The outer halos of elliptical galaxies

The outer halos of galaxies are known to store vital information
about the formation history and merger-induced evolution of their
central galaxies, since the relaxation timescales are much larger
than in the innermost parts and thus the memory of the events is
conserved over a long period. This information provides fundamental
insights into the processes of mass growth and morphological
changes, broadening our understanding of the different mechanisms
of structure formation. Additionally, the radius regime where the
stellar component starts to dominate over the dark matter component
is the perfect place to study the interplay between dark matter and
stars. This interaction between the collisionless components of a
galaxy, although much slower than the gas-induced processes,
significantly alters the appearance of a galaxy in the long term. A
better understanding of those processes can help to shed light on
the dark sides of the galaxies. In this work, we use the combined
strength of idealized high-resolution simulations of individual
galaxies and large cosmological simulations to unveil some of the
information encoded in the outer halos of galaxies. The high
resolution simulations allow us to disentangle the impact of
selected physics on the formation and evolution of galaxies in
particular, while the large cosmological simulations provide a
statistically meaningful sample of galaxies covering a large range
in masses and environments. The first part of this thesis focuses
on the interplay between dark matter and stars, revealing that both
parts actually do interact through their common potential by
re-ordering into a stable state where the total halo is isothermal
and its density distribution follows a $\rho \propto r^{-2}$
profile. The gas, which dissipates energy and sinks towards the
center on much shorter timescales, disturbs this process, forcing
the total halo into a more compact state with approximately $\rho
\propto r^{-3}$. Therefore, as long as gas is present, the
collisionless attractor state can not be reached, but every dry
merger evolves the system towards it. This is also apparent by the
fact that more evolved halos have higher central dark matter
fractions and smaller amounts of stars formed in situ, and that the
slopes are generally steeper at high redshifts. We conclude that
the equilibrium attractor state of dry merging systems provides a
new test case for $\Lambda$CDM and prove that, if $\Lambda$CDM is
correct, the dark matter and the stars do communicate through their
common gravitational potential. The second part of this thesis
deals with the information provided by the stellar halo. We show
that the radial density profiles of all stellar halos have a
universal shape which can be described by a curved exponential,
independent of the morphology of their central galaxy. The strength
of the curvature appears to be an indication for the amount of
merging a galaxy suffered, since the stellar halo mostly grows
through merging as the cold gas density in the halo region is much
too low to cause a significant amount of star formation at all
redshifts. With such a universal shape at hand, it is possible to
study the deviations from this shape to learn about the details of
the stellar accretion history of a galaxy, since different types of
events leave distinct signatures. We suggest that more emphasis on
the understanding of those different signatures is needed in the
future to fully exploit the rich information contained in the outer
halos, to learn more about the accretion driven but also the
secular evolution of galaxies.