Galaxies grow their bulges and black holes in diverse ways
  Galaxies with Milky Way-like stellar masses have a wide range of bulge and
black hole masses; in turn, these correlate with other properties such as star
formation history. While many processes may drive bulge formation, major and
minor mergers are expected to play a crucial role. Stellar halos offer a novel
and robust measurement of galactic merger history; cosmologically-motivated
models predict that mergers with larger satellites produce more massive, higher
metallicity stellar halos, reproducing the recently-observed stellar halo
metallicity-mass relation. We quantify the relationship between stellar halo
mass and bulge or black hole prominence using a sample of eighteen Milky
Way-mass galaxies with newly-available measurements of (or limits on) stellar
halo properties. There is an order of magnitude range in bulge mass, and two
orders of magnitude in black hole mass, at a given stellar halo mass (or,
equivalently, merger history). Galaxies with low mass bulges show a wide range
of quiet merger histories, implying formation mechanisms that do not require
intense merging activity. Galaxies with massive 'classical' bulges and central
black holes also show a wide range of merger histories. While three of these
galaxies have massive stellar halos consistent with a merger origin, two do not
- merging appears to have had little impact in making these two massive
'classical' bulges. Such galaxies may be ideal laboratories to study massive
bulge formation through pathways such as early gas-rich accretion, violent disk
instabilities or misaligned infall of gas throughout cosmic time.
