The effect of metal enrichment and galactic winds on galaxy formation in cosmological zoom simulations

We investigate the differential effects of metal cooling and galactic stellar winds on the cosmological formation of individual galaxies with three sets of cosmological, hydrodynamical zoom simulations of 45 haloes in the mass range 1011 <Mhalo < 1013 M⊙. Models including both galactic winds and metal cooling (i) suppress early star formation at z ≳ 1 and predict reasonable star formation histories for galaxies in present-day haloes of ≲1012 M⊙, (ii) produce galaxies with high cold gas fractions (30-60 per cent) at high redshift, (iii) significantly reduce the galaxy formation efficiencies for haloes (Mhalo ≲ 1012 M⊙) at all redshifts in overall good agreement with recent observational data and constraints from abundance matching, (iv) result in high-redshift galaxies with reduced circular velocities in agreement with the observed Tully-Fisher relation at z ˜ 2 and (v) significantly increase the sizes of low-mass galaxies (Mstellar ≲ 3 × 1010 M⊙) at high redshift resulting in a weak size evolution - a trend in agreement with observations. However, the low-redshift (z <0.5) star formation rates of more massive galaxies are higher than observed (up to 10 times). No tested model predicts the observed size evolution for low-mass and high-mass galaxies simultaneously. Without winds the sizes of low-mass galaxies evolve rapidly, and with winds the size evolution of massive galaxies is too weak. Due to the delayed onset of star formation in the wind models, the metal enrichment of gas and stars is delayed and agrees well with observational constraints. Metal cooling and stellar winds are both found to increase the ratio of in situ formed to accreted stars - the relative importance of dissipative versus dissipationless assembly. For halo masses below ˜1012 M⊙, this is mainly caused by less stellar accretion and compares well to predictions from semi-analytical models and but differs from abundance matching models as the in situ formed fractions of stellar mass are still too low in the simulations. For higher masses, however, the fraction of in situ stars is overpredicted due to the unrealistically high star formation rates at low redshifts.