Angular momentum evolution of bulge stars in disc galaxies in NIHAO
  We study the origin of bulge stars and their angular momentum (AM) evolution
in 10 spiral galaxies with baryonic masses above $10^{10}$M$_\odot$ in the
NIHAO galaxy formation simulations. The simulated galaxies are in good
agreement with observations of the relation between specific AM and mass of the
baryonic component and the stellar bulge-to-total ratio ($B/T$). We divide the
star particles at $z=0$ into disc and bulge components using a hybrid
photometric/kinematic decomposition method that identifies all central mass
above an exponential disc profile as the `bulge'. By tracking the bulge star
particles back in time, we find that on average 95\% of the bulge stars formed
{\it in situ}, 3\% formed {\it ex situ} in satellites of the same halo, and
only 2\% formed {\it ex situ} in external galaxies. The evolution of the AM
distribution of the bulge stars paints an interesting picture: the higher the
final $B/T$ ratio, the more the specific AM remains preserved during the bulge
formation. In all cases, bulge stars migrate significantly towards the central
region, reducing their average galactocentric radius by roughly a factor 2,
independently of the final $B/T$ value. However, in the higher $B/T$
($\gtrsim0.2$) objects, the velocity of the bulge stars increases and the AM of
the bulge is almost conserved, whereas at lower $B/T$ values, the velocity of
the bulge stars decreases and the AM of bulge reduces. The correlation between
the evolution of the AM and $B/T$ suggests that bulge and disc formation are
closely linked and cannot be treated as independent processes.
