Bulge Formation via Mergers in Cosmological Simulations
  The latest generation of cosmological simulations are on the verge of being
able to resolve the structure of bulges for the first time. Hence, we review
the current state of bulge formation in cosmological simulations, and discuss
open questions that can be addressed in the near future by simulators, with a
particular focus on merger-driven bulge growth. Galaxy mergers have long been
assumed to produce classical bulges in disk galaxies. Under this
bulge-formation model, though, the high rates of mergers in Cold Dark Matter
galaxy formation theory predict many more classical bulges than are observed.
Furthermore, simulations of galaxy formation continue to generally produce too
massive of bulges. Feedback offers a promising avenue for reducing
merger-driven bulge growth by maintaining high gas fractions in galaxies and
ejecting low-angular momentum gas driven to the centers of galaxies. After
reviewing the results of relevant research that has been published to date, we
use cosmological simulations to explore the ability of feedback to reduce or
even prevent bulge growth during mergers. In dwarf galaxies, mergers actually
reduce the central concentration of galaxies as the induced burst of star
formation drives out low angular momentum material. This result shows the
potential for feedback to reduce central mass growth. However, we also
demonstrate that it is very difficult for current stellar feedback models to
reproduce the small bulges observed in more massive disk galaxies like the
Milky Way. We argue that feedback models need to be improved, or an additional
source of feedback such as AGN is necessary to generate the required outflows.
