  The dominant feedback mechanism in low mass haloes is usually assumed to take
the form of massive stars exploding as supernovae (SNe). We perform very high
resolution cosmological zoom-in simulations of five dwarf galaxies to z = 4
with our mechanical SN feedback model. This delivers the correct amount of
momentum corresponding to the stage of the SN remnant evolution resolved, and
has been shown to lead to realistic dwarf properties in isolated simulations.
We find that in 4 out of our 5 simulated cosmological dwarfs, SN feedback has
insufficient impact resulting in excessive stellar masses, extremely compact
sizes and central super-solar stellar metallicities. The failure of the SN
feedback in our dwarfs is physical in nature within our model and is the result
of the build up of very dense gas in the early universe due to mergers and
cosmic inflows prior to the first SN occurring. We demonstrate that our results
are insensitive to resolution (provided that it is high enough), details of the
(spatially uniform) UV background and reasonable alterations within our star
formation prescription. We therefore conclude that the ability of SNe to
regulate dwarf galaxy properties is dependent on other physical processes, such
as turbulent pressure support, clustering and runaway of SN progenitors and
other sources of stellar feedback.
