Black Hole Growth and Feedback in Isolated ROMULUS25 Dwarf Galaxies

We investigate the effects of massive black hole growth on the structural evolution of dwarf galaxies within the ROMULUS25 cosmological hydrodynamical simulation. We study a sample of 205 central, isolated dwarf galaxies with stellar masses ${M}_{\mathrm{star}}\lt {10}^{10}{M}_{\odot }$ and a central BH. We find that the local M_BH-M_star relation exhibits a high degree of scatter below M_star < 10¹⁰M_⊙, which we use to classify BHs as overmassive or undermassive relative to their host M_star. Within isolated dwarf galaxies, only 8% of undermassive BHs ever undergo a BH merger, while 95% of overmassive BHs grow through a mixture of BH mergers and accretion. We find that isolated dwarf galaxies that host overmassive BHs also follow different evolutionary tracks relative to their undermassive BH counterparts, building up their stars and dark matter earlier and experiencing star formation suppression starting around z = 2. By z = 0.05, overmassive BH hosts above M_star > 10⁹M_⊙ are more likely to exhibit lower central stellar mass density, lower H I gas content, and lower star formation rates than their undermassive BH counterparts. Our results suggest that overmassive BHs in isolated galaxies above M_star > 10⁹M_⊙ are capable of driving feedback, in many cases suppressing and even quenching star formation by late times.