This image shows Hercules A, a massive galaxy at the center of the Hercules A cluster of galaxies. It combines optical data from Hubble (color image), X-ray data from Chandra (purple overlay), and radio data from the Very Large Array (blue overlay). This image highlights the complex interplay between the central galaxy, the radio jets from its supermassive black hole, and the X-ray-bright intracluster medium. Credit: X-ray: NASA/CXC/SAO, Optical: NASA/STScI, Radio: NSF/NRAO/VLA)

In the centers of galaxy groups and clusters, the hot diffuse gas is dense enough that it ought to cool on timescales of tens to hundreds of millions of years, funneling cool gas toward the center of the cluster. This phenomenon, known as a “cooling flow”, should lead to massive starbursts at the centers of clusters, which we generally do not observe. It is now understood that energetic jets launched from the supermassive black hole at the center of each cluster are likely responsible for heating the surrounding medium and preventing cooling flows from forming (see picture to the right). These jets appear to be acting as a thermostat for the hot gas on large scales, moderating cooling with incredible precision.

There are many open questions remaining in how active galactic nuclei (AGN) moderate cooling in clusters. Our group is working on addressing many of these questions, including:

• When did black hole jets first begin to moderate cooling in cluster cores? Was there an early epoch of runaway cooling before the feedback/cooling cycle was set up?
• What physical conditions dictate the development of thermal instabilities in the hot medium, and trigger condensation, accretion, and subsequent episodes of feedback?
• How does gas cycle in the cores of clusters, and how does it fuel both star formation and black hole accretion?
• What are the details of the heating process? Is gas heated from low temperature, or kept hot? Or both?

Below are some relevant papers and links to ongoing collaborations/efforts.

• “Observational Evidence for Enhanced Black Hole Accretion in Giant Elliptical Galaxies” McDonald M., et al., 2021, ApJ, 908, 85
• “Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster” McDonald M., et al., 2019, ApJ, 885, 63
• “Revisiting the Cooling Flow Problem in Galaxies, Groups, and Clusters of Galaxies” McDonald M., et al., 2018, ApJ, 858, 45
• “A massive, cooling-flow-induced starburst in the core of a luminous cluster of galaxies” McDonald M., Bayliss M., et al., 2012, Nature, 488, 349