Suppressing star formation in quiescent galaxies with supermassive black hole winds

TL;DR

This study provides evidence that low-luminosity active galactic nuclei in quiescent galaxies generate winds capable of heating gas and suppressing star formation, explaining the maintenance of galaxy quiescence.

Contribution

It demonstrates that typical quiescent galaxies host centrally-driven winds powered by low-level black hole activity, a mechanism previously unconfirmed observationally.

Findings

Centrally-driven winds are common in quiescent galaxies with low-luminosity nuclei.

The energy from these nuclei can heat ambient gas, preventing star formation.

Winds contain enough mechanical energy to influence galaxy evolution.

Abstract

Quiescent galaxies with little or no ongoing star formation dominate the galaxy population above $M_{*}\sim 2 \times 10^{10}~M_{\odot}$, where their numbers have increased by a factor of $\sim25$ since $z\sim2$. Once star formation is initially shut down, perhaps during the quasar phase of rapid accretion onto a supermassive black hole, an unknown mechanism must remove or heat subsequently accreted gas from stellar mass loss or mergers that would otherwise cool to form stars. Energy output from a black hole accreting at a low rate has been proposed, but observational evidence for this in the form of expanding hot gas shells is indirect and limited to radio galaxies at the centers of clusters, which are too rare to explain the vast majority of the quiescent population. Here we report bisymmetric emission features co-aligned with strong ionized gas velocity gradients from which we infer the presence of centrally-driven winds in typical quiescent galaxies that host low-luminosity active nuclei. These galaxies are surprisingly common, accounting for as much as $10\%$ of the population at $M_* \sim 2 \times 10^{10}~ M_{\odot}$. In a prototypical example, we calculate that the energy input from the galaxy's low-level active nucleus is capable of driving the observed wind, which contains sufficient mechanical energy to heat ambient, cooler gas (also detected) and thereby suppress star formation.