NIHAO XXI: The emergence of Low Surface Brightness galaxies
  The existence of galaxies with a surface brightness $\mu$ lower than the
night sky has been known since three decades. Yet, their formation mechanism
and emergence within a $\rm\Lambda CDM$ universe has remained largely
undetermined. For the first time, we investigated the origin of Low Surface
Brightness (LSB) galaxies with M$_{\star}$$\sim$10$^{9.5-10}$M$_{\odot}$, which
we are able to reproduce within hydrodynamical cosmological simulations from
the NIHAO suite. The simulated and observed LSBs share similar properties,
having large HI reservoir, extended star formation histories and effective
radii, low S\'{e}rsic index and slowly rising rotation curves. The formation
mechanism of these objects is explored: simulated LSBs form as a result of
co-planar co-rotating mergers and aligned accretion of gas at early times,
while perpendicular mergers and mis-aligned gas accretion result in higher
$\mu$ galaxies by $z$=0. The larger the merger, the stronger the correlation
between merger orbital configuration and final $\mu$. While the halo spin
parameter is consistently high in simulated LSB galaxies, the impact of halo
concentration, feedback-driven gas outflows and merger time only plays a
minor-to-no role in determining $\mu$. Interestingly, the formation scenario of
such `classical' LSBs differs from the one of less massive,
M$_{\star}$$\sim$10$^{7-9}$M$_{\odot}$, Ultra-Diffuse Galaxies, the latter
resulting from the effects of SNae driven gas outflows: a M$_{\star}$ of
$\sim$10$^9$M$_{\odot}$ thus represents the transition regime between a
feedback-dominated to an angular momentum-dominated formation scenario in the
LSB realm. Observational predictions are offered regarding spatially resolved
star formation rates through LSB discs: these, together with upcoming surveys,
can be used to verify the proposed emergence scenario of LSB galaxies.
