The Relative Role of Galaxy Mergers and Cosmic Flows in Feeding Black
  Holes
  Using a set of zoomed-in cosmological simulations of high-redshift
progenitors of massive galaxies, we isolate and trace the history of gas that
is accreted by central supermassive black holes. We determine the origins of
the accreted gas, in terms of whether it entered the galaxy during a merger
event or was smoothly accreted. Furthermore, we designate whether the smoothly
accreted gas is accreted via a cold flow or is shocked upon entry into the
halo. For moderate-mass (10^6 - 10^7 Msun) black holes at z ~ 4, there is a
preference to accrete cold flow gas than gas of shocked or merger origin.
However, this result is a consequence of the fact that the entire galaxy has a
higher fraction of gas from cold flows. In general, each black hole tends to
accrete the same fractions of smooth- and merger-accreted gas as is contained
in its host galaxy, suggesting that once gas enters a halo it becomes
well-mixed, and its origins are erased. We find that the angular momentum of
the gas upon halo entry is a more important factor; black holes preferentially
accrete gas that had low angular momentum when it entered the galaxy,
regardless of whether it was accreted smoothly or through mergers.
