Beasts at the Heart of Galaxies: Beck Smethurst on Supermassive Black Holes and Galaxy Evolution

In this conversation, Dr Becky Smethurst explains how every galaxy hosts a supermassive black hole and how the black hole mass correlates with the galaxy’s stars and dynamics. The discussion covers how mergers and internal feeding channels shape galaxies, the role of accretion disks and jets, and the surprising existence of massive black holes in seemingly undisturbed disk galaxies. They also touch on JWST discoveries of disk galaxies far in the early universe and the implications for black hole growth, the dark matter puzzle, and the future of astronomical surveys. The talk blends cutting edge science with big questions about the fate of galaxies and the cosmos.

Introduction: black holes and galaxy coevolution

The interview begins by framing a central idea in modern astrophysics: almost every galaxy harbors a supermassive black hole at its center, and the black hole’s mass is tightly linked to the galaxy’s stellar mass, central star density, and the stars’ kinematic state. Becky Smethurst emphasizes that galaxies and their central black holes appear to grow together, a process scientists describe as co evolution. The discussion then moves beyond simple pictures of a calm, stable disk to a dynamic history in which mergers can scramble stellar orbits, redistribute angular momentum, and potentially feed the black hole as gas is driven toward the center.

Growth mechanisms: mergers versus internal feeding

Traditionally, galaxy mergers were considered a primary driver of black hole growth: during a merger, gas inflows and the coalescence of the two central black holes could bolster accretion and star formation, reshaping the galaxy into a bulge dominated, chaotic sphere. The host galaxy’s morphology would reflect this tumult, with a central bulge and less prominent spiral structure. However, observational and simulation evidence now indicates mergers are not the sole channel for building supermassive black holes. Smethurst discusses how isolated, left-alone disk galaxies with pristine spiral structures can still host very massive black holes, sometimes exceeding a million solar masses and even reaching billions, challenging the idea that only mergers build the heaviest black holes.

Internal fueling: bars, spiral arms, and the cosmic web

The channel for feeding a black hole inside an isolated disk galaxy is multifaceted. Spiral arms can funnel gas toward the center, while a central bar structure, a long straight feature within many galaxies, can act as an efficient accelerator of gas inflow. Some models also explore gas accretion from the cosmic web that channels material along filaments into galaxies. The relative efficiency of these processes, and how they might dominate at different cosmic times, remains an active area of research. Observers seek observational signatures to identify which mechanism dominates in different galaxies and epochs.

Quasars, accretion disks, and feedback

When material spirals toward a black hole, it forms an accretion disk that heats up and emits prodigious radiation. Quasars are extreme examples of this process, often outshining their host galaxies. The physics of accretion is not limitless; processes such as radiation pressure, magnetohydrodynamics, and self-gravity impose limits on how fast a black hole can grow and how the surrounding gas behaves. Smethurst describes how accretion disks can launch winds and jets, which can interact with the galaxy's gas reservoir, potentially heating or expelling gas, and thus suppressing or triggering star formation. The complex interplay between black hole growth and galaxy evolution—sometimes quenching star formation while at other times stimulating it along shocks—remains a major focus of current research.

Black hole mass scales and the JWST revolution

Questions about how big black holes can get in the local universe lead to discussions of limits on growth and the role of spin in setting those limits. Observationally, Ton 618 is a famously massive black hole, and the idea that black holes may be spinning can push the theoretical mass limit higher. The James Webb Space Telescope (JWST) has opened a new window on the early universe, revealing disk-like galaxies at epochs where mergers were expected to be common, and exposing very massive black holes forming earlier than models predicted. These observations compel theorists to revisit growth channels, including the possibility of direct collapse black holes that bypass the star formation stage altogether.

Dark matter, gravity, and the broader cosmological context

The conversation also touches on dark matter and the ways in which we infer its presence—from gravitational lensing to simulations of galaxy formation. There is an acknowledgment that while Einstein's general relativity remains the best description of gravity given current data, some researchers explore alternative theories. Dark matter remains a central pillar of the cosmological model, but the evidence continues to evolve with observations that could refine or revise our understanding of gravity and structure formation in the universe.

The black hole zoo and the mass gap

Beyond supermassive black holes, there exists a spectrum of black holes ranging from stellar mass to hypothetical intermediate mass holes. The space between roughly 1000 and 100,000 solar masses remains a so-called mass gap, with little robust evidence for black holes in that range. Primordial black holes are also discussed as a potential component of dark matter, should they exist, and their potential observational signatures—such as microlensing events or Hawking radiation—are explored as exciting, yet unresolved possibilities.

Observational futures: Rubin, Euclid, and multi‑messenger astronomy

Future surveys and observatories promise to transform our understanding of black holes and galaxy evolution. The Rubin Observatory will provide time-domain data that captures variability in accretion disks and jet activity across vast numbers of galaxies. Euclid will help resolve galaxy morphologies and trace evolution across cosmic time. The continued development of IFU data, integral field spectroscopy, enables spatially resolved studies of gas flows and star formation. Together with JWST and other facilities, these instruments will illuminate how black holes grow, how they influence their hosts, and how galaxies transition from blue, star-forming systems to red, quiescent ones.

Public outreach and the role of science communication

Smethurst shares her experience communicating science through YouTube and public outreach, highlighting how accessible explanations help cultivate curiosity and support for fundamental research. The conversation reflects on the social and technological benefits that arise from astrophysical research, from imaging techniques to data processing, and the broader impact on education and culture.

The Milky Way’s destiny and the human perspective

Looking to the future of our own galaxy, the anticipated Milky Way Andromeda merger in about five billion years is discussed. The fate of the Milky Way’s central black hole, the Sun's position in the evolving galaxy, and possible high-energy radiation from black hole activity during the merger all become part of a grand cosmic narrative that situates our own existence within a dynamic universe.

Conclusion

Ultimately the interview frames black holes as extreme laboratories for testing gravity, quantum processes, and the physics of accretion and feedback. The pursuit of answers to these questions advances not only our understanding of cosmic history but also the tools, technologies, and scientific culture that shape our world.