects the visibility of clumps.

Clumps disperse in about 100 Myr with minimal impact on bulge growth.

Abstract

Many massive star forming disc galaxies in the redshift range 3 to 0.5 are observed to have a clumpy morphology showing giant clumps of size $\sim$1 kpc and masses of about $10^7M_{\odot}$ to $10^{10} M_{\odot}$. The nature and fate of these giant clumps is still under debate. In this work we use 19 high-resolution simulations of disc galaxies from the NIHAO sample to study the formation and the evolution of clumps in the discs of high redshift galaxies. We use mock HST - CANDELS observations created with the radiative transfer code GRASIL-3D to carry out, for the first time, a quantitative comparison of the observed fraction of clumpy galaxies and its evolution with redshift with simulations. We find a good agreement between the observed clumpy fraction and the one of the NIHAO galaxies. We find that dust attenuation can suppress intrinsically bright clumps and enhance less luminous ones. In our galaxy sample we only find clumps in light (u-band) from young stars but not in stellar mass surface density maps. This means that the NIHAO sample does not show clumpy stellar discs but rather a clumpy light distribution originating from clumpy star formation events. The clumps found in the NIHAO sample match observed age/color gradients as a function of distance from the galaxy center but they show no sign of inward migration. Clumps in our simulations disperse on timescales of a about a hundred Myr and their contribution to bulge growth is negligible.