Abstract
We perform a systematic study of the effect of sub-grid physics, resolution
and structural parameters on the fragmentation of gas-rich galaxy discs into
massive star forming clumps due to gravitational instability. We use the
state-of-the-art zoom-in cosmological hydrodynamical simulation ARGO (Fiacconi
et al. 2015) to set up the initial conditions of our models, and then carry out
26 high resolution controlled SPH simulations of high-z galaxies. We find that
when blast-wave feedback is included, the formation of long-lived,
gravitationally bound clumps is difficult, requiring disc gas fractions of at
least 50% and massive discs, which should have $V_max > 200$ km/s at $z \sim
2$, more massive than the typical galaxies expected at those redshifts. Clumps
have typical masses $10^7 M_ødot$. Clumps with mass $10^8
M_ødot$ are rare, as they require clump-clump merging and sustained mass
accretion for a few orbital times, while normally clumps migrate inward and are
tidally disrupted on the way on shorter timescales. Clump sizes are in the
range $100-400$ pc. Giant clumps identified in observations ($10^8-10^9
M_ødot$) might either have a different origin, such as minor mergers and
clumpy gas accretion, or their sizes and masses may be overestimated due to
blending caused by insufficient resolution. Using an analytical model
originally developed to explain the fragmentation scale in gravitationally
unstable 3D protoplanetary discs, we can predict the characteristic masses of
clumps soon after fragmentation much better than using the conventional Toomre
mass. Due to their modest size, clumps have little effect on bulge growth as
they migrate to the center. In our unstable discs a small bulge can form
irrespective of the presence of long-lived clumps since it is triggered by
efficient gas inflows due to global non-axisymmetric instabilities.
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