Inside the Galactic Centre: Astronomers Map a Black Hole Ecosystem in Space and Time

At the heart of the Milky Way, some 26,000 light-years from Earth, lies the closest galactic nucleus we can study in detail: a crowded, violent and endlessly instructive region where a supermassive black hole weighing more than four million Suns binds together dense stellar populations, streams of multiphase gas and the relentless churn of galactic evolution. That region, the Galactic Centre, was the focus of the International Astronomical Union’s Symposium 405, hosted in Brno, Czech Republic, where astronomers from around the world gathered to consolidate a striking shift in perspective. Rather than treating the central black hole, the nuclear star cluster and the surrounding gas reservoirs as separate objects of study, the symposium framed them as a single connected ecosystem, evolving together across spatial scales from the event horizon out to the circumnuclear disk, and across time from the star-forming episodes of tens of millions of years ago to the flare-driven echoes still rippling through the interstellar medium today.

The observational foundation of this ecosystem view rests on decades of precision astrometry. Near-infrared monitoring of the innermost parsec, most notably by the GRAVITY instrument on the European Southern Observatory’s Very Large Telescope Interferometer, has tracked stars on bound orbits around the compact radio source Sagittarius A star, delivering the most convincing dynamical evidence that this object is indeed a supermassive black hole as described by general relativity. The S-cluster of stars, dominated by the luminous star S2 on its sixteen-year elliptical orbit, has been used to test relativistic effects including gravitational redshift and the Schwarzschild precession, while also pinning down the distance to the Galactic Centre to within a few percent. These measurements transform the central parsec into a precision laboratory where the interplay between stellar dynamics, black hole mass and relativistic gravity can be examined against exact predictions rather than qualitative expectations.

Yet the S-cluster presents one of the field’s most persistent puzzles: the so-called paradox of youth. The central parsec contains massive, luminous B-type and Wolf-Rayet stars whose lifetimes are measured in millions of years, far too short for them to have migrated from their presumed birth sites in the surrounding disk through standard dynamical friction. At the Brno symposium, participants revisited candidate solutions, including in-situ star formation within the dense accretion disk fragments of the past, tidal disruption of binary stars passing close to the black hole which leaves one captured star in a tight orbit, and exchange interactions in which the black hole swaps into a hard massive binary. Each mechanism leaves a different fingerprint in the distribution of orbital eccentricities and inclinations, and the latest modeling of the S-cluster’s phase-space structure suggests that no single channel explains every observed orbit, hinting at a layered formation history stretching back many millions of years.

Beyond the S-cluster lies the nuclear star cluster, a dense concentration of roughly ten million solar masses packed within about ten parsecs of the black hole. New analyses presented at the meeting explored how this cluster’s complex structure, with young stars preferentially rotating in a disk-like configuration at larger radii and older populations dominating closer in, encodes the history of gas inflow episodes triggered by the galactic bar. The nuclear cluster also serves as a gravitational anchor for less massive objects, and speakers examined the fate of stellar-mass black holes, neutron stars and white dwarfs that segregate toward the centre through mass segregation. Their predicted number densities, potentially tens of thousands of stellar remnants within the central parsec, carry consequences for gravitational-wave predictions and for the rates of tidal disruption events, in which stars venturing too close to Sagittarius A star are torn apart and briefly outshine much of the surrounding galaxy.

Gas dynamics formed the second pillar of the symposium. The circumnuclear disk, a ring of dense molecular material at radii of a few parsecs, regulates the flow of gas toward the central engine and episodically feeds, or starves, both accretion onto the black hole and star formation in the central molecular zone. Recent work on the ionized and neutral gas kinematics within the central parsec, including the mini-spiral structures that thread hot plasma through the cavity inside the circumnuclear disk, illustrated how inflow is inefficient and turbulent, with only a small fraction of the supplied material ever reaching the accretion flow. Sagittarius A star today accretes at a rate roughly a billion times below the Eddington limit, producing the faint, radiatively inefficient glow observed by near-infrared polarimetry and by the Event Horizon Telescope, which resolved the ring-like shadow of the black hole’s photon emission region in 2022.

