Nine Galaxies With Feeding Black Holes Share the Same Pattern of Gas

Astronomers have mapped the gas in nine nearby galaxies whose central black holes are actively pulling in matter, and found the same arrangement in every one: rings of new stars well out from the middle, cones of gas lit up by the black hole, and fast shock waves at the very center.

Whether a feeding black hole encourages or shuts down star formation around it is unsettled. The authors say that is partly because new stars, the black hole's own radiation and shocked gas heated by collisions all make a galaxy's center glow in similar ways. Writing in The Astrophysical Journal on Sept. 14, Peixin Zhu of the Center for Astrophysics, Harvard and Smithsonian, and colleagues report that central shocks are a common feature of these galaxies, and that studies of how black holes affect their host galaxies have to account for them.

The team used MUSE, an instrument on the Very Large Telescope that records a spectrum at every point in an image, and sorted the light with a purpose-built diagnostic. In all nine galaxies it found rings or arcs of star formation at about 0.8 to 6 kiloparsecs from the center, roughly 2,600 to 20,000 light-years. The same galaxies show cones of gas lit up by the black hole's radiation, reaching out to comparable scales, and regions dominated by fast shocks at the center, often running at right angles to the cones. Deep X-ray images from the Chandra observatory support the same division, the authors report.

The paper traces the rings to the galaxies themselves rather than to their black holes. Zhu and colleagues report that the rings are consistent with bar-driven resonances (gas gathering at particular distances under the pull of a bar of stars across the galaxy's middle), "although positive AGN feedback may also contribute." Positive feedback means a black hole encouraging star formation rather than suppressing it. The central fast shocks, the authors write, are broadly consistent with the black hole's jets plowing into surrounding gas.