  We study the evolution of giant clumps in high-z disc galaxies using AMR
cosmological simulations at redshifts z=6-1. Our sample consists of 34
galaxies, of halo masses 10^{11}-10^{12}M_s at z=2, run with and without
radiation pressure (RP) feedback from young stars. While RP has little effect
on the sizes and global stability of discs, it reduces the amount of
star-forming gas by a factor of ~2, leading to a decrease in stellar mass by a
similar factor by z~2. Both samples undergo violent disc instability (VDI) and
form giant clumps of masses 10^7-10^9M_s at a similar rate, though RP
significantly reduces the number of long-lived clumps. When RP is (not)
included, clumps with circular velocity <40(20)km/s, baryonic surface density
<200(100)M_s/pc^2 and baryonic mass <10^{8.2}(10^{7.3})M_s are short-lived,
disrupted in a few free-fall times. The more massive and dense clumps survive
and migrate toward the disc centre over a few disc orbital times. In the RP
simulations, the distribution of clump masses and star-formation rates (SFRs)
normalized to their host disc is very similar at all redshifts. They exhibit a
truncated power-law with a slope slightly shallower than -2. Short-lived clumps
preferentially have young stellar ages, low masses, high gas fractions and
specific SFRs (sSFR), and they tend to populate the outer disc. The sSFR of
massive, long-lived clumps declines with age as they migrate towards the disc
centre, producing gradients in mass, stellar age, gas fraction, sSFR and
metallicity that distinguish them from short-lived clumps. Ex situ mergers make
up ~37% of the mass in clumps and ~29% of the SFR. They are more massive and
with older stellar ages than the in situ clumps, especially near the disc edge.
Roughly half the galaxies at redshifts z=4-1 are clumpy over a wide range of
stellar mass, with clumps accounting for ~3-30% of the SFR but ~0.1-3% of the
stellar mass.
