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