A lower fragmentation mass scale in high redshift galaxies and its implications on giant clumps: a systematic numerical study

TL;DR

This study uses numerical simulations to explore how various factors influence gas fragmentation into star-forming clumps in high-redshift galaxies, revealing that giant clumps are rarer and typically formed by mergers rather than in-situ fragmentation.

Contribution

It provides a systematic analysis of the effects of physics, galaxy properties, and resolution on clump formation, and introduces an analytical model to predict clump masses more accurately than traditional methods.

Findings

Giant clumps ($>10^8 M_{\u2299}$) are rare and mainly formed by mergers.

Clump sizes range from 100-400 pc, matching observations.

An analytical model predicts initial clump masses accurately, outperforming the Toomre mass estimate.

Abstract

We study the effect of sub-grid physics, galaxy mass, structural parameters and resolution on the fragmentation of gas-rich galaxy discs into massive star forming clumps. The initial conditions are set up with the aid of the ARGO cosmological hydrodynamical simulation. Blast-wave feedback does not suppress fragmentation, but reduces both the number of clumps and the duration of the unstable phase. Once formed, bound clumps cannot be destroyed by our feedback model. Widespread fragmentation is promoted by high gas fractions and low halo concentrations. Yet giant clumps $M > 10^8 M_{\odot}$ lasting several hundred Myr are rare and mainly produced by clump-clump mergers. They occur in massive discs with maximum rotational velocities $V_{max} > 250$ km/s at $z \sim 2$, at the high mass end of the observed galaxy population at those redshifts. The typical gaseous and stellar masses of clumps in all runs are in the range $\sim 10^7-10^8 M_{\odot}$ for galaxies with disc mass in the range $10^{10}-8\times 10^{10} M_{\odot}$. Clumps sizes are usually in the range $100-400$ pc, in agreement with recent clump observations in lensed high-z galaxies. \\ We argue that many of the giant clumps identified in observations are not due to in-situ fragmetation, or are the result of blending of smaller structures owing to insufficient resolution. Using an analytical model describing local collapse inside spiral arms, we can predict the characteristic gaseous masses of clumps at the onset of fragmentation ($\sim 3-5 \times 10^7 M_{\odot}$) quite accurately, while the conventional Toomre mass overestimates them. Due to their moderate masses, clumps which migrate to the centre have marginal effect on bulge growth.