An Enhanced Massive Black Hole Occupation Fraction Predicted in Cluster Dwarf Galaxies

The occupation fraction of massive black holes (MBHs) in low mass galaxies offers interesting insights into initial black hole seeding mechanisms and their mass assembly history, though disentangling these two effects remains challenging. Using the {\sc Romulus} cosmological simulations we examine the impact of environment on the occupation fraction of MBHs in low mass galaxies. Unlike most modern cosmological simulations, {\sc Romulus} seeds MBHs based on local gas properties, selecting dense (n>3 cm^-3, 15 times the threshold for star formation), pristine (Z<3e-4 Zsol), and rapidly collapsing regions in the early Universe as sites to host MBHs without assuming anything about MBH occupation as a function of galaxy stellar mass, or halo mass, {\it a priori}. The simulations predict that dwarf galaxies with M<1e9 Msol in cluster environments are ~2 times more likely to host a MBH compared to those in the field. The predicted occupation fractions are remarkably consistent with those of nuclear star clusters. Across cluster and field environments, dwarf galaxies with earlier formation times are more likely to host a MBH. Thus, while the MBH occupation function is similar between cluster and field environments at high redshift (z>3), a difference arises as late-forming dwarfs – which do not exist in the cluster environment – begin to dominate in the field and pull the MBH occupation fraction down for low mass galaxies. Additionally, prior to in-fall some cluster dwarfs are similar to progenitors of massive, isolated galaxies, indicating that they might have grown to higher masses had they not been impeded by the cluster environment. While the population of MBHs in dwarf galaxies is already widely understood to be important for understanding MBH formation, this work demonstrates that environmental dependence is important to consider as future observations search for low mass black holes in dwarf galaxies.