Harvard Astronomers Mapped Nine Galaxies With Feeding Black Holes and Found Rings of Newborn Stars 2,600 to 20,000 Light-Years Out. The Shock Waves Always Run Sideways to the Black Hole's Outflow.

Supermassive black holes that are actively swallowing gas may help their host galaxies make new stars rather than simply shutting star formation down, according to a study of nine nearby galaxies published Monday in The Astrophysical Journal. The work, led by Peixin Zhu, a graduate student at the Center for Astrophysics | Harvard & Smithsonian, used the MUSE instrument on the European Southern Observatory's Very Large Telescope to map the light from each galaxy's core and found a recurring set of features: rings or arcs of star formation, cones of gas lit up by the black hole's radiation, and fast shocks running perpendicular to those cones.

The galaxies are all Seyferts, meaning their central black holes are accreting material and shining as active galactic nuclei. The standard picture of "AGN feedback" holds that the energy pouring out of an active nucleus heats or blows away the gas a galaxy needs to form stars. The new maps complicate that story. "Once we resolved them, we could see that they not only accrete things, but they also eject things," Zhu said. "The injection and accretion are linked with each other."

The team's key tool is a three-dimensional diagnostic that assigns each pixel of the MUSE data to one of three excitation sources: young stars, radiation from the black hole, or shocks, which occur when high-speed outflows slam into the surrounding interstellar medium. Across the sample, star-forming rings or arcs appeared at distances of roughly 0.8 to 6 kiloparsecs from the galactic center, about 2,600 to 20,000 light-years. Ionized cones of black-hole radiation extended out of the galaxies' disks, while shock-dominated regions near the center often ran at right angles to those bicones.

"The most interesting phenomenon about shocks is that they always go perpendicular to where the black hole's injected outflows go," Zhu said. "It is very common, and we see it consistently appearing across all nine galaxies." Zhu says the pattern is broadly consistent with jets from the nucleus plowing into the interstellar medium, though winds driven by the black hole may also contribute, especially in galaxies with weaker jets. Chandra X-ray observations independently supported the interpretation.

The theoretical models used to interpret the spectra were built by Zhu with her adviser Lisa Kewley, director of the Center for Astrophysics, and Ralph Sutherland of the Australian National University. "We're seeing that black holes are not just consuming material at the centers of galaxies, but they're actively reshaping their surroundings," Kewley said. "This work helps us understand a complex feedback cycle that plays an important role in galaxy evolution."

The paper, "Common Excitation Patterns of Star Formation, Active Galactic Nuclei, and Shocks in Seyfert Galaxies," argues that accretion, outflow and interaction with the host form a single cycle rather than a one-way process of suppression. Separating the three sources of excitation cleanly, the authors say, gives astronomers a sharper way to test how that cycle shapes galaxies over time. One of the galaxies in the sample, NGC 1386, shows the full pattern: star formation in a ring, black-hole radiation in a cone, and shocks lying crosswise between them.