That quiescent present contrasts dramatically with a far more active past, and the evidence is written in the largest structures of the Galactic Centre region. The Fermi bubbles, two gamma-ray-emitting lobes extending tens of thousands of light-years above and below the Galactic plane, together with the softer X-ray counterpart bubbles detected by the eROSITA instrument, testify to an energetic event several million years ago, plausibly a phase of rapid accretion onto Sagittarius A star or an intense nuclear starburst. Meeting discussions emphasized how the spectral gradients across these bubbles constrain the timing, energy budget and particle acceleration mechanisms of the outburst, with estimates of total injected energy in the range of tens of millions of supernova equivalents, sufficient to reshape the thermal history of the entire Galactic halo.

On smaller scales, X-ray observations of light echoes provide a time-lapse record of more recent activity. Reflections of X-ray photons from past flares, scattered by dense molecular clouds such as the famous MC2 complex near the Sagittarius B region, have been mapped as moving fronts of fluorescing iron, indicating that the central engine brightened substantially within the last few centuries. Modeling of these reverberation signals, combined with polarization measurements that can distinguish forward-scattered from back-scattered radiation, suggests flare luminosities that transiently approached a significant fraction of the Eddington limit, a striking reversal of the present-day faintness. Speakers noted that continuous monitoring of these echoes offers an essentially forensic technique for reconstructing the accretion history of a low-luminosity galactic nucleus, a technique now being extended to nearby external galaxies where analogous echoes betray dormant black holes in action.

The Galactic Centre also functions as the Rosetta stone for galactic nuclei everywhere. Because the Milky Way’s central black hole is roughly ten thousand times less massive than those powering the most luminous quasars, phenomena observed at different scales and timescales in active galaxies can be scaled and tested locally. Symposium sessions drew explicit connections between the scaling of accretion flows, the physics of relativistic jets, the coupling between black hole feedback and star formation, and the secular evolution of gas driven inward by galactic bars. Comparative studies of nearby low-luminosity nuclei, combined with the Milky Way’s uniquely resolvable environment, allow astronomers to trace how a galactic nucleus transitions between quiescence and activity, and how feedback from the central engine regulates the fuel supply in a self-limiting loop that shapes the growth of galaxies over cosmic time.

The meeting also spotlighted the computational and observational infrastructure driving the field forward. General relativistic magnetohydrodynamic simulations of the accretion flow around Sagittarius A star, radiative transfer modeling of its polarization variability, and N-body models of the nuclear star cluster’s formation now operate at resolutions and statistical sophistication that can be directly confronted with GRAVITY, the Event Horizon Telescope, ALMA, Chandra, XRISM and the upcoming ELT datasets. High-cadence monitoring campaigns of the black hole’s infrared and submillimeter flares are converging on a picture in which magnetic reconnection and orbiting plasma instabilities generate the observed variability, linking microphysical processes near the horizon to macroscopic structures visible across the electromagnetic spectrum. As these instruments mature over the coming decade, the Galactic Centre ecosystem that the Brno symposium so comprehensively surveyed will be tested not as a collection of separate puzzles, but as a single evolutionary system whose past activity, present faintness and future episodes of renewed feeding can be read directly from the stars, gas and echoes it leaves behind.

Subject of Research: The Milky Way Galactic Centre as a laboratory for studying supermassive black holes, stellar dynamics and multiphase gas in galactic nuclei.

Article Title: Traversing the Galactic Centre in space and time

Article References: Zajaček, M., Czerny, B., Mondek, M., Mitra, S., Labaj, M., Ondro, T., Janík, J., & Dušek, J. (2026). Traversing the Galactic Centre in space and time. Nature Astronomy. https://doi.org/10.1038/s41550-026-02958-8

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02958-8

Keywords: Galactic Centre, Sagittarius A*, supermassive black hole, nuclear star cluster, GRAVITY, stellar dynamics, circumnuclear disk, Fermi bubbles, X-ray light echoes, IAU Symposium 405, black hole accretion, paradox of youth