NIHAO XXVI: Nature versus nurture, the Star Formation Main Sequence and
  the origin of its scatter
  We investigate how the NIHAO galaxies match the observed star formation main
sequence (SFMS) and what the origin of its scatter is. The NIHAO galaxies
reproduce the SFMS and generally agree with observations, but the slope is
about unity and thus significantly larger than observed values. This is because
observed galaxies at large stellar masses, although still being part of the
SFMS, are already influenced by quenching. This partial suppression of star
formation by AGN feedback leads to lower star formation rates and therefore to
lower observed slopes. We confirm that including the effects of AGN in our
galaxies leads to slopes in agreement with observations. We find the deviation
of a galaxy from the SFMS is correlated with its $z=0$ dark matter halo
concentration and thus with its halo formation time. This means galaxies with a
higher-than-average star formation rate (SFR) form later and vice versa. We
explain this apparent correlation with the SFR by re-interpreting galaxies that
lie above the SFMS (higher-than-average SFR) as lying to the left of the SFMS
(lower-than-average stellar mass) and vice versa. Thus later forming haloes
have a lower-than-average stellar mass, this is simply because they have had
less-than-average time to form stars, and vice versa. It is thus the nature,
i.e. how and when these galaxies form, that sets the path of a galaxy in the
SFR versus stellar mass plane.
