The Impact of Supermassive Black Hole Feedback on Star Formation in Galaxies

Abstract

Observations have revealed the significant growth of a population of 'quiescent' galaxies with reduced star formation activity since z ~ 2. Mounting evidence from across the electromagnetic spectrum indicates that supermassive black hole feedback plays a critical role in suppressing the ability of gas to cool and condense to form fuel for star formation in massive galaxies. However, the link between the galaxy-scale suppression of star formation and relatively small-scale black hole physics is not well understood. In order to explore this link, my dissertation work focuses on the question: How can observable correlations between black holes and the properties of their host galaxies reveal the underlying mechanism behind quiescence? In my work, I find that the latest version of the L-Galaxies semi-analytic model features a black hole mass threshold above which galaxies are quiescent due to the heating rate from black hole feedback overcoming the cooling rate of the gaseous atmosphere (Terrazas et al., 2016a). I find similar behavior in the real universe using a diverse sample of 91 local galaxies with dynamical black hole masses (Terrazas et al., 2016b, 2017). In particular, galaxies with more massive black holes at a given stellar mass exhibit progressively less star formation. Using five galaxy formation models, I find that variations between implementations of black hole growth, accretion physics, and feedback prescriptions predict systematically different joint distributions of stellar mass, black hole mass, and star formation rate. Only models that continuously suppress the cooling of gas onto the galaxy via low accretion rate black hole feedback are able to qualitatively reproduce the trends seen in the observations at z = 0. When quantitatively comparing the distributions of stellar mass, black hole mass, and star formation rate, none of the models agree with one another or with the observations. This largely results from differences in how black hole feedback affects gas as well as differences in how black hole and stellar mass couple to one another. These results indicate the importance of taking into account the scattered relationship between black hole and stellar mass and its dependence on star formation activity in order to adequately model black holes and the relationship to their host galaxies.