Giant black holes could sometimes contribute to the birth of new stars instead of merely slowing it down.
At the center of many galaxies lies a supermassive black hole. When it attracts a great deal of matter, its surroundings become extremely bright and also release energy into the galaxy. Astronomers have long sought to determine whether this activity prevents gas from forming stars or whether, under certain conditions, it can instead promote their birth.
The galaxy NGC 1386 and its central region. Star formation appears in red, gas ionized by the active nucleus in blue, and shocks in yellow.
Credit: Peixin Zhu / Center for Astrophysics | Harvard & Smithsonian
Peixin Zhu and colleagues studied nine nearby galaxies containing this type of active black hole. They used MUSE, an instrument installed on the Very Large Telescope in Chile. It makes it possible to examine light coming from different regions of a galaxy and thus distinguish several phenomena that overlap in conventional images.
The same arrangement appears in all nine galaxies. Around the center, the researchers identify rings or arcs where new stars are forming. They lie about 2,600 to 20,000 light-years from the galactic center. Closer to the black hole, regions also appear where the gas is undergoing violent disturbances.
To understand their origin, we need to look at what matter does before disappearing into the black hole. Some of it forms a very hot disk and produces intense radiation. Matter and energy can also move outward in the form of jets or winds. When these outflows encounter the galaxy's gas, they can cause shocks.
These shocks were found in all nine observed galaxies. X-ray observations carried out with the Chandra space telescope support this interpretation.
The link with the birth of stars nevertheless calls for caution. The observed rings may be produced by the galaxies' own dynamics, particularly by their central structure. The study therefore indicates that black hole activity could contribute to star formation, but it does not demonstrate that it creates these rings on its own.
This distinction matters because the opposite effect is well known: energy released near a black hole can heat or disperse the gas that stars need to form. The new observations indicate that the outcome is not always the same. Depending on where and how the energy encounters the gas, it could also compress it and create conditions favorable to the formation of new stars.
The sample remains limited to nine nearby Seyfert galaxies, selected in particular because detailed observations were available. More galaxies will therefore need to be examined to determine whether this arrangement is common elsewhere. The researchers now have a method for more precisely separating the light from young stars, the active black hole, and shocks.